Insect-Plant Interactions (1993)

Volume V

Elizabeth A. Bernays

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9781138560406
• eBook ISBN: 9781351361286
• Topic: Scienze biologiche
• Subtopic: Biologia

Volume 5 of "Insect-Plant Interactions" is a volume in a series that presents research in the field. Topics covered include chemical changes in plants as a result of insects feeding on their leaves, dynamic elements of the use and avoidance of host plants by tephritid flies as a result of the presence of other flies, floral volatiles in insect biology, endophytic fungi as mediators of plant insect interactions, the cost of chemical defence against herbivory, and life history traits on insect herbivores in relation to host quality. The book also presents the first available review on physicochemical conditions of the gut lumen from an ecological perspective.

Chapter 1
CHEMICAL CHANGES RAPIDLY INDUCED BY FOLIYORY

Ian T. Baldwin

TABLE OF CONTENTS

I. Introduction

II. Chemical Defense Theories and Their Application to Herbivory-Induced Responses in Plants

A. Philosophical Issues

B. C/N Theory

C. Optimal Defense Theory

III. Patterns of Rapidly Induced Chemical Changes

A. Secondary Metabolites: Biosynthetic Patterns

B. Secondary Metabolites: Temporal Patterns

C. Secondary Metabolites: Mechanisms

D. Primary Metabolites

E. Interactions between Induced Primary and Secondary Metabolites

IV. Functional Roles for Rapidly Induced Chemical Changes

A. Ecological Roles

B. Physiological Roles

1. Carbon and Nitrogen Storage and Transport

2. Protection of the Photosystem

V. Synthesis and Future Research Directions

Acknowledgments

References

I Introduction

The chemical characteristics of leaves are remarkably protean. Concentrations of primary and secondary metabolites change throughout ontogeny as leaves develop photosynthetic competence, mature, and senesce.^{42 120 141 142} These ontogenetic chemical changes can be influenced by biotic and abiotic environmental factors affecting plant growth. The observation that the concentrations of many secondary metabolites increase after folivory has captured the imagination of the ecological community; at least superficially, the fact of their inducibility appears to support the notion that these metabolites function defensively. Moreover, the inducibility of these metabolites is consistent with the economic framework proposed to explain the patterns of secondary metabolites: inducibility may minimize the costs of producing a chemical defense.^{11,73,89,94,168}

Herbivory-induced responses have been the subject of many excellent reviews,^{18, 27, 36, 63, 70, 91, 93, 109, 133, 166, 167 and 168, 176, 179,203} including one in this series²¹⁷ and a recent book.194a Induced responses can be studied on three time scales: preformed-induced responses, which occur immediately upon damage, are restricted to the damaged tissues, and usually result from the mixing of previously isolated enzymes and substrates (e.g., cyanogenesis); rapidly- induced responses, which occur within hours or days of the injury and can be systemic or localized to the damaged leaf; and delayed-induced responses, which occur in the next season's foliage or later. The delayed-induced responses have received the most ecological attention because of their potential importance in regulating herbivore populations.⁹⁵

This review takes a phytocentric view of induced responses,^42,96 recognizing that folivory results not only in defense-related changes in secondary metabolism but also in changes in the “civilian” chemistry of a leaf. These changes in primary metabolites result from a suite of physiological responses that contribute to a plant's resilience to herbivory. This review will be restricted to the rapidly-induced responses, because they occur when a growing plant reconfigures its methods of acquiring and partitioning resources in response to folivory.

Folivory is, in many ways, a tolerable form of herbivory. Leaves are engineered for obsolescence, and if an herbivore removes productive leaves prematurely, plants have at their disposal a battery of physiological responses that help them regrow lost tissues. One of these physiological responses — the changes in gas exchange after herbivory — has been thoroughly reviewed in this series.²¹⁸ These physiological responses are not limited to the resource-acquiring leaves and roots; they are part of a whole-plant response to damage that influences the patterns of resource allocation and partitioning. Considering induced “defense” responses in the context of the functional reorganization of a plant that occurs after herbivory is important for both ecological and mechanistic reasons. The physiological responses to herbivory may provide mechanisms for the induced changes in secondary metabolism: the “civilian” physiological responses may alter the pools of metabolites that supply “defensive” metabolic pathways. Moreover, since the food quality of a leaf for an herbivore is determined by the interaction of both its primary and secondary metabolites,^48,101,207 understanding folivory- induced changes in plant function is essential for understanding herbivore resistance induced by chemical changes.

