1 Physiological Basis of Phloem Transport of Agrichemicals
Bioregulators for Pest Control Downloaded from pubs.acs.org by 185.101.68.27 on 01/05/19. For personal use only.
ROBERT T. GIAQUINTA Experimental Station, Central Research and Development Department, Ε. I. du Pont de Nemours & Co., Inc., Wilmington, DE 19898 Agrichemical transport in the phloem is discussed in terms of the physiological, biochemical, and struc tural bases of assimilate translocation. Specifically, the cellular pathways and mechanisms of phloem loading in source leaves, long distance transport, and phloem unloading in sinks, are used as a framework for examin ing the biological basis of the systemic mobility of agrichemicals. Phloem transport is the process responsible for the systemic mobility of agrichemicals in plants. From a practical viewpoint, knowledge of the structural and chemical properties of molecules that are necessary for phloem mobility should have considerable impact on the rational design of systemic agrichemicals with im proved efficacy. Unfortunately, l i t t l e practical information exists on structure-activity relationships of agrichemicals with respect to phloem mobility. That is, what is it about a molecule that governs its ability to be translocated in the phloem? In general, several factors ultimately determine whether an agrichemical moves to its site of action in the plant. These include: (1) efficient chemical penetration through the cuticle of the leaves and stems; (2) the ability of the chemical to enter the symplast or metabolic compartment of the cell ( i . e . , crossing the cell membrane); (3) short- and long-distance transport, either cell-to-cell via plasmodesmata or in the xylem, or phloem; (4) metabolism or conju gation of an agrichemical to an inactive form; and (5) immobiliza tion of the agrichemical at non-active sites (e.g., binding at the cell walls, sequestering in the vacuole, or adsorption to cellular protein). No single factor will dictate whether a chemical is translocated in a l l cases and a complex interrelationship probably exists between a l l of these. The reader is referred to several recent and comprehensive reviews on various aspects of xenobiotic entry and transport within plants (1-5). In this review I address the phloem mobility of agrichemicals from the viewpoint of a phloem physiologist. First, I will present an overview of the physiological basis of translocation by using 0097-6156/85/0276-0007$06.00/0 © 1985 American Chemical Society
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what we know about the i n v i v o t r a n s p o r t o f s u c r o s e as a way o f d e s c r i b i n g the c h a r a c t e r i s t i c s o f the t r a n s l o c a t i o n system which are r e l e v a n t t o a g r i c h e m i c a l t r a n s p o r t . T h i s i s i m p o r t a n t because i f we d e s i r e t o r a t i o n a l l y d e s i g n m o l e c u l e s w i t h improved phloem m o b i l i t y , we need t o be aware o f the s t r u c t u r a l , b i o c h e m i c a l , b i o p h y s i c a l , and p h y s i o l o g i c a l a s p e c t s o f the t r a n s p o r t p r o c e s s i t s e l f . The second p a r t o f t h i s r e v i e w w i l l h i g h l i g h t the p r o p e r t i e s o f x e n o b i o t i c movement i n p l a n t s . A l t h o u g h space c o n s t r a i n t s i n e v i t a b l y l i m i t an i n - d e p t h treatment o f t h i s s u b j e c t , I hope the b r o a d - s t r o k e approach p r e s e n t e d here w i l l g e n e r a t e a b e t t e r u n d e r s t a n d i n g o f the b i o l o g i c a l b a s i s f o r t r a n s l o c a t i o n and, more i m p o r t a n t l y , spur a d d i t i o n a l r e s e a r c h i n the r e l a t i v e l y u n c h a r t e d a r e a o f phloem p h y s i o l o g y and agrichemical transport. P h y s i o l o g i c a l B a s i s o f Phloem T r a n s l o c a t i o n The t r a n s l o c a t i o n system i s u s u a l l y d i v i d e d i n t o t h r e e s t r u c t u r a l l y and p h y s i o l o g i c a l l y d i s t i n c t r e g i o n s : (1) the s o u r c e , u s u a l l y the p h o t o s y n t h e t i c l e a v e s p r o d u c i n g s u g a r s ; (2) the p a t h , a s e r i e s o f c o n n e c t i n g s i e v e elements which comprise the c o n d u i t s f o r a s s i m i l a t e f l o w ; and (3) the s i n k , which i s comprised o f a s s i m i l a t e - c o n s u m i n g o r t a r g e t c e l l s ( e . g . , growing, u t i l i z i n g , o r s t o r a g e r e g i o n s ) . In s o u r c e l e a v e s , phloem l o a d i n g i s the p r o c e s s whereby photos y n t h e t i c a l l y - d e r i v e d s u c r o s e produced i n the m e s o p h y l l i s accumu l a t e d i n t o the minor v e i n network o f the phloem. This sucrose accu m u l a t i o n i n c r e a s e s the s o l u t e p o t e n t i a l o f the s i e v e t u b e s , c a u s i n g water from the s u r r o u n d i n g t i s s u e s t o e n t e r the phloem t o produce h y d r o s t a t i c p r e s s u r e (6^, _7). At the s i n k end, a s s i m i l a t e s e x i t the s i e v e tubes by a v a r i e t y o f mechanisms (see below) t h e r e b y r e d u c i n g the s u c r o s e c o n c e n t r a t i o n i n t h i s p a r t o f the system ( 8 ) . Because o f t h i s p r e s s u r e and c o n c e n t r a t i o n d i f f e r e n c e , water, s u c r o s e , and any o t h e r s u b s t a n c e ( i n c l u d i n g a g r i c h e m i c a l s ) p r e s e n t i n the phloem w i l l move i n b u l k o r mass f l o w from s o u r c e t o s i n k . The d i r e c t i o n o f t h i s o s m o t i c a l l y - d r i v e n flow i n the phloem i s governed s o l e l y by the p o s i t i o n o f s o u r c e s and s i n k s i n the p l a n t . However, the p o s i t i o n o f these s o u r c e s and s i n k s can d i f f e r a t d i f f e r e n t s t a g e s o f l e a f development o r p l a n t ontogeny (_7). Phloem L o a d i n g . How does s u c r o s e which i s produced i n the m e s o p h y l l c e l l s o f s o u r c e l e a v e s e n t e r the t r a n s l o c a t i o n stream and how does t h i s s u c r o s e e x i t from the t r a n s l o c a t i o n stream i n the s i n k r e g i o n s ? More i m p o r t a n t l y , can t h i s t e l l us a n y t h i n g about a g r i c h e m i c a l transport? F i g u r e 1A shows an a u t o r a d i o g r a p h o f a source l e a f f o l l o w i n g the a c c u m u l a t i o n o f - ^ C - s u c r o s e ( l ^ C - l a b e l i s i n w h i t e ) . The l ^ C - l a b e l i s accumulated markedly i n t o the v e i n network com p r i s e d o f the minor v e i n phloem. T h i s i s an e x t e n s i v e network about 70 cm veins/cm^ l e a f - and thus i t r e p r e s e n t s an e f f i c i e n t c o l l e c t i n g system f o r b o t h s u c r o s e which i s produced i n the meso p h y l l and f o r c h e m i c a l s e n t e r i n g the l e a f . F i g u r e IB i l l u s t r a t e s d i a g r a m m a t i c a l l y a c r o s s s e c t i o n o f a s i n g l e minor v e i n t r a c e d from an e l e c t r o n m i c r o g r a p h . The v a s c u l a r bundle i s composed o f a s i n g l e xylem element and the phloem bundle which c o n t a i n s two c e n t r a l l y l o c a t e d s i e v e tubes surrounded by s p e c i a l i z e d phloem c e l l s c a l l e d e i t h e r companion c e l l s or t r a n s f e r c e l l s depending on the p r e s e n c e
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Phloem Transport of Agrichemicals
F i g u r e 1. Source l e a f minor v e i n phloem. (A) A u t o r a d i o g r a p h o f l e a f t i s s u e s f o l l o w i n g -^C-sucrose a c c u m u l a t i o n showing r a d i o a c t i v i t y (white) i n v e i n s . (B) T r a c i n g o f an e l e c t r o n m i c r o graph o f a c r o s s s e c t i o n o f minor v e i n , x, xylem, vp, v a s c u l a r parenchyma; c c , companion c e l l ; se, s i e v e element; pp, phloem parenchyma, mc, m e s o p h y l l c e l l . Reproduced w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1983. Annual Reviews.
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o f c e l l w a l l ingrowths ( t r a n s f e r c e l l s have w a l l i n g r o w t h s ) . The e n t i r e bundle i s surrounded by m e s o p h y l l c e l l s and phloem parenchyma c e l l s (6). Substances can e n t e r the phloem v i a s e v e r a l r o u t e s , f o r example,from the a p o p l a s t , and c e l l - t o - c e l l (see arrows i n F i g u r e IB). S e v e r a l l i n e s o f e v i d e n c e show t h a t sugars do not s i m p l y d i f f u s e down a c o n c e n t r a t i o n g r a d i e n t from the m e s o p h y l l to phloem. I n s t e a d t h e r e i s a marked c o n c e n t r a t i o n o f sugars i n the s i e v e element-companion c e l l complex i n d i c a t i n g t h a t a c o n c e n t r a t i n g mechanism e x i s t s a t the m e s o p h y l l - p h l o e m