Phytopathogenic Toxins from Fungi: An Overview - Advances in

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Phytopathogenic Toxins from Fungi: An Overview H. H . LUKE U.S. Department of Agriculture, Plant Pathology Dept., University of Florida, Gainesville, Fla. 32611 R. H . BIGGS Fruit Crops Department, University of Florida, Gainesville, Fla. 32611 Phytopathogenic toxins that are produced by plant pathogens are classified into two types: nonspecific, those that affect a greater number of plant species than the pathogen that produces them; and specific, those that affect only the same hosts as the pathogen. Evidence indicating that plant pathogens induce disease by toxigenic action has been established by using specific toxins produced in vitro. It is difficult to isolate toxins from infected plants and therefore it is hard to show that toxins produced in vivo incite disease. Most phytopathogenic toxins are produced by species of Alternaria or Bipolaris (formerly Helminthosporium). Similarities in the molecular structure of mycotoxins and some phytopathogenic toxins indicate that the latter may be toxic also to animals. Moreover some phytotoxins occur in higher concentrations in infected plants than mycotoxins.

Phytopathogenic toxins are produced by plant pathogens, induce disease development, and may be considered as pathogenic agents (2). Many fungal metabolites are toxic to plants but do not initiate disease development. Toxins that do not initiate disease or that have a minor influence on disease are referred to as phytotoxins (2). Phytotoxins are usually produced during the later part of the disease syndrome. Some phytotoxins that are produced in vitro have not been found in vivo. The 296 Rodricks; Mycotoxins and Other Fungal Related Food Problems Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

13.

LUXE AND BIGGS

Phytopathogenic

297

Toxins

i d e a t h a t p l a n t pathogens p r o d u c e toxins t h a t c a u s e p l a n t disease o r i g i n a t e d a b o u t a c e n t u r y ago ( I ) . C u r r e n t l y o n l y 10 k n o w n toxins i n i t i a t e disease, a n d t h e i r p r e c i s e m o d e o f a c t i o n is n o t k n o w n . O n e o f t h e m a j o r reasons f o r l i m i t e d progress o n this t o p i c is t h e l a c k o f k n o w l e d g e o f t h e c h e m i c a l s t r u c t u r e o f these toxins. T h e exact s t r u c t u r e o f o n l y a f e w toxins is k n o w n , a n d u n f o r t u n a t e l y m o s t o f those t h a t h a v e b e e n d e s c r i b e d c h e m i c a l l y e i t h e r d o n o t cause disease o r h a v e o n l y a m i n o r i n f l u e n c e o n


disease d e v e l o p m e n t .

W e h o p e i n this d i s c o u r s e t o a l e r t chemists t o t h e

n e e d to d e t e r m i n e t h e structures o f p h y t o p a t h o g e n i c toxins. M o s t toxins t h a t i n c i t e diseases o f h i g h e r p l a n t s a r e p r o d u c e d b y species o f Bipolaris

( f o r m e r l y Helminthosporium)

o r Alternaria.

Some

toxins p r o d u c e d b y these t w o g e n e r a a r e specific a n d s o m e a r e n o n specific.

A f e w f u n g i other t h a n species o f Bipolaris

o r Alternaria

also

p r o d u c e toxins. T h e r e f o r e this discourse is s u b d i v i d e d i n t o six c a t e g o r i e s : specific a n d n o n s p e c i f i c Bipolaris

toxins, specific a n d n o n s p e c i f i c

Alter-

naria t o x i n s , a n d o t h e r toxins. I n t h e s i x t h s e c t i o n w e discuss s i m i l a r i t i e s b e t w e e n m y c o t o x i n s a n d p h y t o p a t h o g e n i c toxins.

S e v e r a l toxins f r o m

each category are discussed. Specific Bipolaris Toxins Specific toxins a r e those t h a t affect t h e same hosts as t h e p a t h o g e n t h a t p r o d u c e s t h e m . S p e c i f i c toxins h a v e b e e n t e r m e d p a t h o t o x i n s ( 2 ) a n d host-specific toxins ( 3 ) . T h e s e terms a r e s y n o n y m o u s b e c a u s e t h e y d e n o t e a substance p r o d u c e d b y a p l a n t p a t h o g e n t h a t initiates disease. T h e m o s t c o n c l u s i v e e v i d e n c e t h a t toxins p r o d u c e d b y p l a n t p a t h o g e n i c f u n g i i n c i t e disease has b e e n o b t a i n e d w i t h specific toxins ( 2 , 3 2 ) . W e b e l i e v e t h a t t h e r e a r e m a n y specific toxins t h a t o c c u r o n l y in vivo, b u t i t is difficult t o extract t h e m f r o m t h e i n f e c t e d host.

Therefore t h e most

c o n c l u s i v e e v i d e n c e t h a t toxins i n i t i a t e p l a n t disease has b e e n o b t a i n e d b y u s i n g specific toxins p r o d u c e d i n a r t i f i c i a l m e d i a . Bipolaris victoriae. T h i s f u n g u s ( f o r m e r l y Helminthosporium toriae)

vic-

causes a d e v a s t a t i n g disease o f o a t c u l t i v a r s t h a t h a v e t h e V i c t o r i a

gene f o r c r o w n r u s t resistance (4).

T h e f u n g u s causes necrosis a t t h e

base o f t h e stem a n d s t r i p i n g o r r e d d e n i n g o f leaves. L e a f s t r i p i n g a n d discoloration

progress

upward from

the lower

leaves.

Plants

show

b l i g h t i n g a n d d i s c o l o r a t i o n a t n o d a l areas a n d severe l o d g i n g at t h e base a n d u p p e r nodes.

Because the pathogen could not be isolated from the

d i s c o l o r e d leaves, i t w a s suggested t h a t this s y m p t o m r e s u l t e d f r o m a t o x i n t h a t o r i g i n a t e d at t h e site o f i n f e c t i o n ( 5 ) . T h i s o b s e r v a t i o n w a s c o n f i r m e d a n d e x t e n d e d to s h o w t h a t c u l t u r e

filtrates

c u l t i v a r s t h a t w e r e s u s c e p t i b l e to t h e p a t h o g e n .

O a t c u l t i v a r s resistant

were toxic to

to t h e p a t h o g e n w e r e u n i f o r m l y resistant t o t h e t o x i n ( 6 ) . F u r t h e r s t u d y

Rodricks; Mycotoxins and Other Fungal Related Food Problems Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

298

MYCOTOXINS

r e v e a l e d that n o n p a t h o g e n i c cultures of the fungus d i d not p r o d u c e

the

t o x i n . Differences i n d e g r e e of p a t h o g e n i c i t y a m o n g cultures w e r e p o s i t i v e l y c o r r e l a t e d w i t h differences i n t o x i n p r o d u c t i o n , a n d t o x i n p r o d u c t i o n was d i r e c t l y r e l a t e d to t h e g r o w t h r a t e of t h e f u n g u s (7).

T h u s this

t o x i n , g i v e n the t r i v i a l n a m e v i c t o r i n ( 8 ) , w a s e s t a b l i s h e d as a specific t o x i n that is the c a u s a l agent of V i c t o r i a b l i g h t of oats. T h e intact victorin molecule

does n o t r e a c t w i t h n i n h y d r i n ,

but


h y d r o l y s i s y i e l d s t w o c o m p o u n d s t h a t d o react w i t h n i n h y d r i n . O n e w a s r e p o r t e d to b e a t r i c y c l i c s e c o n d a r y a m i n e ( v i c t o x i n i n e , e m p i r i c a l f o r m u l a of

C17H29NO), a n d the other w a s

(9).

a p e p t i d e c o m p o s e d of five a m i n o acids

V i c t o x i n i n e w a s s a i d to b e the t o x i c p r i n c i p l e , a n d t h e s m a l l p e p t i d e

supposedly conveyed

specificity to t h e i n t a c t m o l e c u l e

(10).

Another

r e p o r t h o w e v e r stated t h a t v i c t o x i n i n e w a s n o t the t o x i c m o i e t y of v i c t o r i n (3).

I n a later d i s c u s s i o n P r i n g l e a p p e a r e d r e l u c t a n t to c o n c e d e that a

p e p t i d e l i n k a g e occurs i n the i n t a c t t o x i n m o l e c u l e ( I I ) . t h e c h e m i s t r y of t h e i n t a c t t o x i n has n o t b e e n

Unfortunately

studied

successfully.

E m p h a s i s was p l a c e d o n v i c t o x i n i n e w h i c h m a y n o t b e i n v o l v e d i n t h e disease caused b y B. victoriae.

A s a r e s u l t , the c h e m i s t r y of v i c t o x i n i n e

is not presented i n this discourse.

A reliable molecular structure for

v i c t o r i n w o u l d c e r t a i n l y b e u s e f u l to d e t e r m i n e the m o d e of a c t i o n of this t o x i n . A l t h o u g h the precise m o d e of a c t i o n of v i c t o r i n is n o t k n o w n , v a r i o u s forms of c i r c u m s t a n t i a l e v i d e n c e suggest t h a t the t o x i n causes a n i r r e v e r s i b l e p h y s i o l o g i c a l m a l f u n c t i o n of the p l a s m a m e m b r a n e

(12,

13).

