Fungal Elicitors of Phytoalexins and Their Potential Use in Agriculture

Chapter 8

Fungal Elicitors of Phytoalexins and Their Potential Use in Agriculture

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 16, 2016 | http://pubs.acs.org Publication Date: November 28, 1988 | doi: 10.1021/bk-1988-0380.ch008

Jack D. Paxton Department of Plant Pathology, University of Illinois, Urbana, IL 61801

Fungal elicitors of plant phytoalexins [natural plant antibiotics] (1) have the potential of becoming a new class of plant protectants and herbicides. Progress has been made recently to characterize elicitors chemically and further study their application to agriculture. The best characterized elicitor at present is from the cell walls of the Oomycete Phytophthora megasperma f. sp. glycinea. The smallest active, and potentially the most useful, fragment of this elicitor is a heptaglucan with specific structural requirements (2). Other fungal polysaccharides also elicit phytoalexin production in plants but their structures are not as well characterized. Fungal pectinases also have been implicated in phytoalexin elicitation. These enzymes appear to e l i c i t phytoalexin production by releasing pectic fragments from the cell walls of plants (3). The use of elicitors in agriculture holds exciting promise. Because phytoalexins have an important role in plant disease and pest resistance, their controlled elicitation could be used to stimulate natural disease and pest resistance without the use of environmentally damaging compounds. The t o x i c i t y of phytoalexins toward the plant in which they are produced might also be used to create a new class of herbicides, elicitors that cause the plant to 'self destruct'. Plant diseases are caused by a range of organisms including bacteria, fungi, insects, nematodes and viruses. The diseases caused by these pathogens 0097-6156/88/0380-0109$06.00/0 1988 American Chemical Society c

Cutler; Biologically Active Natural Products ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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BIOLOGICALLY ACTIVE NATURAL PRODUCTS

c o l l e c t i v e l y cause s i g n i f i c a n t y i e l d l o s s e s on crops around the world. Plant pathogens have evolved h i g h l y s p e c i f i c m e c h a n i s m s t o r e c o g n i z e a n d a t t a c k many o f o u r crop p l a n t s and even circumvent our attempts a t c o n t r o l l i n g them. S e v e r a l methods of c o n t r o l l i n g plant diseases have been devised. These include breeding f o r plant disease resistance, chemical control either before or after t h e plant i s attacked, a g r i - c u l t u r a l practices, and b i o l o g i c a l control. Each method o f disease c o n t r o l has problems a s s o c i a t e d with i t . New r a c e s o f pathogens emerge t o d e f e a t genes for r e s i s t a n c e . Some fungicides and p e s t i c i d e s have resulted i n t h e build-up o f pathogens and pests resistant to these chemicals. Adverse environments can stress a normally r e s i s t a n t p l a n t and cause i t t o become s u s c e p t i b l e t o s p e c i f i c microorganisms and pests. Therefore, new methods o f d i s e a s e c o n t r o l a r e needed t o reduce crop losses. The mechanisms o f p l a n t disease r e s i s t a n c e have been studied e x t e n s i v e l y t o improve c o n t r o l o f p l a n t diseases. For t h i s purpose a model system o f an important disease, Phytophthora root r o t , on an important U.S. crop, s o y b e a n s , was d e v e l o p e d . Soybean i s a major export crop w o r t h $10 b i l l i o n p e r y e a r i n t h e U.S., a n d i t h a s b e e n variously estimated that Phytophthora root r o t can reduce t h e y i e l d o f t h i s c r o p b y 1 t o 4 8 % ( 4 ) .T h i s r e p r e s e n t s a $100 m i l l i o n t o $1 b i l l i o n loss per year! The pathogen which causes t h i s severe disease i s quite variable; 25 races of t h e pathogen have been reported (5). Other Phytophthora species have developed resistance t o the best chemical control, metalaxyl ( 6 ) . Therefore, i t becomes i m p e r a t i v e t o understand plant disease and pest resistance b e t t e r , a n d t o l e a r n how fungal e l i c i t o r s might be used i n p l a n t disease and pest control. By s t u d y i n g P h y t o p h t h o r a root r o t o f soybean, a b e t t e r understanding h a s b e e n g a i n e d o f how p l a n t s c a n r e c o g n i z e pathogens (by t h e i r e l i c i t o r s ) a n d what they often do a f t e r a pathogen i s recognized (produce p h y t o a l e x i n s ) . Phytoalexins

