The relationship between ß-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 ß-amylase genes. In addition to the known plastid-type ß-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, ß-amylase activity gradually increased after 21:00, reaching a maximum during the early morning. These results suggest that in rice leaf blades, ß-amylase plays an important role in starch degradation by being highly active from midnight to dawn.

Inhibition of ammonium assimilation restores elongation of seminal rice roots repressed by high levels of exogenous ammonium.

Elongation of seminal and lateral roots of rice seedlings was markedly inhibited by high ammonium levels in growth medium. However, high exogenous nitrate concentrations had little inhibitory effect on root growth. The objective of this study was to elucidate the relationship between inhibition of rice root growth induced by high ammonium conditions and ammonium assimilation in the seedlings. Activity of glutamine synthetase (GS) was kept at a low level in the seminal roots of the seedlings grown under high nitrate levels. In contrast, high ammonium levels significantly enhanced the GS activity in the roots, so that Gln abundantly accumulated in the shoots. These results indicate that ammonium assimilation may be activated in the seminal roots under high ammonium conditions. Application of methionine sulfoximine (MSO), an inhibitor of GS, relieved the repression of the seminal root elongation induced by high ammonium concentrations. However, the elongation of lateral roots remained inhibited even under the same condition. Furthermore, MSO drastically increased ammonium level and remarkably decreased Gln level in the shoots grown under high ammonium conditions. These results show that, for rice seedlings, an assimilatory product of ammonium, and not ammonium itself, may serve as an endogenous indicator of the nitrogen status involved in the inhibition of seminal root elongation induced by high levels of exogenous ammonium.