Sugar-modulated gene expression of sucrose synthase in suspension-cultured cells of rice

In suspension‐cultured cells of rice (Oryza sativa L. cv. Tainung 67), two sucrose synthase genes, RSus1 and RSus2, were found to be differentially regulated by sugars. Exogenously provided sugars triggered marked accumulation of the RSus1 transcripts and this required de novo synthesized proteins. In addition to being regulated at the transcriptional level, other regulatory mechanisms might be involved in the sugar‐modulated expression of RSus1 since the RSuS1 proteins were extremely stable in sucrose‐depleted cells. Glucose analogues, 3‐O‐methyl glucose, 6‐deoxyglucose and mannose, did not trigger the induction of RSus1 expression, suggesting that hexose uptake per se and hexokinase are not involved in the sugar‐sensing pathway controlling RSus1 expression. The accumulation of RSus2 mRNA was observed regardless of the presence or absence of sugars. However, newly synthesized proteins induced by sucrose and/or starvation were involved in the regulation of RSus2 expression at transcriptional and/or post‐transcriptional levels. The protein level of RSuS2 was elevated under prolonged starvation condition but a reciprocal change was observed in the sucrose‐fed cell. The constant presence of RSuS1 and RSuS2 proteins in both sucrose‐starved and sucrose‐provided cells may allow the cells to respond to sugar availability immediately.