Chemical Defenses of Cotton Plants and Facultative Fungal Endophytes: Induction by Caterpillar Herbivory

Abstract

Facultative fungal endophytes (FFEs) are unspecialized plant-associated fungi capable of colonizing plant tissues without causing symptoms of disease. Studies of cotton (Gossypium hirsutum L.) treated with FFEs suggest that they may enhance plant defense against insect herbivores. Chemical defenses are typically classified as direct or indirect, with direct defenses toxic or repellant to herbivores, and indirect defenses attractive to natural enemies of herbivores. I examined volatile organic compound (VOC) emissions and extrafloral (EF) nectar production in the presence and absence of herbivores to determine whether indirect defenses were affected by FFE treatments. Cotton produces a class of chemicals known as terpenoid aldehydes that are responsible for much of this plant’s direct defense against chewing pests such as caterpillars, and I examined those chemicals similarly. I found clear evidence that changes in these chemical defenses are involved in cotton’s induced response to caterpillar herbivory, which corroborates many other studies in this field. I did not find overarching evidence that the FFE treatments influenced plant production of these chemical defenses, but this is likely the result of low colonization rates throughout my experiments. Furthermore, despite low colonization rates, I found that emissions of the acyclic monoterpene ocimene appeared to be enhanced by FFE treatments, and this finding warrants further investigation. I conclude that the various techniques used throughout these chapters should be combined into a single comprehensive design that might provide a clearer examination of FFE-mediated herbivore resistance in cotton.