Loss of FLOWERING LOCUS C activity eliminates the late-flowering phenotype of FRIGIDA and autonomous pathway mutations but not responsiveness to vernalization.

The MADS domain--containing transcription factor FLOWERING LOCUS C (FLC) acts as an inhibitor of flowering and is a convergence point for several pathways that regulate flowering time in Arabidopsis. In naturally occurring late-flowering ecotypes, the FRIGIDA (FRI) gene acts to increase FLC levels, whereas the autonomous floral promotion pathway and vernalization act to reduce FLC expression. Previous work has shown that the Landsberg erecta allele of FLC, which is not a null allele, is able to partially suppress the late-flowering phenotype of FRIGIDA and mutations in the autonomous pathway. In this study, using a null allele of FLC, we show that the late-flowering phenotype of FRIGIDA and autonomous pathway mutants are eliminated in the absence of FLC activity. In addition, we have found that the downregulation of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 by FRI and autonomous pathway mutants also is mediated by FLC. Complete loss of FLC function, however, does not eliminate the effect of vernalization. Thus, FRI and the autonomous pathway may act solely to regulate FLC expression, whereas vernalization is able to promote flowering via FLC-dependent and FLC-independent mechanisms.