and their

e l i c i t o r s

Phytoalexins a r e low molecular weight, a n t i m i c r o b compounds t h a t a r e both s y n t h e s i z e d by and accumulated plants after exposure t o microorganisms [_L] • Seve lines of evidence suggest that these compounds have important role i n plant disease and pest r e s i s t a

i a l i n ral an n c e

The accumulation of phytoalexins i n plants c a n be evoked by b i o l o g i c or non-biologic treatments [10-131 . Examples of non-biologic treatments that lead t o the accumulation o f p h y t o a l e x i n s a r e treatment w i t h u l t r a v i o l e t light, s a l t s o f heavy metals, and freezing of tissues. E l i c i t o r s a r e compounds that a r e capable of evoking the accumulation of phytoalexins i n plants



8. PAXTON

Fungal Elicitors of Phytoalexins

111

f 1 4 — 19 1 . B i o l o g i c e l i c i t o r s c a n be d i v i d e d into endogenous, such as fragments o f p e c t i n m o l e c u l e s from plant c e l l walls , and exogenous, s u c h a s c a r b o h y d r a t e and glycoprotein molecules from fungal cell walls f20-241. In t h i s paper I w i l l c o n f i n e the d i s c u s s i o n t o r e c e n t work on f u n g a l b i o l o g i c e l i c i t o r s o f p h y t o a l e x i n s . P h y t o a l e x i n s from a l a r g e number o f d i f f e r e n t p l a n t s have b e e n c h e m i c a l l y c h a r a c t e r i z e d . T h e s e phytoalexins include isoflavanoid-derived pterocarpan compounds characteristic o f t h e Leguminosae, sesquiterpenoid compounds c h a r a c t e r i s t i c o f t h e S o l a n a c e a e , p h e n a n t h r e n e compounds characteristic o f t h e Orchidaceae and a c e t y l e n i c compounds c h a r a c t e r i s t i c o f t h e C o m p o s i t a e ( F i g u r e 1) . G l y c e o l l i n , a p h y t o a l e x i n o f soybeans, o c c u r s as a s e r i e s o f i s o m e r s f i r s t i d e n t i f i e d b y Lyne e t a l . [2 51 ; t h e t h r e e most common i s o m e r s a r e shown i n F i g u r e 2 . P l a n t s f r e q u e n t l y produce a s e r i e s o f a c t i v e isomers and r e l a t e d p h y t o a l e x i n s [2_£] .

