Light regulation and differential tissue-specific expression of phototropin homologues from rice (Oryza sativa ssp. indica)

Isolation and characterization of rice (Oryza sativa L. ssp. indica) OsPHOT1 cDNA, encoding a blue light photoreceptor phototropin 1, is reported. The predicted amino acid sequence of OsPHOT1 corresponds to a protein of 103.6 kDa harboring two LOV domains (LOV1 and LOV2) and a C-terminal protein kinase domain conserved in other phototropins. The structural part of the gene is interrupted by 22 introns. The OsPHOT1 gene is represented by two copies in the rice genome (designated as OsPHOT1a and OsPHOT1b), one each on the duplicated segments of chromosomes 11 and 12. BLASTN and TBLASTN searches also led to the identification of another phototropin homologue, OsPHOT2, present on chromosome 4. The real-time PCR analysis showed that both the OsPHOT1 and OsPHOT2 are expressed at significantly higher levels in mature leaves. The mRNA levels of OsPHOT1 were higher in the coleoptiles and that too in the apical portion, whereas OsPHOT2 was strongly expressed in leaves of etiolated rice seedlings. The transcript levels of OsPHOT1 and OsPHOT2 were differentially regulated by a broad spectrum of light, including white, blue, red, and far-red light; the transcript levels of OsPHOT1 declined, whereas those of OsPHOT2 were upregulated. These results indicate the participation of red/far-red sensing phytochromes and blue light (perceived through either cryptochromes or phototropins) in differential regulation of transcript abundance of phototropins, OsPHOT1 and OsPHOT2.