Up- and Down-regulated Expression of OsCPK25/26 Results in Increased Number of Stamens in Rice

Calcium-dependent protein kinases play important roles in the regulation of plant growth and development. Rice genome contains 31 CDPK genes, and the biological functions of most of these CDPK genes remain unclear. To elucidate the biological function of OsCPK25/26, we first analyzed the expression profiles for OsCPK25/26. Transcripts of OsCPK25/26 were observed in the panicles, hulls, and stamens, with significantly higher expression in stamens. Plasmid constructs of either overexpression of OsCPK25 or RNAi knockdown of OsCPK25/26 were created and introduced into the Zhonghua11 (Oryza sativa L. ssp. japonica) rice variety, and single-copy homozygous lines of the resultant transgenic plants were characterized. The expression levels of OsCPK25/26 in panicles were increased 11-15-fold in the overexpressing plants and decreased 1.5-1.7-fold in the plants expressing RNAi against OsCPK25/26 compared with that in the control plants. The numbers of stamens in some florets increased with varying degrees, ranging from six to 13 stamens in both the overexpressing and RNAi plants. Compared with the control plants, the proportion of florets with increased stamen numbers in T-2 generation was 30 % in the overexpressing plants and 36 % in RNAi plants and that in T-3 generation was 50 and 51 %, respectively. Our results suggest that OsCPK25/26 are involved in the regulation of stamen development, and changes (increases or decreases) of OsCPK25/26 expression levels result in an increased number of stamens. Thus, the optimal expression levels of OsCPK25/26 are a key regulating factor in maintaining normal stamen numbers in rice.