Phytotoxins from Plant Pathogens of Weedy Plants - American

Chapter 46

Phytotoxins from Plant Pathogens of Weedy Plants Gary Strobel , Fumio Sugawara , and Jon Clardy 1

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Department of Plant Pathology, Montana State University, Bozeman, M T 59717-0002 Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301

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Weeds are host plants to an array of plant pathogenic fungi and bacteria and there is current interest in the use of these organisms for weed control. Commonly, associated with pathogens are one or more phytotoxins involved in causing disease. Thus, one alternative approach in weed control is the use of phytotoxins or their derivatives for direct application to the noxious plant. A necessary prelude to this approach is the isolation, characterization and biological testing of phytotoxic metabolites. Recently, we have done such experiments by isolating alteichin from Alternaria eichorniae, a pathogen of water hyacinth; and bipolaroxin,a host-selective phytotoxin from Bipolaris cynodontis, a pathogen of Bermuda grass. X-ray crystallography was used to determine the structures of these phytotoxins. In the past few decades food production has increased through plant breeding, f e r t i l i z e r use, mechanized farming, and chemical pest control. The most recent U.N. study, the FAO World Food Report, indicated that 1984 food production was k% greater than 1983- But the report noted that further improvements w i l l be required i f food production is to keep pace with population growth. One area for potential improvement is the control of weeds. We have been working on a novel approach to weed control, and find the preliminary results encouraging. The approach was quite simple conceptually and can be b r i e f l y summarized. Many pathogens of crop plants produce symptoms on their hosts through the production of phytotoxins . It seemed logical to assume that weed pathogens would also produce compounds acting as phytotoxins, and that these phytotoxins could serve as novel agents for the control of weeds. We began with a target l i s t of the world's worst weeds (2), and searched for their fungal or bacterial 3

Current address: Institute of Physical and Chemical Research, Rikagaku, Kenkyusho, Wako-Shi, Saitama 351-01, Japan 0097-6156/87/0330-0516$06.00/0 © 1987 American Chemical Society

Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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pathogens. Searching involved direct f i e l d observation, examination o f the r e l e v a n t l i t e r a t u r e , and d i s c u s s i o n s w i t h p l a n t p a t h o l o g i s t s . Once p r o m i s i n g p a t h o g e n s w e r e l o c a t e d , t h e y w e r e c u l t u r e d i n t h e l a b o r a t o r y , and t h e i r e x t r a c t s examined f o r p h y t o t o x i c a c t i v i t y . Then s t a n d a r d i s o l a t i o n and s t r u c t u r e d e t e r m i n a t i o n t e c h n i q u e s w e r e used to i d e n t i f y the compound(s) r e s p o n s i b l e for the p h y t o t o x i c activi ty. T h e r e was p r e c e d e n t f o r t h i s a p p r o a c h . P l a n t pathogens o b t a i n e d f r o m s p e c i f i c w e e d y p l a n t s h a v e r e c e n t l y b e e n u s e d as w e e d c o n t r o l a g e n t s (jO , and very r e c e n t l y m e t a b o l i t e s o f s o i l borne m i c r o o r g a n i s m s h a v e b e e n u s e d f o r t h e same p u r p o s e (4_) . We h o p e d t h a t by f o c u s i n g o n w e e d p a t h o g e n s , we w o u l d f i n d n o v e l a n d m o r e s e l e c t i v e chemical agents. At the very l e a s t the chemical agents w i l l s u g g e s t s t r u c t u r a l types and arrangements o f f u n c t i o n a l groups t h a t w i l l s e r v e as s t a r t i n g p o i n t s f o r t h e d e v e l o p m e n t o f improved herbicides. U n t i l now, s u c h a n a p p r o a c h h a s n o t b e e n s e r i o u s l y c o n s i d e r e d , and weed pathogens a r e not w e l l r e p r e s e n t e d i n t h e world's culture collections. Many d e s c r i b e d e a r l i e r h a v e s u f f e r e d t h e f a t e o f d e s i c c a t i o n and d e a t h on t h e s h e l v e s o f p l a n t p a t h o l o g i s t s w h o s e w o r k may n o t h a v e b e e n a p p r e c i a t e d a t a n e a r l i e r t i m e . The Weed H o s t - P a t h o g e n

