GIBBERELLIN PROMOTES HISTONE H1 KINASE-ACTIVITY AND THE EXPRESSION OFCDC2 AND CYCLIN GENES DURING THE INDUCTION OF RAPID GROWTH IN DEEP-WATER RICE INTERNODES
M. Sauter et al., GIBBERELLIN PROMOTES HISTONE H1 KINASE-ACTIVITY AND THE EXPRESSION OFCDC2 AND CYCLIN GENES DURING THE INDUCTION OF RAPID GROWTH IN DEEP-WATER RICE INTERNODES, Plant journal, 7(4), 1995, pp. 623-632
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.