i n t e r f a c e . The c u r r e n t body o f e v i d e n c e i n d i c a t e s t h a t p h o t o s y n t h e t i c a l l y d e r i v e d s u c r o s e t r a v e l s s y m p l a s t i c a l l y ( v i a plasmodesmata) to the phloem r e g i o n , then e x i t s the symplasm i n t o the a p o p l a s t where i t i s t h e n a c t i v e l y accumulated a c r o s s the phloem c e l l membranes ( 8 ) . The c u r r e n t w o r k i n g model f o r the mechanism o f s u c r o s e uptake a c r o s s the phloem membranes i s i l l u s t r a t e d i n F i g u r e 2. In t h i s model, s u c r o s e which i s the f r e e space, i n t e r a c t s w i t h a s u c r o s e - s p e c i f i c c a r r i e r on the membrane ( 6 ) . We know v e r y l i t t l e about t h i s p u t a t i v e s u c r o s y l c a r r i e r o t h e r than t h a t i t c o n t a i n s e s s e n t i a l s u l f h y d r y l groups and i s h i g h l y s e l e c t i v e f o r sucrose. The c h a r a c t e r i s t i c s o f the phloem i t s e l f f i g u r e p r o m i n e n t l y i n t h i s proposed mechanism. The phloem i n t e r i o r has a low p r o t o n con c e n t r a t i o n (pH 7.5 t o 8.0) r e l a t i v e t o the a p o p l a s t which has a h i g h p r o t o n c o n c e n t r a t i o n (pH 5.5). Thus, a s u b s t a n t i a l p r o t o n g r a d i e n t o f up t o 2-3 pH u n i t s e x i s t s a c r o s s the phloem membranes. More c o r r e c t l y , t h e r e i s an e l e c t r o c h e m i c a l p o t e n t i a l g r a d i e n t a c r o s s the phloem membrane which g i v e s r i s e to an i n t e r i o r n e g a t i v e membrane p o t e n t i a l o f about -150 mv. It i s believed that t h i s electrochem i c a l p o t e n t i a l o f p r o t o n s which e x i s t s a c r o s s the phloem membranes i s the d r i v i n g f o r c e f o r s u c r o s e l o a d i n g . The g r a d i e n t i s e s t a b l i s h e d by a "metabolism-dependent" p r o t o n pump, presumably an ATPase enzyme which i s l o c a t e d on the phloem membrane. It i s envisioned t h a t s u c r o s e uptake i s c o u p l e d t o the c o - t r a n s p o r t o f p r o t o n s where by the e n e r g e t i c a l l y " d o w n h i l l " movement o f p r o t o n s i n t o the phloem i s c o u p l e d t o the " s e c o n d a r y " a c t i v e t r a n s p o r t o f s u c r o s e i n t o the phloem (6^). As d i s c u s s e d below, t h e s e c h e m i c a l and e l e c t r i c a l prop e r t i e s o f the phloem can i n f l u e n c e the a b i l i t y o f a g r i c h e m i c a l s t o e n t e r the phloem. The c h a r a c t e r i s t i c s o f the phloem l o a d i n g system can be sum m a r i z e d as f o l l o w s . Sucrose l o a d i n g i s : (1) dependent on metabo l i s m ; (2) c a r r i e r - m e d i a t e d ; (3) s e l e c t i v e f o r s u c r o s e ; (4) m a i n t a i n s a h i g h c o n c e n t r a t i o n i n s i d e the phloem which i s the b a s i s f o r the o s m o t i c a l l y - d r i v e n mass flow o f s o l u t i o n s ; and (5) dependent on the f a c t o r s which c o n t r o l a s s i m i l a t e s u p p l y t o the l o a d i n g s i t e s ( e . g . , p h o t o s y n t h e s i s , s u c r o s e s y n t h e s i s , and s u c r o s e movement between l e a f c e l l s , and w i t h i n s u b c e l l u l a r compartments such as the c y t o plasm and v a c u o l e ) (^, 7^). V a s c u l a r Anatomy. One a s p e c t o f the t r a n s l o c a t i o n system t h a t i s o f t e n o v e r l o o k e d i s the i n f l u e n c e o f the s t r u c t u r a l f e a t u r e s o f the p l a n t ' s v a s c u l a r system on s o l u t e t r a n s p o r t . F o r example, a l t h o u g h much o f what i s known about phloem l o a d i n g has been d e r i v e d from a few d i c o t y l e d o n l e a v e s , a l l d i c o t y l e d o n l e a v e s are not s i m i l a r . A n o t a b l e example i s the soybean l e a f . Soybean l e a v e s a r e s p e c i a l i z e d i n t h a t they have a unique c e l l type c a l l e d the p a r a v e i n a l m e s o p h y l l
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GIAQUINTA
Phloem Transport of Agrichemicals
SIEVE ELEMENT COMPANION CELL MEMBRANE
SUCROSE f SUCROSE
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ATPost
pH 5.5 LOWK* LOW SUCROSE
pH 8.5 HIGH K* HIGH SUCROSE
(-)
Figure 2. Model for phloem loading of sucrose. See text for details. Reproduced with permission from Ref. 8. Copyright 1980. Academic Press.
