Journal of Chemical Ecology, Vol. 21, No. 7, 1995

TURNABOUT

IS F A I R P L A Y : S E C O N D A R Y PRIMARY COMPOUNDS

ROLES FOR

MAY R. B E R E N B A U M Department of Entomology University of Illinois at Urbana-Champaign 320 Morrill Hall, 505,7. Goodwin, Urbana. Illinois 61801-3785 (Received February 14, 1995: accepted March 10, 1995)

Abstract--Chemically based resistance of plants to herbivorous insects is today essentially synonymous with allelochemically based resistance; the importance of plant secondary compounds in determining patterns of hostplant utilization has been established in a wide variety of insect-plant interactions, In contrast, primary metabolites, those involved in fundamental plant physiological processes, are rarely considered to be major determinants of host-plant resistance despite the fact that, as insect nutrients, they can have profound effects on behavior and physiology, The degree to which variation in plant primary metabolism results from the selective impact of herbivory may be greatly underestimated in that the biosynthetic and structural diversity of primary metabolites and the consequences of that diversity on herbivores are rarely taken into account in most studies of insect preference and performance. Qualitative and quantitative variation in the production of primary metabolites can result from herbivore selection pressure if production of primars' metabolites is under genetic control and if plant fitness in the presence of herbivores is associated in a predictable way with genetically based primary metabolite variation. Variation in primary metabolism is likely to be particularly effective as a defense against highly oligophagous herbivores with limited mobility, especially those confined to structures containing allelochemicals that could neutralize the benefits associated with compensatory feeding. Key Words--Secondary compounds, primary compounds, plant resistance, herbivory.

INTRODUCTION

Over the last 40 years, as chemical techniques for the separation, identification, and quantification of secondary chemicals have become increasingly more sophisticated and accessible, interest on the part of ecologists in the chemistry 925 (X)08~033)/95/0700-0925507.50/0 ~, 1995PJenmnPublishingCorporation

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of primary constituents of plants has virtually disappeared. With few exceptions, chemical ecologists have, for the most part, relegated the precise identification and quantification of primary plant metabolites (with the exception of elemental nutrients) to physiologists. In point of fact, however, amino acids, vitamins, sterols, and other herbivore nutrients are chemical constituents of plants that are synthesized, sequestered, or metabolized as are alkaloids, coumarins, phenylpropanoids, and the other more "secondary" metabolites. Moreover, considerable evidence exists that they play an essential role in determining host-plant suitability for herbivores and that herbivores detect and process them in ways very similar to the manner in which secondary metabolites are detected and processed. There is a clear need, then to differentiate between the terms "secondary substance" and "atlelochemical"--the two terms are not really synonymous or interchangeable. Just as some secondary substances may have primary physiological functions (Seigler, 1977), some primary substances may function as allelochemicals. By the same token, it must be emphasized that the terms "nutrient" and "allelochemical" are not necessarily exclusive and opposite (Slansky, 1992); whether a substance is nutritive or toxic is entirely dependent upon context (Reese, 1979; Janzen, 1979) and not upon biosynthetic origin. Although it is now widely accepted that allelochemicals may function in some instances as nutrients (Bernays and Woodhead, 1982; Rosenthal and Bell, 1979), the allelochemical function of erstwhile nutrients is rarely considered. To a great extent, the relative lack of emphasis on primary" metabolites of plants in current discussions of plant-insect interactions is largely the result of historical precedent; methodologies and technologies now exist, however, that make the analysis of primary metabolism in an ecological context more tractable. Such analyses stand to make substantial contributions to understanding the chemical bases for host-plant choice in insects and the evolutionary responses of plants to herbivory. HISTORICAL BACKGROUND

The chemical characterization of plants began in earnest in the nineteenth century, and it rapidly became apparent that, while certain plant constituents, such as chlorophyll or sucrose, are virtually universal in distribution, other types of plant constituents are restricted in distribution to individual families, individual genera within families, individual species within genera, and even individual populations within species. Such idiosyncratic distributions suggested that compounds that are not universally distributed are not essential to the physiological function of the plant. Kossel (1891, as translated by Mothes, 1980) proposed "calling . . . essential components of the cell primary components, and those which are not found in every cell capable of developing, secondary."

