Commercial Maize Hybrids and Mutant Genotypes Reveal Complex Protective Roles for Inducible Terpenoid Defenses

Despite long-term efforts to characterize inducible biochemical defenses in maize, the presence of predominant acidic phytoalexins has only recently been established with the discovery of kauralexins and zealexins. To elucidate further defenses, we profiled terpenoids in maize tissues infected with fungi and identified a novel zealexin, termed zealexin A4 (ZA4). Evaluation of zealexins, diterpenoids, and other defense metabolites in commercial hybrid maize revealed distinct defense accumulation patterns in pathogen-infected tissues. ZA4 was strongly elicited by Cochliobolus heterostrophus but only weekly induced by Fusarium graminearum and Colletotrichum graminicola. ZA4 had potent antimicrobial activity against F. graminearum, and contrastingly, promoted C. heterostrophus and C. graminicola growth in liquid culture. Overall, a negative correlation was observed between total phytoalexin production and fungal growth. Statistical analysis supported a role for kauralexin A3 (KA3) and the drought inducible hormone abscisic acid (ABA) to have the strongest impacts on fungal growth suppression. Anther Ear 2 mutants deficient in kauralexins demonstrated significantly improved C. heterostrophus and F. verticillioides growth compared to wild-types. Drought-induced ABA moderately reduced fungal proliferation in stems. Current results highlight the widely occurring defense functions for maize terpenoids in diverse commercial lines and highly selective activities on different fungal pathogens.