Maize Kernel Development Stage the Primary Factor in Differential Gene Expression in Response to Two Methods of Field Inoculation with Aspergillus flavus

Abstract

BackgroundAflatoxins, produced by the fungus Aspergillus flavus, often contaminate preharvest maize (Zea mays L.) grain under heat and drought stresses, posing serious health hazards to humans and livestock, and resulting in significant costs to identify and dispose of contaminated grain. This study was designed to investigate the changes in differential gene expression during seed morphogenesis and maturation in the \"aflatoxin resistant\" Argentinian inbred line Tx772 when challenged by the introduction of A. flavus through two different methods of ear inoculation; non-wounding (silk channel, used to select Tx772), wounding (side needle) and a non-inoculated control.\n\nMethod and FindingsGrain maturity had the largest effect on overall RNA-Seq differential gene expression (DGE) as measured by edgeR of the Bioconductor platform. However, within each stage of development, ranging from blister to dent, similar up-regulation in expression of many maize genes following inoculation with either method was observed; a total of 16 genes previously associated with resistance to pathogens were identified among the transcripts differentially expressed (DE) at p [≤] .05, FDR [≤] .10, and fold change [≥] 2.0 over all stages. The side needle technique produced a larger effect of infection as evidenced by 6,324 fungal reads versus 518 in silk channel and a higher level of aflatoxin. Correlations between approximately 7,000 fungal reads and the number of maize DE genes for each of the eight treatment groups was 0.56 (p = .152) and was 0.65 (p < .001) with levels of aflatoxin ranging from 0 to 137 ng g-1.\n\nConclusionThese correlations provided an internal measure of effectiveness of inoculation methods that were associated with the mostly up-regulation of defense-related genes in response to the presence of Aspergillus flavus in a unique maize genetic background.