T h i s c o n c l u s i o n is b a s e d o n five different lines of e v i d e n c e : ( a ) S m a l l q u a n t i t i e s of t o x i n c a u s e d electrolyte leakage w i t h i n 5 m i n after treatment, a n d t h e r e was a s t r o n g p o s i t i v e c o r r e l a t i o n b e t w e e n electrolyte leakage a n d t o x i n c o n c e n t r a t i o n (14). ( b ) Electron micrographs showed that victorin caused partial sepa r a t i o n of the p l a s m a m e m b r a n e f r o m t h e c e l l w a l l r e s u l t i n g i n b l i s t e r - l i k e formations. S u c h s e p a r a t i o n d i s r u p t e d the o s m o t i c p r o p e r t i e s of the m e m b r a n e a n d c a u s e d the loss of c e l l u l a r c o m p o n e n t s a n d c e l l t u r g o r (12.15). ( c ) W h e n c e l l w a l l s w e r e r e m o v e d , v i c t o r i n c a u s e d b u r s t i n g of protoplasts f r o m susceptible c u l t i v a r s , b u t protoplasts f r o m resistant c u l t i v a r s seemed to b u r s t at a s l o w e r r a t e (16). ( d ) T r e a t m e n t w i t h t o x i n c a u s e d l e a k a g e of p h o s p h o r y l a t e d h e x oses f r o m susceptible tissue. P h o s p h o r y l a t e d sugars d o not pass t h r o u g h m e m b r a n e s that f u n c t i o n n o r m a l l y , a n d t h e r e f o r e v i c t o r i n appears to d i s r u p t the p h y s i o l o g i c a l f u n c t i o n of t h e p l a s m a m e m b r a n e (13). ( e ) V i c t o r i n i n d u c e d l e a k a g e of R b and C a f r o m susceptible b u t not f r o m resistant root tissue. I f v i c t o r i n a n d c a l c i u m c o m p e t e for n e g a t i v e l y c h a r g e d sites o n t h e p l a s m a m e m b r a n e , t h e r e m o v a l of c a l c i u m f r o m n e g a t i v e l y c h a r g e d sites m a y r e s u l t i n a r e p u l s i o n b e t w e e n n e g a t i v e charges o n t h e m e m b r a n e a n d the c e l l w a l l (17). Repulsion 8 6

4 5


13.

L U X E A N D BIGGS

Phytopathogenic

299

Toxins

forces w o u l d r e s u l t i n b l i s t e r - l i k e f o r m a t i o n s o b s e r v e d w i t h t h e electron microscope ( 1 2 ) . T h e exact m a n n e r b y w h i c h v i c t o r i n causes d y s f u n c t i o n o f t h e p l a s m a m e m b r a n e is n o t k n o w n , n o r is i t k n o w n i f t h e a c t i o n o n t h e p l a s m a m e m b r a n e is a p r i m a r y o r s e c o n d a r y one. T h e s p e e d ( 5 m i n ) w i t h w h i c h v i c t o r i n d i s r u p t s t h e f u n c t i o n o f t h e p l a s m a m e m b r a n e suggests t h a t t h e a c t i o n is d i r e c t , b u t a firm c o n c l u s i o n to this effect s h o u l d b e s u p p o r t e d Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

b y direct evidence.

T h e unstable nature of the purified toxin has pre

v e n t e d successful l a b e l i n g experiments that c o u l d s h o w t h e site o f a c t i o n of v i c t o r i n . Bipolaris zeicola. T h i s f u n g u s

(formerly

Helminthosporium

car-

honum) causes a leaf b l o t c h o n c e r t a i n i n b r e d lines o f c o r n (Zea

mays).

I n t h e e a r l y stages of disease, w a t e r s o a k i n g o f leaf tissues is p r o n o u n c e d . L e s i o n s b e c o m e e l o n g a t e d a n d d e v e l o p a y e l l o w i s h - b r o w n color.

The

m a r g i n s are i r r e g u l a r , a n d i n a d v a n c e d stages t h e lesions s h o w a definite z o n a t e p a t t e r n (IS, 19). A l t h o u g h t h e s t r u c t u r a l c o n f i g u r a t i o n o f t h e B. zeicola t o x i n h a s n o t b e e n d e t e r m i n e d , its e m p i r i c a l f o r m u l a is c o n s i d e r e d to b e C32H50N6O10. T h i s f o r m u l a is c o n s i d e r e d a p p r o x i m a t e b e c a u s e of t h e u n s t a b l e n a t u r e of t h e m o l e c u l e .

D a t a from i o n exclusion columns indicated that the

t o x i n has a m o l e c u l a r w e i g h t o f less t h a n 700 ( I I ) . H o w e v e r t h e u n r e l i a b l e n a t u r e o f d a t a o b t a i n e d f r o m i o n e x c l u s i o n c o l u m n s suggests t h a t m o r e s t u d y o f t h e m o l e c u l a r properties o f this t o x i n is n e e d e d .

I R spec

t r o m e t r y r e v e a l e d that t h e B. zeicola t o x i n is a s u b s t i t u t e d p o l y a m i d .

A

c o m p l e t e a c i d h y d r o l y s i s r e s u l t e d i n five different n i n h y d r i n - r e a c t i n g p r o d u c t s , a l l o f w h i c h a p p e a r e d to b e o n a l i p h a t i c carbons.

Because

n o n e o f these c o m p o u n d s r e a c t e d w i t h p - n i t r o b e n z y l c h l o r i d e , n o n e a p p e a r to h a v e m e t h y l a m i n o groups

(11).

T h e intact toxin d i d n o t react w i t h

n i n h y d r i n o r w i t h 2,4-dinitrofluorobenzene. d o u b l e b o n d s , o r b o t h (11).

T h i s t o x i n contains 11 r i n g s ,

T h u s either t h e i n t a c t t o x i n h a s r i n g s t r u c

t u r e , o r t h e t e r m i n a l a m i n o groups are a c y l a t e d . L o w levels o f t h e t o x i n ( 4 5 / * g / m l ) i n h i b i t root e l o n g a t i o n o f other n o n h o s t p l a n t s . I f the t o x i n c o n c e n t r a t i o n is l o w e n o u g h , c u l t i v a r s suscep t i b l e to B. zeicola a r e m o r e sensitive to t h e t o x i n t h a n c u l t i v a r s t h a t a r e resistant to t h e f u n g u s

(20).

Therefore

the toxin m a y b e

considered

specific i f t h e p r o p e r c o n c e n t r a t i o n is u s e d . A l t h o u g h t h e m o d e o f a c t i o n of t h e B. zeicola t o x i n is n o t k n o w n , some i n f o r m a t i o n o n p h y s i o l o g i c a l changes i n d u c e d b y this t o x i n is a v a i l able.

T h e t o x i n s t i m u l a t e s r e s p i r a t i o n , i n d u c e s e l e c t r o l y t e leakage, a n d

increases d a r k fixation o f C 0 . 2

is decreased.

B u t amino acid and uridine incorporation

T h e u p t a k e o f B. zeicola t o x i n seems to r e q u i r e energy ( 2 0 ) .

Bipolaris may dis. T h i s f u n g u s ( f o r m e r l y Helminthosporium

maydis)

causes a severe l e a f b l i g h t o f c o r n c u l t i v a r s that h a v e t h e T e x a s m a l e -


300

MYCOTOXINS

sterile c y t o p l a s m a n d a m i n o r leaf s p o t o n c u l t i v a r s that d o not h a v e this c y t o p l a s m . T h e s y m p t o m s of the disease are c h a r a c t e r i z e d b y s m a l l , l i g h t - g r e e n , w a t e r - s o a k e d spots.

A f t e r five to six days the s y m p t o m s on

resistant a n d s u s c e p t i b l e c u l t i v a r s differ d i s t i n c t l y . L e s i o n s o n

leaves

of resistant c u l t i v a r s are s m a l l ( 2 - 5 m m ) a n d h a v e b r o w n - t o - t a n n e c r o t i c centers c i r c u m s c r i b e d b y r e d d i s h b o r d e r s a n d c h l o r o t i c m a r g i n s . B l o t c h e s o n leaves of s u s c e p t i b l e c u l t i v a r s r a n g e f r o m 10 to 4 0 m m a n d h a v e l i g h t Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

t a n n e c r o t i c centers w i t h o u t d e f i n e d p i g m e n t e d b o r d e r s .

A f t e r 10 days

blotches o n s u s c e p t i b l e c u l t i v a r s h a v e d a r k b r o w n b o r d e r s b u t l i t t l e or n o chlorosis ( 18,19

).

B . may dis p r o d u c e s f o u r toxins in vitro r e f e r r e d to as toxins I , I I , I I I , a n d I V . T h r e e of these h a v e b e e n i s o l a t e d f r o m leaf tissue of s u s c e p t i b l e c o r n plants i n f e c t e d w i t h the f u n g u s .

T h e t o x i c i t y a n d specificity of a l l

f o u r are s i m i l a r . T o x i n s I , I I , a n d I I I , w h i c h c a n b e b i o l o g i c a l l y d e r i v e d f r o m m e v a l o n i c a c i d , h a v e b e e n r e p o r t e d to b e t w o terpenoids a n d a terpenoid glycoside (21).

T o x i n s I a n d I I are h i g h l y s a t u r a t e d a n d g i v e

p r o t o n m a g n e t i c resonance s p e c t r a s i m i l a r to spectra e m i t t e d b y t e t r a c y c l i c t r i t e r p e n o i d s . T o x i n I I I m a y b e a g l y c o s i d e of a c o m p o u n d s i m i l a r to toxins I a n d I I , b u t t o x i n I I I contains a l a r g e r n u m b e r of h y d r o x y l g r o u p s . T h e m o l e c u l a r w e i g h t of t o x i n I I I , a b o u t 162, is greater t h a n the w e i g h t s of toxins I a n d I I . T o x i n I I I has properties s i m i l a r to those of a r e d u c i n g hexose sugar, i n d i c a t i n g t h a t t o x i n I I I is a glycoside.

Data

seem to i n d i c a t e t h a t the c o m p l e t e l y s a t u r a t e d c a r b o n skeleton is s i m i l a r to t e t r a c y c l i c t r i t e r p e n o i d s or t h e p e n t a c y c l i c t r i t e r p e n o i d s ( 2 1 ) . T h e p r o d u c t i o n of t o x i n b y B . may dis was first r e p o r t e d b y O r s e n i g o and Sina (22). a n d Q u i m o (23)

T o x i n s r e p o r t e d b y t h e m a n d those r e p o r t e d b y Q u i m o w e r e s l i g h t l y t o x i c a n d nonspecific.

ever s h o w e d t h a t B . maydis

L a t e r reports h o w

p r o d u c e d a specific t o x i n (24, 2 5 ) .

researchers i n d i c a t e d that toxins w e r e p r o d u c e d in vivo a n d in

These vitro.

