Hypoxia tolerance as a factor in Aspergillus flavus invasion and toxin contamination of developing maize seeds
S. Chalivendra (1), C. DeRobertis (2), K. Damann (1) (1) Louisana State University, U.S.A.; (2) Louisiana State University, U.S.A.
Aspergillus flavus predominantly colonizes the embryo and peripheral tissues (e.g., pericarp and the aleurone layer) of maize seeds but shows limited or late invasion of the endosperm. This pattern correlates with the O2 levels in seed tissues and is suggestive of a role for hypoxia tolerance in the successful invasion of the fungus in seeds. This proposal was tested by comparing A. flavus isolate responses to O2 deprivation in vitro with those in field inoculated seeds. Hypoxia (3 to 5% O2) allowed conidial germination but retarded radial growth and inhibited conidial and sclerotial development. Anoxia imposed suspended animation in most isolates. Growth differences among isolates correlated with biomass differences in infected kernels. Gene expression studies showed that canonical hypoxia genes (e.g., pdc1) were induced, while key development genes (e.g., brlA) were strongly suppressed by O2 deprivation. Aflatoxin (AF) levels were relatively high in inoculated kernels, although O2 deprivation in vitro decreased AF content in most isolates. Oleic acid addition to the growth medium enhanced toxin levels under hypoxia, consistent with high AF levels observed in seed. Isolate differences in growth, sporulation, gene expression and AF accumulation patterns correlated with their responses during seed colonization, implicating hypoxia tolerance as a key virulence factor in A. flavus.