Omics-based Approach for Cereal Starch Biosynthesis: Toward a Determination of Key Factors for Quality of Rice Grain Affected by High Temperature

High temperature impairs grain filling of rice by inhibiting accumulation of starch in the endosperm, making the grain appear chalky. It affects the molecular structure of a starch component, amylopectin (e.g., elongation of its chains), then hardening the texture of cooked rice. Recent comprehensive analyses such as transcriptome, proteome and metabolome which are assisted by the rice genome information revealed the key metabolic factors involved in this deterioration of rice grain by elevated temperatures. Shortage of starch (defect in starch quantity) could be caused by a combination of decreased expression of starch biosynthesis-related genes such as GBSSI and BEIIb and increased degradation of starch by induced expression of α-amylase genes. Determination of carbohydrate metabolites further suggested that sugar metabolism and ATP production are also impaired under high temperature, leading to the decrease of starch deposition. For the hardening of the texture (defect in starch quality), decreased branching of amylopectin by reduced expression of starch branching enzyme genes, especially short chain-producing BEIIb, would contribute to enrichment of its long chains. In this review, effects of high temperature on grain filling-related metabolism are summarized at the gene level.