Isolated m i t o c h o n d r i a f r o m resistant c u l t i v a r s w e r e not affected B . maydis

by

t o x i n , b u t m i t o c h o n d r i a f r o m Texas m a l e - s t e r i l e c u l t i v a r s w e r e

severely affected b y i t (26).

A later r e p o r t suggested that the m i t o c h o n

d r i a w e r e not the p r i m a r y site of a c t i o n of the t o x i n (27). c l u s i o n was b a s e d o n t w o observations.

This con

First, toxin treatment i n h i b i t e d

r o o t g r o w t h i n 30 m i n , b u t 2 h r of t o x i n t r e a t m e n t w a s r e q u i r e d t o i n h i b i t r e s p i r a t i o n . S e c o n d , p r o l o n g e d treatments w i t h h i g h t o x i n c o n centrations w e r e r e q u i r e d to decrease c e l l u l a r A T P . H o w e v e r a c u r r e n t r e p o r t i n d i c a t e d t h a t the m i t o c h o n d r i a f r o m s u s c e p t i b l e c u l t i v a r s are p e r h a p s the site of a c t i o n of this t o x i n (28).

T h e s e researchers also r e

p o r t e d that the close a g r e e m e n t b e t w e e n s u s c e p t i b i l i t y to B . maydis

and

t h e s u p p r e s s i o n of r e s p i r a t o r y c o n t r o l of i s o l a t e d m i t o c h o n d r i a i n d i c a t e d that either m e a s u r e m e n t c o u l d b e u s e d to p r e d i c t the other (28). c h o n d r i a i s o l a t e d f r o m c u l t i v a r s resistant to B . maydis

Mito

race Τ are n o t


13.

LUKE

affected

Phytopathogenic

A N D BIGGS

b y t o x i n treatment.

T h i s fact raises some q u e s t i o n

m i t o c h o n d r i a affect disease d e v e l o p m e n t m a y n o t react in vitro as t h e y d o in toxin produced

by

on

how

b e c a u s e resistant c u l t i v a r s are

not i m m u n e to the fungus or t h e t o x i n ( 2 9 ) . The

301

Toxins

H o w e v e r , i s o l a t e d organelles

vivo.

B . maydis

causes electrolyte

leakage

of

c u l t i v a r s susceptible to the f u n g u s , b u t resistant c u l t i v a r s also leak e l e c trolytes w h e n h i g h concentrations

of

the t o x i n are u s e d

This

(30).


o b s e r v a t i o n is consistent w i t h the r e a c t i o n of t h e fungus a n d the t o x i n . A later s t u d y w i t h t o x i n o n excised roots d i d n o t i n d i c a t e a l a r g e or r a p i d i o n leakage.

T h e w o r k e r s r e p o r t e d that r o o t g r o w t h w a s i n h i b i t e d i n

30 m i n after t o x i n treatment. T h i s w a s the earliest effect of the t o x i n t h a t c o u l d be detected

(31).

R o o t s d o not l e a k electrolytes

as r a p i d l y as

leaves, a n d therefore root tissue is not a d e q u a t e for p e r m e a b i l i t y studies. I n another s t u d y these researchers c o u l d n o t detect a n y d a m a g e to m e m branes of s u s c e p t i b l e c u l t i v a r s t r e a t e d w i t h the t o x i n (31). searchers h o w e v e r suggested

Other re

that toxins m a y cause p h y s i o l o g i c a l

dis

r u p t i o n s of m e m b r a n e s or t h a t the p h y s i c a l d a m a g e is b e l o w the r e s o l u t i o n of the e l e c t r o n m i c r o s c o p e

( 1 2 , 32).

A r n t z e n et a l . (31)

d e t e r m i n e the site of a c t i o n of t h e B. maydis

could

toxin, but they

not

suggested

t h a t n e i t h e r the m i t o c h o n d r i a n o r t h e p l a s m a m e m b r a n e w a s i n v o l v e d . T h e site a n d m o d e of a c t i o n of the B. maydis r e s o l v e f o r t w o reasons.

toxins h a v e b e e n difficult to

F i r s t , one o r m o r e of the f o u r toxins t h a t t h e

f u n g u s p r o d u c e s m a y act as a nonspecific t o x i n . S e c o n d , n u c l e a r genes m o d i f y the c y t o p l a s m i c a l l y - i n h e r i t e d r e a c t i o n to the t o x i n a n d the f u n g u s (29).

This modification

may

e x p l a i n t h e l a c k of

agreement

v a r i o u s scientists w o r k i n g o n the m o d e of a c t i o n of the B . maydis G r a c e n (29)

among toxins.

s p e c u l a t e d that the site of a c t i o n of this t o x i n is a c o m p o n e n t

t h a t is a s t r u c t u r a l u n i t of m e m b r a n e s .

T h i s f u n g u s appears to i n i t i a t e

disease i n a w i d e v a r i e t y of genotypes, a n d the t o x i n amplifies a n a b n o r m a l i t y i n t h e s t r u c t u r e of m e m b r a n e s i n c u l t i v a r s w i t h t h e T e x a s m a l e sterile c y t o p l a s m . Bipolaris

sac chart.

T h i s fungus

(formerly

Helminthosporium

chart ) causes eyespot disease of c e r t a i n clones of sugarcane.

sac-

Early symp

toms o n leaves consist of e l o n g a t e d lesions w i t h r e d centers c i r c u m s c r i b e d b y n a r r o w c h l o r o t i c m a r g i n s . A f e w days after i n f e c t i o n r e d d i s h - b r o w n streaks m a y l e n g t h e n to 8 c m L e e first suggested

(33).

t h a t a t o x i n w a s i n v o l v e d i n the disease

(34).

T h e absolute s t r u c t u r e of the specific t o x i n p r o d u c e d b y B . sacchari

is n o t

k n o w n ; h o w e v e r , m u c h is k n o w n a b o u t the c h e m i s t r y of this t o x i n w h i c h was n a m e d helminthosporoside s i d e ) (35).

(2-hydroxycyclopropyl-a-D-galactopyrano-

T h e l o c a t i o n of the h y d r o x y l g r o u p o n the c y c l o p r o p a n e

ring

i n d i c a t e d t h a t positions 2 a n d 3 w e r e i d e n t i c a l . T h e c o n c l u s i o n t h a t g a l a c tose is the g l y c o n e p o r t i o n of t h e t o x i n w a s b a s e d o n

chromatography


302

MYCOTOXINS

after a c i d h y d r o l y s i s a n d the i n c o r p o r a t i o n of g a l a c t o s e - l - C

into the

14

g l y c o n e p o r t i o n of the t o x i n .

N M R spectral data indicated that

a g l y c o n e p o r t i o n of t h e t o x i n is 2 - h y d r o x y c y c l o p r o p a n e .

a h y d r o x y l group on the aglycone was confirmed b y detecting as a n e n d p r o d u c t of a c i d h y d r o l y s i s

the

T h e p r e s e n c e of acrolein

(35).

D e t a i l s of the m o d e of a c t i o n of h e l m i n t h o s p o r o s i d e w e r e e s t a b l i s h e d


i n a series of p a p e r s p u b l i s h e d b y S t r o b e l a n d associates. ments i n d i c a t e d t h a t clones s u s c e p t i b l e to B. sacchari

T h e first e x p e r i had a

membrane

p r o t e i n that b i n d s h e l m i n t h o s p o r o s i d e w h e r e a s m e m b r a n e s f r o m resistant clones d i d n o t h a v e b i n d i n g properties.

M o r e o v e r clones that h a d a n

i n t e r m e d i a t e r e a c t i o n to t h e t o x i n also h a d a n i n t e r m e d i a t e capacity.

binding

T h e s e results w e r e c o n f i r m e d in vivo w i t h r a d i o a c t i v e t o x i n .

T h e b i n d i n g protein was isolated, a n d a molecular

weight

of

about

45,000 w a s d e t e r m i n e d . I t consisted of f o u r s u b u n i t s , e a c h w i t h a m o l e c u l a r w e i g h t of about 11,700.

T h e t w o b i n d i n g sites t h a t w e r e

a p p e a r e d to h a v e different b i n d i n g affinities

reported

(36).

L a t e r reports i n d i c a t e d that t h e p r o t e i n f r o m susceptible clones that b i n d s h e l m i n t h o s p o r o s i d e is o n the e x t e r n a l surface of the p l a s m a m e m brane (a)

(37, 3 8 ) .

This conclusion

was b a s e d

o n three k i n d s of

T h e a p p l i c a t i o n of a n t i s e r u m p r e p a r e d to m e m b r a n e s f r o m

data: suscep

t i b l e p l a n t s r e s u l t e d i n the p r o t e c t i o n of leaf tissue susceptible to t h e toxin,

(b)

T h e a p p l i c a t i o n of the a n t i s e r u m to the b i n d i n g p r o t e i n

a g g l u t i n a t e d protoplasts f r o m s u s c e p t i b l e clones,

(c)

p h a t e reacts w i t h the surface of the p l a s m a m e m b r a n e . were 8

treated w i t h p y r i d o x a l phosphate

HNaBH . 4

P y r i d o x a l phos S u g a r c a n e cells

that h a d b e e n r e d u c e d

with

B i n d i n g p r o t e i n w a s i s o l a t e d f r o m t r e a t e d tissue a n d

was

f o u n d to b e l a b e l e d suggesting that the p r o t e i n t h a t b i n d s the t o x i n is o n the surface of the p l a s m a m e m b r a n e

(39).

T h e t o x i n b i n d i n g p r o t e i n b i n d s t h e t o x i n a n d increases activity.

I n c r e a s e d A T P a s e a c t i v i t y causes a net increase i n K

ATPase +

uptake.