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TABLE I. DATA FOR CHEMICAL CHARACTERIZATIONOF REPRESENTATIVE PRIMARY AND SECONDARY METABOL[TES (WINDHOLZ, 1983)

Primary metabolites

Tryptophan Vitamin A Methionine Vitamin E Vitamin B~,

1901 1913 1922 1936 1948

Secondary metabolites

Cocaine Coniine Nicotine Mescaline ~-Pinene

1885 1886 1893 1896 1896

This dichotomy was rapidly embraced and prevailed for the next century, although variously modified over the years; as more constituents were characterized, they were, for the most part, readily characterized as either primary or secondary metabolites. Surprisingly, the chemical characterization of many nutritive substances lagged behind that of many secondary metabolites (Table 1), at least in part because some essential nutrients, such as vitamins, are present in plants in concentrations even lower than are most secondary compounds; the word "vitamin," for example, was not even coined until 1912. In some ways, the choice of the word "secondary" was unfortunate; although it for the most part accurately depicts the biosynthetic dependence of secondary metabolites on intermediates of primary metabolism (referred to by Bonner and Galston (1952) as the byways and highways of plant metabolism, respectively), it also connotes inutility or expendability. Over the next century, various unflattering epithets, including "excretory products" (Goris, 1921, cited by Robinson, 1930), "metabolic waste products" (Muller, 1969), "unnecessary metabolites" (Robinson, 1974), even "flotsam on the metabolic beach" (Mothes, 1980), were applied to these compounds to emphasize the fact that they did not appear to play a role in the daily physiological demands of plant life. Relatively early on, however, Stahl (1888) suggested that such compounds play a vital role in protecting plants against biological stress agents, referring to them as Schutzexcrete, or protective excretions. Staht's suggestion that the function of the so-called secondary compounds is primarily ecological was not widely accepted until 1959, when Fraenkel (1959) reintroduced the idea to the scientific community, accompanied by a compelling body of evidence attesting to the behavioral and physiological effects of these compounds on herbivorous insects. In full recognition of the fact that secondary compounds are essential in the sense that they allow plants to survive in their biotic milieu, Whittaker and Feeny (1971) coined the less judgmental term allelochemic (from allelo, Greek for "one another") to describe compounds with primarily ecological functions; "most allelochemical interactions involve the diverse secondary sub-

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stances . . . which do not occur in all living matter but appear sporadically throughout the living world. They are contrasted with the primary subtances, such as various proteins, carbohydrates, nucleic acids, and fats, which are of general occurrence." It is interesting to note that Whittaker and Feeny (197 i) did not equate "allelochemical" with "'secondary substances" but carefully noted that secondary substances do function as such in " m o s t " cases. Dissatisfaction with the notion that a clear dichotomy of function exists between primary and secondary compounds has arisen many times over the last century; indeed, even Kossel (1891, cited in Mothes, 1980), in proposing the terms primary and secondary metabolite, acknowledged the difficulties of distinguishing between them. Seigler (1977) and Seigler and Price (1976) suggested that secondary compounds may well play a role in primary physiological processes by serving as biosynthetic intermediates, as growth regulators, or as rather elaborate storage molecules for elements in short supply. Consistent with this function is the phenological, ontogenetic, and diurnal variation in abundance that characterizes virtually all secondary compounds. The idea that secondary compounds can function as storage molecules for elements in excess supply is, in fact, at the foundation of the carbon-nutrient balance hypotheses currently in vogue (according to which excesses of nutrients in the form of either carbon or nitrogen allow for the accumulation of either carbon-based or nitrogen-based allelochemicals, respectively) (Bryant et al., 1983; Coley et al., 1985). Primary constituents, however, were not completely forgotten in all of this discussion. Because many biochemical and physiological processes are identical in plants and animals, it was assumed from the beginning that primary compounds produced by plants serve as nutrients for plant-feeding animals (including humans) when they are ingested. That the nutrient content of plants determines in part the suitability of plants to herbivorous insects was clearly recognized by Painter (1936), who stated that the "possible lack of specific food materials, especially proteins" could be critical to plant food quality, leading to, among other things, changes in longevity, size, fecundity, and death rates in herbivorous insects. He also ruefully noted that, at that time, "the development of insects with respect to specific food substances or mineral elements is almost an untouched field." The development of defined artificial diets and aseptic rearing techniques in the next decade allowed for tremendous gains in identifying the essential amino acid, vitamin, and other dietary requirements of insects in general and phytophagous insects in particular (Lipke and Frankel, 1965). By mid-century, some investigators were proposing that both nutrients and secondary chemicals were important in determining patterns of host-plant utilization. This idea was perhaps best stated by Kennedy (1953, 1958), in his "'dual discrimination hypothesis," according to which secondary chemicals are involved principally in host-plant recognition and nutrients principally in feeding-site selection within a plant. That both nutrients and secondary chemicals

