Heat-Shock Protein 70 and Dough-Quality Changes Resulting from Heat Stress During Grain Filling in Wheat

In an attempt to further elucidate the molecular mechanisms that determine the loss of dough strength associated with heat stress of growing wheat, the roles of heat-shock proteins (HSP) and heat-shock elements upstream of glutenin genes were investigated. A range of genotypes differed in the extent of synthesis of high molecular weight glutenin subunits (HMW-GS) and HSP during heat stress. The concentration of HSP 70 remaining in mature grain increased as a result of a few days' heat stress of wheat plants. The amount of HSP 70 in mature grain samples from heat-stressed plants of 45 genotypes was not strongly correlated with loss of dough strength. There was much less evidence for this mechanism than for other molecular hypotheses from the literature, particularly, changes in glutenin-to-gliadin ratio, size distribution of the glutenin polymer, and the involvement of HSP and chaperones during grain-protein synthesis. HSP 70 was purified from heat-stressed grain, and was added to (or incorporated into) dough in the direct-drive mixograph. The HSP behaved similarly to several other hydrophilic proteins when added at a level of 2 mg/2 g of flour. It showed no dramatic effects on dough properties that could constitute a major explanation for the dough-weakening effects of heat stress, even though the level of addition was well above the maximum levels that might be encountered in field-grown, mature grain. Furthermore, sequencing of the genes (upstream of the coding region) for HMW-GS failed to show the presence of heat-shock promoters, even for genotypes that differed considerably in their reactions to heat stress. The findings simplify the range of possibilities that cause heat-related loss of dough strength, focusing attention on the degree of polymerization of the glutenin chains, and on the roles of HSP and chaperones in the developing grain.