D i s r u p t i o n s of e n z y m e s i n t h e p l a s m a m e m b r a n e m a y cause i o n efflux to b e greater t h a n i o n i n f l u x ; thus t h e o s m o l a r i t y of the p l a s m a

membrane

is p a r t i a l l y lost. I n c r e a s e d o s m o t i c pressure or g e n e r a l loss of

membrane

i n t e g r i t y m a y cause cells to b u r s t . T h e s e suppositions n e e d t o b e s u p ported b y direct evidence showing

that membranes

from w h i c h

the

b i n d i n g protein was extracted were indeed pure plasma m e m b r a n e p r e p a rations.

Evidence showing how

the b i n d i n g of t h e t o x i n to b i n d i n g

p r o t e i n activates the p o t a s s i u m - m a g n e s i u m - A T P a s e system is also n e e d e d . A l t h o u g h w e d o n o t k n o w the precise m o d e of a c t i o n of this t o x i n , i t appears t h a t the site of a c t i o n of h e l m i n t h o s p o r o s i d e is o n the p l a s m a m e m b r a n e . T h i s a s s u m p t i o n is consistent w i t h a n e a r l i e r r e p o r t i n d i c a t i n g t h a t the site of a c t i o n of v i c t o r i n is the p l a s m a m e m b r a n e

(12).


There-

13.

LUKE

Phytopathogenic

A N D BIGGS

303

Toxins

f o r e the p l a s m a m e m b r a n e m a y b e the site of a c t i o n of m a n y

specific

toxins t h a t i n c i t e a b r u p t e l e c t r o l y t e leakage. Bipolaris

Nonspecific

Toxms

N o n s p e c i f i c toxins are toxins p r o d u c e d b y p l a n t pathogens t h a t d o not affect the same hosts as the pathogens t h a t p r o d u c e t h e m . E m p h a s i s Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

o n specific toxins has left the i m p r e s s i o n t h a t nonspecific toxins h a v e l i t t l e i m p o r t a n c e i n disease d e v e l o p m e n t .

A l t h o u g h some

nonspecific

toxins h a v e b e e n s h o w n to b e u n r e l a t e d to disease d e v e l o p m e n t , b e l i e v e that some are d i s e a s e - i n d u c i n g agents.

Sometimes,

toxins are v i t a l to the entry of the host b y t h e p a t h o g e n . toxins p r o d u c e d in vitro

we

nonspecific

M o r e o v e r some

m a y n o t s h o w specificity o r a d e q u a t e t o x i c i t y

b e c a u s e c o m p o u n d s t h a t r e g u l a t e e n t r y of the t o x i n i n t o p l a n t cells a r e not p r o d u c e d in vitro.

A l s o s o m e toxins m a y not s h o w s p e c i f i c i t y b e c a u s e

a d m i n i s t e r i n g the correct c o n c e n t r a t i o n of the t o x i n to the host is difficult. Bipolaris

T h i s f u n g u s ( f o r m e r l y Helminthosporium

oryzae.

oryzae)

causes a s e e d l i n g b l i g h t a n d l e a f b l o t c h of r i c e . S m a l l e l l i p t i c a l l e a f spots enlarge a n d h a v e r e d d i s h - b r o w n m a r g i n s w i t h g r a y centers.

During

severe e p i d e m i c s i n f e c t e d leaves d r y o u t b e f o r e plants m a t u r e .

Brown

n e c r o t i c areas result i n s h r i v e l e d kernels a n d b r o k e n p a n i c l e s . W h e n t h e disease is severe, s m a l l b r o w n lesions o c c u r o n bracts a n d seeds c a u s i n g d i s c o l o r a t i o n of the g r a i n ( 3 3 ) . T h e first t o x i n i s o l a t e d f r o m B. oryzae

was called cochiobolin

A n o t h e r g r o u p n a m e d the t o x i n o p h i o b o l i n

p o s e d t h e t r i v i a l names o p h i o b o l i n A , B , C , a n d D . sesquiterpenoid molecule

with

a n e m p i r i c a l f o r m u l a of

(41).

A later report pro

(42).

O p h i o b o h n A is a The

C25H36O4 (43).

appears to h a v e t w o d o u b l e b o n d s a n d f o u r r i n g s , a n d its

m o l c e u l a r structure has b e e n p r o p o s e d .

O p h i o b o l i n w a s t h e first of the

n e w f a m i l y of C25 terpenes to b e d i s c o v e r e d , a n d its biosynthesis is b e i n g d e t e r m i n e d . N o c o r r e l a t i o n w a s possible b e t w e e n o p h i o b o l i n s a n d other n a t u r a l l y - o c c u r r i n g c o m p o u n d s because of t h e i r p e c u l i a r structures.

The

structures of o p h i o b o l i n s A , B , C , a n d D a r e s i m i l a r . E x c e l l e n t r e v i e w s o n the s t r u c t u r e a n d biosynthesis of this u n i q u e g r o u p of c o m p o u n d s n o w a v a i l a b l e (44,

are

45).

I n 1939 i t was first r e p o r t e d t h a t B. oryzae m a y i n i t i a t e disease (46).

produces

a toxin that

A t the onset of i n f e c t i o n t h e p a t h o g e n

duces t w o or m o r e toxins t h a t are s t r u c t u r a l l y s i m i l a r to o p h i o b o l i n .

pro These

toxins k i l l host cells i n a d v a n c e of h y p h a l d e v e l o p m e n t s u g g e s t i n g t h a t analogs of o p h i o b o l i n are r e s p o n s i b l e f o r t h e p e n e t r a t i o n of t h e host b y the p a t h o g e n .

A f t e r i n v a s i o n the p a t h o g e n p r o d u c e s o p h i o b o l i n w h i c h

causes a m a l f u n c t i o n of the p o l y p h e n o l m e t a b o l i s m of t h e h o s t

(47).

A n o t h e r a s s u m p t i o n a b o u t t h e m o d e of a c t i o n of o p h i o b o l i n is t h a t the


304

MYCOTOXINS

t o x i n depresses r e g u l a t o r y

factors

synthesis i n h e a l t h y p l a n t s (48).

that m a i n t a i n the n o r m a l

b o l i n i r r e v e r s i b l y damages c y t o p l a s m i c m e m b r a n e s Bipolaris sativum)

sorokinianum.

T h i s fungus

(41).

(formerly

Helminthosporium

causes a s e e d l i n g b l i g h t a n d spot b l o t c h of b a r l e y a n d attacks

other g r a m i n e o u s species. a n d progress

inward.

D a r k b r o w n lesions o c c u r o n the

S e e d l i n g leaves

is r e t a r d e d , a n d t i l l e r i n g b e c o m e s excessive. tissue rots at or b e l o w the s o i l surface

coleoptile

b e c o m e d a r k green w i t h

lesions near the s o i l t h a t e x t e n d i n t o the leaf b l a d e . S e e d l i n g Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

enzyme

A n o t h e r w o r k e r suggested that o p h i o

dark

development

I n the later stages of disease,

(33).

A l t h o u g h t h e role of the t o x i n h e l m i n t h o s p o r a l i n p l a n t disease is d u b i o u s , m u c h is k n o w n a b o u t the c h e m i s t r y of toxins p r o d u c e d b y sorokinianum.

B.

F i r s t attempts to i d e n t i f y h e l m i n t h o s p o r a l i n d i c a t e d that

i t is a s e s q u i t e r p e n o i d w i t h a n e m p i r i c a l f o r m u l a of

C15H22O2

(49).

Its

s t r u c t u r e has b e e n d e s c r i b e d ( 4 4 ) , a n d its biosynthesis appears to i n v o l v e m e v a l o n i c a c i d (50).

Another toxin, helminthosporol, w i t h

m o t i n g p r o p e r t i e s has b e e n i s o l a t e d f r o m B. sorokinianum.

growth-pro

T h e structure

of h e l m i n t h o s p o r o l is s i m i l a r to that of h e l m i n t h o s p o r a l , b u t strains of the f u n g u s that p r o d u c e h e l m i n t h o s p o r o l do not p r o d u c e h e l m i n t h o s p o r a l (51). H e l m i n t h o s p o r o l i n h i b i t s r e s p i r a t i o n , a n d the site of a c t i o n

appears

to b e b e t w e e n f l a v o p r o t e i n d e h y d r o g e n a s e ( s ) a n d c y t o c h r o m e C . t o x i n also d i s r u p t s o x i d a t i v e p h o s p h o r y l a t i o n (52).

as a g r o w t h - p r o m o t i n g substance s i m i l a r to t h e c y t o k i n i n s Specific

Alternaria

This

H e l m i n t h o s p o r o l acts (53).

Toxins

T o x i n s p r o d u c e d b y Alternaria

citri, A. kikuchiana,

a n d A . malt h a v e

b e e n r e p o r t e d to h a v e the same host r a n g e as the pathogens t h a t p r o d u c e t h e m . L i t t l e is k n o w n a b o u t t h e toxins p r o d u c e d b y A . citri a n d A . malt, b u t toxins p r o d u c e d b y A. kikuchiana Alternaria

citri.

h a v e b e e n s t u d i e d m o r e extensively.

T h i s fungus causes a d i s t i n c t i v e d a r k b r o w n spot

w i t h a l i g h t t a n center o n y o u n g leaves a n d fruits of E m p e r o r m a n d a r i n , c a l a m o n d i n , a n d Citrus

reticulata

cv. S o v e r e i g n .

L e a v e s of

susceptible

c u l t i v a r s b e c o m e m o r e resistant as t h e y a p p r o a c h m a t u r i t y . M a n y c u l t i vars of c i t r u s are resistant t o A. citri.

W h e n the disease is severe, y o u n g

f r u i t s d r o p f r o m t h e tree w i t h i n t h r e e days after i n f e c t i o n H i s t o l o g i c a l studies s h o w e d

(54).

that A . citn d i d n o t penetrate

young

f r u i t s a n d leaves of s u s c e p t i b l e c u l t i v a r s . N e v e r t h e l e s s i n o c u l a t i o n w i t h the pathogen

resulted i n typical b r o w n

i n i t i a t e d disease.

spots i n d i c a t i n g t h a t a t o x i n

C u l t u r e filtrates f r o m p a t h o g e n i c isolates d i l u t e d 100-

f o l d c a u s e d t y p i c a l b r o w n spot s y m p t o m s . nonpathogenic

O t h e r results s h o w e d

that

strains of A. citri d i d n o t p r o d u c e t h e t o x i n . T h u s three


13.