Elicitors E l i c i t o r s were f i r s t d i s c o v e r e d b y C r u i c k s h a n k [27.] , who f o u n d a p r o t e i n , M o n i l i c o l i n A (Mr=6K), p r o d u c e d b y Monolinia f r u c t i c o l a . This protein e l i c i t s the phytoalexin p h a s e o l l i n i n garden beans b u t n o t t h e related phytoalexins i n pea o r broad bean. Another protein, found only recently t o be p r o d u c e d by P h y t o p h t h o r a p a r a s i t i c a v a r . n i c o t i a n a e . i s a c t i v e a t 20 ng/cotylendon [4x10" -^mole] . T h i s p r o t e i n (Mr=46K) may be a fi 1-4 e n d o x y l a n a s e [2JL] . Many o f t h e e l i c i t o r s studied a r e carbohydrates [2 9,301 . T h a t carbohydrates carry 'recognizable information' i s well known. An example o f s p e c i f i c r e c o g n i t i o n o f s u b t l e changes i n surface carbohydrates i s t h e human b l o o d g r o u p a n t i g e n s (31). ' R e c o g n i t i o n ' o f microorganisms i s b e i n g s t u d i e d i n several plant-microorganism i n t e r a c t i o n s [.3_2] . A n o t h e r a r e a o f i n t e r e s t i s how t h e e l i c i t o r s i g n a l i s t r a n s d u c e d to the p l a n t nucleus f o r d i r e c t e d p r o c e s s i n g o f n u c l e i c a c i d s a n d / o r de novo p r o d u c t i o n o f p r o t e i n s • An e l i c i t o r p r o d u c e d b y P_. mega s p e rma f . sp. g l y c i n e a was i d e n t i f i e d b y F r a n k a n d P a x t o n [3_4] as a g l y c o p r o t e i n . A y e r s e t a l [35-381 i s o l a t e d f o u r d i f f e r e n t carbohydrate f r a c t i o n s f r o m P h y t o p h t h o r a megasperma f . s p . g l y c i n e a t h a t were a c t i v e i n e l i c i t i n g phytoalexin production by soybean suspension cells r2 5 1 a n d cotyledons, using a bioassay developed by Frank and Paxton [341 . An e l i c i t o r from t h i s fungus has been characterized a n d s y n t h e s i z e d [40,411 , a n d i s shown i n F i g u r e 3 . The s o y b e a n p l a n t c o n t a i n s enzymes c a p a b l e o f r e l e a s i n g a c t i v e f r a g m e n t s from t h e w a l l s o f Oomycetes f 4 2 , 4 3 1 , which a l s o e l i c i t i n other plant systems [44,45].



112


Orchinol

S a f

y

n o 1

Figure 1. P h y t o a l e x i n s f r o m : Leguminosae, pisatin from peas; Solanaceae, r i s h i t i n from potato; Orchidaceae, o r c h i n o l from O r c h i s spp.; Compositae, s a f y n o l from safflower.



PAXTON

Fungal Elicitors ofPhytoalexins

Figure 3. megasperma

Elicitor derivative f . sp. glycinea.

from

Phytophthora



114


Anderson [461 found that C o l l e t o t r i c h u m l i n d i m u t h a n e u m , the c a u s a l agent of bean a n t h r a c n o s e , p r o d u c e d a compound i n c u l t u r e w h i c h was active in a s p e c i f i c manner even a t 10 ng/ml. Hamdan and D i x o n r471 found that this fungus produces a mannose-rich p o l y s a c c h a r i d e t h a t e l i c i t s t h r e e enzymes i n p h y t o a l e x i n s y n t h e s i s and f o u n d a p o l y s a c c h a r i d e w i t h l o w e r mannose content, that preferentially induced one of these enzymes, c h a l c o n e s y n t h a s e . Chitin, an i m p o r t a n t component o f nematode e g g s , i n s e c t e x o s k e l e t o n s and t h e c e l l w a l l s o f many p l a n t p a t h o g e n i c f u n g i , a l s o i s an e f f e c t i v e e l i c i t o r . Hadwiger and colleagues [4JL] f o u n d t h a t c h i t o s a n , a deacylated fragment of c h i t i n , at 10 ug/ml e l i c i t s phytoalexin a c c u m u l a t i o n i n p e a s , and a c t s as a f u n g i c i d e against F u s a r i u m s o l a n i , a p a t h o g e n o f p e a s . The heptamer was the s m a l l e s t a c t i v e polymer s i z e . V a r i o u s r e s e a r c h e r s have s t u d i e d t h e e l i c i t a t i o n o f potato phytoalexins by a r a c h i d o n i c and eicosapentaenoic acids f 2 4 , 4 9-51 1 . These compounds are released by Phytophthora infestans, e l i c i t phytoalexin accumulation i n p o t a t o a t 10 u g / t i s s u e s l i c e , and a r e p o t e n t i a t e d by g l u c a n s f r o m t h i s fungus [JIL2] . V e r t i c i l l i u m albo-atrum releases a glucan [Mr

Fungal Elicitors of Phytoalexins and Their Potential Use in Agriculture

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