Developing Research Trends in the Chemistry of Plant Resistance to

1 Developing Research Trends in the Chemistry of Plant Resistance to Pests Paul A . Hedin

Downloaded by 80.82.77.83 on March 3, 2018 | https://pubs.acs.org Publication Date: January 16, 1986 | doi: 10.1021/bk-1986-0296.ch001

Boll Weevil Research Laboratory, USDA, Agricultural Research Service, Mississippi State, MS 39762 The development of crop plant varieties with resistance to insects and diseases has several advantages compared with other approaches to pest control because of economics, environmental considerations, and relative efficiency. In recent years, chemistry has begun to have an impact on selection of resistant lines. Growing amounts of information have been synthesized by chemists and cooperating biologists on the presence and roles of allelochemicals in resistant plants, suggesting that its application to selections may now be a realistic strategy. Developing research trends in the chemistry of plant resistance to pests are identified and reviewed. The development of crop plant v a r i e t i e s with resistance to pests including insects and diseases has several advantages compared with other approaches to pest control. The application of p e s t i cides may be decreased or eliminated, r e s u l t i n g i n decreased costs of production and environmental p o l l u t i o n . The use of resistant v a r i e t i e s also does not require the adoption of complicated technologies by the user. U n t i l now, resistant v a r i e t i e s have been selected from f i e l d evaluations by plant breeders, often i n cooperation with entomolog i s t s , plant pathologists, and/or other b i o l o g i s t s . In recent years, chemists have i d e n t i f i e d a number of compounds conferring resistance that had previously or subsequently been shown to be controlled by single genes or combinations of genes. Investigations In which a r e l a t i v e l y large number of available susceptible and resistant lines or v a r i e t i e s have been analyzed to determine whether correlations between the allelochemical and resistance are s t a t i s t i c a l l y s i g n i f i c a n t have only recently been carried out on a substantive scale. I t i s therefore generally accurate to state that the impact of chemistry (and chemists) on the s e l e c t i o n of resistant lines by plant breeders has been minimal. On the other hand, the growing amount of information that

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

Green and Hedin; Natural Resistance of Plants to Pests ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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has recently been generated by chemists and cooperating b i o l o g i s t s on the presence ana roles of chemicals i n resistant plants suggests that i t s a p p l i c a t i o n to plant breeding may now be r e a l i s t i c . E f f o r t s to define the roles of these chemicals i n various plant-insect and plant-plant interactions governing plant resistance have been reported i n a succession of conferences, symposia, and t h e i r proceedings, e s p e c i a l l y during the last 10 years (1-4), This chapter i s an e f f o r t to i d e n t i f y the various concepts and approaches currently being used by s c i e n t i s t s to chemically define the parameters of plant resistance to pests. Of the t h i r teen approaches that were i d e n t i f i e d , only three, each associated with behavior, were unique to i n s e c t s . The remaining 10 are generally applicable to either insects or diseases, and were therefore not separated. The l i s t i n g of examples of these may suggest emerging trends which are being successfully followed for the e l u c i d a t i o n of the various chemical bases of plant resistance. The examples include not only those presented i n this symposium book, but others recently reported. Origins and Actions of Allelochemicals I d e n t i f i c a t i o n and Roles of Plant Resistance Compounds including Phytoalexlns. Not a l l expressions of plant resistance are chemic a l l y mediated, but of those that are, the so c a l l e d secondary plant compounds appear to be dominant. Among those shown to mediate resistance are the various classes of phenolic compounds including flavonolds and aromatic acids, terpenoids including the sesquiterpene lactones and h e l l o c l d e s , nitrogenous compounds including amino acids and amides, proteinaceous compounds including protease i n h i b i t o r s , glycosidase i n h i b i t o r s , and phytohemagglutinlns, the l e c t i n s , toxic seed l i p i d s including unusual f a t t y acids, acetylenic and a l l e n i c l i p i d s , the f l u o r o l i p i d s and cyanolipids, the saponins, the tannins, and l i g n i n s (1-4). The roles of these compounds are d i f f i c u l t to elucidate for several reaeons. Not a l l compounds toxic to one insect or pathogen are t o x i c to another. In evolutionary time, some pests have evolved mechanisms to detoxify, tolerate, or even u t i l i z e compounds i n plants from which they derive energy and/or other resources whereas others do not. Also, several resistance mechanisms may be operative i n the plant. To the extent that biosynthesis of these compounds i s an expression of genetic information, the e l u c i d a t i o n of these compounds and t h e i r roles can provide a guide to s e l e c tions by plant breeders. The description i n this book of several polyhydroxyl alkaloids from legumes that act as g l y c o s i dase i n h i b i t o r s i n insects and other animals (Fellows et a l . , 5_) i s an unique example. Byers (6) found that 3-nitropropionic acid from crown vetch was a feeding deterrent, and also toxic to some insects and non-ruminant animals. Although this compound serves as a protectant against insects, i t s decrease or removal could improve the use of crown vetch as a forage for some animals. Cutler et a l . (7) reported that several diterpenes and sucrose esters were i n h i b i t o r y to e t i o l a t e d wheat c o l e o p t i l e s and microbes, but o v i p o s i t i o n stimulants for the tobacco budworm, H e l l o t h i s