System

S e v e r a l f a c t o r s o f t h e h o s t - p a t h o g e n s y s t e m must be c o n s i d e r e d i n s e l e c t i n g s p e c i f i c cases to s t u d y . The p a t h o g e n s h o u l d be e x p e c t e d to produce a p h y t o t o x i n . For s e v e r a l p l a n t pathogens — the r u s t s , m i l d e w s , v i r u s e s , a n d n e m a t o d e s — t h e r e i s l i t t l e o r no r e c o r d o f p h y t o t o x i n s b e i n g a s s o c i a t e d w i t h the d i s e a s e s t h a t they c a u s e . The most n o t a b l e g r o u p o f p h y t o t o x i c p r o d u c e r s a r e t h e F u n g i imperfecti a n d some p l a n t p a t h o g e n i c b a c t e r i a ( 5 ) . Our i n i t i a l s t u d i e s h a v e f o c u s e d on the f u n g a l p a t h o g e n s . Geography is a l s o e x p e c t e d to p l a y an i m p o r t a n t r o l e i n s e l e c t i n g p a t h o g e n s . The m o s t e f f e c t i v e pathogens a r e l i k e l y t o be found near the c e n t e r o f o r i g i n o f t h e h o s t p l a n t s i n c e the pathogen w i l l have had the l o n g e s t time to s p e c i a l i z e i n a t t a c k i n g the h o s t . C u l t u r i n g and t o x i n p r o d u c t i o n . Once a p a t h o g e n i s c h o s e n f o r f u r t h e r s t u d y , i t s h o u l d be p l a c e d i n a permanent c u l t u r e c o l l e c t i o n to ensure i t s continued a v a i l a b i l i t y . A l i q u i d c u l t u r e medium must s u p p o r t growth and p r o d u c t i o n o f t o x i c o r damaging s e c o n d a r y metabolites. I d e a l l y , i t w o u l d b e a d e f i n e d medium ( c o n t a i n no extracts of p l a n t s , yeast extract, e t c . ) . A d e f i n e d medium s i m p l i f i e s t h e i s o l a t i o n p r o c e d u r e and e n s u r e s t h a t t h e e x t r a c t i s not c o n t a m i n a t e d w i t h added m e t a b o l i t e s . U n f o r t u n a t e l y t h i s is not always p o s s i b l e . The m e c h a n i s m s t h a t r e g u l a t e t o x i n p r o d u c t i o n i n p a t h o g e n s a r e p o o r l y d e f i n e d , b u t many o r g a n i s m s r e q u i r e t h e presence o f c e r t a i n p l a n t metabolites to maximize t o x i n p r o d u c t i o n (6). These p l a n t m e t a b o l i t e s a r e not p r e c u r s o r s o f the t o x i n s , but somehow a c t i v a t e t h e t o x i n s ' b i o s y n t h e t i c p a t h w a y s ( 6 ) . An e f f i c i e n t way t o c u l t u r e p a t h o g e n s i s t o u s e a s t a n d a r d d e f i n e d m e d i u m w i t h t h e a d d i t i o n o f t h e c r u d e a q u e o u s e x t r a c t o f o n e o r two young host p l a n t s (6). Bioassay. A p r e r e q u i s i t e for p h y t o t o x i n i s o l a t i o n is i t y o f a s u i t a b l e b i o a s s a y (5^,7_,8) . Each s t e p o f t h e

