Isolation and structural elucidation of acidic terpenoid phytoalexins in maize and their interactions with Aspergillus flavus

Plants use a variety of physical and chemical defenses in response to herbivory and pathogen attack. Infection of maize by the fungal pathogen Aspergillus flavus results in the accumulation of aflatoxins, which are among the most detrimental biogenic substances known to man. The majority of maize defenses against fungal pathogens were believed to be proteinaceous until the recent identification of inducible acidic diterpenoids and sesquiterpenoids derived from ent-kaur-15-ene and β-macrocarpene, termed kauralexins and zealexins. Initial reports of these phytoalexins demonstrated inhibition of A. flavus growth with only three members of the zealexin family; however, more than 20 acidic terpenoids co-accumulate in fungal infected maize tissues. In an effort to determine which individual phytoalexins or mixtures have the most significant effects on fungal growth and aflatoxin production, we isolated and characterized many of the additional family members. We report the structures of new zealexins, including those based on a β-bisabolene backbone, and the A. flavus growth inhibition of each. We also report the strong influence these phytoalexins, separately and in mixtures, on aflatoxin production independent from A. flavus growth. Understanding the role of phytoalexins in regulating aflatoxin production provides new mechanistic insights useful in the development of maize lines resistant to pre-harvest mycotoxin accumulation.