Comprehensive analysis of wheat starch synthase III revealed existence of two copies differentially expressed under heat stress

Starch is the most critical source of energy and constitutes 65–80% of the wheat grain. Starch synthase (SS) is a key regulator of starch metabolism. Among its five isoforms, SSIII plays a pivotal role in phytoglycogen accumulation. In wheat, it is associated with starch accumulation but is sensitive to heat stress. In silico characterization indicated two homologous copies of the TaSSIII genes (Triticum aestivum starch synthase III genes), TaSSIIIa and TaSSIIIb. TaSSIIIa shares maximum similarity with HvSSIIIa, whereas TaSSIIIb exhibits maximum similarity with OsSSIIIb. It encodes proteins with an N-terminal transit peptide, an SSIII specific domain, a C-terminal catalytic domain, and a conserved glycosyltransferase domain with variations among the homoeologs. The catalytic domain has N-glycosylation motifs, with different numbers in the two homologs. ADP glucose-binding motifs are present in the SS-CD and GT-1 domains. ADP-binding pockets are present at the C-terminus of the catalytic domains. Maximum heterogen ligands are found in TaSSIIIa1B and minimum in TaSSIIIb2B. TaSSIIIb is expressed in leaf, stem, root, spike, and grain, and in much higher amounts than TaSSIIIa. Pot-grown wheat genotypes showed heat stress-induced upregulation of the genes TaSSIIIa1D and TaSSIIIb2D in the flag leaf. The tolerant genotype showed a significantly higher fold increase in the transcript levels under heat stress. Under the stress in the field, transcript level change for TaSSIIIa1D was higher in peduncle as compared to flag leaf, and that of TaSSIIIb2D was not significantly different between tissues. Homoeologous copies of the gene have tissue and genotype-specific expression, are influenced by heat stress, and thus may play a role in the homeostasis of starch metabolism under stress.