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p r o c e d u r e must be m o n i t o r e d by c h e c k i n g t h e b i o l o g i c a l activity. The m o s t common a n d s i m p l e s t p r o c e d u r e i s t o p l a c e a few m i c r o l i t e r s o f t h e t e s t s o l u t i o n o v e r a s m a l l p u n c t u r e wound o n a d e t a c h e d leaf. The p u n c t u r e wound e n h a n c e s t h e a c c e s s o f t h e t o x i n t o t h e l e a f tissue. The l e a f i s then p l a c e d i n a p e t r i d i s h c o n t a i n i n g a f i l t e r paper s a t u r a t e d w i t h w a t e r . The t o p c o v e r o f t h e p l a t e i s s e a l e d w i t h p a r a f i l m , and t h e p l a t e i s i n c u b a t e d under c o n t r o l l e d l i g h t and t e m p e r a t u r e c o n d i t i o n s . T o x i n a c t i v i t y i s u s u a l l y i n d i c a t e d by c h l o r o t i c , n e c r o t i c , o r c o l o r e d spots on the l e a f . O t h e r methods f o r b i o a s s a y i n v o l v i n g CO2 f i x a t i o n , o r e f f e c t s o n o r g a n e l l e s , whole p l a n t s , p r o t o p l a s t s , t i s s u e c u l t u r e s , o r p l a n t parts a r e o u t l i n e d (_5,7) . The b i o a s s a y i s c o m p l i c a t e d by t h e h i g h l y v a r i a b l e r e s p o n s e o f d i f f e r e n t p l a n t s t o a g i v e n compound, i e . , h o s t s p e c i f i c i t y . Some t o x i n s a r e c o m p l e t e l y n o n s p e c i f i c ; they a f f e c t a l l p l a n t s p e c i e s used i n t h e b i o a s s a y a t v i r t u a l l y a l l c o n c e n t r a t i o n s . At the other extreme a r e t h e h o s t - s p e c i f i c t o x i n s ; they a f f e c t o n l y a c e r t a i n p l a n t s p e c i e s , and a t reduced c o n c e n t r a t i o n s w i l l even d i s c r i m i n a t e among c u l t i v a r s w i t h i n a s p e c i e s . We a r e m o s t i n t e r e s t e d i n f i n d i n g h i g h l y h o s t - s p e c i f i c t o x i n s f o r common w e e d s , b u t t h i s i s n o t a n easy t a s k . Such h o s t - s p e c i f i c t o x i n s a r e n o t l i k e l y t o b e e a s i l y f o u n d i n t h e p l a n t p a t h o g e n s from weed h o s t s b e c a u s e o f t h e w i d e genetic d i v e r s i t y of the hosts in nature. Ironically, highly hosts p e c i f i c t o x i n s a r e w e l l known f o r c r o p p l a n t s . The l a r g e a r e a s o f c r o p p l a n t s i n m o n o c u l t u r e make s e l e c t i o n f o r h i g h l y s p e c i f i c p a t h o g e n s much e a s i e r . The most d r a m a t i c r e c e n t e x a m p l e i s t h e c o r n b l i g h t e p i d e m i c o f 1971 where l a r g e a r e a s o f c o r n p l a n t s c o n t a i n i n g T e x a s m a l e s t e r i l e g e r m p l a s m w e r e h i g h l y v u l n e r a b l e t o H_. m a y d i s (3). F o r t u n a t e l y t h e r e i s an i n t e r m e d i a t e group o f p h y t o t o x i n s t h a t d i s p l a y some h o s t s e l e c t i v i t y . They a r e c o m p l e t e l y i n e f f e c t i v e a t a l l c o n c e n t r a t i o n s a g a i n s t some s p e c i e s , b u t w i l l a f f e c t o t h e r s . S e l e c t i v i t y w i t h t h i s g r o u p o f p h y t o t o x i n s may o c c u r w i t h i n a r a n g e o f t o x i n c o n c e n t r a t i o n s , between t o x i n - s e n s i t i v e p l a n t s , o r even within cultivars of a certain species. Phytotoxin