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(PVM) (9^, 10). In c r o s s s e c t i o n the PVM appears as a s i n g l e , d i s c o n t i n u o u s l a y e r o f c e l l s i n the c e n t e r of the l e a f ( F i g u r e 3A). The s i g n i f i c a n c e o f the c e l l type t o t r a n s p o r t , however, i s i n d i c a t e d i n a paradermal s e c t i o n ( F i g u r e 3B) which shows t h a t the PVM forms an i n t e r c o n n e c t i n g network o f c e l l s i n the c e n t e r o f the l e a f t h a t c o n n e c t s t o the phloem. A l l a s s i m i l a t e s produced i n the p a l i s a d e and spongy m e s o p h y l l appear to pass t h r o u g h the PVM b e f o r e they e n t e r the phloem. I t i s a l s o i n t e r e s t i n g t h a t the v a c u o l e s o f these c e l l s accumulate s u b s t a n t i a l amounts o f a g l y c o p r o t e i n d u r i n g c e r t a i n s t a g e s o f l e a f development ( f l o w e r i n g t o e a r l y p o d - f i l l i n soy beans) (9^, 10). T h i s p r o t e i n , which p r o v i d e s a n i t r o g e n r e s e r v e f o r seed growth, c o u l d cause s e q u e s t e r i n g o f a g r i c h e m i c a l s a t nonactive sites. A n o t h e r example o f v a s c u l a r d i f f e r e n c e s i n p l a n t s i s r e p r e sented by monocotyledon g r a s s e s such as wheat ( F i g u r e 3C). The v a s c u l a r bundle i n wheat and i n many g r a s s e s i s surrounded by a mestome sheath which i s comprised o f an impermeable, s u b e r i z e d l a y e r o f c e l l s (6). T h i s may r e p r e s e n t a f o r m i d a b l e b a r r i e r t o f o l i a r applied hydrophilic agrichemicals. These s t r u c t u r a l f e a t u r e s may need t o be t a k e n i n t o account when s e e k i n g t o r a t i o n a l l y d e s i g n c r o p s p e c i f i c systemic chemicals. There are even s t r u c t u r a l d i f f e r e n c e s among the d i f f e r e n t g r a s s s p e c i e s . F o r example, i n g r a s s e s t h a t have C-3 p h o t o s y n t h e s i s , such as b a r l e y , o a t s , wheat, and f e s c u e , t h e r e are 11-15 m e s o p h y l l c e l l s between each l o n g i t u d i n a l v a s c u l a r b u n d l e , w i t h a d i s t a n c e between each v a s c u l a r bundle o f about 0.30 mm. In c o n t r a s t , g r a s s e s w i t h C-4 p h o t o s y n t h e s i s , l i k e c o r n , s o r g hum, s u g a r c a n e , f o x t a i l , c r a b g r a s s , b a r n y a r d g r a s s , have o n l y 2 m e s o p h y l l c e l l s between each v a s c u l a r bundle and the v a s c u l a r bun d l e s are o n l y 0.1 mm away from each o t h e r (JL1). T h i s s h o r t e r r o u t e o f s u c r o s e t r a n s p o r t from m e s o p h y l l to phloem i n the C-4 s p e c i e s i s thought to be one of the prime reasons why C-4 p l a n t s t r a n s l o c a t e a t a f a s t e r r a t e than C-3 s p e c i e s . I t i s not known i f t h i s i n f l u e n c e s the t r a n s p o r t c h a r a c t e r i s t i c s o f a g r i c h e m i c a l s , but t h e r e are d a t a which suggest t h a t t h e s e s t r u c t u r a l d i f f e r e n c e s may i n f l u e n c e move ment. F o r example, M a r t i n and E d g i n g t o n (12) found t h a t o n l y 3% o f the t o t a l amount o f f e n a r i m o l t h a t was t r a n s p o r t e d i n b a r l e y o c c u r r e d s y m p l a s t i c a l l y , whereas i n soybeans t h i s v a l u e was 43%. Simi l a r l y , the p e r c e n t o f oxamyl t r a n s p o r t w i t h i n the symplasm was 4 and 31% i n b a r l e y and soybean, r e s p e c t i v e l y . The p e r c e n t o f 2 , 4 - d i c h l o r o p h e n o x y a c e t i c a c i d t h a t was t r a n s p o r t e d s y m p l a s t i c a l l y was 65 and 96% i n b a r l e y and soybean, r e s p e c t i v e l y . The reduced amount of s y m p l a s t i c t r a n s p o r t o f these t h r e e c h e m i c a l s i n b a r l e y compared t o soybean may be r e l a t e d t o the d i f f e r e n c e s i n v a s c u l a r anatomy between t h e s e s p e c i e s . Path and S i n k F e a t u r e s . In the t r a n s l o c a t i o n p a t h ( e . g . , stems and p e t i o l e s ) , a s s i m i l a t e s and s o l u t e s move i n mass flow t h r o u g h the c y l i n d r i c a l s i e v e tubes which have open s i e v e p o r e s . The a b i l i t y o f a c h e m i c a l t o l e a k a c r o s s the membrane from the s i e v e tube d u r i n g t r a n s i t w i l l a f f e c t i t s a b i l i t y t o be t r a n s p o r t e d t h r o u g h the e n t i r e pathway. In s i n k r e g i o n s , t h e r e are e s s e n t i a l l y t h r e e i n v i v o pathways by which s u c r o s e e x i t s the s i e v e tubes ( F i g u r e 4 ) . A l l t h r e e are o p e r a t i n g i n d i f f e r e n t t y p e s o f s i n k s and a l l are m e t a b o l i s m depend-
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F i g u r e 3. L e a f v a s c u l a t u r e anatomy. T r a c i n g s from l i g h t m i c r o graphs o f : (A) soybean l e a f c r o s s s e c t i o n showing PVM ( a r r o w s ) ; (B) paradermal s e c t i o n o f soybean l e a f showing i n t e r c o n n e c t i n g PVM (shaded c e l l s ) ; and (C) c r o s s s e c t i o n o f a wheat l e a f showing mestome sheath c e l l s s u r r o u n d i n g v a s c u l a r t i s s u e . Tracings p r o v i d e d by S h i e l a McKelvey.