LUXE

Phytopathogenic

A N D BIGGS

lines of e v i d e n c e suggest t h a t A. citri

305

Toxins causes disease t h r o u g h

toxigenic

a c t i o n . T h e last r e p o r t a v a i l a b l e to us w a s p u b l i s h e d i n 1966 (54), recent

and

correspondence indicated that additional w o r k w i t h this toxin

has not b e e n i n i t i a t e d ( 5 5 ) . r i e d o u t w i t h this system.

H o p e f u l l y additional research w i l l be

car

W e k n o w that the t o x i n p r o d u c e d b y A .

citri

causes disease t h r o u g h t o x i g e n i c a c t i o n a n d that o l d tissue is resistant to the t o x i n . T h e r e f o r e

genetic v a r i a b i l i t y i n the h o s t w o u l d n o t b e


p r o b l e m as i t is i n other toxin-host Alternaria

a

combinations.

T h i s fungus p r o d u c e s a s m a l l , b l a c k n e c r o t i c

kikuchiana.

s p o t s u r r o u n d e d b y a y e l l o w h a l o o n fruits a n d leaves of c e r t a i n c u l t i v a r s of Japanese pears ( 5 6 ) .

I t is h i g h l y p a t h o g e n i c

o n N i j i s s e i k i pears

does not cause disease o n E u r o p e a n a n d N o r t h A m e r i c a n pears

but

(57).

T h r e e different toxins, p h y t o a l t e r n a r i a n A , B , a n d C w e r e i s o l a t e d b y H i r o e a n d A o e (58)

f r o m the c u l t u r e filtrates of this fungus.

p h y t o a l t e r n a r i a n A affects t h e same hosts as A . kikuchiana,

Although

l i t t l e is k n o w n

a b o u t the c h e m i s t r y of these toxins. T h e p h y t o a l t e r n a r i a n s g i v e a n e g a t i v e F e h l i n g r e a c t i o n b u t express a p o s i t i v e n i n h y d r i n r e a c t i o n , i n d i c a t i n g that t h e y h a v e p e p t i d e linkages

(59).

T h e r e is l i t t l e i n f o r m a t i o n c o n c e r n i n g the m o d e of a c t i o n of these toxins. I n f o r m a t i o n r e c e n t l y r e c e i v e d i n d i c a t e d that a r b u t i n a n d c h l o r o g e n i c a c i d m a y be responsible for the b r o w n spot s y m p t o m t h a t is c h a r acteristic of the disease (60).

Another report indicated that the toxin

i n h i b i t e d β-glucosidase i n susceptible c u l t i v a r s b u t n o t i n resistant c u l t i v a r s ; thus β-glucosidase a c t i v i t y is responsible for resistance (61). of other aspects of the p h y t o a l t e r n a r i a n s w e r e r e v i e w e d b y

Details

Templeton

(62). A l t e n i n is another t o x i n that has b e e n isolated f r o m A .

kikuchiana.

L i t t l e has b e e n r e p o r t e d o n its t o x i c i t y a n d specificity. I t has b e e n i d e n t i fied as e t h y l h y d r o x y - 5 - ( l - h y d r o x y e t h y l ) - 4 - o x o t e t r a h y d r o f u r o n a t e e m p i r i c a l f o r m u l a of C H i 0 4

9

6

(63).

with an

T h i s structure w a s also s y n t h e s i z e d ,

a n d the active p o r t i o n of a l t e n i n was r e p o r t e d to b e i n the e n d i o l c a r b o x y l grouping

(64).

Alternaria

malt.

T h i s fungus causes b r o w n n e c r o t i c spots o n I n d o

a n d R a l l s a p p l e varieties, b u t the p a t h o g e n a n d toxic c u l t u r e

filtrate

do

n o t affect J o n a t h a n or M c i n t o s h varieties. A t o x i n p r o d u c e d b y t h e f u n g u s k i l l e d cells of y o u n g a p p l e leaves i n a d v a n c e of the i n v a d i n g p a t h o g e n . Moreover culture

filtrates

f r o m h i g h l y v i r u l e n t strains of A . mali

m o r e t o x i c t h a n those f r o m m o d e r a t e l y v i r u l e n t strains. C u l t u r e f r o m n o n v i r u l e n t strains w e r e n o n t o x i c .

T h e s e t w o observations

suggest t h a t A . mali causes disease b y t o x i g e n i c a c t i o n . i n d i c a t e d t h a t A . mali

also p r o d u c e d

strongly

Other evidence

a nonspecific t o x i n w h i c h

w i l t i n g a n d v e i n - b a n d i n g necrosis i n several plants

were filtrates

caused

(65).

R e c e n t papers i n d i c a t e d t h a t the specific t o x i n a l t e r n a r i o l i d e p r o -


306

MYCOTOXINS

d u c e d b y A. mali has a n e m p i r i c a l f o r m u l a of C23H31N3O6. T h e s t r u c t u r e t h a t has b e e n p r o p o s e d f o r a l t e r n a r i o l i d e i n d i c a t e d t h a t this t o x i n is a c y c l i c t r i p e p t i d e lactone.

T h e three a m i n o acids w e r e r e p o r t e d to

be

2 - h y d r o x y i s o v a l e r i c a c i d , a l a n i n e , a n d one d e s i g n a t e d as a l t e r n a m i c a c i d (66).

N o w that some c h e m i s t r y is k n o w n a b o u t this

phytopathogenic


t o x i n , studies o n its m o d e of a c t i o n s h o u l d y i e l d c o n c l u s i v e results. Nonspecific

Toxins

Alternaria

A r e c e n t r e p o r t lists 19 t o x i c c o m p o u n d s p r o d u c e d b y v a r i o u s species of Alternaria A. tenuis,

H o w e v e r o n l y a f e w toxins p r o d u c e d b y A .

(62).

a n d A . zinniae

w i l l b e discussed.

p r o d u c e d b y these species of Alternaria

solani,

T h e phytotoxic compounds

vary chemically—a carboxylic

a c i d , a c y c l i c p e n t a p e p t i d e , a n d a p e n t a - s u b s t i t u t e d benzene. Alternaria

T h i s fungus causes a d e v a s t a t i n g b l i g h t of

solani.

t o m a t o a n d potato.

S y m p t o m s first a p p e a r o n leaves as s m a l l ( 1 - 4

d a r k b r o w n spots.

L a t e r a c h l o r o t i c r i n g appears a r o u n d t h e

necrotic

spot. S o m e t i m e s leaflets next to the n e c r o t i c spot b e c o m e c h l o r o t i c . t h e disease is severe, leaflets w i t h e r a n d d r o p off

the

mm), When

(67).

A l t h o u g h this t o x i n m a y p l a y o n l y a m i n o r r o l e i n disease, its c h e m i s t r y has b e e n C2iH

3 0

O . 8

studied.

A l t e r n a r i c a c i d has a n e m p i r i c a l f o r m u l a

Its s t r u c t u r e w a s r e p o r t e d to b e

of

12-(5,6-dihydro-4-hydroxy-6-

methyl-2-oxopyran-3-yl)-4,6-dehydroxy-3-methyl-9-methylene-12-oxododec-6-ene-5-carboxylic

a c i d (68,

69).

A l t e r n a r i c a c i d has a l o w l e v e l of

t o x i c i t y , a n d i t is t o x i c to m a n y plants n o t p a r a s i t i z e d b y A . solani.

No

d e t a i l e d d a t a o n its m o d e of a c t i o n h a v e b e e n r e p o r t e d . Alternaria

tenuis.

T h i s f u n g u s causes a v a r i e g a t e d s e e d l i n g chlorosis

o n c o t t o n , c i t r u s , a n d m a n y other p l a n t s . S y m p t o m s are c h a r a c t e r i z e d b y c h l o r o t i c spots s h a r p l y d e l i n e a t e d f r o m n o r m a l green areas. C h l o r o s i s is i r r e v e r s i b l e , a n d s e e d l i n g g r o w t h is r e t a r d e d .

C o t t o n seedlings

with

m o r e t h a n 3 5 % chlorosis u s u a l l y d i e . T h e fungus appears to p r o d u c e a t o x i n w h i l e g r o w i n g o n the seed coat or s o i l refuse. i n t o the c o t y l e d o n s

T h e t o x i n diffuses

either t h r o u g h i n j u r y d u r i n g g e r m i n a t i o n or

the s e e d c o a t r u p t u r e s

when

(70).

T h e t o x i n p r o d u c e d b y A . tenuis, t e n t o x i n , has a m o l e c u l a r w e i g h t of 414.5 a n d a n e m p i r i c a l f o r m u l a of C22H30N4O4. described

Its structure has

been

as c y c l o - N - m e t h y l d e h y d r o p h e n y l a l a n y l - L - l e u c y l g l y c y l - L - 2 V -

m e t h y l a l a n y l (62).

B e c a u s e the s t r u c t u r e is c y c l i c , the i n t a c t t o x i n gives a

n e g a t i v e r e a c t i o n to n i n h y d r i n . N M R d a t a h a v e i n d i c a t e d the a m i n o a c i d s e q u e n c e . T h e o p t i c a l r o t a r y d i s p e r s i o n s p e c t r u m shows o p t i c a l a c t i v i t y . A l t h o u g h the precise m o d e o f a c t i o n of this t o x i n is not k n o w n , i t a p p e a r s t h a t t e n t o x i n i n h i b i t s c h l o r o p h y l l f o r m a t i o n i n specific tissues o f c e r t a i n p l a n t species.

O n l y t h e cotyledons a n d p r i m a r y leaves of sensitive


13.

