Expressed sequence tags of radish flower buds and characterization of a CONSTANS LIKE 1 gene

Expressed sequence tag (EST) analysis was conducted for young flower buds of radish plants. Among a total of 66 ESTs examined, 40 showed a significant similarity to previously identified genes. Twenty-eight ESTs were similar to proteins identified in other plants, 11 were similar to eukaryotic proteins other than plants, and one was similar to a prokaryotic protein. Four clones were selected for further studies. EST clone 81, which showed a homology to germin-like proteins was expressed more abundantly in leaves and roots as compared to flower buds. Clone 105 was highly homologous to the translation inhibitor protein and was expressed in all three organs, but the expression level was higher in flower buds and roots. Another EST clone, 133, which shared a significant similarity with the Ran-binding protein, hybridized to two different size transcripts that were detectable only in flower buds. Clone 39 was a homolog of CONSTANS, which is a gene involved in controlling the flowering time in Arabidopsis. The cDNA clone of EST clone 39 containing the entire open reading frame was obtained and designated as RsCOL1 (Raphanus sativus CONSTANS LIKE 1). It was 1049 bp long and contained an open reading frame of 307 amino acid residues (calculated molecular mass = 33.1 kDa). The RsCOL1 protein contained two putative zinc finger motifs in the amino terminal region which were 59% identical to the corresponding region of the Arabidopsis CO protein. The radish protein also contained a predicted nuclear localization domain in the carboxyl terminal region which was 87% identical to the corresponding region of CO. DNA blot analysis revealed that the radish genome contained several genes similar to RsCOL1. RNA blot analysis showed that RsCOL1 was strongly expressed in flower buds at the early bolting stage, and the expression level declined as the flower bud matured. The transcript was also detectable in leaves and roots. In mature flowers, the RsCOL1 transcript was present primarily in carpels.