Isolation

and C h a r a c t e r i z a t i o n

Most c h a r a c t e r i z e d p h y t o t o x i n s a r e from c r o p p l a n t s and i n c l u d e a large array o f chemical f a m i l i e s . Proteins, peptides, glycopeptides, g l y c o s i d e s , p h e n o l i c s , t e r p e n o i d s , m a c r o l i d e s , and o t h e r s have been c h a r a c t e r i z e d (5_ J) . T h u s , no g e n e r a l i s o l a t i o n s c h e m e i s a v a i l a b l e , a n d each c a s e must be h a n d l e d s e p a r a t e l y . To d a t e t h e m a j o r i t y o f p l a n t p a t h o g e n s t h a t we h a v e e x a m i n e d h a s p r o d u c e d r e l a t i v e l y s m a l l m o l e c u l a r w e i g h t compounds - - compounds t h a t e a s i l y l e n d t h e m s e l v e s t o e x t r a c t i o n f r o m c u l t u r e f l u i d s by o r g a n i c solvents. This c o u l d r e f l e c t o u r c u r r e n t technology and not apply g e n e r a l l y t o weed p a t h o g e n s . Further p u r i f i c a t i o n is accomplished by t e c h n i q u e s s u c h a s f l a s h c o l u m n c h r o m a t o g r a p h y , p r e p a r a t i v e t h i n l a y e r chromatography, and p r e p a r a t i v e h i g h performance liquid chromatography. The u l t i m a t e goal i s t o d e f i n e t h e p r e c i s e m o l e c u l a r structure of the phytotoxins(s). S p e c t r o s c o p i c t e c h n i q u e s such as mass s p e c t r o m e t r y , Η NMR a n d NMR, a n d UV h a v e b e e n e x t r e m e l y useful. Since the quantities of the phytotoxins are t y p i c a l l y l i m i t e d and t h e range o f s t r u c t u r a l types r a t h e r l a r g e , x - r a y 9

