Characterization of the starch synthase under terminal heat stress and its effect on grain quality of wheat

Terminal heat stress (HS) is a key barrier for wheat grain yield and quality. Various physiochemical and molecular parameters such as photosynthetic rate, expression analysis and activity of starch synthase (SS), total starch, amylose and amylopectin content, total amylolytic activity, and total antioxidant capacity (TAC) were analysed in wheat cvs.HD3059 (thermotolerant) and BT-Schomburgk (thermosusceptible) at grain-filling stage under HS (32 °C and 40 °C, 1 h). The decrease in photosynthetic rate was observed under HS. Expression analysis of the SS gene at transcript level showed downregulation in both the wheat cvs.HD3059 and BT-Schombugk under HS (32 °C and 40 ºC, 1 h) as compared to the control. Although the downregulation of SS gene transcript expression was less in HD3059 than BT-Schombugk. Both the cultivars showed decrease in starch synthase activity and starch content under HS and the overall content was higher in HD3059, compared to BT-Schomburgk. Higher total amylolytic activity and amylose content were observed in BT-Schomburgk. Scanning electron microscopy (SEM) showed un-structured starch granules under HS. Total antioxidant capacity (TAC) was found higher in HD3059 (14.07 mM FeSO₄ gm^-1 FW) compared to BT-Schomburgk (8.89 mM FeSO₄ gm^-1 FW) under HS (40 ºC, 1 h). Findings suggest that HS during grain filling stage had more severe impact on the overall physiochemical properties of the wheat grain. Thus the starch bisynthesis pathway associated gene(s) could be exploit to enhance the yield and quality of wheat under heat stress.