Effects of heat stress on enzyme activities and transcript levels in developing maize kernels grown in culture

Heat stress can cause decreased grain yields and increased susceptibility in maize to the aflatoxin-producing fungus, Aspergillus flavus. We hypothesized that heat stress may affect gene expression in developing kernels, thereby affecting enzyme activities and storage product accumulation. To test this hypothesis, maize (Zea mays L. hybrid B73xMo17) kernels were cultured in vitro and grown at 25 (control), 30, and 35 degrees C. At 5-day intervals, developing kernels were analyzed for levels of gene expression and enzyme activity. Message levels of all genes evaluated, including Shrunken-1, Shrunken-2, Brittle-2, Aldolase, Waxy, alpha-Zein, beta-Zein, and Opaque-2 were reduced 65 to 80% after 5 days at 35 degrees C. In contrast, of the 17 enzymes assayed, only the activities of adenosine diphosphate glucose (ADP-Glc) pyrophosphorylase, aldolase, aspartate aminotransferase, add invertase, and acid phosphatase were decreased by heat-stress. Although the decrease in ADP-Glc pyrophosphorylase activity corresponded well with its decreased mRNA, aldolase activity only loosely corresponded to its mRNA levels, and sucrose synthase activity was not affected by growth temperature, although its transcript was reduced by 80%. We suggest that the declines in enzyme activities in the heat-stressed kernels are a result of declines in transcript. The differing responses df enzyme activities to the heat stress may reflect differences in protein turnover. In a subsequent experiment, we tested the hypothesis that heat stress may interfere with the ability of a maize kernel to generate pathogenesis-response proteins. We found that the activity of the pathogenesis-related protein, chitinase, was increased two-fold in cultured kernels grown at 25 degrees C by treatment with salicylic acid. However, the salicylate-induced increase in chitinase activity was inhibited in kernels grown at 35 degrees C. These results suggest that heat stress inhibits the induction of pathogenesis-related proteins in the maize kernel, and may, thereby, interfere with the ability of the kernel to defeat pathogen invasion.