Tissue-specific and light-dependent regulation of phytochrome gene expression in rice.

Phytochromes are red- and far red light photoreceptors in higher plants. Rice (Oryza sativa L.) has three phytochromes (phyA, phyB and phyC), which play distinct as well as cooperative roles in light perception. To gain a better understanding of individual phytochrome functions in rice, expression patterns of three phytochrome genes were characterized using promoter-GUS fusion constructs. The phytochrome genes PHYA and PHYB showed distinct patterns of tissue- and developmental stage-specific expression in rice. The PHYA promoter-GUS was expressed in all leaf tissues in etiolated seedlings, while its expression was restricted to vascular bundles in expanded leaves of light-grown seedlings. These observations suggest that light represses the expression of the PHYA gene in all cells except vascular bundle cells in rice seedlings. Red light was effective, but far red light was ineffective in gene repression, and red light-induced repression was not observed in phyB mutants. These results indicate that phyB is involved in light-dependent and tissue-specific repression of the PHYA gene in rice.