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virescens, while combinations of duvanes and sucrose esters i n h i bited budworm l a r v a l development. Thus, multiple, divergent roles are evident. Harborne (8) and others have shown that phytoalexins are natural fungitoxlc substances produced de_ novo i n plants as a dynamic response to microbial i n f e c t i o n . As many as 25 structures may be formed i n a single i n t e r a c t i o n . The compounds which r e f l e c t a wide d i v e r s i t y including those l i s t e d above have been i s o l a t e d from a large number of plant f a m i l i e s , generally i n response to Infection of the plant by an appropriate microorganism. Bailey (9) reported the biosynthesis of phytoalexins by plants i n response to i n f e c t i o n by microorganisms and viruses, but also i n response to a number of chemicals including carbohydrates, glycoproteins, and peptides i s o l a t e d from microorganisms. He reported that the formation of phytoalexins involves the coordinated synthesis of several enzymes, and appears to be mediated i n l i v i n g c e l l s by metabolites released from adjacent, dying c e l l s . Wain (10) has found that the phytoalexins often occur i n disease free plants, but b u i l d up i n response to i n f e c t i o n , or e x i s t i n a conjugated form, and are released when the plant i s i n f e c t e d . Also, i n f e c t i o n may cause the a c t i v a t i o n of enzymes, often oxidases that mediate the production of phytoalexins. Mechanical and heat treatments have also been shown to promote phytoalexln synthesis. Gustine (11) reported that the biosynthes i s of the major phytoalexln, medlcarpin, i n ladlno clover i s s t i mulated by bacteria, fungi, and c e r t a i n chemicals. Studies with s u l f h y d r y l reagents suggest medlcarpin accumulation i s regulated by SH-containing components i n the c e l l membrane. Location and D i f f e r e n t i a t i o n of Secondary Constituents Within Plants. For greatest e f f e c t , the allomone should be located at the appropriate concentration i n the plant tissue which i s attacked by the pest at the most vulnerable stage i n the pest's l i f e c y c l e . The content of an allomone may be s i m i l a r i n ground, whole plant samples of susceptible and r e s i s t a n t v a r i e t i e s , but considerably d i f f e r e n t i n s p e c i f i c t i s s u e s . The content at the surface may be d i f f e r e n t i n some s i t u a t i o n s , but this difference may not be noted i f the whole plant tissue i s homogenized and analyzed. For example, c u t i c u l a r components of tobacco, Nlcotiana tabacum (L.), leaf have been reported by Cutler, et a l . (12) to possess various a n t i b i o t i c properties. In cotton, Gossyplum spp., the germ plasm pool i s now large enough to select for allelochemicale that are concentrated c h i e f l y In s p e c i f i c t i s s u e s . For example, the promising red marker with demonstrated a n t i b i o t i c properties, cryeanthemin (13), could be located almost exclusively i n the calyx or the bracts with no loss i n impact on pests. Up u n t i l now, the red cottons have not been grown extensively because they y i e l d about 10Z less than isogenic green l i n e s , presumably because of the biosynthetic costs to the plant of producing the anthocyanln i n a l l of i t s surface t i s s u e s . Precision i n locating ailelochemicals i n the preferred feeding s i t e s of insects could r e t a i n resistance properties without concomitant y i e l d decreases U4). Gregory, et a l . (15) reported on potato glandular trichome chemistry as i t a f f e c t s several i n s e c t s . The trichomes exude a