LUKE

Phytopathogenic

A N D BIGGS

307

Toxins

species b e c o m e c h l o r o t i c w h e n seeds or v e r y y o u n g p l a n t s are t r e a t e d w i t h the t o x i n . E x c e p t i n t o m a t o a n d the c r u c i f e r s , o l d p r i m a r y leaves a r e n o t affected b y the t o x i n . M o s t d i c o t y l e d o n o u s plants are sensitive to t h e t o x i n , b u t s o r g h u m a n d crabgrass are the o n l y m o n o c o t s t h a t h a v e b e e n f o u n d to b e sensitive. T e n t o x i n does n o t i n t e r f e r e w i t h the c o n v e r s i o n of p r o t o c h l o r o p h y l l to c h l o r o p h y l l . M o r e o v e r i t appears t h a t t h i s t o x i n causes a r e d u c t i o n i n c h l o r o p h y l l synthesis w i t h i n e a c h c h l o r o p l a s t Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

r a t h e r t h a n a n o n u n i f o r m expression of t o x i c i t y a m o n g p l a s t i d s Alternaria

T h i s f u n g u s causes

zinniae.

b l i g h t of z i n n i a , sunflower, a n d m a r i g o l d s (72, o c c u r o n cotyledons, leaves, stems, a n d

(71).

a leaf spot a n d s e e d l i n g 73).

flowers.

S m a l l b r o w n spots

Initial symptoms

are

c h a r a c t e r i z e d b y a n e c r o t i c fleck s u r r o u n d e d b y a c h l o r o t i c h a l o . L a t e r spots enlarge a n d b e c o m e i r r e g u l a r a n d r e d d i s h b r o w n . P l a n t s t h a t h a v e severe stem lesions w i l t r a p i d l y e v e n t h o u g h the l e s i o n does n o t g i r d l e the stem.

T h i s r e a c t i o n seems to suggest t h a t a t o x i n is i n v o l v e d i n t h e

disease s y n d r o m e

(74).

T h e t o x i n p r o d u c e d b y A. zinniae,

z i n n i o l , is a p e n t a - s u b s t i t u t e d

b e n z e n e w i t h a n e m p i r i c a l f o r m u l a of C H i 0 . 9

has b e e n suggested

(73).

2

4

A s t r u c t u r e of z i n n i o l

Moreover two isomeric phthalides obtained

f r o m z i n n i o l h a v e b e e n u s e d to resynthesize a n d c o n f i r m its structure. T h e m o d e of a c t i o n of z i n n i o l is n o t k n o w n . I t does n o t a p p e a r to i n c i t e disease d e v e l o p m e n t , b u t i t m a y b e i n v o l v e d i n s o m e of the sec o n d a r y s y m p t o m s b e c a u s e 5 0 0 - 1 0 0 0 p p m of the t o x i n are r e q u i r e d to cause severe s y m p t o m s ; also i t is n o t specific. Other

Toxins

N u m e r o u s toxins h a v e b e e n r e p o r t e d ; therefore i t w o u l d b e u n r e a l i s t i c to i n c l u d e the r e m a i n d e r of the p h y t o p a t h o g e n i c toxins i n this section. O n l y a f e w that are k n o w n to i n i t i a t e disease a n d those t h a t h a v e b e e n r e c e n t l y d i s c o v e r e d are i n c l u d e d .

M o r e o v e r the presence of a t o x i n i n

this section does n o t i m p l y t h a t i t is r e l a t i v e l y u n i m p o r t a n t . T h e toxins d i s c u s s e d i n this section are p r o d u c e d b y Periconia, metta, a n d Periconia

Phyllosticta,

Didy-

Fusicoccum. T h i s f u n g u s is a s o i l i n h a b i t a n t t h a t i n v a d e s

circinata.

the s u b t e r r a n e a n parts of m i l o - t y p e sorghums ( 7 5 ) .

T h e f o l i a g e of d i s

eased p l a n t s turns y e l l o w , w i l t s , a n d shows t y p i c a l b l i g h t s y m p t o m s .

If

t h e i n f e c t i o n is severe, the p l a n t s c o m m o n l y b l o o m e a r l y , g r o w s t u n t e d , a n d d i e p r e m a t u r e l y (76). (77).

C u l m s a p p e a r r e d n e a r the base of the p l a n t

L e u k e l ( 7 7 ) first r e p o r t e d t h a t a t o x i n p r o d u c e d the same s y m p

toms as the p a t h o g e n

a n d t h a t t h e y h a d the same host range.

observations w e r e c o n f i r m e d i n 1961

The

(78).

A l t h o u g h t w o toxins w e r e i s o l a t e d f r o m c u l t u r e filtrate of P .


circinata,

308

MYCOTOXINS

o n l y one of these has b e e n s t u d i e d , a n d l i t t l e i n f o r m a t i o n o n its n a t u r e has b e e n r e p o r t e d . I t has a m o l e c u l a r w e i g h t of less t h a n 2000 a n d reacts p o s i t i v e l y to n i n h y d r i n .

T h e first r e p o r t i n d i c a t e d t h a t a c i d h y d r o l y s i s

y i e l d e d five a m i n o acids ( a s p a r t i c , g l u t a m i c , a l a n i n e , serine, a n d one of the leucines)

(3).

A l a t e r p a p e r r e p o r t e d f o u r a m i n o acids ( a s p a r t i c ,

g l u t a m i c , a l a n i n e , a n d s e r i n e ) (11).

A s s u m i n g t h a t l e u c i n e does n o t o c c u r

i n the m o l e c u l e , i t appears t h a t t h e t o x i n m a y b e a s m a l l p e p t i d e t h a t Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

y i e l d s 6 moles a l a n i n e , 4 moles aspartic, a n d 2 moles e a c h of g l u t a m i c a c i d a n d serine

(11).

L i m i t e d studies of the m o d e of a c t i o n of the t o x i n p r o d u c e d b y P . r e v e a l e d that the t o x i n a n d t h e fungus cause i n c r e a s e d r e s p i r a

circinata

t i o n , electrolyte leakage, a n d decreased a b i l i t y to i n c o r p o r a t e a m i n o acids a n d u r i d i n e (79).

T h e t o x i n d i d not affect the activities of m i t o c h o n d r i a

i n cell-free p r e p a r a t i o n s . T h e p h y s i o l o g i c a l d i s r u p t i o n s c a u s e d b y t o x i n p r o d u c e d b y P . circinata

the

are s i m i l a r to those c a u s e d b y v i c t o r i n t h u s

s u g g e s t i n g t h a t the site o f a c t i o n is the p l a s m a m e m b r a n e (79).

These

conclusions w e r e n o t a d e q u a t e l y s u b s t a n t i a t e d ; therefore the site of a c t i o n a n d a n e x p l a n a t i o n for host-specificity n e e d to b e d e t e r m i n e d b e f o r e the m o d e of a c t i o n c a n b e a s c e r t a i n e d . Phyllosticta

T h i s f u n g u s p r o d u c e s a y e l l o w leaf b l i g h t of

maydis.

c o r n w h i c h first appears as y e l l o w blotches o n the l o w e r leaves. expand r a p i d l y resulting i n general yellowing.

Necrotic,

Blotches

buff-colored

lesions are e l l i p t i c a l ( 7 - 1 0 X 1 5 - 2 5 m m ) r u n n i n g p a r a l l e l w i t h , b u t n o t e n t i r e l y d e l i n e a t e d b y , the veins. S e v e r e l y diseased leaves d i e a n d t u r n b r o w n ; e v e n t u a l l y a l l b u t t h e t o p m o s t leaves b e c o m e severely b l i g h t e d . L e s i o n s o n the u p p e r leaves are l o n g a n d n a r r o w a n d are u s u a l l y c o n c e n t r a t e d n e a r t h e m i d v e i n . T h e n e c r o t i c lesions of y e l l o w l e a f b l i g h t are s i m i l a r to those of s o u t h e r n leaf b l i g h t ( 8 0 ) , a n d the presence of T e x a s m a l e - s t e r i l e c y t o p l a s m g r e a t l y increases s u s c e p t i b i l i t y to P .

maydis.

N o t h i n g is k n o w n a b o u t the c h e m i s t r y of the t o x i n ( s ) p r o d u c e d b y P . maydis,

b u t electrolyte l e a k a g e , d i s r u p t i o n of m i t o c h o n d r i a l f u n c t i o n ,

a n d root i n h i b i t i o n c a u s e d b y the P . maydis c a l m a l f u n c t i o n s i n c i t e d b y Bipolaris Didymella

applanata.

t o x i n are s i m i l a r to p h y s i o l o g i

maydis

race Τ toxin (81, 82).

T h i s fungus causes a stem a n d b u d disease

of the r a s p b e r r y c h a r a c t e r i z e d b y d e f e c t i v e b u d d i n g i n t h e axils of the l e a f stalks. T h e fungus enters the stem t h r o u g h insect w o u n d s a n d causes d a r k d i s c o l o r a t i o n near t h e b u d s . D i s c o l o r e d spots b e c o m e l i g h t e r i n the s u m m e r . D a m a g e to stems k i l l s b u d s a n d severely d a m a g e s f r u i t s ( 8 3 ) . T h e t o x i n p r o d u c e d b y D. applanata

is a m o n o s a c c h a r i d e d e r i v a t i v e

o l i g o s a c c h a r i d e w i t h a m o l e c u l a r w e i g h t of 1682 db 150, a n d n o or v e r y little r e d u c i n g p o w e r .

O n e of the m o n o s a c c h a r i d e s is glucose.

Mono

saccharides w e r e f o u n d after t r i m e t h y l s i y l a t i o n . G r o u p s other t h a n m o n o -


13.

LUKE

saccharides nucleus

Phytopathogenic

A N D BIGGS

are

a t t r a c t e d to

some

309

Toxins members

of

the

monosaccharide

(S3).

A l t h o u g h the t o x i n appears

to

cause

symptoms

s i m i l a r to

those

c a u s e d b y the p a t h o g e n , one e x p e r i m e n t f a i l e d to y i e l d i n f o r m a t i o n o n its phytotoxicity (83).

