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 deter mine 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 di ffered in the extent of synthesis of high molecular weight glutenin su bunits (HMW-GS) and HSP during heat stress. The concentration of HSP 7 0 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 f rom heat-stressed plants of 45 genotypes was not strongly correlated w ith loss of dough strength. There was much less evidence for this mech anism than for other molecular hypotheses from the literature, particu larly, changes in glutenin-to-gliadin ratio, size distribution of the glutenin polymer, and the involvement of HSP and chaperones during gra in-protein synthesis. HSP 70 was purified from heat-stressed grain, an d was added to (or incorporated into) dough in the direct-drive mixogr aph. The HSP behaved similarly to several other hydrophilic proteins w hen added at a level of 2 mg/2 g of flour. It showed no dramatic effec ts on dough properties that could constitute a major explanation for t he dough-weakening effects of heat stress, even though the level of ad dition was well above the maximum levels that might be encountered in field-grown, mature grain. Furthermore, sequencing of the genes (upstr eam of the coding region) for HMW-GS failed to show the presence of he at-shock promoters, even for genotypes that differed considerably in t heir reactions to heat stress. The findings simplify the range of poss ibilities that cause heat-related loss of dough strength, focusing att ention on the degree of polymerization of the glutenin chains, and on the roles of HSP and chaperones in the developing grain.