4 Mode of Action of Herbicides D O N A L D E. M O R E L A N D
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U. S. Department of Agriculture, Agricultural Research Service Crop Science Department, North Carolina State University, Raleigh, N . C . 27607
The primary biochemical sites of action of some herbicides have been i d e n t i f i e d and an appreciation i s being gained on how these same herbicides express phytotoxicity by i n t e r f e r i n g with the plant's biochemistry. The progress being made in this area of research accompanies the increased comprehension that i s be ing achieved on the basic biochemistry of plant growth and on the endogenous control systems that regulate growth and development. Corbett (1) recently summarized the current status of b i o chemical knowledge on the mode of action of herbicides in the general form shown i n Figure 1. Interference with the processes i d e n t i f i e d in the left-hand column has been documented for the action of one or more herbicides (1, 2, 3, 4 ) . Interferences are indicated as affecting various interrelated processes (structural organization, energy supply, and growth and reproduction). I f the interference i s extreme, the treated plant dies. Thiolcarbamates have been shown to interfere with lipid syn thesis and, thereby, to a l t e r the i n t e g r i t y of membranes. Some of the pyridazinones interfere not only with lipid synthesis, but also with the Hill reaction and carotenoid synthesis. The bipy ridiliums intercept photoinduced electron flow i n the chloroplasts and undergo one-electron reduction to form free r a d i c a l s . When the radicals are oxidized, hydrogen peroxide i s formed, which i s thought to react with unsaturated membrane lipids. Membrane per meability i s increased and, subsequently, c e l l u l a r structure i s destroyed. Mitochondrial electron transport and oxidative phos phorylation are affected by a large group of herbicides, including the N-phenylcarbamates, a c y l a n i l i d e s , phenols, and halogenated b e n z o n i t r i l e s . Most of the herbicides that interfere with the mitochondrial reactions also i n h i b i t photosynthetic electron transport as do the phenylureas, s - t r i a z i n e s , and u r a c i l s . The N-phenylcarbamates and d i n i t r o a n i l i n e s , in addition to affecting the mitochondrial and chloroplast reactions, arrest cell d i v i s i o n . Glyphosate has been reported to interfere with protein synthesis 56
In Pesticide Chemistry in the 20th Century; Plimmer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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Action
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i n Lerrma (5) . I d e n t i f i c a t i o n o f t h e b i o c h e m i c a l mechanisms i n v o l v e d i n t h e a c t i o n of the phenoxies continues t o challenge i n v e s t i g a t o r s . These h e r b i c i d e s a r e d e p i c t e d , i n F i g u r e 1 , as a f f e c t i n g t h e un known s i t e a t w h i c h t h e n a t i v e hormone, i n d o l e a c e t i c a c i d , e x presses i t s growth-controlling a c t i o n ( 1 ) . Of t h e v a r i o u s b i o c h e m i c a l p a t h w a y s i d e n t i f i e d as b e i n g a f f e c t e d by h e r b i c i d e s , the c h l o r o p l a s t - m e d i a t e d r e a c t i o n s have r e c e i v e d t h e g r e a t e s t a t t e n t i o n . A p p r o x i m a t e l y 70 p e r c e n t o f t h e c u r r e n t c o m m e r c i a l h e r b i c i d e s , w h i l e t h e y may a l s o a f f e c t o t h e r systems, i n t e r f e r e w i t h c h l o r o p l a s t r e a c t i o n s . Hence, the o b j e c t i v e s o f t h i s p a p e r a r e t o r e v i e w some o f t h e w o r k c o n d u c t e d w i t h i s o l a t e d c h l o r o p l a s t s , e v a l u a t e the s t a t u s o f these s t u d i e s , and r e l a t e the observed i n t e r f e r e n c e s t o the expression o f p h y t o t o x i city. Chloroplas t-mediated
Reactions.
