Expression profiling of genes related to starch synthesis in rice leaf sheaths during the heading period

Abstract

The stems (leaf sheaths and culms) of rice plants (Oryza sativa L.) accumulate high levels of starch before heading, which is subsequently remobilized after heading to provide a carbon source for grain filling. To elucidate the molecular mechanism of starch synthesis in rice leaf sheath, a comprehensive expression analysis was conducted on the gene families encoding starch synthesis–related enzymes, ADP‐glucose pyrophosphorylase (EC 2.7.7.27), starch synthase (EC 2.4.1.21) and branching enzyme (EC 2.4.1.18). The changes in the activities of these enzymes in leaf sheaths during the heading period were shown to be accompanied by a coordinated change in the transcript levels of particular members of the corresponding gene families. A similar change before and after heading was also found in the messenger RNA level of GPT2, one of the two genes encoding for a plastidial glucose‐6‐phosphate/phosphate translocator, suggesting that the capacity of both starch synthesis and its substrate import is under coordinated transcriptional regulation. The members of the gene families predominantly expressed in leaf sheaths are mostly different from those actively expressed in the developing endosperm. The time course for the transcript level of some of these genes appears to correlate to hexose level in the leaf sheaths, suggesting that hexose levels may be a controlling factor in their expression.