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should play a role in determining host-plant preferences was not an unreasonable supposition, according to Thorsteinson (1958), because "physiologically there is little basis for distinguishing between perception of token stimuli and nutrients since both are manifestations of the chemotactic sense." In his work with Plutella xylostella, Thorsteinson himself had demonstrated that the feeding-stimulant properties of sinigrin, a secondary compound typical of crucifers, were greatly enhanced in the presence of sucrose (Thorsteinson, 1960). This synthetic view was largely forgotten, however, when Fraenkel proposed that allelochemicals, rather than nutrients, are exclusively responsible for determining plant utilization patterns by herbivorous insects--stating boldly that "'there is little reason to suppose that differences in chemical composition with respect to the 'primary' substances (which occur in all living matter) can be responsible for the choice of food plant on the part of the insect" (Fraenkel, 1959). Although Fraenkel eventually softened this view (somewhat) in a later paper (Fraenkel, 1969, " . . . it would be illogical to expect that extreme changes in nutrients would leave the insect entirely unaffected"), the later paper never had the impact of the first, which went on to become a classic cited over 200 times between 1959 and 1984 (Current Contents, March 12, 1984). During that critical period in the development of insect-plant interaction theory, discussions of the impact of insect herbivory on plant evolution focused primarily on plant secondary metabolism.

LOW N U T R I E N T Q U A L I T Y AS DEFENSE

Plant nutritional quality reemerged as an important element of coevolutionary studies in 1976, when both Feeny (1976) and Rhoades and Cates (1976) described allelochemicals with antinutritive, or digestibility-reducing, properties. The suggestion was made that low nutrient quality, arising either through antinutritive alleochemicals, such as tannins and resins, or through reduced amounts of essential nutrients, such as proteins, could function as a plant defense. There were perceived theoretical shortcomings with this position, however, articulated by both Moran and Hamilton (1980) and Price et al. (1980)--namely, that reduced nutrient quality could potentially lead to increased damage to plants due to the propensity of certain herbivores to compensate for low nutrient levels by ingesting some food. Moran and Hamilton (1980) conceded that low nutritive quality could function as a defense under special conditions--if herbivores are able to detect low nutrient quality and are sufficiently mobile to move to avoid it, if successive generations of extremely sedentary herbivores on the same host experience reductions in size as a result of reduced nutrient quality, or if prolonged development in early stages as a result of low nutrient quality increases the risk of predation or disease.