I n t e r f e r e n c e by c e r t a i n p h e n y l u r e a and #-phenylcarbamate h e r b i c i d e s with the photochemical reactions o f i s o l a t e d c h l o r o p l a s t s was f i r s t r e p o r t e d i n 1956 ( 2 ) . O v e r t h e n e x t few y e a r s , i n h i b i t i o n by t h e s - t r i a z i n e s , u r a c i l s , b e n z i m i d a z o l e s , and benz o n i t r i l e s was r e p o r t e d (2^, _3, 6) . C h l o r o p l a s t s o f h i g h e r p l a n t s a r e s a u c e r - s h a p e d , and f r o m 4 t o 10 y m i n d i a m e t e r and 1 t o 3 ym t h i c k . The c h l o r o p h y l l i s concentrated i n bodies w i t h i n the c h l o r o p l a s t s c a l l e d grana, w h i c h a r e a b o u t 0.4 ym i n d i a m e t e r . Under t h e e l e c t r o n m i c r o scope, t h e grana appear as h i g h l y o r g a n i z e d , p r e c i s e l y s t a c k e d l a m e l l a e , t o w h i c h t h e c h l o r o p h y l l i s b o u n d , imbedded i n a s t r o m a matrix. The l i g h t a n d a s s o c i a t e d e l e c t r o n t r a n s p o r t r e a c t i o n s t a k e p l a c e i n t h e l a m e l l a e , w h e r e a s enzymes i n v o l v e d i n c a r b o n d i o x i d e f i x a t i o n a r e l o c a t e d i n the stroma. P h o t o i n d u c e d e l e c t r o n t r a n s p o r t and the c o u p l e d p h o s p h o r y l a t i o n r e a c t i o n s as they a r e p o s t u l a t e d t o o c c u r i n c h l o r o p l a s t s a r e p r e s e n t e d s c h e m a t i c a l l y i n F i g u r e 2. N o t a l l i n v e s t i g a t o r s a g r e e o n t h e d e t a i l s o f t h i s scheme, a n d some e v e n q u e s t i o n t h e sequence o f t h e i n t e r m e d i a t e s . The numbers a n d l o c a t i o n s o f t h e p h o s p h o r y l a t i o n s i t e s a l s o remain t o be i d e n t i f i e d p r e c i s e l y . However, t h e scheme i s a r e a s o n a b l e a p p r o x i m a t i o n b a s e d o n a v a i l able information. Reactions that occur i n the l i g h t a r e r e p r e s e n t e d b y t h e o p e n arrows, a n d t h e s o l i d a r r o w s r e p r e s e n t e l e c t r o n t r a n s f e r s t h a t occur i n the dark. Through a s e r i e s o f o x i d a t i o n - r e d u c t i o n r e a c t i o n s d r i v e n by two l i g h t r e a c t i o n s o p e r a t i n g i n s e r i e s a n d i n v o l v i n g s e v e r a l h u n d r e d c h l o r o p h y l l m o l e c u l e s , e l e c t r o n s f l o w f r o m w a t e r t o NADP. P a r t i c i p a t i n g i n t h e o v e r a l l r e a c t i o n i s a w a t e r - s p l i t t i n g com p l e x t h a t i n c l u d e s a m a n g a n o - p r o t e i n a n d c h l o r i d e i o n s . An un i d e n t i f i e d c h l o r o p h y l l α molecule serves as the r e a c t i o n center o f p h o t o s y s t e m I I , w i t h Q as t h e p r i m a r y e l e c t r o n a c c e p t o r . I n v o l v e d s e q u e n t i a l l y on the e l e c t r o n t r a n s p o r t c h a i n a r e p l a s t o -
In Pesticide Chemistry in the 20th Century; Plimmer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
In Pesticide Chemistry in the 20th Century; Plimmer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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PHOSPHORYLATION
ELECTRON TRANSPORT
ELECTRON TRANSPORT
AT
Figure 1.
COMBINATION
SITE
—
MODIFIED
GROWTH AND REPRODUCTION
ENERGY SUPPLY
STRUCTURAL ORGANIZATION
FUNCTION
Summary diagram of the mode of action of pesticides [adapted from Corbett (1)]
IAA
DIVERTED
DESTRUCTION OF PIGMENTS
SYNTHESIS
RNA,
DNA,
PROTEIN
OR NUCLEAR DIVISION
SYNTHESIS—>
CELL
CAROTENOID
PHOTOSYNTHETIC ELECTRON TRANSPORT ( H I L L REACTION)
OXIDATIVE
MITOCHONDRIAL
PHOTOSYNTHETIC
INTEGRITY-
SYNTHESIS—
MEMBRANE
LIPID
INTERFERENCE WITH
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DEATH
or
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4.
MORELAND
Action of
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Herbicides
-0.6 r-
-0.4 NADP+
Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0037.ch004
-0.2
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Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions; solid arrows indicate dark reactions; and the narrow dashed line represents the cyclic pathway. Abbreviations used: PS J, photosystem I; PS 11, photosystem 11; Ύ, postulated electron donor for photosystem 11; Q, unknown primary electron acceptor for photosystem 11; PQ, plastoquinones; cyt b, b-type cyto chromes; cyt f, cytochrome f; P C , plastocyanin; P , reaction center chlorophyll of photosystem 1; F R S , ferredoxin-reducing substance; Fd, ferredoxin; Fp, ferreaoxinNADP oxidoreductase; FeCy, ferricyanide; asc, ascorbate; and DPIP, 2,6-dichlorophenolindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by herbicides. See text for details. 70fl
In Pesticide Chemistry in the 20th Century; Plimmer, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
PESTICIDE C H E M I S T R Y IN
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q u i n o n e , a b-type c y t o c h r o m e , c y t o c h r o m e / ( a
