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

Abstract

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.

Figure 1.

Dependence of the Late-Flowering Phenotype of FRI and Certain Late-Flowering Mutants on FLC. Plants at left are homozygous for an flc null allele. Plants at right contain wild-type FLC alleles and are shown as controls. Plants were grown under LD conditions.

Figure 2.

Effect of FLC on Flowering Time under LD Conditions. Black bars represent plants homozygous for wild-type FLC, and white bars represent plants homozygous for an flc null allele. All analyses were performed in the Col background except for those involving fca. For fca, an F₂ population was generated from a cross between flc-3 in Col and an fca allele in Wassilewskija. Because of the different backgrounds, a minimum of 10 plants of each of the following genotypes was isolated in the F₂ generation, and their leaf numbers were averaged: black bar, fca/fca FLC/FLC; white bar, fca/fca flc/flc; gray bar, FCA/FCA flc/flc. The white and gray bars show the effect of the FCA in the flc null background. Error bars indicate ±sd. The y axis indicates the number of rosette leaves formed prior to flowering.

Figure 3.

Effect of FLC on Flowering Time under SD Conditions. Black bars represent plants homozygous for wild-type FLC, and white bars represent plants homozygous for an flc null allele. All analyses were performed in the Col background except for those involving fca. For fca, F₃ lines were grown from each of the F₂ plants described in Figure 2, and their leaf numbers were averaged: white bar, fca/fca flc/flc; gray bar, FCA/FCA flc/flc. The white and gray bars show the effect of the FCA in the flc null background. Lines containing FRI, fve, ld, and fpa with wild-type FLC alleles did not flower after forming >80 leaves (data not shown). Error bars indicate ±sd. The y axis indicates the number of rosette leaves formed prior to flowering.

Figure 4.

Effect of Vernalization in an flc Null Mutant. Black bars represent Col (homozygous for wild-type FLC), and white bars represent an flc null allele. Plants were cold treated for 2 or 70 days before growth under SD conditions. Error bars indicate ±sd. The y axis indicates the number of rosette leaves formed prior to flowering.

Figure 5.

RNA Gel Blot Analysis of SOC1 Expression in Various Genotypes. RNA was isolated from 12-day-old plants grown under continuous light. FLC expression is shown for comparison. Blots were probed with 18S rDNA as a control for loading.

Figure 6.

Local Model for the Interaction of FLC, FRI, the Autonomous Pathway, and Vernalization in the Regulation of Flowering Time.