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sticky substance, then sesquiterpenoids are released which cause enhanced insect movement* Subsequently, the trlchomes release polyphenol oxidase (PPO) and phenol substrates which react to form quinones* The resultant effect on the insect i s cessation of feeding and death* The glandular trlchomes thus contribute to a concentrated, evidently cost e f f e c t i v e , defense system* Stipanovic (16) reviewed the l i t e r a t u r e on the function and chemistry of plant trlchomes and glands from a number of plants, and t h e i r r e l a t i o n to insect resistance* There i s a great divers i t y of allelochemicals found i n trlchomes, glands, nectaries; c h i e f l y a l k a l o i d s , flavonoids, terpenes, and phenols* Rodriguez (17) also has found trlchomes to be important storage structures f o r terpenoids, a l k a l o i d s , phenols and amines i n the a r i d land plants that have been the focus of h i s research* He and h i s associates have i s o l a t e d several chromenes, prenylated quinones, and sesquiterpenes e s t e r l f i e d with phenolic acids which have repellent and/or cytotoxic a c t i v i t y . Multiple Factor Contributions* There i s increasing evidence f o r multiple factor contributions to plant resistance* In laboratory studies, i t has been shown that the growth of tobacco budworm l a r vae i s retarded by a number of compounds Isolated from the cotton plant including gossypol and related compounds, several f l a v o noids, catechln condensed tannins, cyanidln, delphinidin, and t h e i r glucosides (13, 14, 18, 19)* F i e l d studies have shown that v a r i e t i e s high i n several components possess the greatest resistance (20). Thus, there i s the p o t e n t i a l to breed f o r v a r i e t i e s high i n several components, the biosynthesis of which may be controlled by separate genes* Improved gene i n s e r t i o n techniques should eventually be applicable to these objectives* Multiple factor resistance may be less susceptible to adaptation by pests* Cutler et a l * (7_) have reported that combinations of diterpene duvanes and sucrose esters of C 3 - C 7 f a t t y acids are o v i p o s i t i o n stimulants f o r the tobacco budworm, suggesting that a p r o f i l e of components i s required* Saxena (21) i n h i s studies on r i c e plant allelochemicals as o v i p o s i t i o n deterrents f o r several insects has implicated the Importance of v o l a t i l e s , amino acids, and t r a n s - a c o n i t l c acid, again Indicating the requirement f o r a p r o f i l e of components. Kennedy (22) also has shown the requirement f o r multiple components; phenols, a l k a l o i d s , and methyl ketones, i n resistance of tomato plants, Lycopersicon, to several insect pests. Secondary Metabolites from Higher Plants as Pest Control Agents An a c t i v i t y related to various e f f o r t s to i d e n t i f y compounds i n plants with resistance to pests, and subsequently to attempt s e l e c t i o n of l i n e s with higher concentrations, i s the u t i l i z a t i o n of these compounds (or the synthetic species) i n f i e l d pest c o n t r o l . The following includes a few examples from a very large s c i e n t i f i c a c t i v i t y . Jacobson (23) has tested azadirachtin, a sesquiterpenoid from the neem tree, Azadirachta l n d l c a (A. Juss), against several Insects and nematodes. I t i s e f f e c t i v e at concentrations as low as


