Conservation and divergence of FCA function between Arabidopsis and rice

Although several genes have been identified in rice which are functionally equivalent to the flowering time genes in Arabidopsis, primarily genes involved in the photoperiod pathway, little data is available regarding the genes that function in the autonomous pathway in rice. In order to acquire further insight into the control of heading dates in rice, we isolated and conducted an expression analysis on OsFCA, which exhibited 38% sequence homology with Arabidopsis FCA. The N-terminal region of the OsFCA protein appears to be unusually rich in glycine-residues, unlike the N-terminal region found in FCA. However, the genetic structure of OsFCA is, in general, similar to that of FCA. RT-PCR and in silico analyses also showed that alternative splicing and polyadenylation at intron3 were conserved in the genetic expression of OsFCA. We were able to detect alpha, beta, and gamma transcripts, but not the delta transcript, of the OsFCA gene. The beta and gamma transcripts of the OsFCA gene were detected via Northern analysis in the leaves, roots, and flowers of the plant. Flowers in younger stages exhibited higher transcript levels. These data suggest that intron3 may constitute a primary control point in the OsFCA pre-mRNA processing of rice. The overexpression of OsFCA cDNA, driven by the 35S promoter, was shown to partially rescue the late flowering phenotype of the fca mutant, suggesting that the functions of the OsFCA and the FCA are partially overlapped, despite the lack of an apparent FLC homologue in the rice genome. The constitutive expression of OsFCA resulted in no downregulation of FLC, but did result in the weak upregulation of SOC1 in the transgenic Arabidopsis. OsFCA overexpression did not result in a reduction of the gamma transcript levels of FCA in the transgenic Arabidopsis either, thereby suggesting that OsFCA had no effects on the autoregulation of Arabidopsis FCA. All of these results imply conservation and divergence in the functions of FCA between rice and Arabidopsis.