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Over the past decade, although the effects of nutrient variation on herbivore preference and performance have been studied extensively (Slansky and Rodriguez, 1987; Slansky, 1992; Simpson and Simpson, 1990), the idea that low nutrient quality can evolve as a plant defense has garnered little attention. Neuvonen and Haukioja (1984) attempted to examine the hypothesis in the context of delayed inducible resistance in Betula pubescens in response to repeated defoliation by Epirrita autumnata; resistance in this system took the form of increased levels of foliar phenolics and decreased amounts of foliar nitrogen. These authors concluded that, due to the presence of multiple confounding variables, "one cannot say that low nutritive quality in itself is defensive." Lundberg and Astrom (1990) elaborated upon the idea of low nutritive value as a plant defense in a theoretical paper focused on repercussions of variable nutrient quality on movements of vertebrate herbivores. These authors assumed for purposes of discussion that low nutritive quality could be defined as " N primarily total nitrogen weight per unit of biomass," which they considered to be "a positive currency common to both the plant and the herbivore." In none of these more recent studies, however, is nutrient quality ever assessed in a rigorous manner. Purported tests of the idea that low nutritive quality can confer resistance against herbivores have completely ignored the tremendous chemical variability that exists among primary metabolites; that secondary chemicals, such as tannins, phenolics, or resins, may have antinutritive effects is almost beside the point. Plant nutritional quality in ecological studies is almost invariably measured as or equated with total protein (or, even less definitively, total nitrogen content) and water content (Scriber, 1984). While these values may well be correlated with herbivore performance, it is extremely unlikely that herbivores are capable of responding physiologically to "total nitrogen.'" Nitrogen is at best a surrogate for nutritional value (Simpson and Simpson, 1990) and, for nitrogen in particular, it can be argued that it is not at all the same currency as far as plants and herbivores are concerned, since plants take in nitrogen in forms that are completely unutilizable by herbivores, Even for plants, which can utilize atmospheric nitrogen with the assistance of symbiotic microbes, the form in which nitrogen enters the system affects its utilizability (Fitter and Hay, 1983). As for herbivores, the efficiency with which nitrogen can be utilized depends critically on the form in which it is ingested; free amino acids, for example, are for more readily utilized than are complex proteins (Cockfield, 1988) and ureides, frequently found in legumes symbiotically associated with nitrogen-fixing bacteria, are even less readily utilized by insects (Wilson and Stinner, 1984), Due to differential needs, the efficiency with which amino acids can be utilized depends critically on their relative abundance and proportional representation (Rock, 1972; Reese, 1979). As well, vitamins, carbohydrates, lipids, and a host of other synthesized materials are as important

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as, if not more important than, nitrogen per se for insect nutrition. The hypothesis that plant primary metabolism can be influenced by herbivore selection pressure effectively remains untested. TESTING THE HYPOTHESIS

That plant primary metabolites can confer resistance against herbivores, or, altematively, that insect selection pressure can effect changes in the content and composition of plant primary metabolites, is an eminently testable hypothesis. In order to demonstrate that plant defense is a "raison d'6tre," as it were, for quantitative or qualitative variation in the production of particular primary metabolites, certain conditions must be met: (1) primary metabolite content must be variable among potential host plants; (2) variation in primary metabolite content must be at least partly genetically based and thus available for selection; and (3) plant fitness (reproductive success) in the presence of the herbivore must be associated in a predictable way with that genetically based primary metabolite variability (Haukioja et al., 1991). If these three conditions are met, then the potential exists for insects, by differential herbivory, to effect changes in the content or composition of primary metabolites in their host plants. There is one other criterion to consider as well. Insect herbivores may act as selective agents on plant primary metabolism only indirectly if variation in the content of a particular primary metabolite is genetically correlated with variation in the content of a secondary metabolite, which is the trait under direct selection, for example. Direct selection can act efficiently on plant primary metabolites only if those metabolites are themselves responsible for a change in herbivore physiology or behavior that in turn alters plant fitness. Although it may seem a daunting task to identify particular nutrients likely to respond to herbivore pressure, it is probably less daunting than identifying particular allelochemicals likely to respond to herbivore pressure, at least in part because there may be fewer primary metabolites of ecological relevance to insects in plants than there are secondary metabolites. According to Fraenkel (1969), "Thirty to forty such substances [necessary nutrients] (according to whether one counts the amino acids individually, and making allowance for the discovery of new factors) are involved in the nutrition of insects"; by contrast, essential oils of individual plants may contain hundreds of terpenoid and phenylpropanoid constituents (Guenther, 1948). The task may be simplified further because many phytophagous insects appear to have similar nutritional requirements. All phytophagous insects, for example, have an absolute sterol requirement, although there is variation in the extent to which different plant sterols can be utilized (Chapman, 1972; Grunwald and Kogan, 1981). Although carbohydrates occur in many forms throughout the plant kingdom, simple sugars

