Mutagenesis of plants overexpressing CONSTANS demonstrates novel interactions among Arabidopsis flowering-time genes

CONSTANS (CO) promotes flowering of Arabidopsis in response to long photope riods. Transgenic plants carrying CO fused with the cauliflower mosaic viru s 35S promoter (35S::CO) flowered earlier than did the wild type and were a lmost completely insensitive to length of day. Genes required for CO to pro mote flowering were identified by screening for mutations that suppress the effect of 35S::CO. Four mutations were identified that partially suppresse d the early-flowering phenotype caused by 35S::CO. One of these mutations, suppressor of overexpression of CO 1 (soc1), defines a new locus, demonstra ting that the mutagenesis approach is effective in identifying novel flower ing-time mutations. The other three suppressor mutations are allelic with p reviously described mutations that cause late flowering. Two of them are al leles of ft, indicating that FT is required for CO to promote early floweri ng and most likely acts after CO in the hierarchy of flowering-time genes. The fourth suppressor mutation is an allele of fwa, and fwa soc1 35S::CO pl ants flowered at approximately the same time as co mutants, suggesting that a combination of fwa and soc1 abolishes the promotion of flowering by CO. Besides delaying flowering, fwa acted synergistically with 35S::CO to repre ss floral development after bolting. The latter phenotype was not shown by any of the progenitors and was most probably caused by a reduction in the f unction of LEAFY. These genetic interactions suggest models for how CO, FWA , FT, and SOC1 interact during the transition to flowering.