II CHEMICAL DEFENSE THEORIES AND THEIR APPLICATION TO HERBIVORY-INDUCED RESPONSES IN PLANTS

A Philosophical Issues

Two classes of theories address the intraspecific patterns in secondary chemistry: 1) the carbon-nutrient theory (C/N) and the growth-differentiation theory, which emphasize respectively the physiological and ontogenetic constraints imposed by resource availability on the allocation of resources to growth, reproduction, and defense;^{13,24,43, 127, 201, 202} and 2) the optimal defense theory, which emphasizes the adaptive value of secondary metabolites based on considerations of tissue value,^132,133 apparency to herbivores,^61,169 or the optimization of defense allocation.^18,168 Although these two classes of hypotheses both make predictions about the interspecific, intraspecific, and within-individual patterns of secondary metabolites, they are not alternative hypotheses, for they are posed at different levels of analysis (see Sherman, Reference 185). Alternatives to the C/N and growth-differentiation theories and the optimal defense theories need to be posed at mechanistic and functional levels, respectively. While the C/N and growth-differentiation theories are not strictly functional-level hypotheses, they have been used as a null model for the defensive, functional-level hypotheses by proposing that damage results in “incidental” changes in secondary metabolism.²⁶ However, as Tuomi et al. and Haslam have pointed out, the two mechanistic-level hypotheses do not preclude the defensive tailoring of “incidentally” increased “overflow” metabolites.^92,203 This “defensive” view of a damage-induced chemical response derived from the C/N theory differs from the “defensive” view derived from the optimal defense theory on one important point: no tradeoffs are predicted to occur between the allocation to defensively-functioning metabolites and other plant functions, and therefore, the economic argument that inducible defenses evolved as a cost-savings measure would not apply. Other models for the evolution of inducibility that do not invoke the trade-off argument have been proposed.^67,109 For example, an inducible plant presents herbivores and pathogens with a protean, heterogeneous target, to which adaptation might be more difficult than to a static, constitutively defended target.¹⁰⁹

B C/N Theory

This theory argues that allocation of a plant's resources to growth is its highest priority, and allocation to secondary metabolites increases in response to an imbalance in resources needed for growth. Hence plants with an amount of carbon in excess of growth requirements (which, by definition, have a high C to N ratio) would be predicted to produce more carbon-intensive secondary metabolites — such as tannins, phenols, and terpenes — than plants with a low C to N ratio. This theory has successfully predicted the shifts in constitutive levels of carbonintensive metabolites after nitrogen fertilization, shading, and watering regimes;^{27,139,159,202,217} however, other manipulations such as exposure to different CO₂ concentrations have not been consistent with the mechanism set forth by the C/N theory.¹⁰⁵ In addition, many of the patterns of delayed-induced alterations in the phenolic contents of trees are consistent with the C/N theory.^27,200,203 Most of the studies that have examined this model have defined the C to N ratio as the availability of these resources in the plant's external environment. However, it is the internal levels of these resources that determine their availability to a plant,¹⁴³ and these internal leve...

Table of contents

• Cover Page
• Half Title
• Title Page
• Copyright
• Preface
• The Editor
• Advisory Board
• Contributors
• Contents
• 1. Chemical Changes Rapidly Induced by Foliyory
• 2. Use And Avoidance Of Occupied Hosts As A Dynamic Process In Tephritid Flies
• 3. Floral Volatiles in Insect Biology
• 4. Endophytic Fungi as Mediators of Plant-Insect Interactions
• 5. The Cost Of Plant Chemical Defense Against Herbivory: A Biochemical Perspective
• 6. Life History Traits of Insect Herbivores in Relation to Host Quality
• 7. The Chewing Herbivore Gut Lumen: Physicochemical Conditions And Their Impact On Plant Nutrients, Allelochemicals, And Insect Pathogens
• Subject Index
• Taxonomic Index