GIBBERELLIN PROMOTES HISTONE H1 KINASE-ACTIVITY AND THE EXPRESSION OFCDC2 AND CYCLIN GENES DURING THE INDUCTION OF RAPID GROWTH IN DEEP-WATER RICE INTERNODES

Partial submergence or treatment with either ethylene or gibberellin ( GA) promotes rapid internodal growth in deepwater rice (Oryza sativa L .). Earlier work has shown that GA is the immediate hormonal signal fo r this growth response, which involves induction of the cell cycle at the G2/M phase transition and subsequent enhancement in the rate of DN A synthesis. In all eukaryotes, onset of mitosis is regulated by the p 34(cdc2/CDC28) protein kinase, whose activity is assayed by in vitro p hosphorylation of histone H1. It was found that GA enhanced the activi ty of p34(cdc2/CDC28)-like histone H1 kinase in the intercalary merist em of rice internodes. The enzyme activity showed a sharp peak that co rrelated with a decrease in the population of cells in the G2 phase du ring the first 4 h of GA treatment but not with changes in DNA synthes is. The level of histone H1 kinase activity increased again when cell division activity in the intercalary meristem is known to be high. The expression of two cdc2 homologs was examined. The mRNA level of one o f these, cdc20s-2, was increased after 1 h of GA treatment, whereas th e mRNA level of the other, cdc20s-1, was not affected. Two cDNAs, cycO s1 and cycOs2, which show high homology to cyclin cDNAs, were cloned f rom rice. They share 75.1% sequence identity at the amino acid level, and both of them are encoded by mRNAs of 1.6 kb. Expression of the two corresponding cyclin genes was enhanced by GA, and the time course of the induction was compatible with a role for both cyclins in regulati ng the G2/M phase transition. The cyclins were expressed in the interc alary meristem and the elongation zone of the internode, but the GA-in duced increase in transcript levels was restricted to the meristem onl y. The results support the hypothesis that induction of mitosis by GA is brought about by increased p34(cdc2/CDC28) protein kinase activity, which may be the result of transcriptional activation of the cdc20s-2 , cycOs1 and cycOs2 genes.