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0·1 ppm, and appears safe for f i s h , animals and crop plants. The insect feeding deterrents i s o l a t e d from semiarid and a r i d land plants by Rodriguez (17) may also f i n d eventual commercial a p p l i c a t i o n . Whitehead (24) has reported on the prognosis for using natural products and their analogues for combating pests i n A f r i c a , He has suggested that stored grains may be protected from pests with t h i n coatings of vegetable o i l s , often smoked, i n situations where sophisticated, often expensive procedures are not f e a s i b l e . Cutler (25) and associates have i d e n t i f i e d a number of secondary metabolites that have allelochemical e f f e c t s i n plants, fungi, bact e r i a , and vertebrates, Carboxyatractyloslde, i s o l a t e d from the plant Xanthium strumarium, i s both a hypoglycemic and phytotoxic agent. It also has f u n g i s t a t i c a c t i v i t y . The s e l e c t i v e action of t h i s compound suggests that i t might be developed as a herbicide to control grasses i n corn. P h o t o s e n s i t i z e s , E l i c i t o r s , and Primitive Immune Systems. Plants synthesize chemicals which possess a wide range of b i o l o g i c a l a c t i v i t i e s . Natural products that require l i g h t for expression of t h e i r toxic b i o l o g i c a l consequences are among the most i n t e r e s t i n g of these b i o l o g i c a l a c t i v i t i e s . Such photosensitizers or phototoxins are s t r u c t u r a l l y variable, are produced by widely divergent flowering plant f a m i l i e s , and mediate broad-spectrum t o x i c react i o n s . A l k a l o i d s , acetophenones, furocoumarlns, furochromes, and polyinee are among the classes of compounds i d e n t i f i e d by Downum (26) and associates. Amas on et a l . (27) have reported on other photosensitizers; polyacetylenes, 0r-carbolines, and extended quinones with high a c t i v i t y to i n s e c t s . ot-Terthienyl was found to be more toxic to mosquito than DDT, The t o x i c i t y of some of these compounds i s mediated by the production of singlet oxygen although free r a d i cals may be involved, Furanocoumarins and furanoquinolines, on the other hand, interact with DNA. The furanocoumarin, 8-methoxy-psoralen, was found highly toxic to Spodoptera larvae. Kogan and Paxton (28) reported that there are a number of natural inducers of plant resistance to insects such as c l i m a t i c f a c t o r s , pesticides, growth regulators, and microbial i n f e c t i o n s . They concluded that resistance of plants to arthropods generally resulted from genetically controlled, injury-independent accumulat i o n of allomonal agents, whereas for many pathogens, phytoalexln formation i s the result of a post-challenge response. However, they provided several examples where herbivore attacks did e l i c i t resistance i n plants, an example of which i s the role of phytoalexins as antiherbivory agents i n soybeans against the Mexican bean beetle, Epllachna v a r i v e s t i s . An example of a so-called primitive immune system, somewhat p a r a l l e l to that i n animals, i s provided by the f i n d i n g by Nelson et a l . (29) that two proteinase i n h i b i t o r s accumulate i n leaves of tomato plants when attacked by chewing insects or mechanically wounded. The accumulation of the two antinutrient proteins i s apparently a defense response and i s i n i t i a t e d by the release of a putative wound hormone c a l l e d the proteinase i n h i b i t o r inducing


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f a c t o r . A second wound at nine hours l a t e r t r i p l e s the steady rate of i n h i b i t o r accumulation. Phytochemical Disruption of Hormonal Processes. Hormones are u n i que regulators of insect and plant growth and other l i f e processes. Allelochemicals which operate by disrupting hormonal processes can be v i s u a l i z e d as important components of plant defensive mechanisms. Bowers (30) has reviewed the elucidation by himself and others of compounds i d e n t i f i e d as disrupting hormonal processes i n insects. Juvabione, isolated from balsam f i r , induced the linden bug and other insects to molt into nymphal-adult i n t e r mediates or supernumerary nymphs which eventually died without completing metamorphosis. Several other compounds i s o l a t e d from protected plants or trees with quasi-juvenile hormone a c t i v i t y were also i d e n t i f i e d including farnesol, dehydrojuvabione, sesamin, sesamolin, and the juvocimones. Molting and d i f f e r e n t i a t i o n i n insects are controlled by molting hormones c a l l e d ecdysones. Subsequently, a s t r u c t u r a l l y s i m i l a r polyhydroxysteroid, ponasterone A, was isolated from the plant Podocarpus nakail and demonstrated to have molting hormone a c t i v i t y . This has led to the i d e n t i f i c a t i o n of nearly 70 additional plant ecdysterolds with molting hormone a c t i v i t y . Ecdysterolds have been shown to have anti-insect a c t i v i t i e s , and were thus assumed to play a role i n plant defensive strategy. Bowers (30) and coworkers have also discovered that certain plants contained compounds which i n i t i a t e d behavioral responses i n the American cockroach, i d e n t i c a l to those induced by the natural sex pheromone of the v i r g i n female. The active p r i n c i p l e i n several evergreen species was isolated and found to be D-bornyl acetate. Later, Germacrene D was Isolated from the plant Erigeron annus and shown also to have sexual a c t i v i t y for this insect. Subsequently, the true pheromone of this insect was i s o l a t e d from the midgut and shown to be Periplanone B, a derivative of Germacrene D. It was speculated that the plant's biosynthesis of these compounds may disrupt pheromonal communication of insects that feed on the plant. Biorational Approaches to the Development of Pest Control Agents. As the roles of natural products i n the control of pests has increased, there has been a concomitant growth i n the understanding of biochemical processes. Knowledge of metabolism, biosynthetlc processes, neurochemistry, regulatory mechanisms, and many other aspects of plant, animal, and pest biochemistry has provided a more complete basis for understanding the modes of action of pest control agents including allelochemicals. The exploitation of knowledge of the chemical bases of b i o l o g i c a l interactions ( i . e . , b i o r a t i o n a l approaches) may lead to the recognition and u t i l i z a t i o n ( i . e . , synthesis, chemically directed selections, gene insertions) of new molecules designed to act at a p a r t i c u l a r s i t e or to block a key step i n a biochemical process. Attempts to develop resistant l i n e s or c u l t i v a r s by employing analyses to determine whether correlations between the a1lelochemical and resistance are s t a t i s t i c a l l y s i g n i f i c a n t , has only recently been explored on a substantive scale. E f f o r t s to make selections based on the high (or low) concentration of an allelochemical depend on the a v a i l a b i l i t y of a large, diverse germ