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tend to be of greater importance for folivorous insects than are complex sugars or starches (Dadd, 1963). Although not universal by any means, the dietary requirement for ascorbic acid is practically a defining trait for phytophages, among other insects (as is the case in mammals, in which frugivorous species and other species with high dietary intakes of ascorbic acid have apparently lost the capacity to synthesize it). All insects also share a dietary requirement for unsaturated fatty acids, which, for phytophagous insects, is generally satisfied by unsaturated C~s fatty acids, particularly linolenic acid (Thompson, 1973). Proteins and their constituent amino acids are among the primary metabolites most likely to be influenced by insect herbivory. The protein content and composition of plants are dramatically different from that of herbivores and, in fact, this difference is widely regarded as a major nutritional barrier to utilizing plant food (Hinton, 1976). Most phytophagous insects have an absolute requirement for the same 10 amino acids that are required by the vast majority of animal species: these are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (Chen, 1966). According to Chen (1966), some insects contain levels of amino acids upwards of 30 times higher than levels recorded from other animals. Why their amino acid demands are so great is unclear--amino acids appear to play a role in osmoregulation, in detoxification (e.g., conjugation with glycine for export), and in energy conservation. Certain activities may impose higher amino acid demands on certain insects, relative to other species. The parsnip webworm Depressaria pastinacella, for example, invests approximately 20% of its nitrogen intake into the production of a silken web. The amino acid composition of the silk is radically different from that of parsnip fruits, the principal food of the webworms; levels of glutamic acid, serine, and glycine in silk far exceed proportionately the levels in fruits. In turn, the composition of silk is radically different from that of the body of the webworm itself (Figure 1). The quantities of histidine, arginine, and methionine, all essential amino acids, are notably low

20

~

larva

0, asx glx set his" gly thr" aia arg" tyr val'met'phe'Jle" leu" lys" pro

AMINO ACID

FIG. 1. Amino acid composition of parsnip webworm silk, webworm bodies with silk glands removed, and green fruits of wild parsnip. Amino acids essential for insects are marked with an asterisk (Rock, 1972).

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in parsnip fruits relative to the amino acid profile of the webworm. While larval body contents may not necessarily reflect dietary needs, these disproportions between insect and plant proteins may impose a considerable nutritional stress on the insect and leave it vulnerable to deficiencies in its food plant. The effects of variability in the amino acid composition of protein on plant suitability to insects have been demonstrated by Broadway and Duffey (1988), who found a 16-fold variation in larval weights of Spodoptera exigua, depending on the source of dietary protein. The most significant influence on larval growth rates was amino acid profile (in particular, the content of lysine + arginine and of sulfhydryl-containing amino acids). As storage molecules, many plant proteins appear to be effectively physiologically inactive--it is only when they are hydrolyzed to component amino acids that they enter into plant primary physiological processes (Mothes, 1980). It seems likely, then, that the amino acid composition of these storage proteins could vary among genotypes with little physiological impact on the plant. Moreover, genetic variations is known to exist for protein concentration (Moser and Frey, 1993; McFerson and Frey, 1991; Hansen et al., 1992; Sullivan and Bliss, 1983; Caradus, t992) and composition (Axtell, 1981) and thus would potentially be available for selection by herbivores. Evidence also exists that there is a predictable association between quantitative variation in amino acid content and plant resistance to insects in some systems. Maltais and Auclair (1957) demonstrated that Pisum sativum lines resistant to Acvrtosiphon pisum were 11.5-37.1% lower in amino acids and 1647 % higher in free sugars (such that the ratio of sugar to nitrogen in resistant lines was 23-64% higher, a critical factor for a phloem-feeding insect that relies on high throughput for extracting sufficient amounts of protein from a proteinpoor food resource). Auclair et al. (1957) also demonstrated that amino acid content tended to be higher in susceptible varieties; Weibull (1994) showed that Hordeum vulgare resistance to Rhopalosiphum padi is associated with variation in glutamic acid content in the sense that low glutamic acid levels are associated with reduced aphid weights. Amino acid imbalance was also implicated by Janzen (1977) in the inability of Callosbruchus maculatus to thrive on diets high in tryptophan, cystine, and methionine, relative to levels in their normal host Vigna unguiculata. While these studies involve effectively sedentary specialists [stipulated by Moran and Hamilton (1980) as particularly prone to nutritional defenses], Waldbauer et al. (1984) have shown that in at least one more mobile species, Heliothis (=Helicoverpa) zea, insects can detect and respond to protein deficiencies in their diet by self-selection and avoidance of nutritionally inadequate food. The content and composition of plant sugars appear to play an important role in resistance of plants to insects as well. In studies of Douglas fir, Pseudotsuga menziesii, resistance to Choristoneura occidentalis, Clancy (1992)