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ent ( 8 , 1 3 ) . S u c r o s e c a n e x i t t h e s i e v e tube and: (1) e n t e r t h e f r e e space where i t i s h y d r o l y z e d by a c e l l w a l l i n v e r t a s e t o hexose p r i o r t o uptake ( t h i s r o u t e o c c u r s i n c o r n k e r n e l s and sugarcane s t o r a g e s t a l k s ; (2) e n t e r t h e f r e e space and then be accumulated as t h e i n t a c t m o l e c u l e ( e . g . , s u g a r b e e t r o o t , soybean seeds, wheat s e e d s ) ; and (3) e n t e r s i n k c e l l s v i a plasmodesmata c o n n e c t i o n s w i t h o u t l e a v i n g t h e symplasm o r m e t a b o l i c space (growing r o o t s and young l e a v e s ) . D i f f e r e n t pathways e x i s t i n d i f f e r e n t organs and t h i s s h o u l d be r e c o g n i z e d when c o n s i d e r i n g t h e t a r g e t - s p e c i f i c phloem m o b i l i t y o f a g r i c h e m i c a l s . Thus, based on t h e c h a r a c t e r i s t i c s o f the t r a n s p o r t system, s e v e r a l f a c t o r s can be i d e n t i f i e d t h a t i n f l u e n c e t h e e n t r y o f a compound i n t o c e l l s and i t s subsequent t r a n s l o c a t i o n i n t h e phloem. F i r s t , t h e b i n d i n g o f compound t o t h e c e l l w a l l c a n p r e v e n t i n i t i a l e n t r y i n t o the c y t o p l a s m . I n g e n e r a l , because o f the nega t i v e charge o f t h e c e l l w a l l s , p o s i t i v e l y charged compounds w i l l b i n d more than uncharged o r n e g a t i v e l y charged ones. Second, p e r m e a b i l i t y b a r r i e r s , such as t h e s u b e r i z e d l a y e r s s u r r o u n d i n g t h e v a s c u l a r system i n c e r t a i n g r a s s e s , can p r e v e n t t h e movement o f a h y d r o p h i l i c c h e m i c a l d i r e c t l y t o phloem. Third, structural features o f t h e v a s c u l a t u r e , such as v e i n d i s t a n c e o r the p a r a v e i n a l meso p h y l l i n soybeans, may a l s o i n f l u e n c e movement o f a c h e m i c a l t o t h e vein. Fourth, increased l i p o p h i l i c i t y i s u s u a l l y a p r e r e q u i s i t e f o r phloem m o b i l i t y b u t , as w i l l be d i s c u s s e d below, h i g h l y p e n e t r a t i n g c h e m i c a l s can show v e r y l i t t l e s y s t e m i c movement. F i f t h , t h e chemi c a l cannot be a s h o r t - t e r m i n h i b i t o r o f m e t a b o l i s m because phloem l o a d i n g and t r a n s l o c a t i o n a r e h i g h l y dependent on m e t a b o l i c energy. Compounds such as u n c o u p l e r s o f p h o s p h o r y l a t i o n , photosynthetic i n h i b i t o r s , and compounds which i n c r e a s e t h e p e r m e a b i l i t y o f c e l l membranes w i l l a l l i n h i b i t t r a n s l o c a t i o n and thus p r e v e n t compound movement. S i x t h , t h e o v e r a l l d i r e c t i o n o f a s s i m i l a t e movement i s d e t e r m i n e d by t h e r e l a t i v e p o s i t i o n o f s o u r c e s and s i n k s on a p l a n t . Seventh, e n v i r o n m e n t a l f a c t o r s such as l i g h t , t e m p e r a t u r e , and water s t r e s s a f f e c t t r a n s l o c a t i o n . These f a c t o r s s h o u l d be t a k e n i n t o account, p a r t i c u l a r l y during a p p l i c a t i o n of a chemical i n order t o maximize t r a n s l o c a t i o n e f f i c i e n c y . Agrichemical
Transport
The s y s t e m i c m o b i l i t y o f an e f f i c a c i o u s a g r i c h e m i c a l depends on: (1) e f f e c t i v e p e n e t r a t i o n o f the c u t i c l e ; (2) l o n g d i s t a n c e movement w i t h i n t h e p l a n t ; (3) m e t a b o l i c s t a b i l i t y ; and (4) s e l e c t i v e t o x i city. There a r e two components o f t h e p l a n t ' s s t r u c t u r e and volume t h a t a r e important t o l o n g d i s t a n c e x e n o b i o t i c movement: t h e apo p l a s t and t h e s y m p l a s t . The a p o p l a s t i s e s s e n t i a l l y t h e nonm e t a b o l i c space r e s i d i n g o u t s i d e t h e c e l l membrane and c o n s i s t s o f the c e l l w a l l s , xylem, and n o n - l i v i n g f i b e r s . I t i s bounded by t h e c u t i c l e on b o t h l e a f s u r f a c e s . S o l u t e movement i n t h e a p o p l a s t i s s t r o n g l y d i r e c t i o n a l and movement i s u s u a l l y by d i f f u s i o n o r by mass flow i n t h e t r a n s p i r a t i o n stream. A l l c h e m i c a l s e n t e r t h e p l a n t v i a the a p o p l a s t . The symplast i s d e f i n e d as t h e m e t a b o l i c o r c y t o p l a s m i c space r e s i d i n g i n s i d e t h e plasmamembrane. I t a l s o i n c l u d e s the phloem. C h e m i c a l s e n t e r t h e symplast by c r o s s i n g t h e c e l l membrane.
