Photomorphogenesis of rice seedlings: a mutant impaired in phytochrome-mediated inhibition of coleoptile growth.

A mutant showing a long coleoptile phenotype under white light was isolated from gamma-ray-mutagenized rice (cv. Nihonmasari). This mutant, named cpm1 (coleoptile photomorphogenesis 1), has been found to be impaired in phytochrome-mediated inhibition of coleoptile growth. Another outstanding feature of the mutant is impaired anthesis. Under red light (R), cpm1 coleoptiles elongate at a higher rate than wild-type (WT) coleoptiles, owing to substantially reduced responsiveness to R. This phenotype occurs in an age-dependent manner, and cpm1 coleoptiles become responsive to R as they elongate. The impairment was found in both very-low-fluence and low-fluence responses. Mutant coleoptiles also elongate longer than WT coleoptiles in darkness, but in this case the long coleoptile results from an extended elongation period. The cpm1 mutation does not affect the following phytochrome responses: the growth stimulation in submerged coleoptiles (uncovered in this study), potentiation of greening, and down-regulation of PHYA transcription. The cpm1 mutation does not significantly affect the level of spectroscopically detectable phytochrome and the transcription levels of three phytochrome genes (PHYA-C). It is concluded that the CPM1 gene is involved in the phytochrome signal transduction that specifically leads to growth inhibition. Some aspects of rice seedling photomorphogenesis are discussed in relation to the results obtained.