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plasm pool that contains entries from many countries, both from temperate zones and p a r t i c u l a r l y from the tropics where they have endured severe challenges by pests. Subsequently, chemical and biochemical evaluations may provide guidance as breeders make crosses to develop resistant l i n e s with required commercial a t t r i butes. This approach i s now being i n i t i a t e d at several laborat o r i e s including ours at M i s s i s s i p p i State where we have monitored 20 cotton Gos8ypium spp. l i n e s (13, 14) and 15 corn Zea mays L. l i n e s (31) f o r allelochemicals. The results have formed the basis for subsequent s e l e c t i o n s . As biotechnological techniques improve so that gene t r a i t s for allelochemicals can be transferred to crop plants, the b i o r a t i o n a l approach w i l l a i d , and perhaps control, the gene s e l e c t i o n process. Genetic Manipulation of Plants for Resistance. Biotechnology promisee to play a s i g n i f i c a n t role i n crop improvement and prod u c t i v i t y i n the coming years. The improvement of crop plants using recombinant DNA techniques w i l l require the development of transformation vectors and regeneration technology. With the development of these techniques, I d e n t i f i c a t i o n of the genes to be transferred w i l l be required. Marvel (32) has recently reviewed the prognosis f o r biotechnology i n crop improvement. Kennedy (22) has observed that although there are a number of b i o l o g i c a l l y active chemicals which have potential f o r genetic manipulation, the manipulations are l i k e l y to be complex i n that the actual l e v e l of resistance often depends upon the larger chemical context within which that compound occurs i n the plant. A further complicating factor i s that the introduction of a p a r t i c u l a r biochemically-mediated resistance to one insect species may have unanticipated and undesirable e f f e c t s on other non-target insects. A thorough understanding of the biochemical interactions w i l l be required to develop plants with high levels of allélochemical by mediated resistance. The potential f o r , (and some disadvantages of) genetic manipulation i s i l l u s t r a t e d by investigations of Rosenthal (33) and Dahlman and Berge (34) with L-Canavanine, the non-protein amino acid biosyntheslzed by Dloclea megacarpa and other legumes· L-canavanlne, i n this instance, exists i n the free form, but i s incorporated by insects into t h e i r protein whence t o x i c effects occur. In another scenario, L-canavanine might be inserted genetically into plant protein, and thus exert i t s t o x i c i t y . However, i f canavanine containing proteins were also toxic to mammals, the purpose would be defeated. In any event, there are large numbers of allelochemicals from which selections f o r i n s e r t i o n might be made. Of course, the e c o l o g i c a l and agronomic fitneee of the transformed plant w i l l have to be evaluated, and i t may take several years to produce a commercially promising c u l t i ver. Mechanisms of Plant-Pest Interactions Coevolntlon of Plants and Pests. Plants and t h e i r herbivores and pathogens (including pests) coevolve, and the continuing adjustments of one to the other r e f l e c t the biosynthesis of defensive compounds by the plant and the development of d e t o x i f i c a t i o n



