Chapter 3
Biochemistry of Non-Host-Selective
Phytotoxins
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Richard D. Durbin Agricultural Research Service, U.S. Department of Agriculture and Department of Plant Pathology, University of Wisconsin—Madison, Madison, WI 53706
Many plant pathogens produce non host-selection toxins as part of their attack mechanisms. The benefical application of these toxins to agricultural problems requires knowledge about their structure, metabolism, regulation and mechanism of action. Such knowledge is still quite meager except for structural studies and, to a lesser extent, mechanism of action. The majority of bacterial toxins are small, modified peptides, plant growth hormones, or high molecular weight polysaccharide-like molecules. Fungal toxins are structurally more diverse and, correspondingly, involve a wide variety of biogenetic pathways. Some toxins are activated in the host by hydrolytic cleavage; whether this is commonplace or not is unknown. Even though these toxins have no plant selectivity, they are are highly selective in how they act (i. e., inhibit specific enzymes). Since many pathogens also contain these targets, they possess multiple mechanisms for protecting themselves against their own toxins. P r a c t i c a l l y a l l phytotoxins are non-selective, that i s , although the pathogens t h a t produce them may s e l e c t i v e l y a t t a c k o n l y c e r t a i n p l a n t s p e c i e s , t h i s i s not determined by t h e t o x i n s . U s u a l l y , i n f a c t , p r e p a r a t i o n s o f these t o x i n s w i l l a f f e c t a l m o s t any h i g h e r p l a n t s p e c i e s as w e l l as o t h e r e u k a r y o t i c and p r o k a r y o t i c organisms. On t h e o t h e r hand, the s u b c e l l u l a r s i t e s where these t o x i n s a c t a r e i n most cases h i g h l y s p e c i f i c and a r e unique f o r each t o x i n . I n f a c t , t h e degree o f b i o c h e m i c a l s p e c i f i c i t y they e x h i b i t i s every b i t as g r e a t as t h a t o f t h e h o s t - s e l e c t i v e t o x i n s . The o n l y d i f f e r e n c e between the two groups i s t h a t t h e t a r g e t s o f non h o s t - s e l e c t i v e t o x i n s a r e more w i d e l y d i s t r i b u t e d among p l a n t s p e c i e s (Table I ) . This chapter not subject to U.S. copyright Published 1990 American Chemical Society
Hoagland; Microbes and Microbial Products as Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
64
MICROBES AND MICROBIAL PRODUCTS AS HERBICIDES
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Table I.
A C l a s s i f i c a t i o n Scheme o f P h y t o t o x i n s on T h e i r Proposed S i t e o f A c t i o n
Based
Phytotoxin
Proposed s i t e o f a c t i o n
Fumaric a c i d Tabtox i n i ne-β-lac tarn Fusicoccin Tentoxin T-toxins AAL-toxins
OH, NH , SH groups glutamine synthetase plasmalemma AT Pa se coupling factor 1 m i t o c h o n d r i a l membrane aspartate carbamoyltransferase
3
2
SOURCE: Reproduced from reference 29.
a
T h e s e examples c o v e r the range i n non h o s t - s e l e c t i v e t o x i n s from no s p e c i f i c i t y ( f u m a r i c a c i d ) through those t h a t a r e s i t e s p e c i f i c , but have t a r g e t s i n microbes and a n i m a l s too ( T - 0 - L ) , only plants ( f u s i c o c c i n ) , or selected p l a n t species ( t e n t o x i n ) . H o s t - s e l e c t i v e t o x i n s (T- and AAL-- t o x i n s ) have t a r g e t s b i o c h e m i c a l l y s i m i l a r t o those o f advanced non h o s t - s e l e c t i v e t o x i n s , but t h e i r d i s t r i b u t i o n i s more h i g h l y r e s t r i c t e d .
The u s u a l symptoms e l i c i t e d i n the h o s t by p h y t o t o x i n s , t o name the most common, a r e c h l o r o s i s , n e c r o s i s , w i l t i n g , w a t e r s o a k i n g , s t u n t i n g , growth a b n o r m a l i t i e s and r o o t f o r m a t i o n . A s p e c i a l word about p h y t o t o x i n s t h a t induce r o o t f o r m a t i o n and growth a b n o r m a l i t i e s . I mean t o i n c l u d e here not o n l y those compounds t h a t are s y n t h e s i z e d s o l e l y by m i c r o o r g a n i s m s , but a l s o those t h a t a r e a l s o p r o d u c t s o f h i g h e r p l a n t s (e. g., a u x i n s , g i b b e r e l l i n s and c y t o k i n i n s ) . The c r i t i c a l p o i n t s here a r e t h a t t h e i r s y n t h e s i s must be c a r r i e d out by the pathogen and t h a t i t i s t h i s p r o d u c t i o n t h a t i s r e s p o n s i b l e f o r the growth a b n o r m a l i t y . Thus, j u s t because a compound i s s y n t h e s i z e d by a h i g h e r p l a n t i s no r e a s o n a p r i o r i t o d i s a l l o w i t as a p h y t o t o x i n . U n f o r t u n a t e l y , symptom e x p r e s s i o n can t e l l us very l i t t l e about a t o x i n ' s mode o f a c t i o n , s i n c e each symptom can r e s u l t from t o x i n s a c t i n g a t very d i f f e r e n t l o c a t i o n s i n the complex p h y s i o - c h e m i c a l events o f metabolism. Furthermore, the b i o c h e m i c a l means by which any one o f t h e s e symptoms can be produced are many and complex, and can i n v o l v e q u i t e d i f f e r e n t pathways. Me are most aware o f symptoms t h a t a r e v i s i b l e above ground, hence a l m o s t a l l o f our a t t e n t i o n has been on t o x i n s o f a e r i a l pathogens. I t ' s q u i t e c l e a r though t h a t s o i l - b o r n e microorganisms a l s o produce t o x i n s . Some o f t h e s e a r e now being examined i n more d e t a i l f o r t h e i r a b i l i t y t o s e l e c t i v e l y k i l l o r slow the growth o f weedy p l a n t s p e c i e s (1 and C h a p t e r 1 5 ) . I suspect t h a t t h e i r study w i l l u n v e i l some q u i t e e x c i t i n g and unusual s t r u c t u r e s and mechanisms o f a c t i o n .
Hoagland; Microbes and Microbial Products as Herbicides ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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3.
DURBIN
Biochemistry cfN