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d i f f r a c t i o n h a s p r o v e d t o b e a most p o w e r f u l t o o l . Most, but not a l l , o f t h e p h y t o t o x i n s t h a t we h a v e i n v e s t i g a t e d h a v e u l t i m a t e l y been c h a r a c t e r i z e d by x - r a y d i f f r a c t i o n . It i s not unusual f o r weed p a t h o g e n s t o p r o d u c e p h y t o t o x i n s t h a t a r e i d e n t i c a l o r c l o s e l y related to phytotoxins that a t t a c k crop p l a n t s . Some e x a m p l e s o f t h i s a r e z i n n i o l ( l ) p r o d u c e d b y a number o f A l t e r n a r i a s p p . ( 1 0 ) , and m o n o c e r i n ( l l ) from E x s e r o h i l u m t u r c i c u m , a p a t h o g e n o f J o h n s o n g r a s s ( Γ1_) · Thus t h e i n v e s t i g a t o r s h o u l d b e w e l l a c q u a i n t e d w i t h t h e p h y t o t o x i n l i t e r a t u r e a n d h a v e a c c e s s t o r e f e r e n c e s a m p l e s and/or spectra. R e c e n t l y , we h a v e f o u n d t h a t p a t h o g e n s w i t h a r e s t r i c t e d h o s t range and u n r e l a t e d t o common c r o p p a t h o g e n s do p r o d u c e n o v e l phytotoxins. These novel p h y t o t o x i n s have both unusual c h e m i c a l s t r u c t u r e s a n d s u r p r i s i n g , i n some c a s e s u n p r e c e d e n t e d , b i o l o g i c a l activities. We w i l l s u m m a r i z e t h e s e s t u d i e s b e l o w , b e g i n n i n g w i t h the l e a s t s e l e c t i v e p h y t o t o x i n s and p r o g r e s s i n g to p h y t o t o x i n s w i t h some h o s t s e l e c t i v i t y . Water h y a c i n t h i s an e c o n o m i c a l l y s i g n i f i c a n t weed n a t i v e t o t h e Amazon b a s i n . In t h e r e c e n t p a s t , i t h a s become a w i d e s p r e a d p e s t t h r o u g h o u t much o f t h e w o r l d . Water h y a c i n t h i s a s e r i o u s p r o b l e m i n paddy c r o p s s u c h a s r i c e a n d t a r o , a n d i n some a r e a s i t has become s o p r o l i f i c t h a t i t b l o c k s f o r m e r l y n a v i g a b l e w a t e r w a y s , i r r i g a t i o n c a n a l s , and drainage d i t c h e s . Alternaria eichorniae a t t a c k s the leaves o f water h y a c i n t h , and t h e r e s u l t i n g l e s i o n s sometimes cause l e a f d e a t h . B e c a u s e o f t h i s f o r m o f symptom e x p r e s s i o n and t h e r e l a t i v e l y narrow host range o f t h e fungus, A . e i c h o r n i a e was e x a m i n e d f o r i t s p r o d u c t i o n o f p h y t o t o x i c metabolites. B o s t r y c i n , a reduced anthraquinone w i t h p h y t o t o x i c a c t i v i t y , h a d p r e v i o u s l y b e e n i s o l a t e d f r o m t h i s s o u r c e (J_2) . O u r r e i n v e s t i g a t i o n l e d to the i s o l a t i o n o f a l t e i c h i n ( i l l ) , a doubly h y d r a t e d form o f h , 9 ~ d i h y d r o x y p e r y 1 e n e - 3 , 1 0 - q u i n o n e . The s t r u c t u r e o f t h i s r a t h e r s e n s i t i v e m o l e c u l e was d e d u c e d by x - r a y c r y s t a l l o g r a p h i c a n a l y s i s (_1_3) . A t l e a s t o n e o t h e r p h y t o t o x i n , c l o s e l y r e l a t e d t o a l t e i c h i n , was a l s o p r e s e n t i n t h e c u l t u r e f l u i d s . T h e p r e l i m i n a r y s p e c t r a l a n a l y s i s o f t h i s a d d i t i o n a l component s u g g e s t s that i t is the t r i p l y hydrated version o f the parent quinone, but a d d i t i o n a l work w i l l be r e q u i r e d to f u l l y e l u c i d a t e t h e s t e r e o chemistry. A l t e i c h i n i s , not s u r p r i s i n g l y , very s e n s i t i v e to a c i d and i s r e a d i l y converted to the water-insolub1e parent quinone. The p a r e n t q u i n o n e may w e l l b e t h e u l t i m a t e p h y t o t o x i n . Alteichin d o e s n o t show h o s t s e l e c t i v i t y i n e i t h e r t h e w h o l e l e a f o r p r o t o plast assays. In a l l t e s t p l a n t s i t c a u s e s n e c r o t i c f l e c k s i n l e a f p u n c t u r e w o u n d s ( 2 . 7 mM (2% e t h a n o l ) ) w i t h i n 12 h o f a p p l i c a t i o n . The a l t e i c h i n - i n d u c e d l e s i o n s r e s e m b l e t h o s e c a u s e d b y A . e i c h o r n i a e on w a t e r h y a c i n t h . Exserohi1 one, from E x s e r o h i l i u m h o l m i i , i s a fungal leaf pathogen o f Dacty1octeniurn aegyptium (crowfoot g r a s s ) , a s e r i o u s weed i n a l l m a j o r t r o p i c a l a n d s e m i t r o p i c a l a g r i c u l t u r a l a r e a s o f the w o r l d . W h i l e a m a j o r p r o d u c t was i d e n t i f i e d a s m o n o c e r i n (I I) by s p e c t r o m e t r i c m e a n s , t w o n o v e l p h y t o t o x i n s w e r e a l s o i s o l a t e d from l i q u i d shake c u l t u r e s o f t h i s fungus. T h e s e new p h y t o t o x i n s w e r e s h o w n t o b e d i k e t o p i p e r a z i n é s ( I V ) a n d (V) b y s t a n d a r d s p e c t r o s c o p i c and x - r a y c r y s t a l l o g r a p h i c a n a l y s e s ( 1 4 ) . Both o f t h e s e compounds c a u s e n e c r o t i c l e s i o n s a t 1 0 " ^ - 1 0 ^ M . T h e


