Transgenic complementation of the hy3 phytochrome B mutation and response to PHYB gene copy number in Arabidopsis

A recombinant PHYB minigene (mPHYB) consisting of the complete Arabidopsis PHYB cDNA sequence fused to 2.3 kb of upstream PHYB promoter sequence has been introduced into wild-type Arabidopsis and into a strain containing the Bo64 allele of the hy3 mutation. The Bo64 mutant has previously been shown to contain a nonsense mutation in the PHYB coding sequence. Transformation of this strain with the mPHYB gene results in complementation of all of the mutant phenotypic characteristics tested including hypocotyl length and hypocotyl cell size, response to end-of-day far-red light, leaf morphology, chlorophyll level, and flowering time. Presence of the mPHYB transgene in a wild-type genetic background causes exaggeration of this same set of phenotypic characteristics, indicating that these diverse photo-morphogenic responses are sensitive to the copy number of the PHYB gene. The transgene inserts in the Bo64(mPHYB) and WT(mPHYB) lines are shown to be single locus and single copy and the immunologically detectable level of phytochrome B is shown to vary linearly with PHYB gene copy number. These results demonstrate a complex role for phytochrome B in Arabidopsis photo-morphogenesis and suggest that the expression level of this phytochrome gene is an important determinant of the intensity of light-induced plant responses.