Analysis of inducible terpenoids in cotton leaves to test for indirect plant-endophyte-herbivore interactions

Authors: GALE, CODY - Texas A&M University; Suh, Charles; Puckhaber, Lorraine; Perez, Jose; SWORD, GREGORY - Texas A&M University

Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2021
Publication Date: 2/11/2021
Citation: Gale, C.C., Suh, C.P., Puckhaber, L.S., Perez, J.L., Sword, G.A. 2021. Analysis of inducible terpenoids in cotton leaves to test for indirect plant-endophyte-herbivore interactions. Journal of Entomological Science. 57(1):114-118. https://doi.org/10.18474/JES21-07.
DOI: https://doi.org/10.18474/JES21-07

Interpretive Summary: Fungi that colonize plant tissues without causing symptoms of disease are commonly known as facultative fungal endophytes. Previous research showed that corn earworm larval survival and feeding were significantly reduced on cotton plants that were treated as seeds with certain fungal endophytes, but the mechanism responsible for the reduction in larval survival and feeding is not known. One hypothesis is that these fungal endophytes cause cotton plants to increase production of defensive chemicals known at terpene aldehydes (e.g., gossypol). We quantified terpene aldehyde levels in cotton plants treated with and without fungal endophytes, and found no significant difference in terpene aldehyde levels between fungal-treated and non-treated plants. Thus, our findings suggest some other factor is responsible for the reduction in larval survial and feeding previously observed on cotton plants treated with fungal endophytes.

Technical Abstract: Cotton (Gossypium hirsutum) produces a suite of terpenoid aldehydes that are responsible for much of the plant’s direct defense against chewing herbivores. A previous study of cotton treated with horizontally transmitted facultative fungal endophytes (FFEs) showed reduced survivorship of Helicoverpa zea larvae feeding on those plants. One of these fungi was Beauveria bassiana, an entomopathogen capable of parasitizing insects, but mycosis was never observed as a cause of reduced H. zea survivorship. An alternative mechanistic hypothesis is that the FFE treatments indirectly influenced the plant-insect interactions by eliciting a change in terpenoid aldehyde chemistry. To test this, terpenoid aldehydes were quantified in greenhouse grown cotton treated as seeds with and without the horizontally transmitted FFEs, B. bassiana and Phialemonium inflatum. Terpenoid aldehyde production in cotton is known to increase significantly in response to herbivory, so quantification was performed in both the absence of herbivores and in response to H. zea feeding using a fully-factorial design. A significant difference in terpenoid aldehyde content was detected in the presence of herbivory, corroborating previous studies that demonstrated the herbivore-inducibility of these compounds. FFE treatments did not produce any significant differences in terpenoid aldehyde content, regardless of herbivory. These findings suggest that previous observations of reduced H. zea survivorship on cotton treated with either B. bassiana or P. inflatum are likely not due to changes in terpenoid aldehyde content, and further investigation into the underlying mechanism is warranted.