The relationship between β-amylase and the degradation of starch temporarily stored in rice leaf blades.

Starch is stored temporarily in the leaves during the day but degraded during the night. In this study, we investigated the relationship between diurnal changes in starch content in rice leaf blades and the mRNA levels of β-amylase genes. In addition to the known plastid-type β-amylases OsBAM2 and OsBAM3, OsBAM4 and OsBAM5 were also identified as plastid targeted proteins. In the leaf blades, starch contents, which reached its maximum at the end of day, showed two periods of marked decrease: from 18:00 to 21:00 and from 24:00 to 6:00. The expression of OsBAM2, OsBAM3, OsBAM4, and OsBAM5 was maintained at a low level from 18:00 to 21:00 but increased strongly after midnight. Furthermore, β-amylase activity gradually increased after 21:00, reaching a maximum during the early morning. These results suggest that in rice leaf blades, β-amylase plays an important role in starch degradation by being highly active from midnight to dawn.