1.
GIAQUINTA
Phloem Transport ofAgrichemicals
15
A g r i c h e m i c a l s which t r a v e l m a i n l y i n t h e a p o p l a s t c h a r a c t e r i s t i c a l l y accumulate a t t h e l e a f t i p s and margins o f mature l e a v e s , whereas compounds t h a t t r a v e l i n t h e phloem accumulate a t growing r e g i o n s ( i . e . , new l e a v e s , buds, r o o t t i p s , and s t o r a g e organs). Based on t h e o v e r a l l d i s t r i b u t i o n p a t t e r n i n p l a n t s , c h e m i c a l t r a n s p o r t h i s t o r i c a l l y has been c h a r a c t e r i z e d as b e i n g a p o p l a s t i c o r symplastic. S i n c e t h e mid-1970's i t has been i n c r e a s i n g l y c l e a r t h a t many compounds a r e ambimobile ( 4 ) , i n t h a t t h e s e c h e m i c a l s t r a v e l i n b o t h t h e a p o p l a s t and symplast depending on t h e p h y s i c a l c h a r a c t e r i s t i c s o f the molecule. I n f a c t , most o f t h e c h e m i c a l s t h a t were p r e v i o u s l y c h a r a c t e r i z e d as moving o n l y i n t h e a p o p l a s t o r xylem a r e now r e g a r d e d as ambimobile because they p e n e t r a t e mem branes q u i t e r e a d i l y ( 4 ) . The f i r s t c l u e s t h a t " a p o p l a s t i c " o r xylem m o b i l e c h e m i c a l s were n o t l i m i t e d t o t h e xylem came from s e v e r a l a n o m o l i e s . These included the following observations: (1) many a p o p l a s t i c chemicals have s y m p l a s t i c s i t e s o f a c t i o n ( t h e p h o t o s y n t h e s i s inhibitors like d i u r o n and a t r a z i n e have t o t r a n s v e r s e n o t o n l y t h e c e l l membrane, but a l s o t h e d o u b l e membrane o f t h e c h l o r o p l a s t and t h e i n t e r n a l t h y l a k o i d membrane); (2) many xylem t r a n s p o r t e d i n s e c t i c i d e s a r e t o x i c t o a p h i d s which f e e d e x c l u s i v e l y on t h e phloem; ( 3 ) b e n z i m i d a z o l e f u n g i c i d e s have c y t o k i n i n - l i k e a c t i v t y s u g g e s t i n g i n t e r a c t i o n w i t h t h e symplasm; (4) many " a p o p l a s t i c " compounds a r e e i t h e r metab o l i z e d t o CO2 o r c o n j u g a t e d w i t h amino a c i d s o r s u g a r s ; ( 5 ) "apo p l a s t i c " c h e m i c a l s t h a t t r a n s p o r t a c r o s s t h e water impermeable c a s p a r i a n s t r i p s i n t h e r o o t s ; and ( 6 ) t h e b a s i p e t a l t r a n s p o r t o f certain fungicides (4). E d g i n g t o n and P e t e r s o n ( 4 ) have s u b d i v i d e d a p o p l a s t i c xenob i o t i c s i n t o two c l a s s e s . Euapoplastic (only transported i n the a p o p l a s t ) and p s e u d o a p o p l a s t i c ( t r a n s p o r t o c c u r s m a i n l y i n t h e xylem but e n t r y i n t o t h e symplast o c c u r s ) . Most t r a d i t i o n a l " a p o p l a s t i c " c h e m i c a l s a r e now known t o r e a l l y be p s e u d o a p o p l a s t i c chemicals, e.g., a t r a z i n e , d i u r o n , oxamyl, e t c . The u n r e s o l v e d q u e s t i o n i s why don't t h e s e p s e u d o a p o p l a s t i c c h e m i c a l s which c r o s s t h e c e l l mem branes and e n t e r t h e symplast remain i n t h e symplasm o f t h e phloem? There have been numerous s t u d i e s f o c u s i n g on t h e m o l e c u l a r r e q u i r e ments f o r phloem m o b i l i t y ( 1 - 5 ) , I n g e n e r a l , t h e r e i s n o t a good c o r r e l a t i o n between phloem m o b i l i t y and water s o l u b i l i t y , m e t a b o l i s m o f t h e x e n o b i o t i c , o r t h e p r e s e n c e o f v a r i o u s s u b s t i t u t i o n groups i n a molecule. "Weak A c i d " and " I n t e r m e d i a t e - D i f f u s i o n " Hypotheses. Two h