Modulation of amylose content by structure‐based modification of OsGBSS1 activity in rice (Oryza sativa L.)

Summary The rice Waxy ( Wx ) gene encodes granule‐bound starch synthase 1 ( EC 2.4.1.242), Os GBSS 1, which is responsible for amylose synthesis in rice seed endosperm. In this study, we determined the functional contribution of eight amino acids on the activity of Os GBSS 1 by introducing site‐directed mutated Wx gene constructs into the wx mutant glutinous rice. The eight amino acid residues are suspected to play roles in Os GBSS 1 structure maintenance or function based on homologous enzyme sequence alignment and homology modelling. Both Os GBSS 1 activity and amylose content were analysed in homozygous transgenic lines carrying the mutated Os GBSS 1 ( Wx ) genes. Our results indicate that mutations at diverse sites in Os GBSS 1 reduces its activity by affecting its starch‐binding capacity, its ADP ‐glucose‐binding capability or its protein stability. Our results shed new light on the structural basis of Os GBSS 1 activity and the mechanisms of Os GBSS 1 activity on amylose synthesis in vivo . This study also demonstrates that it is feasible to finely modulate amylose content in rice grains by modifying the Os GBSS 1 activity.