Novel molecular and cell biological insights into function of rice α-amylase

Abstract

Abstract α-Amylases have been of interest in diverse fields for many years because of their importance in basic biology, agriculture, and industry. Starch hydrolysis in plants has been studied extensively in germinating cereal seeds. It is generally accepted that α-amylases are secretory enzymes with a pivotal role in the breakdown of starch reserves in the endosperm. Intriguingly, however, recent investigations reveal that some α-amylases degrade starch in the plastids of living cells. The recent solving of the crystal structure of rice AmyI-1 isoform shows that the binding pocket of starch binding site 1 situated outside of the active site cleft interacts with the substances other than oligosaccharides. These findings provided novel insights into structural and cell biological aspects of α-amylase functions in intracellular transport, organelle targeting, and organ-specific actions. Under global warming, abnormal high temperatures during rice grain filling increase grain chalkiness, resulting in yield loss. Intensive “omics” analyses of developing caryopses and mature grains grown under heat stress showed the downregulation of starch synthesis enzymes and the upregulation of α-amylases. Transgenic studies using ectopic overexpression and suppression of α-amylase revealed that α-amylase is a key factor in grain chalkiness. Here we discuss unique new functions of α-amylase in rice cells.