Interactive effects of elevated [CO2] and drought on the maize phytochemical defense response against mycotoxigenic Fusarium verticillioides

Authors: Vaughan, Martha; Huffaker, Alisa; Schmelz, Eric; Dafoe, Nicole; Christensen, Shawn; MCAUSLANE, HEATHER - University Of Florida; Alborn, Hans; HARTWELL ALLEN, LEON - Retired ARS Employee; Teal, Peter

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2016
Publication Date: 7/13/2016
Publication URL: https://handle.nal.usda.gov/10113/5695385
Citation: Vaughan, M.M., Huffaker, A., Schmelz, E.A., Dafoe, N.J., Christensen, S.A., McAuslane, H.J., Alborn, H.T., Allen, L.H., Teal, P.E.A. 2016. Interactive effects of elevated [CO2] and drought on the maize phytochemical defense response against mycotoxigenic Fusarium verticillioides. PLoS One. 11(7):e0159270. doi: 10.1371/journal.pone.0159270.

Interpretive Summary: Fusarium verticillioides (Fv) is an economically important fungal pathogen of maize that reduces grain yield by causing tissue rot and producing fumonisin mycotoxins that contaminate grain rendering it unsafe for consumption. Growth at elevated atmospheric carbon dioxide ([CO2]) makes maize more vulnerable to Fv. However, climate change models predict that future maize crops will experience elevated [CO2] and drought simultaneously, and we don’t know what these combined conditions will do. Scientists at the USDA-ARS Mycotoxin Prevention and Applied Microbiology Research Unit at the National Center for Agricultural Utilization Research in collaboration with the Chemistry Unit at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Florida, found that maize exposed to both elevated [CO2] and drought are more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. However, the increase in toxin is likely due to increased pathogen biomass rather than an increase in fungal toxin production triggered by changes in plant metabolites. The increase in vulnerability to Fv can be explained by the compromised immune response in plants exposed to simultaneous elevated [CO2] and drought, but this change in the immune response intensity may be due to a diversion of resources to the roots which accumulate higher levels of defense metabolites under conditions of drought. These data indicate that new disease management strategies may be necessary to improve the production and safety of the future maize crop at higher [CO2].

Technical Abstract: Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. Elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels are unaltered. In this study we demonstrate that plants simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Following Fv inoculation, the amount fumonisin relative to pathogen biomass is reduced in comparison to plants grown at ambient [CO2], and drought does not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. Therefore, the increase in fumonisin contamination is likely due to greater pathogen biomass rather than an increase in host derived stimulates. The accumulation of terpenoid phytoalexins in inoculated stems at elevated [CO2] when combined with drought was weaker which is consistent with greater Fv biomass. However, since elevated [CO2] does not influence the drought induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the stifled defense response in maize stems may be a consequence of resource redirection to the roots.