Foliar herbivory increases sucrose concentration in bracteal extrafloral nectar of cotton

Authors: GALE, CODY - Texas A&M International Unviersity; LESNE, PIERRE - Texas A&M University; WILSON, CAROLINE - Texas A&M University; DICKENS, C.MICHAEL - Texas A&M University; HELMS, ANJEL - Texas A&M University; Suh, Charles; SWORD, GREGORY - Texas A&M University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2021
Publication Date: 10/29/2021
Citation: Gale, C.C., Lesne, Pierre, Wilson, C., Dickens, C., Helms, A., Suh, C.P., Sword, G.A. 2021. Foliar herbivory increases sucrose concentration in bracteal extrafloral nectar of cotton. PLoS ONE. 16(10). Article e0258836. https://doi.org/10.1371/journal.pone.0258836.
DOI: https://doi.org/10.1371/journal.pone.0258836

Interpretive Summary: Cotton plants produce nectar to attract parasitoids and predators, such as ants, to help protect plants from insect pests. Previous work revealed that cotton plants increased nectar production in response to herbivory as well as following treatment by certain fungi known as facultative fungal endophytes. However, whether fungal endophytes or herbivory influences the sugar composition of cotton nectar is not known. Because sucrose concentration is a determining factor in the foraging behavior of ants, we investigated the effects of fungal endopyhtes and herbivory on the composition of sugars (glucose, fructose, and sucrose) in cotton nectar. We found that only herbivory significantly altered the sucrose concentration of cotton nectar, but not at a level that affected the attraction and foraging behavior of ants. Our findings raise new questions regarding the role and production of cotton nectar, which has been widely hypothesized to play a key role in the attraction of ants that help protect cotton plants against insect pests.

Technical Abstract: Cotton plants (Gossypium spp.) produce extrafloral (EF) nectar as an indirect anti-herbivore defense. In exchange for this carbohydrate-rich substance, predatory insects such as ants, protect the plant against herbivorous insects. Previous work indicates EF nectar-bearing plants respond to herbivory by increasing EF nectar production. Plant interactions with facultative fungal endophytes (FFEs), plant-inhabiting fungi that do not cause disease and, in some cases, improve plant resistance to herbivores, can also increase EF nectar production. This study investigated whether treatments with FFEs, in combination with herbivory, influenced the carbohydrate composition of cotton EF nectar. Herbivory significantly increased the sucrose concentration of the EF nectar, while FFE treatments had no significant effect on EF nectar carbohydrate composition. Sucrose concentration is a determining factor in the foraging behavior of generalist ants, so as a follow-up, ant preference was tested in the field. Although no difference in ant preference was detected at the concentrations tested, our findings raise new questions about cotton’s inducible response to herbivory in EF nectar. This study, to the best of our knowledge, is the first to report a significant difference in EF nectar carbohydrate composition in response to herbivory. Further research is needed to determine whether a change in EF nectar carbohydrate composition is common in Gossypium spp., and if these changes are a byproduct of systemic sucrose induction, or a potentially adaptive mechanism which enhances ant attraction to the plant.