Relationship between Starch Accumulation and Activities of the Related Enzymes in the Leaf Sheath as a Temporary Sink Organ in Rice (Oryza sativa)

Abstract

The starch stored temporarily in the leaf sheath of rice plants is translocated to the grain, contributing to the grain yield. In this paper, the relationship between starch accumulation and the activities of enzymes involved in the starch biosynthetic pathway in the leaf sheaths of rice plants was examined to elucidate the regulation mechanism of starch accumulation in the temporary sink organ. When the starch content was compared between different leaf sheath positions on the main stem, the 14th leaf sheath counted from the bottom, which elongated just before anthesis, showed about a four-fold higher value than the 10th leaf sheath. Among the enzymes involved in carbohydrate metabolism, the activity of starch branching enzyme (BE, EC2.4.1.18) was greatly higher in the 14th leaf sheath than in the 10th leaf sheath, while that of ADPglucose pyrophosphorylase (EC2.7.7.27) was similar between the two leaf sheaths. The starch content increased rapidly in the period around anthesis in the 12th and 14th leaf sheaths, but did not in the 10th and 11th leaf sheath. In the higher leaf sheath position, the activity of BE changed with noticeably similar trend to the starch content. Soluble starch synthase (SSS, EC2.4.1.21), granule-bound starch synthase (GBSS, EC2.4.1.21) and plastidial fructose- 1,6-bisphosphatase (FBPase, EC3.1.3.11) were also significantly correlated with the starch content. These results suggest that BE is involved in regulation of starch metabolism, possibly in collaboration with other enzymes such as SSS, GBSS and plastidial FBPase in temporary sink organs like the leaf sheath.