N e v e r t h e l e s s these w o r k e r s i n d i c a t e d t h a t u n p u b

l i s h e d results of K e r l i n g a n d S c h i p p e r e s t a b l i s h e d that D. applanata

pro

duces a p h y t o t o x i n i n c u l t u r e t h a t causes s m a l l i n t e r v e i n a l n e c r o t i c spots Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

o n the leaves a n d sprout w i l t i n g . Fusicoccum

T h i s fungus causes a leaf b l i g h t a n d w i l t of

amygdalt.

a l m o n d a n d p e a c h trees. T h e f u n g u s enters t h r o u g h a b u d or leaf scar a n d causes, some distance f r o m t h e p o i n t of i n f e c t i o n , w i l t w i t h i n a f e w days.

E i g h t to 10 days after i n v a s i o n , g u m m a t e r i a l s o c c u r i n the b a r k

a n d x y l e m tissue next to the l e s i o n site. W h e n g u m m o s i s occurs,

the

p a t h o g e n behaves as a l o c a l p a r a s i t e a n d b e c o m e s l i m i t e d to the tissue that was first i n v a d e d . T h e n c o r k y layers that f o r m e d a r o u n d the l e s i o n cause c a n k e r f o r m a t i o n

(84).

T h e t o x i n f u s i c o c c i n p r o d u c e d b y F. amygdali, w i t h a n e m p i r i c a l f o r m u l a of C Labeled

mevalonalactone

3 6

is a s m a l l d i t e r p e n o i d

H 6 0 i 2 a n d a m o l e c u l a r w e i g h t of 680. 5

incorporated

into fusicoccin

f u s i c o c c i n a g l y c o n e has a d i t e r p e n i c t r i c y c l i c skeleton. is a g l u c o s i d e

of a c a r b o t r i c y c l i c t e r p e n e

(85).

indicated that Thus fusicoccin

F u s i c o c c i n has

s y n t h e s i z e d , a n d its absolute structure has b e e n d e t e r m i n e d .

been

Moreover

16 different d e r i v a t i v e s of f u s i c o c c i n h a v e b e e n p r e p a r e d . P e r h a p s m o r e is k n o w n a b o u t the c h e m i s t r y of the p h y t o p a t h o g e n i c t o x i n t h a n a b o u t a n y of the others that h a v e b e e n s t u d i e d (44,

85).

F u s i c o c c i n is n o t specific a n d is h i g h l y t o x i c to m a n y p l a n t species at 2 j u g / m l , b u t i t appears to cause w i l t i n g at some distance f r o m the p o i n t of i n v a s i o n . T h e r e f o r e i f the w a t e r - t r a n s p o r t system is n o t p h y s i c a l l y b l o c k e d , f u s i c o c c i n w o u l d a p p e a r to b e a n e x c e p t i o n a l non-specific t o x i n that is i m p o r t a n t i n disease. V a r i o u s d e r i v a t i v e s w e r e p r e p a r e d to s t u d y the p o r t i o n of the m o l e c u l e responsible for the t o x i c i t y of f u s i c o c c i n (86).

R e m o v a l of the text-

p e n t e n y l or the g l u c o s i d i c m o i e t y d r a s t i c a l l y r e d u c e d t o x i c i t y . F u s i c o c c i n increases w a t e r u p t a k e of p e a seedlings, a n d d e r i v a t i v e s f r o m w h i c h t h e f e r f - p e n t e n y l g r o u p has b e e n r e m o v e d also s t i m u l a t e w a t e r u p t a k e of p e a seedlings.

T h i s u p t a k e is significant because the

terf-pentenyl

g r o u p is

n e e d e d for t o x i c i t y . T h e r e f o r e f u s i c o c c i n appears to b e a c t i v e i n b o t h the w a t e r - u p t a k e a n d w a t e r - t r a n s p o r t systems. M o r e e v i d e n c e i n d i c a t e d t h a t f u s i c o c c i n causes i r r e v e r s i b l e extension of the c e l l w a l l

(87).

A l s o f u s i c o c c i n causes a b n o r m a l o p e n i n g of t h e stomata. T h i s obser v a t i o n l e d to the c o n c l u s i o n that u n u s u a l s t o m a t a l o p e n i n g r e s u l t e d i n excessive w a t e r loss ( 8 8 ) .

I t has also b e e n suggested

that fusicoccin

causes p e r m e a b i l i t y changes i n the p l a s m a m e m b r a n e of p l a n t cells


(89).

310

MYCOTOXINS

Others observed

that stomata treated w i t h fusicoccin c o u l d be

closed

w i t h a b s c i s i n , b u t t o m a t o cuttings after t h i s t r e a t m e n t w i l t e d .

It was

t h e r e f o r e c o n c l u d e d t h a t t h e t o x i n decreases the resistance of t h e p l a s m a m e m b r a n e to w a t e r passage a n d t h a t s t o m a t a l o p e n i n g is a effect ( 9 0 ) .

secondary

I t appears t h a t d i s r u p t i o n of the f u n c t i o n of t h e p l a s m a

m e m b r a n e of s u b e p i d e r m a l cells w o u l d r e s u l t i n loss of c e l l t u r g o r .

Loss

of c e l l t u r g o r of these cells w o u l d r e s u l t i n a loss of pressure o n g u a r d


cells, a loss t h a t c o u l d c u l m i n a t e i n s t o m a t a l o p e n i n g .

Fusicoccin could

cause w i l t i n g i n this m a n n e r .

Ftisarial Phytotoxic Mycotoxins species p r o d u c e n u m e r o u s toxins, b u t a f e w species p r o d u c e

Fusarium both

mycotoxins

a n d phytotoxins.

A l t h o u g h some toxins affect

both

p l a n t s a n d a n i m a l s , n o n e of the m y c o t o x i n s that affect p l a n t s i n i t i a t e p l a n t disease. M a n y of the m y c o t o x i n s ( trichothecenes ) p r o d u c e d b y

Fusarium

species h a v e a m o l e c u l a r s t r u c t u r e s i m i l a r to t h a t of the s e s q u i t e r p e n o i d s . M o r e o v e r s o m e p h y t o p a t h o g e n i c toxins p r o d u c e d b y different f u n g i are d i t e r p e n o i d s or s e s q u i t e r p e n o i d s .

U n f o r t n u a t e l y l i t t l e is k n o w n

about

t h e a n i m a l t o x i c i t y of t h e p h y t o p a t h o g e n i c t e r p e n o i d s t h a t are i m p o r t a n t i n p l a n t disease. T h e o b j e c t i v e of this s e c t i o n is to discuss the r e l a t i o n s h i p between mycotoxins a n d phytotoxins. T h e p h y t o t o x i c m a t e r i a l s p r o d u c e d b y f u n g i are r e f e r r e d to as p h y t o toxins (2).

P h y t o p a t h o g e n i c toxins differ f r o m p h y t o t o x i n s i n i n i t i a t i n g

p l a n t disease.

P h y t o t o x i n s d o n o t i n i t i a t e disease, b u t t h e y are i n v o l v e d

i n a m i n o r w a y d u r i n g t h e l a t e r p a r t of the disease s y n d r o m e .

I n this

d i s c o u r s e a m y c o t o x i n is c o n s i d e r e d a t o x i n p r o d u c e d b y a f u n g u s t h a t is t o x i c to a n i m a l s . B r i a n et a l . ( 9 1 )

first

observed

that compounds

(trichothecenes)

p r o d u c e d b y p l a n t p a t h o g e n s are t o x i c to p l a n t s a n d a n i m a l s . T h e same trichothecenes that caused skin irritation, nerve damage, a n d hemorrhage i n a n i m a l s also cause w i l t i n g a n d necrosis i n p e a seedlings

The

(92).

t r i c h o t h e c e n e s also i n h i b i t p l a n t g r o w t h b y i n t e r f e r i n g w i t h the a c t i o n o f i n d o l e a c t i c a c i d (91).

T h e interaction between the trichothecenes a n d

g r o w t h h o r m o n e s has n o t b e e n r e p o r t e d i n a n i m a l s . M o r e o v e r the t r i c h o t h e c e n e s a r e v e r y t o x i c t o b o t h p l a n t s a n d a n i m a l s . C o n c e n t r a t i o n s as l o w as 0.1 f t g / m l i n h i b i t g r o w t h of c e r t a i n p l a n t species formin

is a t o x i n r e c e n t l y i s o l a t e d f r o m

(Fusarium

moniliforme)

(91).

a plant pathogenic

that is t o x i c to p l a n t s a n d a n i m a l s

Monilifungus

(93).

S o m e p h y t o t o x i n s cause p h y s i o l o g i c a l d i s t u r b a n c e s i n p l a n t s s i m i l a r t o those t h a t m y c o t o x i n s cause i n a n i m a l s . F o r e x a m p l e f u s i c o c c i n diterpenoid)

causes p e r m e a b i l i t y changes

(a

a n d necrosis i n m a n y p l a n t

species w h e r e a s c e r t a i n s e s q u i t e r p e n o i d s cause i n c r e a s e d v a s c u l a r p e r -


13.

LUKE

A N D BIGGS

m e a b i l i t y i n rats (94).

