Circadian regulation of rice (Oryza sativa L.) CONSTANS-like gene transcripts

We identified three rice cDNA clones showing amino acid similarity to the Arabidopsis CONSTANS-like proteins from a database search (S12569, S3574, and C60910), to examine if their transcript abundances were under circadian control. Unlike the other two proteins, the protein encoded by the S12569 cDNA contains only one CONSTANS-like zinc finger B box, and a CCT region. We found that the transcript levels of these rice CONSTANS-like (COL) genes were under circadian control. The oscillation phase of the S12569 gene transcript was more or less opposite to those of OsGI (rice GIGANTEA homolog) and Hd1 (rice COSTANS homolog), whereas the phases of the other two gene transcripts were similar to that of the Hd1 transcript. S12569 mRNA started to increase about 3 h after the onset of the dark period, with a peak about 3 h after its end. The S3574 and C60910 genes were expressed to similar extents during the vegetative and reproductive phases, like OsGI. Higher levels of S12569 transcripts, however, like those of Hd1, were detected in the earlier stages of panicle development. Unlike Hd1 transcripts, S12569, S3574, and C60910 transcripts were present at similar levels in the aerial parts of plants and in their roots during the vegetative phase. In conclusion, the rice COL genes showed distinctive expression patterns from the CO and COL genes, as well as Hd1, a rice CO homolog.