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showed in artificial diet studies that optimal sugar concentrations were about 6% and survivorship actually declined when sugar levels exceeded this amount even slightly; natural levels range from 5.7 to 18.4 %, thus potentially exceeding optimal concentrations by a large margin. Excess amounts of sugar may hinder growth because its processing diverts energy away from growth (Waldbauer et al., 1984). In this system, sugar composition plays a role as well; Zou and Cates (1994), upon noting that 78% of total sugar in Douglas fir was galactose, determined that 6% galactose increased mortality and decreased growth relative to comparable amounts of glucose and fructose in diets. Sugar content and composition are known to be under genetic control in certain crop plants (Simon et al., 1982, Freeman and Simon, 1983; Poehlman, 1979) and have been extensively modified by artificial selection (Stommel and Simon, 1989). Of all plant prinlary constituents, vitamins are probably among the most variable in terms of distributive and quantity. Ascorbic acid content, for example, differs by an order of magnitude or more among cultivars of squash, peppers, tomatoes, muskmelons, apples, pears, and mangoes (Mozafar, 1994), and /3-carotene varies by almost two orders of magnitude among varieties of carrots, sweet potato, and cassava (Mozafar, 1994). Many cases can be cited of successful breeding programs designed to increase the vitamin content of fruits or foliage of crop plants (Bittenbender and Kelly, 1988). There is at least one example of an association between vitamin content of plant tissue and susceptibility to an insect herbivore--ascorbic acid content of fruits is positively correlated with amount of damage inflicted by Heliothis arraigera (Singh et al., 1982, cited in Mozafar, 1994)--and numerous other studies demonstrate an association between ascorbic acid content and resistance to microbial pathogens. Felton and Summers (1993) have cogently argued that ascorbate oxidase, a plant enzyme that can convert ascorbic acid into the unstable and potentially toxic oxidized derivative dehydro-L-ascorbic acid in the insect gut, is effectively a defense-related protein in that it renders ascorbic acid unusable by the insect as a dietary nutrient or antioxidant. Low levels of ascorbic acid in plant tissues, independent of the activity of ascorbate oxidase, may serve a similar defensive function. That phytophagous insects are capable of responding behaviorally as well as physiologically to vitamin imbalances in their food was demonstrated by Schiff et al. (1988), who showed that Heliothis zea larvae can self-select optimally between diets complete except for lipids or vitamins. INTERACTIONS BETWEEN PRIMARY AND SECONDARY METABOLITES In that a herbivorous insect almost inescapably ingests both primary and secondary metabolites when it feeds, the potential exists for toxicological and and physiological interactions between these classes of compounds. Examples

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of mitigation of allelochemical toxicity by primary metabolites are numerous. High levels of protein, for example, can reduce the toxicity of trypsin inhibitors of glycoalkaloids (Slansky, 1992); antioxidants such as vitamins A, C, and E can reduce the phototoxicity of photosensitizing altetochemicals (Aucoin et al., 1990; Green and Berenbaum, 1994). Allelochemicals can also exacerbate the effects of low nutrient concentrations. Diet dilution can elicit compensatory feeding behavior, which in the presence of a toxin can lead to ingestion of a lethal dose (Slansky and Wheeler, 1992). In addition, the vitamin content of the diet can influence the detoxification capacity of an herbivore; in other systems, for example, there have been suggestions that vitamin C inhibits or otherwise alters metabolic transformations catalyzed by cytochrome P-450 (Sinclair et al., 1993; Suzuki et al., 1993), the principal enzyme system involved in insect detoxification of plant secondary metabolites (Brattsten, 1992). BENEFITS AND COSTS OF NUTRIENT-BASED DEFENSE TO T H E PLANT

