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 Arabido psis PHYB cDNA sequence fused to 2.3 kb of upstream PHYB promoter sequ ence has been introduced into wild-type Arabidopsis and into a strain containing the Bo64 allele of the hy3 mutation. The Bo64 mutant has pr eviously 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 teste d including hypocotyl length and hypocotyl cell size, response to end- of-day far-red light, leaf morphology, chlorophyll level, and flowerin g time. Presence of the mPHYB transgene in a wild-type genetic backgro und causes exaggeration of this same set of phenotypic characteristics , indicating that these diverse photomorphogenic responses are sensiti ve to the copy number of the PHYB gene. The transgene inserts in the B o64(mPHYB) and WT(mPHYB) lines are shown td be single locus and single copy and the immunologically detectable level of phytochrome B is sho wn to vary linearly with PHYB gene copy number. These results demonstr ate a complex role for phytochrome B in Arabidopsis photomorphogenesis and suggest that the expression level of this phytochrome gene is an important determinant of the intensity of light induced plant response s.