Phytopathogenic

311

Toxins

T h i s o b s e r v a t i o n m a y e x p l a i n the e d e m a a n d

h e m o r r h a g i n g i n a n i m a l s w h i c h is c h a r a c t e r i s t i c of the t r i c h o t h e c e n e - t y p e toxins. M o r e o v e r the trichothecenes are c h e m i c a l l y s i m i l a r to f u s i c o c c i n . T h e specific m e c h a n i s m b y w h i c h the t r i c h o t h e c e n e toxins affect m e m b r a n e systems i n a n i m a l s is n o t k n o w n . T h e s i m i l a r i t i e s i n m o l e c u l a r s t r u c t u r e a n d p h y s i o l o g i c a l response b e t w e e n c e r t a i n p h y t o p a t h o g e n i c toxins a n d s o m e m y c o t o x i n s i n d i c a t e Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0149.ch013

t h a t some p h y t o p a t h o g e n i c toxins m a y also b e t o x i c to a n i m a l s . S o m e of t h e p h y t o p a t h o g e n i c toxins h a v e n o t b e e n i s o l a t e d f r o m diseased p l a n t s b e c a u s e of t h e i r l a b i l e n a t u r e or because of l o w c o n c e n t r a t i o n s ; therefore these types of toxins w o u l d n o t i n j u r e a n i m a l s i n n a t u r e . S o m e toxins t h a t cause p l a n t disease ( f u s i c o c c i n , h e l m i n t h o s p o r o s i d e ) h a v e b e e n i s o l a t e d f r o m i n f e c t e d p l a n t s . M o r e o v e r some of these h a v e c h e m i c a l s t r u c tures s i m i l a r to those of v a r i o u s m y c o t o x i n s , a n d the p h y t o p a t h o g e n i c toxins o c c u r r e d i n r e l a t i v e l y l a r g e q u a n t i t i e s . O n the other h a n d c e r t a i n mycotoxins (trichothecenes)

h a v e not b e e n i s o l a t e d f r o m n a t u r a l l y i n

f e c t e d p l a n t s o r s t o r e d g r a i n . B e c a u s e i t has n o t b e e n e s t a b l i s h e d t h a t a g i v e n t r i c h o t h e c e n e w a s r e s p o n s i b l e for m y c o t o x i c o s i s i n t h e field, i t has b e e n suggested t h a t n a t u r a l l y o c c u r r i n g mycotoxicosis m a y r e s u l t f r o m different substances p r o d u c e d b y s e v e r a l f u n g i (95).

This assumption

a d d s significance w h e n i t is r e a l i z e d that some of t h e p h y t o t o x i n s a n d p h y t o p a t h o g e n i c toxins o c c u r i n h i g h e r concentrations i n n a t u r a l l y i n f e c t e d p l a n t s t h a n d o the m y c o t o x i n s . T h e r e f o r e some of these p h y t o t o x i c compounds complex.

m a y b e i n v o l v e d i n the n a t u r a l l y o c c u r r i n g m y c o t o x i c o s i s

T h i s i n v o l v e m e n t w o u l d suggest t h a t the a n i m a l t o x i c i t y of

phytotoxic compounds isolated f r o m infected plants should be studied. L i t t l e a t t e n t i o n has b e e n g i v e n to this subject.

Discussion A l t h o u g h toxins h a v e b e e n i m p l i c a t e d i n p l a n t disease for a l m o s t a c e n t u r y , o n l y 10 are c o n s i d e r e d to i n d u c e disease. E v i d e n c e t h a t these 10 i n c i t e disease hinges o n t w o forms of c i r c u m s t a n t i a l e v i d e n c e :

the

t o x i n has t h e same host r a n g e as the p a t h o g e n , a n d a p o s i t i v e c o r r e l a t i o n exists b e t w e e n t o x i n p r o d u c t i o n a n d p a t h o g e n i c i t y . A l t h o u g h i n d i r e c t e v i d e n c e suggests t h a t a f e w toxins i n d u c e p l a n t disease, t h e i r p r e c i s e m o d e of a c t i o n is u n k n o w n . P l a n t scientists h a v e h a d l i t t l e success i n this area. T h e e x p l a n a t i o n , m a i n l y conjecture, rests o n three m a j o r p o i n t s . T h e first concerns concentrations a n d s t a b i l i t y of t h e t o x i n . M a n y toxins are p r o d u c e d i n s m a l l q u a n t i t i e s m vivo, a n d s o m e of m a j o r i m p o r t a n c e a r e n o t stable w h e n p u r i f i e d .

Sometimes, t w o or

m o r e toxins of s i m i l a r c o n s t i t u t i o n are p r o d u c e d , a n d thus i t is d i f f i c u l t to a s c e r t a i n w h i c h t o x i n i n c i t e s disease d e v e l o p m e n t .


312

MYCOTOXINS


A

Β

Figure 1. Chemical structure of thosporol A and helminthosporol

helmin Β

T h e s e c o n d p o i n t i n v o l v e s interactions b e t w e e n toxin(s).

the host a n d

S o m e t i m e s t h e host p r o d u c e s toxins t h a t cause necrosis.

the Such

toxins are p r o d u c e d i n response to i n f e c t i o n or i n j u r y . W h e n toxins p r o d u c e d b y the host a n d the p a t h o g e n are p r o d u c e d o n l y in vivo, i t is diffi c u l t to d e t e r m i n e the o r i g i n of a g i v e n t o x i n or one t h a t triggers disease. O c c a s i o n a l l y b o t h resistant a n d susceptible c u l t i v a r s i n a c t i v a t e toxins at different rates.

M a n y c o m p o u n d s are n o t r e a d i l y t r a n s p o r t e d i n p l a n t s ,

a n d i t is difficult to detect c e l l u l a r d a m a g e s o m e w h a t r e m o v e d f r o m the p o i n t of i n f e c t i o n . S u c h d e t e c t i o n ( 5 - 1 0 m m ) f r o m the i n v a d i n g p a t h o g e n s h o u l d be u s e d as a p r i m a r y c r i t e r i o n t o i m p l i c a t e a t o x i n as a diseasei n d u c i n g agent. produced

T h i s c r i t e r i o n w o u l d b e m o s t u s e f u l w h e n t h e t o x i n is

o n l y in vivo.

W e believe that m a n y disease-inducing

toxins

are c o m m o n l y p r o d u c e d o n l y in vivo a n d t h a t scientists w h o are i n t e r e s t e d i n the t o x i g e n i c n a t u r e of p h y t o p a t h o g e n s toxins p r o d u c e d in

s h o u l d t u r n t h e i r a t t e n t i o n to

vivo.

T h e t h i r d a n d p e r h a p s the f o r e m o s t reason for t h e s l o w w i t h phytopathogenic

toxins is t h e l a c k of k n o w l e d g e

structures of these c o m p o u n d s .

progress

of t h e c h e m i c a l

U n t i l recently v e r y little was k n o w n about

t h e structures of p h y t o p a t h o g e n i c toxins. M o s t of the toxins t h a t a t t r a c t e d the a t t e n t i o n of chemists w e r e n o t the c a u s a l agents of p l a n t disease. M u c h c r e d i t to e l u c i d a t e t h e m o d e s of a c t i o n of a n t i b i o t i c s a n d m y c o t o x i n s m u s t b e g i v e n to chemists w h o d e t e r m i n e d the structures a n d f u n c t i o n a l groups of these c o m p o u n d s .

O n c e these a r e k n o w n , the site of


action

13.

LUKE

A N D BIGGS

Phytopathogenic

313

Toxins

is m o r e easily ascertained. K n o w l e d g e of this k i n d w o u l d b e h e l p f u l to biologists w h o w i s h to d e t e r m i n e t h e m o d e of a c t i o n of t o x i g e n i c c o m p o u n d s . H o p e f u l l y this s y m p o s i u m w i l l s t i m u l a t e the interest of chemists i n p h y t o p a t h o g e n i c toxins. T h e i r expertise is s o r e l y n e e d e d . S i n c e this s y m p o s i u m e m p h a s i z e s m y c o t o x i n s or toxins t h a t a n i m a l s , i t is i n t e r e s t i n g to note s i m i l a r i t i e s b e t w e e n

affect

mycotoxins

toxins t h a t affect p l a n t s . S o m e of t h e i m p o r t a n t m y c o t o x i n s

and

(trichothe


c e n e s ) are k n o w n to b e the p r i m a r y agents t h a t i n c i t e p l a n t disease. T h a t is, some m y c o t o x i n s development

are of o n l y m i n o r i m p o r t a n c e to

symptom

i n p l a n t s , or t h e i r t o x i c i t y to p l a n t s m a y b e c o i n c i d e n t a l .

T h i s p o i n t of v i e w m a y b e i n error because some m y c o t o x i n s cause severe reactions a n d d e a t h i n c e r t a i n p l a n t s at v e r y l o w concentrations.

Several

p h y t o t o x i n s a n d m y c o t o x i n s h a v e c h e m i c a l structures t h a t are t e r p e n i c i n origin.

S t r u c t u r a l r e l a t i o n s h i p s a n d p a t h w a y s of synthesis a l l o w classifi

c a t i o n as s e s q u i t e r p e n o i d , d i t e r p e n o i d , a n d sesterterpenoid. S e v e r a l w e l l k n o w n sesquiterpenoids s p o r a l , h e l m i n t h o s p o r o l (see

are f o m a n n o s i n ,

helmintho

F i g u r e 1), and diacetoxyscirpenol.

c o m p o u n d s t h a t h a v e the 12,13-epoxytrichothecene

Some

nucleus have similar

c h e m i c a l structures a n d b i o l o g i c a l p r o p e r t i e s of t o x i c i t y to p l a n t a n d a n i m a l cells (44).

I n a recent r e v i e w , 22 n a t u r a l l y o c c u r r i n g

12,13-epoxy-

trichothecenes w e r e fisted, a n d t h e i r c h e m i c a l structures w e r e (95).

Experiments w i t h labeled compounds

compared

s h o w e d t h a t the n u c l e i of

these substances are d e r i v e d f r o m m e v a l o n i c a c i d (96, 9 7 ) . A w e l l - k n o w n , d i t e r p e n o i d , p h y t o p a t h o g e n i c t o x i n is f u s i c o c c i n . s t r u c t u r e is k n o w n a n d is w e l l d o c u m e n t e d i n a recent r e v i e w (44).

Its The

absolute s t r u c t u r e f o r f u s i c o c c i n is s h o w n i n F i g u r e 2. I n c o r p o r a t i o n of

HO

^CHpCH

Figure 2.

3

Chemical structure of

fusicoccin


314

MYCOTOXINS

OH

ν


0=

Phytopathogenic Toxins from Fungi: An Overview - Advances in

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