y p o t h e s e s (which a r e n o t n e c e s s a r i l y m u t u a l l y e x c l u s i v e ) have been p r o p o s e d f o r t h e e n t r y and s y s t e m i c m o b i l i t y o f c h e m i c a l s i n t h e phloem: t h e "weak-acid" h y p o t h e s i s proposed by C r i s p and c o l l e a g u e s (_5), and t h e " i n t e r m e d i a t e - d i f f u s i o n " h y p o t h e s i s proposed by E d g i n g t o n and Peterson (4). These a r e i l l u s t r a t e d i n F i g u r e 5. The weak-acid h y p o t h e s i s proposes t h a t compounds which have a f r e e COOH group on t h e m o l e c u l e w i l l be i n t h e p r o t o n a t e d s t a t e i n the a p o p l a s t because o f t h e low pH o f t h e a p o p l a s t (pH 5 t o 5.5). The uncharged m o l e c u l e w i l l c r o s s t h e phloem membranes because o f increased l i p i d s o l u b i l i t y . Once i n s i d e t h e phloem, where t h e pH i s a l k a l i n e (pH 8) the COOH group w i l l be i o n i z e d . The i o n i z e d s p e c i e s w i l l be r e l a t i v e l y impermeable t o t h e phloem membrane b o t h
16
BIOREGULATORS FOR PEST CONTROL
SUCROSE
VACUOLE
PHLOEM
SINK
F i g u r e 4. Pathways o f phloem u n l o a d i n g i n s i n k r e g i o n s . See text f o r d e t a i l s . Reproduced w i t h p e r m i s s i o n from R e f . 13. C o p y r i g h t 1983. American S o c i e t y o f P l a n t P h y s i o l o g i s t s .
"Weak-Acid" H y p o t h e s i s R«COOH-
R-COO"
Lipid soluble Uncharged
"Intermediate" Diffusion Hypothesis R
APOPLAST P
H 5
F i g u r e 5. W e a k - a c i d a n d i n t e r m e d i a t e d i f f u s i o n h y p o t h e s e s f o r the e n t r y and s y s t e m i c m o b i l i t y o f c h e m i c a l s i n t h e phloem.
1.
GIAQUINTA
17
Phloem Transport of Agrichemicals
because o f i t s charge and i t s reduced s o l u b i l i t y i n t h e membrane. T h i s i o n i z e d s p e c i e s c a n t h e n move i n mass flow w i t h t h e t r a n s l o c a t i o n stream. Testable features of t h i s hypothesis are that the a g r i c h e m i c a l w i l l tend t o accumulate i n t h e phloem above i t s e x t e r n a l c o n c e n t r a t i o n because o f t h i s " t r a p p i n g " mechanism and t h a t a g r i c h e m i c a l uptake w i l l be ρ_Η dependent ( h i g h e r a t more a c i d i c pH). There i s q u a l i f i e d support f o r t h e weak-acid h y p o t h e s i s , p a r t i c u l a r l y f o r compounds such as 2 , 4 - d i c h l o r o p h e n o x y a c e t i c acid. Crisp and Look (_5) compared t h e phloem m o b i l i t y o f s e v e r a l s y n t h e t i c 4c h l o r o p h e n o x y d e r i v a t i v e s . The c a r b o x y l d e r i v a t i v e was loaded and t r a n s p o r t e d i n t h e phloem, whereas d e r i v a t i v e s i n which t h e COOH group was r e p l a c e d by an e t h y l e s t e r , amide, k e t o n e , a l c o h o l , o r amino group were n o t t r a n s l o c a t e d . A l t h o u g h t h e weak a c i d h y p o t h e s i s appears t o e x p l a i n t h e mobi l i t y o f compounds such as c h l o r o p h e n o x y d e r i v a t i v e s , t h e r e a r e s e v e r a l e x c e p t i o n s t o t h e weak-acid h y p o t h e s i s (4, JL4, L 5 ) . F o r example, some x e n o b i o t i c s a r e phloem m o b i l e b u t a r e n o t weak a c i d s and do n o t appear t o be c o n v e r t e d t o a weak a c i d p r i o r t o t r a n s p o r t ( e . g . , a m i t r o l e , oxamyl). A l s o , some x e n o b i o t i c s ( e . g . , g l y p h o s a t e ) which have an i o n i z a b l e COOH group a r e loaded i n t o t h e phloem i n d e p e n d e n t l y o f a p o p l a s t pH. These s h o u l d l o s e t h e i r m o b i l i t y under pH c o n d i t i o n s which i o n i z e t h e c h e m i c a l i n t h e f r e e space. Further m o r e , a c c u m u l a t i o n o f t h e weak a c i d g l y p h o s a t e a g a i n s t a c o n c e n t r a t i o n g r a d i e n t does n o t o c c u r ( 1 4 ) . The " i n t e r m e d i a t e d i f f u s i o n " h y p o t h e s i s proposes t h a t t h e c r i t i c a l d e t e r m i n a n t o f phloem m o b i l i t y i s t h e optimum p e r m e a b i l i t y c o e f f i c i e n t o f a g i v e n m o l e c u l e , P. Ρ i s c a l c u l a t e d a s :
