Induction of cell growth and cell division in the intercalary meristem of submerged deepwater rice (Oryza sativa L.)

In deepwater rice (Oryza sativa L.), the youngest internode can be induced to grow rapidly with submergence. This growth response is mediated by gibberellic acid (GA). It is based on both increased cell growth and increased cell division activity. In order to understand whether acceleration of the cell division cycle is a result of increased cell size, we have analyzed various growth and division parameters at the cellular and molecular levels. Flow-cytometric analysis of cells from the intercalary meristem showed that the S phase population increased within 4-6 h after submergence and the G2 phase population increased within 6-8 h, indicating activation of the cell division cycle at the G1 --> S phase transition. Expression of the G1 and S phase-specific cdc20s-2 and histone H3 genes was induced prior to S phase induction and also prior to induction of growth of meristematic cells which started with a lag phase of 4 h, indicating that transcript levels increased in response to submergence directly and not as a result of the submergence-induced accelerated growth of cells. Transcripts of the two mitotic cyclins cycOs1 and cycOs2 accumulated to significant levels in G2 phase. In parallel, activity of the histone H1 kinase which represents the G2/M phase-specific cyclin-dependent cdc2 kinase complex increased, supporting earlier findings that the activity of this kinase complex may, at least in part, be regulated at the level of cyclin expression. The molecular evidence presented therefore indicates that induction of cell division activity at G1 --> S occurs independently of cell growth. It also indicates that control of cell division through cell size is exerted ata level other than cdc2 gene expression.