INDUCTION OF CELL-GROWTH AND CELL-DIVISION IN THE INTERCALARY MERISTEM OF SUBMERGED DEEP-WATER RICE (ORYZA-SATIVA L.)

In deepwater rice (Oryza sativa L.), the youngest internode can be ind uced to grow rapidly with submergence. This growth response is mediate d by gibberellic acid (GA). It is based on both increased cell growth and increased cell division activity. In order to understand whether a cceleration of the cell division cycle is a result of increased cell s ize, we have analyzed various growth and division parameters at the ce llular and molecular levels. Flow-cytometric analysis of cells from th e intercalary meristem showed that the S phase population increased wi thin 4-6 h after submergence and the G2 phase population increased wit hin 6-8 h, indicating activation of the cell division cycle at the G1 --> S phase transition. Expression of the G1 and S phase-specific cdc2 0s-2 and histone H3 genes was induced prior to S phase induction and a lso prior to induction of growth of meristematic cells which started w ith a lag phase of 4 h, indicating that transcript levels increased in response to submergence directly and not as a result of the submergen ce-induced accelerated growth of cells. Transcripts of the two mitotic cyclins cycOs1 and cycOs2 accumulated to significant levels in G2 pha se. In parallel, activity of the histone H1 kinase which represents th e G2/M phase-specific cyclin-dependent cdc2 kinase complex increased, supporting earlier findings that the activity of this kinase complex m ay, 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 growt h. It also indicates that control of cell division through cell size i s exerted ata level other than cdc2 gene expression.