p
= fr
l
n
1 1
- o k
]
where L i s t h e l e n g t h o f t h e v a s c u l a r system*, 1, t h e l e n g t h o f t h e source o r l o a d i n g r e g i o n ; r , t h e r a d i u s o f t h e s i e v e t u b e s ; and V, t h e average d a i l y t r a n s l o c a t i o n v e l o c i t y . A compound t h a t i s permeable enough t o e n t e r t h e phloem w i l l be t r a n s p o r t e d as l o n g as t h e compound i s n o t so permeable t o t h e phloem membrane t h a t i t l e a k s back out o f t h e phloem i n t o t h e a d j a c e n t and o p p o s i n g xylem stream. The compound has t o have a r e t e n t i o n time l o n g enough t o be c a r r i e d i n t h e phloem. Each compound and each p l a n t have t h e i r own optimum Ρ and Tyree e t a l . (15) propose t h a t i t i s n o t t h e o r e t i c a l l y p o s s i b l e t o d e v i s e a x e n o b i o t i c which i s o p t i m a l l y ambimobile f o r p l a n t s o f a l l s i z e s . In summary, t h e i n t e n t o f t h i s r e v i e w was t o examine t h e s y s t e m i c t r a n s p o r t o f x e n o b i o t i c s from t h e v i e w p o i n t o f a phloem p h y s i o l o g i s t i n order t o h i g h l i g h t c e r t a i n biochemical, p h y s i o l o g i c a l , and s t r u c t u r a l f e a t u r e s o f t h e t r a n s l o c a t i o n system t h a t may be r e l e v a n t t o t h e f u t u r e d e s i g n o f phloem m o b i l e c h e m i c a l s . I hope the r e v i e w has t a k e n a modest s t e p i n t h a t d i r e c t i o n . Acknowledgment The t y p i n g o f t h e m a n u s c r i p t by T. S p a r r e and m i c r o g r a p h t r a c i n g s by S h i e l a McKelvey a r e g r e a t l y a p p r e c i a t e d .
BIOREGULATORS FOR PEST CONTROL
18
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Christ, R. A. In "Advances in Pesticide Science"; Geissbühler, H., Ed.; Pergammon Press: New York, 1979; Vol. III, p. 420-9. Kirkwood, R. C. In "Herbicides and Fungicides"; McFarlane, N. R., Ed.; Burlington House: London, 1977; p. 67-80. Price, C. E. In "Herbicides and Fungicides"; McFarlane, N. R., Ed.; Burlington House: London, 1977; p. 426. Edgington, L. V.; Peterson, C. A. In "Antifungal Compounds"; Siegal, M. R.; Sisler, H. D.; Eds.; Marcel Dekker, Inc.: New York, 1977; Vol. II, Chap. 2, p. 51-89. Crisp, C. E . ; Look, M. In "Advances in Pesticide Science"; Geissbühler, H., Ed.; Pergammon Press: New York, 1979; Vol. III. p. 430-7. Giaquinta, R. T., Annu. Rev. Plant Physiol. 1983; 34, 347-87. Geiger, D. R.; Giaquinta, R. T. In "Photosynthesis: CO2 Assimilation and Plant Productivity"; Govindjee, Ed.; Academic Press: New York, 1982; Vol. II, p. 345-86. Giaquinta, R. T. In "The Biochemistry of plants"; Preiss, J., Ed.; Academic Press: New York, 1980; Vol. III, p. 271-320. Franceschi, V. R.; Giaquinta, R. T. Planta, 1983a, 157, 411-21. Franceschi, V. R.; Giaquinta, R. T. Planta, 1983b, 157, 422-31. Crookston, K. R.; Moss, D. N. Crop Sci. 1974, 14, 123-5. Martin, R. Α.; Edgington, L. V. Pest. Biochem. Physiol. 1981, 16, 87-96. Giaquinta, R. T.; Lin, W.; Sadler, N. L.; Franceschi, V. R. Plant Physiol. 1983, 72, 362-7. Gougler, J . Α.; Geiger, D. R., Plant Physiol. 1981, 68, 668-72. Tyree, M. T.; Peterson, C. Α.; Edgington, L. V. Plant Physiol. 1979, 63, 367-74
RECEIVED
November 15,
1984
