Independent Regulation of Flowering by Phytochrome B and Gibberellins in Arabidopsis

Phytochromes and gibberellins (GAs) coordinately regulate multiple aspects of Arabidopsis development. Phytochrome B (PHYB) promotes seed germination by increasing GA biosynthesis, but inhibits hypocotyl elongation by decreasing the responsiveness to GAs. Later in the life cycle of the plant, PHYB and GAs have opposite effects on flowering. PHYB delays flowering, while GAs promote flowering, particularly under noninductive photoperiods. To learn how PHYB and GAs interact in the control of flowering, we have analyzed the effect of a phyB mutation on flowering time and on the expression of the floral meristem-identity gene LFY (LEAFY). We show that the early flowering caused by phyB correlated with an increase in LFY expression, which complements our previous finding that GAs are required for activation of LFY under noninductive photoperiods (M.A. Blázquez, R. Green, O. Nilsson, M.R. Sussman, D. Weigel [1998] Plant Cell 10: 791−800). Since phyB did not change the GA responsiveness of the LFY promoter and suppressed the lack of flowering of severe GA-deficient mutants under short days, we propose that PHYB modulates flowering time at least partially through a GA-independent pathway. Interestingly, the effects of PHYB on flowering do not seem to be mediated by transcriptional up-regulation of genes such as CO (CONSTANS) and FT (Flowering locus T), which are known to mediate the effects of the photoperiod-dependent floral-induction pathway.