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mechanisms by the pest. The dynamic nature of this relationship i s I l l u s t r a t e d by the a b i l i t y of insects to induce detoxifying enzymes within 24 h. when challenged by a toxic agent. Plant injury, i n turn, can e l i c i t the biosynthesis of additional quan t i t i e s of resistance agents. Cates (35) i n a study of Douglas-fir/budworm interactions showed changing responses of one to the other as a consequence of plant toxins, tree n u t r i t i o n a l l e v e l s , geographical location, and climate stresses. Fellows et a l . (5) and others i d e n t i f i e d several polyhydroxy alkaloids i n higher plants which s t r u c t u r a l l y resemble monosaccharides and are potent i n h i b i t o r s of glycosidaee a c t i v i t y i n insects, animals, and microorganisms. Because these compounds also i n h i b i t trehalose, and trehalose i s an important storage carbohydrate i n insects, incorporation of the alkaloids into the legume pods on which bruchid beetles (Callosobruchus spp.) feed could provide a defense mechanism. However, the t o x i c i t y to other strains of the bruchid beetle, and to other Insect species d i f f e r , suggesting that adap tations have occurred. Biochemical Mechanisms. The role of natural products In the control of pests has Increased i n recent decades as the chemist has acquired more sophisticated tools with which to elucidate complex structures. This, i n turn, has led to explosive growth i n the understanding of biochemical processes. Knowledge of metabo lism, blosynthetlc processes, neurochemistry, regulatory mecha nisms, and many other aspects of plant, animal, and insect biochemistry has provided a more complete basis for understanding the mechanisms of plant resistance. B e l l et a l . (36) reported on the biochemistry of cotton resistance to b a c t e r i a l and fungal i n f e c t i o n s . Cotton plants synthesize both sequiterpenoid phytoalexins and proanthocyanidins i n response to microbial i n f e c t i o n s . From their continuing research, they have i d e n t i f i e d a large number of terpenoid com pounds, associated with gossypol, and have suggested the mecha nisms by which they are b i o s y n t h e t i c a l l y formed from one another. Gustine (11), using s u l f h y d r y l reagents, showed that medlcarpin accumulation i s regulated by SH-containing components i n the c e l l membrane. Fellows et a l . (5) elucidated the mechanism by which glycosidaees from insects, mammals, and microorganisms are i n h i bited by polyhydroxyalkaloids, and t h e i r possible e c o l o g i c a l r o l e . Gregory et a l . (15) showed that an oxidation reaction, I n i t i a t e d upon disruption of potato trlchomes, between a unique polyphenoloxldaee enzyme and phenolic substrate(s), resulted In qulnone formation and subsequent insect immobilization. N u t r i t i o n a l Antagonisms. Some plants possess the capability to present a n u t r i t i o n a l l y undesirable diet to pests apart from t h e i r biosynthesis (or f a i l u r e to biosynthesize) of allelochemicals. Rosenthal (33) and Dahlman and Berge (34) have shown that L-Canavanine, a non-protein amino acid of certain leguminous plants, i s incorporated into proteins of some insects with toxic r e s u l t s . The tobacco hornworm i s s e n s i t i v e to the formed protein, however the tobacco budworm, H e l i o t h i s virescens (Fab.), and the bruchid beetle, Caryedes b r a s 1 l i e n s i s , are able to tolerate and