One conceivable advantage to a plant of a defense system based on altered levels of plant primary metabolites is that autotoxicity may not pose as great a risk as it does in defense systems based on secondary metabolites. Secondary metabolites often target fundamental physiological processes, and, as such, absent a system for localization or sequestration, pose as much of a danger to the plants producing them as they do to insects consuming them. Presumably, systems for the rapid synthesis or degradation of primary metabolites are already in place to meet the physiological demands of the plant; such systems may require little modification to permit plants to store comparatively high levels of a nutrient that, if ingested by a herbivore, due to its differing physiological needs, could create a lethal nutritional imbalance. Nutritional defenses can be expected to arise most frequently in those cases in which the nutritional needs of plant and animal diverge most dramatically. Nutrient-based defenses would be unlikely to arise in developing seeds, for example. Developing seeds utilize plant nutrients in much the same way that heterotrophic organisms do, whereas photosynthetic source tissues can synthesize needed components. Indeed, seedling success in terms of competitive ability, growth rate, or germination rate is often correlated with nutrient content of the seed (Vails and George, 1985; Rahman and Goodman, 1983; Parrish and Bazzaz, 1985). Even in those cases in which plant and animal nutritional requirements differ dramatically, plant nutrient content cannot decrease to such a point that plant function is impaired. Plants require proteins, lipids, carbohydrates, vitamins, and micronutrients to grow and reproduce, and reductions in the amounts of these constituents may compromise a plant's physiological capabilities at the same time they decrease attractiveness to herbivores. To date, there is abundant

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evidence of the impact of quantitative variation in plant elemental nutrients such as nitrogen on photosynthesis (Field and Mooney, 1986; Sinclair and Horie, 1989) and seed phosphorus on seedling success (Austin and Longden, 1965; Bolland and Paynter, 1990; Zhang et al., 1990), but little information exists as to whether similar limitations exist with respect to primary metabolites synthesized by the plant. All things considered, low nutrient value is unlikely to evolve as a defense under many, possibly even most, circumstances, and it is certainly unlikely to be selected for by herbivores that can compensate for low nutrient content by increasing consumption rates (in the absence of toxins) or by herbivores whose susceptibility to predators or pathogens is unaffected by growth rates or nutritional state. However, low nutrient value may well be selected for by highly oligophagous herbivores with physiological requirements for certain nutrients that differ dramatically from the requirements of their host plants or with limited mobility such that they must complete development on a single plant, or even a single plant part, with no opportunity to compensate for nutritional deficiencies by switching host plants. Primary metabolites acting as nutrients under these circumstances may be most effective in conferring resistance against herbivores in the presence of toxic secondary metabolites, in that such toxins may offset compensatory feeding responses by increasing toxicity as intake increases (Slansky and Wheeler, 1992). There are undoubtedly innumerable plant-insect relationships in which these conditions pertain, but in the absence of any systematic investigation of either the primary metabolite content of host plants or nutrient requirements of herbivores it is impossible to speculate whether nutrient-based defenses are rare or common.

APPLICATIONS

Demonstrations of the efficacy of nutrient-based defense have implications beyond theories of insect-plant interactions. There is currently tremendous interest in developing novel mechanisms to reduce pest problems in agroecosystems and at the same time reduce chemical inputs (both synthetic and natural) without reducing yields. Because humans do not feed exclusively on a single plant species for sustenance, as do many phytophagous insects, selective breeding for altered nutrient levels may lead to increased resistance to insects in the absence of chemical inputs with little or no adverse impact on human consumers (Slansky, 1990). This strategy has already been exploited in controlling pests of stored products that, because they are consumed by humans, are not ideally protected with synthetic organic insecticides. Prepared food, such as pudding mixes, have been formulated such that key vitamins, fatty acids, and other insect nutrients are in short supply; this strategy has proved successful in causing mortality of

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flour beetles ( M a n s i n g h , 1981). W h e t h e r s u c h a s t r a t e g y will w o r k u n d e r field c o n d i t i o n s , for a suite o f h e r b i v o r e s that m a y h a v e differing n u t r i e n t requirem e n t s , r e m a i n s to be s e e n , Acknowledgments--I thank Arthur Zangerl for his considerable insights into all matters botanical as well as for comments on the manuscript and assistance with the figure. I also thank Francis Webster for the opportunity to contribute to this volume: this manuscript provides me with an opportunity to thank Robert Silverstein and John Simeone for a publication that has provided me with endless hours of thought-provoking and often utterly absorbing reading over the past 16 years. This work was supported by NSF DEB91-19162.

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