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l e s i o n s a r e u s u a l l y s u r r o u n d e d by a r e d d i s h brown b o r d e r on a variety of plant species. D i h y d r o p y r e n o p h o r i n , from D r e c h s l e r a avenae, is a l e a f pathogen o f b o t h w i l d and c u l t i v a t e d o a t s . It causes r e d d i s h brown l e s i o n s w i t h a n e c r o t i c sunken c e n t e r . A t l e a s t o n e compound i s o l a t e d f r o m b r o t h c u l t u r e s o f t h i s fungus caused comparable l e s i o n s on o a t s and a v a r i e t y o f o t h e r p l a n t s a t 3.2 χ 10"^ M (_1j>). T h e p h y t o t o x i n was c h a r a c t e r i z e d by s p e c t r o m e t r i c a n a l y s e s a n d c h e m i c a l c o n v e r s i o n a s (-)-dihydropyrenophorin ( V I ) , an i m p o r t a n t d i l a c t o n e m a c r o l i d e (15). However, the major p r o d u c t o b t a i n e d i n our e x t r a c t i o n procedure u s e d t o i s o l a t e ( - ) - d i h y d r o p y r e n o p h o r i η was t h e d i o l V I I (_^6) , w h i c h was n o t a c t i v e i n o u r b i o a s s a y t e s t s . We h a v e n o t y e t a s s i g n e d t h e s t e r e o c h e m i s t r y o f t h e s e c o n d a r y a l c o h o l c e n t e r s i n VII o r V I I I . The s i m p l i c i t y o f t h e NMR s p e c t r u m of VIII indicates a symmetrical molecule. T h i s w o u l d most p l a u s i b l y b e t h e m o l e c u l e w i t h t w o f o l d s y m m e t r y , b u t we c a n n o t y e t r u l e o u t t h e meso f o r m . Upon o x i d a t i o n V I I c o u l d b e c o n v e r t e d t o p y r e n o p h o r in ( V I I I ) . P y r e n o p h o r i n was a l s o a c t i v e i n t h e b i o l o g i c a l a s s a y s , b u t was n o t f o u n d i n avenae. I t i s p r o d u c e d by s e v e r a l other fungi (J7). B i o l o g i c a l l y , t h e most i n t e r e s t i n g a s p e c t o f (-)-dihydropyrenop h o r i n i s t h a t i t causes r e d d i s h l e s i o n s on Johnson g r a s s a t 10*6, 10~7, a n d 10~& M w h e r e i n no o t h e r p l a n t s p e c i e s t e s t e d shows a n y s e n s i t i v i t y whatever at these c o n c e n t r a t i o n s . Thus, i t would appear t h a t VI i s h o s t s e l e c t i v e . To o u r k n o w l e d g e J o h n s o n g r a s s i s n o t a h o s t o f D_. a v e n a e . Bipolaroxin (IX) i s t h e m o s t h o s t - s e l e c t i v e p h y t o t o x i n . It was i s o l a t e d f r o m B i p o l a r i s c y n o d o n t i s , a f u n g a l p a t h o g e n o n B e r m u d a g r a s s (18) . The t o x i n p r o d u c e s r e d d i s h l e s i o n s and r u n n e r s on t r e a t e d l e a v e s o f Bermuda g r a s s and J o h n s o n g r a s s ( h o s t p l a n t s o f t h e p a t h o g e n ) a t 10"5 M , b u t a t t h e s e c o n c e n t r a t i o n s no o t h e r p l a n t species tested is affected. B i p o l a r o x i n is a h i g h l y oxygenated member o f t h e e r e m o p h i l a n e f a m i l y . When t h e a l d e h y d e g r o u p i s r e d u c e d ( y i e l d i n g d i h y d r o b i p o l a r o x i n (X)) a l l p h y t o t o x i c i t y a p p e a r s t o b e l o s t e v e n a t c o n c e n t r a t i o n s a s h i g h a s 10~3 M ( 1 5 ) . Bermuda g r a s s i s one o f t h e most n o t o r i o u s weeds i n t h e g r a s s f a m i l y s i n c e i t h a s b e e n l i s t e d a s a p r o b l e m i n a t l e a s t 40 d i f f e r e n t crops. The z o n a t e l e s i o n s p r o d u c e d by B_. c y n o d o n t i s g r e a t l y r e s e m b l e t h o s e c a u s e d by b i p o l a r o x i n . Mode o f a c t i o n s t u d i e s o n b i p o l a r o x i n may be f a c i l i t a t e d by t h e u s e o f t h e ^ C - l a b e l e d compound. R e c e n t l y , we have learned t h a t [^C] mevalonate administered to c u l t u r e s of the f u n g u s s e r v e s a s a n e x c e l l e n t p r e c u r s o r t o b o t h IX a n d X . M

