Identification of Aspergillus Fungal Resistance Factors in a Plant Model System

Aspergillus flavus is a saprophytic, mycotoxigenic fungus that contaminates agriculturally important seeds with the potently toxic and carcinogenic secondary metabolite, aflatoxin. Seed infection by fungi is often prevented by intact seed coats. Although Arabidopsis thaliana is naturally resistant to Aspergillus infection, certain mutants in the flavonoid biosynthetic pathway are compromised in seed coat integrity. We hypothesized that these mutants might also permit Aspergillus infection. To that end, we systematically tested infectibility of mutants in the flavonoid biosynthetic pathway to identify those lacking resistance to Aspergillus fungal infection. Susceptible seeds included those mutated in the genes encoding for synthesis of the first flavonoid pathway precursor, chalcone, through leucocyanidin (CHS, F3'H, and DFR), indicating that the requisite compound is either leucocyanidin or a derivative of that compound. While preliminary observations suggested that older chs seeds might be more susceptible to A. nidulans than younger seeds, an experiment testing infectibility of seeds harvested at specific ages failed to reproduce the infection rates previously observed. Further investigation revealed that chs seed batches dominated by non-viable seeds are more infectible, as expected from a saprophytic fungus. A novel finding was that chs seeds formed during the final weeks of the parent plant's development are more highly susceptible to A. nidulans. Our results suggest that wildtype Arabidopsis seeds have a barrier to infection, which may be either mechanical or chemical.