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metabolize high concentrations. The biochemical d e t o x i f i c a t i o n routes have been elucidated. Reese (37) has studied nutrient-allelochemical interactions using diets supplemented by plant extracts. He has concluded that many of the deleterious e f f e c t s may be due primarily to various interactions between these allélochemical and e s s e n t i a l n u t r i e n t s . It i s Important to not only consider the presence of nutrients, but also the b i o a v a i l a b i l i t y of these nutrients to the phytophagous i n s e c t . He c i t e d findings where neonate larvae were found more s e n s i t i v e to plant allelochemicals than older larvae, and Interpreted the greater resistance of the older larvae In terms of increasing c a p a b i l i t y to u t i l i z e a v a i l a b l e nutrients, probably because of the induction of enzymatic d e t o x i f i c a t i o n systems. Blum (38) observed that many insects have the c a p a b i l i t y to feed on "so-called" t o x i c plants, having developed the required detoxif i c a t i o n systems. N u t r i t i o n a l factors may become c r i t i c a l as a consequence of stresses to the insect involved with the energy requirements devoted to d e t o x i f i c a t i o n , deactivation, or sequestration. F i n a l l y , there are general n u t r i t i o n a l requirements f o r e s s e n t i a l amino acids, fatty acids, s t e r o l s , and vitamins by pests. The absence or deficiency of one or more of these may l i m i t growth or development. D i f f e r e n t i a l Sensory Perceptions of Plant Compounds by Insects. These perceptions are a function of the chemical composition of the plant and the chemical sensory (neurological) resources (including the brain) of the i n s e c t . The result of these i n t e r a c tions i s a behavioral response. The understanding that not a l l insects respond i d e n t i c a l l y to a stimulus brings d i v e r s i t y to these i n v e s t i g a t i o n s . Narahashi (39) and others have established that insects may possess several neurochemical targets. In pheromone perception by Insects, at least two d i f f e r e n t receptor c e l l s have been found to be involved. These receptors are s u f f i c i e n t l y f l e x i b l e to perceive more than one compound and are s e n s i t i v e to the precise r a t i o s of compounds. Receptor s p e c i f i c i t y i s further demonstrated i n the d i f f e r e n t i a l responses to c h i r a l forms of pheromones and i n the responses from receptors on the antennae of males and females. Visser (40) has studied insect perception of plant characters with regard to s e l e c t i o n of a plant for feeding. By means of gustatory receptors, an insect i s informed about the plant's n u t r i t i o n a l q u a l i t y . Plant odors are the chemical messengers for the insect's o r i e n t a t i o n . The d i f f e r e n t i a l percept i o n of green odor, being composed of C-6 alcohols, aldehydes, and derivative acetates i s a common feature i n the s e n s i t i v i t y and s e l e c t i v i t y to phytophagous insects with the s e n s i t i v i t y of o l f a c tory receptors for the i n d i v i d u a l components i n the complex varying. Other compounds also are involved i n host plant s e l e c tion. Psychomanipulation of Insects. Murdock et a l . (41) have developed the hypothesis that certain plant chemicals consumed by feeding insects Interfere with information processing i n the insect's central nervous system and thereby modify behavior. Two plant


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components, nicotine and lobeline, act as antagonists of the nico t i n i c cholinergic receptor, Gamma-aminobutyric acid (GABA) recep tors are blocked by two chemicals of plant o r i g i n , bicueculine and p i c r o t o x i n . Scopolamine i s an excellent blocking agent f o r muscarinic cholinergic receptors, Physostigmine, another active plant chemical that i s a synaptic clearance antagonist, i s also a strong cholinesterase i n h i b i t o r . This i n h i b i t i o n results i n i n t e n s i f i e d stimulation of the postsynaptic c e l l . A number of s i m i l a r e f f e c t s have been established with plant chemicals. The o v e r a l l e f f e c t i s that plants obtain a measure of protection from insects by accumulating secondary substances which act i n the CNS to change the insect's behavior i n ways that reduce or prevent further feeding such as suppressing appetite, i n t e r f e r i n g with CNS integrative procedures, blocking learning or memory, or d i s t o r t i n g vision. Summary. This review has been an attempt to i d e n t i f y developing trends by which research to elucidate the chemistry of plant resistance to pests i s evolving. Although the impact of chemical knowledge on the development of resistant v a r i e t i e s has been hitherto l i m i t e d , developing techniques and insights concerning these and other approaches may now make a much larger, more vigorous contribution, even a d i r e c t i o n , to this a c t i v i t y . Acknowledgments and Disclaimers Mention of a trademark, proprietary product or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not Imply Its approval to the exclusion of other products or vendors that may also be s u i t a b l e . This chapter was prepared by a U.S. Government employee as part of h i s o f f i c i a l duties and l e g a l l y cannot be copyrighted.

Literature Cited 1. 2. 3. 4. 5.

6.

Wallace, J . W.; Mansell, R. L. "Biochemical Interaction Between Plants and Insects"; Recent Advances in Phytochemistry, 1976; Vol. 10, Plenum Press, N.Y., 425 pp. Rosenthal, G. A.; Janzen, D. H. "Herbivores: Their Interaction With Secondary Plant Metabolites"; Academic Press, N.Y.; 1979, 718 pp. Hedin, P. A. "Host Plant Resistance to Pests"; ACS SYMPOSIUM SERIES No. 62, American Chemical Society, Washington, D.C., 1977, 286 pp. Hedin, P. A. "Plant Resistance to Insects"; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, 375 pp. Fellows, L. E . ; Evans, S. V.; Nash, R. J.; Bell, E. A. Ch. 6, In Natural Resistance of Plants: Roles of Allelochemicals; Green, M. B; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Byers, R. Α.; Gustine, D. L . ; Moyer, B. G.; Bierlein, D. L. Ch. 8, In Natural Resistance of Plants: Roles of Allelochemicals ; Green, M. B.; Hedin, P. Α., Eds.; ACS