Future

Work

The m a j o r i t y o f t h e n o v e l p h y t o t o x i n s known f r o m w e e d p a t h o g e n s are summarized i n t h i s r e p o r t . S i n c e o n l y a few p a t h o g e n s h a v e b e e n s t u d i e d t o d a t e , t h e r e a r e many m o r e o r g a n i s m s y e t t o b e e x a m i n e d . F o r e x a m p l e , t h e r e i s no r e p o r t o f a p h y t o t o x i n f r o m a b a c t e r i a l w e e d p a t h o g e n , a l t h o u g h s u c h compounds s u r e l y e x i s t . I t seems u n l i k e l y that extremely h o s t - s e l e c t i v e (host-specific) phytotoxins w i l l b e r e a d i l y f o u n d f r o m weed p a t h o g e n s s i n c e t h e r e i s r e l a t i v e l y l i t t l e g e n e t i c s e l e c t i o n p r e s s u r e on t h e p a t h o g e n . This arises f r o m t h e h i g h l y d i v e r s e g e n e t i c b a c k g r o u n d o f any weed p o p u l a t i o n .


, ;

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46.

STROBEL ET AL.

Phytotoxins

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Nevertheless, we are encouraged by the findings on bipolaroxin (18) to believe that host-selective compounds do exist. Thus far, there have been no biochemical/physiological studies showing the effects or modes of action of any of the phytotoxins from weed pathogens on their hosts. Also, chemical modification (5) of the toxins should shed light on the bioactive portion of the molecule. The intrinsic biological activity of these phytotoxins certainly warrants considerable study, since there is potential for them to serve as herbicides or models for herbicides, and because they possess somewhat unexpected bîological activities. Acknowledgments We thank Dr. E. S. Luttrell of the University of Georgia for supplying cultures of many of the weed pathogens. The work at Bozeman was funded by a grant from Rohm ε Haas Co. and the Montana Agricultural Experiment Station. The work at Cornell was partially supported by a grant, NIH CA 24487. Literature Cited 1. "Index of Plant Diseases in the United States"; USDA: Washington, DC, 1969. 2. Holm, L. G.; Plucknett, D.L.; Pancho, J.V.; Herberger, J.P. "The World's Worst Weeds, Biology and Distributions"; Univ. Press of Hawaii: Honolulu, 1977. 3. Charudattan, R.; Walker, H.L. "Biological Control of Weeds with Plant Pathogens"; Wiley-Interscience: New York, 1982. 4. Takematsu, T.; Konnai, M.; Tachibana, K.; Tsuruoka, T.; Inoue, S.; Watanabe, T. (Melji Seika Kaisha, Ltd.) U.S. 4,448,601 (1984). 5. Strobel, G.A. Ann. Rev. Biochem. 1982, 51, 309-329. 6. Pinkerton, F.; Strobel, G.A. Proc. Natl. Acad. Sci. USA, 1976, 73, 4007-4011. 7. Durbin, R.D. "Toxins in Plant Disease"; Academic Press: New York, 1981. 8. Strange, R.N.; Pippard, D.J.; Strobel, G.A. Physiol. Plant Pathol. 1982, 20, 359-364. 9. Tatum, L.A. Science 1971, 171, 1113-1115. 10. Robeson, D.J.; Strobel, G.A. Phytochemistry 1984, 23, 1597-1599. 11. Robeson, D.J.; Strobel, G.A. Agric. Biol. Chem. 1982, 46, 26812683. 12. Stevens , K.L.; Bader-Ud-Din, Α.Α.; Admad, M. Phytochemistry 1979, 18, 1579-1580. 13. Robeson, D.J.; Strobel, G.A.; Matsumoto, G.K.; Fisher, L.E.; Chen, M.H.; Clardy, J. Experientia 1984, 40, 1248-1250 and references therein. 14. Sugawara, K.; Sugawara, F.; Strobel, G.A.; Fu, Y.; Cun-Heng, H.; Clardy, J. J. Org. Chem. 1985, in press. 15. Sugawara, F.; Strobel, G.A. Plant Sci. 1985, in press. 16. Kis, Z.; Furgernard, P.; Sigg, H.P. Experientia 1969, 25, 123124. 17. Grove, J.F. Tetrahedron Lett. 1965, 4675-4677. 18. Sugawara, F.; Strobel, G.A.; Fisher, L.E.; Van Duyne, G.D.; Clardy, J. Proc. Natl. Acad. Sci. USA 1985, in press. RECEIVED January 3, 1986 Waller; Allelochemicals: Role in Agriculture and Forestry ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Phytotoxins from Plant Pathogens of Weedy Plants - American

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