Insect-Plant Interactions

Elizabeth A. Bernays

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9781000697551
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

First Published in 1989, this book explores the relationship between plants and insects and the ways in which they interact with each other. Carefully compiled and filled with a vast repertoire of notes, diagrams, and references this book serves as a useful reference for students of oncology, and other practitioners in their respective fields.

1	On the Role of Microbial Symbiotes in Herbivorous Insects

Bruce C. Campbell

U.S. Department of Agriculture-Agricultural Research Service
Western Regional Research Center
Albany, California

TABLE OF CONTENTS

I.	Introduction
II.	Extracellular Symbiotes
	A.	Internal Extracellular Symbiotes
	B.	External or Cuticular Symbiotes
III.	Endosymbiotes
	A.	Histology, Ultrastructure, and Host Nutrition
	B.	Molecular Biology of Aphid Endosymbiotes
	C.	Endosymbiotes and Insect Biotypes
IV.	Conclusions and Summary
Acknowledgment
References

I. INTRODUCTION

For almost a century it has been known that many insects, across a broad taxonomic range, have an obligate association with prokaryotic and/or eukaryotic microorganisms which are harbored extra- or intracellularly. The most comprehensive review on the subject involving these symbiotes in insects was written by Buchner more than 30 years ago.⁷⁹ Whereas much is known about the taxonomy and biochemistry of the extracellular symbiotes, a corresponding understanding of the biology of intracellular symbiotes (i.e., endosymbiotes) has yet to be achieved. This is primarily because insect endosymbiotes have never been successfully cultured outside their host. However, recent efforts in the study of insect endosymbiotes have employed genetic engineering techniques that permit the study of genes and/or gene products which designate the taxonomy and biochemistry of the endosymbiotes.

This chapter presents information on the role of symbiotes in insect-plant interactions, from a biochemical perspective, with emphasis on the endosymbiotes. A consideration of the role of insect symbiotes in the ecology of insect-plant interactions has already been evaluated by Jones.²⁹³ Unlike that of extracellular symbiotes, it is difficult to come to definitive conclusions as to the role of endosymbiotes in the interaction of insects with their host-plants without a thorough understanding of their biochemistry. There is a large body of information that considers the biochemistry of insect endosymbiotes indirectly. Studies have compared the biosynthetic products and/or physiological performances between insects that possess endosymbiotes with those whose symbiotes have been eliminated by antibiotics or some other treatment (e.g., heat). However, because of the potential of these treatments to affect host tissue, one should be cautious with the conclusions drawn from such studies. The available knowledge on extracellular symbiotes may be relevant to our understanding of the biological role of insect endosymbiotes. Hence, some of the pertinent literature on these extracellular symbiotes is reviewed prior to that of insect endosymbiotes.

II. EXTRACELLULAR SYMBIOTES

Extracellular symbiotes between microorganisms and insects has developed both internally and externally (viz., extracuticularly) in insects. By far, the most widely studied extracellular symbiotes are the protozoa that inhabit the hindgut of termites.^{120,121,262,263,568} Additionally, there are numerous reviews which emphasize the literature on extracellular microorganisms that inhabit insects in general.^{47,162,238,521, 522, 523, 524,536} There are other less well-defined examples of extracellular microorganisms inhabiting the alimentary tract of insects (e.g., bacteria of the esophageal bulb in tephritid larvae) or residing on the cuticle (e.g., saprophytic or mycangial fungi of wasps and/or xylophagous beetles, respectively).

A. INTERNAL EXTRACELLULAR SYMBIOTES

1. Termites

Reviews are available on the biochemical,^60,410,442 taxonomic,^43,59,228 and overall biological roles of termite symbiotes.^325,326 The most widely studied and best-defined role of termite symbiotic microbiota is by far their production of cellulolytic enzymes for the digestion of wood. Almost 50 years ago, it was discovered that certain protozoa in the hindgut of Zootermopsis produced enzymes capable of fermenting cellulose.^262,263,265 More-recent studies have greatly expanded the known number of taxa where cellulase, carboxymethylcellulase, or cellobiase activities in termites are produced either by protozoan flagellates or bacteria inhabiting the hindgut.^{248,387,451,532,533,569,571} Some termites (e.g., Nastutitermes and Coptotermes) are capable of producing their own cellulolytic enzymes in addition to those supplemented by bacterial symbiotes.²⁴⁸ In many cases, the kinetic and physical characteristics of termite cellulolytic enzymes have been defined.^465,466 It has also been suggested that the symbiotic microflora of termites produce ligninases⁸³ and chitinases.⁵⁴⁸

The above-mentioned studies establish that one of the biological roles of termite symbiotes is to produce enzymes which digest wood and/or cultivated fungi. Such enzymes enable subterranean termites to meet carbon-source requirements. However, numerous studies also show that these symbiotes play a second, vital role in the synthesis of termite nutritional requisites. Hungate suggested that termite-gut symbiotes were involved in fixation of atmospheric nitrogen.²⁶⁴ Recent studies confirm that nitrogen is fixed by bacteria in the hindgut of many species of termites.^{62,196,430,460,469,470} Moreover, the nutritional status of termite colonies is dependent upon the bioavailability of a nitrogen source from either fixed nitrogen or from the metabolism of uric acid by gut symbiotes.^{32,352,461, 462, 463, 464} In addition to nitrogen metabolism, termite-gut symbiotes play a role in the synthesis and metabolism of fatty acids.^247,384,443 Lipid metabolism by symbiotic bacteria in termites is also involved in the production of precursors for synthesis of amino acids.³⁸⁵ The symbiotic relationship in termites is not limited to termite-gut symbiote interactions. Certain hindgut symbiotes in termites interact interdependently in the synthesis and metabolism of lactates.⁴⁹⁷ Bacterial symbiotes in termites maintain a specific redox potential in the hindgut that inhibits the growth of nonsymbiotic or potentially pathogenic bacteria.⁵⁴⁶ The symbiotes may also be involved in the synthesis of noxious chemicals used in defense of the colony against ants or termite predators⁴⁶⁸ and contribute to atmospheric methane.⁴⁷⁷

The ability to culture and describe physical characteristics of termite symbiotes outside of their host has greatly facilitated their definitive identification and classification. Many of the protozoan symbiotes^{61,249,343,570} and bacterial symbiotes have been classified.^{37,39,137,183,184,343,496,534} The specialized morphology and epithelial tissue of the alimentary tract of termites which enables the attachment or establishment of symbiotic microflora has also been noted.^38,40,46

2. Other Insects

The symbiotic microflora of the alimentary tract of a variety of other xylophagous insects (e.g., wood-inhabiting cockroaches and beetles) has been studied almost to the same extent as that of termites. To a lesser degree, the biological role of the extracellular gut symbiotes of certain Diptera (e.g., larval symbiotes of various tephritids and the screwworm fly), Lepidoptera, and chrysopids have been studied as well. However, in many cases with these latter insects, the contribution that the symbiote provides towards host survival is not clear.

a. Cockroaches

As with...

Table of contents

• Cover
• Title Page
• Copyright Page
• PREFACE
• THE EDITOR
• ADVISORY BOARD
• Table of Contents
• Chapter 1 On the Role of Microbial Symbiotes in Herbivorous Insects
• Chapter 2 The Relative Importance of Vertebrate and Invertebrate Herbivores in Plant Population Dynamics
• Chapter 3 Air Pollution and Insect Herbivores: Observed Interactions and Possible Mechanisms
• Chapter 4 Extrinsic Factors Influencing Production of Secondary Metabolites in Plants
• Chapter 5 Arthropod Impact on Plant Gas Exchange
• Index