Genome wide response of Aspergillus flavus to developing maize kernels and the functional analysis of the phytase gene phy1

Aspergillus flavus infects maize kernels in the field and contaminates them with aflatoxin. Aflatoxin is a potent liver toxin and carcinogen that impacts the health of humans and animals. The goal of this research was to better understand how the fungus responds to the kernel environment during infection. Gene transcription in A. flavus was analyzed by microarrays after growth on kernels at the four developmental stages blister (R2), milk (R3), dough (R4), and dent (R5). Five days after inoculation, total RNA was isolated from kernels and hybridized to Affymetrix Gene Chip arrays containing probes representing 14,163 A. flavus genes. Statistical comparisons of the expression profile data revealed significant differences (P\u3c0.001) that included unique sets of up-regulated genes for each kernel stage and six patterns of expression over the four stages. Among the genes expressed in colonized dent kernels, were a phytase gene and six putative genes involved in zinc acquisition. A knockout-mutant of the phytase gene, phy1, was obtained. Although growth of the mutant on medium containing inorganic phosphate was the same as the wild type, growth of the mutant was restricted when phytate was the sole source of phosphate. After five days on wound-inoculated maize ears, growth of the mutant was less compared to the wild type. Total aflatoxin produced by the Δphy1 mutant also was less than the wild type; however, based on the amount of aflatoxin relative to the amount of fungal growth, the mutation appears to have no direct effect on aflatoxin production. The results suggest that phytase in A. flavus has an important role in pathogenesis