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SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Cutler, H. G.; Severson, R. F.; Cole, P. D.; Jackson, D.M. Ch. 15, In Natural Resistance of Plants: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Harborne, J . B. Ch. 3, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Bailey, J . A. "Abstracts of Papers"; 189th National Meeting of the American Chemical Society, Miami Beach, Fl., April 1985; American Chemical Society, Washington, D.C.; PEST 10. Wain, R. L. Ch. 2, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Gustine, D. L. Ch. 5, In Natural Resistance of Plants to Pests: Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Cutler, H. G.; Reid, W. W.; Deletang, J . Plant Cell Physiol. 1977, 18, 711. Hedin, P. Α.; Jenkins, J . N.; Collum, D. H.; White, W. H.; Parrott, W. L . ; MacGown, M. W. Experientia 1983, 39, 799. Hedin, P. Α.; Jenkins, J . N.; Collum, D. H.; White, W. H.; Parrott, W. L. Ch. 20, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C. 1982. p. 347. Gregory, P.; Tingey, W. M.; Ave, D. Α., Bouthyette, P. Y. Ch. 13, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Stipanovic, R. D. Ch. 5, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington D.C., 1982, p. 69. Rodriguez, E. Ch. 31, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C. 1985, p. 447. Bell, Α. Α.; Stipanovic, R. D. Proc. Beltwide Cotton Production Res. Conf. 1977, Jan. 10-12, Atlanta, Ga., 244. Chan, B. G.; Wales, A. C.; Binder, R. G.; and Elliger, C. A. Entomologia Exp. Appl. 1978, 24, 94. Jenkins, J . Ν., unpublished data. Saxena, R. C. Ch. 12, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, Μ. Β.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Kennedy, G. G. Ch. 11, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986.


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28. 29.

30. 31. 32. 33. 34.

35.

36.

37. 38.

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Jacobson, M. Ch. 18, In Natural Resistance of Plants to Pests; Green, Μ. Β.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Whitehead, D. L. Ch. 29, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C. 1985, p. 409. Cutler, H. G. Ch. 32, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C. 1985, p. 455. Downum, K. R. Ch. 16, In Natural Resistance of Plants to Pests; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Arnason, T.; Towers, G. H. N.; Philogene, B. J . R.; Lambert J . D. H. Ch. 9, In "Plant Resistance to Insects"; Hedin, Ρ Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 139. Kogan, M.; Paxton, J . Ch. 9, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 153. Nelson, C. E . ; Walker-Simmons, M.; Makus, D.; Zuroske, G.; Graham, J.; Ryan, C. A. Ch. 6, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 103. Bowers, W. S. Ch. 15, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society Washington, D.C., 1985, p. 225. Hedin, P. Α.; Davis, F. M.; Williams, W. P.; Salin, M. L. J. Agr. Food Chem. 1984, 32, 262. Marvel, J . T. Ch. 34, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C., 1985, p. 477. Rosenthal, G. A. Ch. 16, In "Plant Resistance of Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 279. Dahlman, D. L . ; Berge, M. A. Ch. 10, In Natural Resistance of Plant to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Cates, R. G. Ch. 9, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. A. Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Bell, Α. Α.; Mace, M. E . ; Stipanovic, R. D. Ch. 4, In Natural Resistance of Plants to Pests: Roles of Allelochemicals; Green, M. B.; Hedin, P. Α., Eds.; ACS SYMPOSIUM SERIES, American Chemical Society, Washington, D.C., 1986. Reese, J . C. Ch. 13, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 231. Blum, M. S. Ch. 15, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C., 1985, p. 265.



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Narahashi, T. "Neurotoxicology of Insecticides and Pheromones", Narashi, T., Ed.; Plenum Press, N.Y., 1979. Visser, J . H. Ch. 12, In "Plant Resistance to Insects"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 208, American Chemical Society, Washington, D.C., 1982, p. 215. Murdock, L. L . ; Brookhart, G.; Edgecomb, R. S.; Long, T. F.; Sudlow, L. Ch. 24, In "Bioregulators for Pest Control"; Hedin, P. Α., Ed.; ACS SYMPOSIUM SERIES No. 276, American Chemical Society, Washington, D.C., 1985, p. 337.


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Developing Research Trends in the Chemistry of Plant Resistance to

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