Lepidopteran Larval Salivary Secretions and their Effect on Tomato Plant Defenses
Open Access
- Author:
- Shoemaker, Erica Beth
- Graduate Program:
- Entomology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Gary Felton, Thesis Advisor/Co-Advisor
- Keywords:
- Lepidopteran Salivary Secretions
Tomato Plant Defenses - Abstract:
- ABSTRACT Forty-five percent of the over one million described insect species feed on plants. Agricultural losses due to arthropod pests account for about 100 billion dollars annually. Investigation of the role of caterpillar saliva is motivated by the long-term coevolutionary relationship between plants and the herbivores that consume them. Over the past 35 years it has become increasingly apparent that a wounded plant undergoes a variety of significant changes in plant chemistry and these changes affect the palatability of the plant as well as the fitness of the herbivore. Many plant defenses are activated and these include the accumulation of protease inhibitor genes. The cost related to these defenses provides the selection pressure behind the evolution of inducible defenses. Using genetic engineering in conjunction with these naturally occurring genes can lead to a more environmentally friendly pest management solution. This work is an important contribution towards investigating a cost-effective, environmentally safe solution to the insect herbivores’ role in the loss of crops in agriculture. In this research I have investigated seven lepidopteran larvae: H. zea, H. virescens, S. frugiperda, S. exigua, O. nubilalis, T. ni and M. sexta. Saliva initiates digestive processes and may also be the first line of defense against microbes as well as plant defenses. Salivary glucose oxidase is primarily produced in the labial glands, and many entomologists currently believe it helps to suppress toxins produced by plants that are triggered by herbivore feeding. Herbivores acquire their amino acid nutrition from plant proteins. Many plant defense genes such as protease inhibitors block amino acid uptake in the feeding herbivore depriving it of the nutrition it needs. Protease inhibitor proteins are found in all forms of life and are one of the most plentiful classes of proteins found in the living world. In 1972, Green and Ryan were the first to discover the accumulation of protease inhibitors after wounding. Tomato has been widely studied as a model system for investigating the mechanism of induced resistance based on its’ response to wounding and herbivory. Tomato leaf inhibitor II is a protein that acts as a potent inhibitor of endopeptidases displaying specificity towards trypsin and chymotrypsin proteases; it accumulates in the leaf as a result of insect attack. Jasmonates are a class of oxylipin molecules that are the best-characterized class of elicitors involved in the defensive responses of plants to wounding and herbivory. Ethylene, hydrogen peroxide (H2O2), oligogalacturonides (OGAs), fatty acid-amino acid conjugates (FACs), and abscisic acid (ABA) along with UV light activate jasmonic acid (JA) formation. Salicylate and nitric oxide (NO) repress JA formation. Local wounding along with prosystemin causes systemin to bind to the SR160 receptor. This activation of systemin along with the activation of JA leads to pin2 formation via the octadecanoid pathway. Protease inhibitor genes provide a new perspective on improving plant defense systems through two routes: either through using traditional breeding methods in order to select lines with enhanced expression or through the use of DNA technologies for the direct transfer of inhibitor genes to target specific digestive enzymes of pests and/or pathogens of plants. The systemic nature of induced plant responses can be compared with the vertebrate immune response based on the manner in which endocrine signals target several tissues through the circulatory system. The polypeptide systemin has been isolated from tomato leaves and was found to be a powerful signal for systemic defense as well as inducing and regulating over 20 defense genes. Choice tests have been used to answer a broad array of research questions pertaining to behavior. Researchers try to obtain an understanding of insect feeding preference because understanding it is critical to the fields of entomology and ecology.