Vfld containing FLD

Figure 5.16 Regulation of FLOWERING LOCUS C (FLC), a floral repressor of Arabidopsis, by FLD (FLOWERING LOCUS D). In the wild type, FLC expression is suppressed by deacetylation of histones in the vicinity of FLC, conducted by an HDAC, of which FLD is a part. If FLD is mutated or if a 294 bp region in the first intron of FLC is deleted, HDAC is no longer able to deacetylate the FLC histones, alleviating FLC from transcription suppression and causing late flowering. Reprinted with permission from Bastow and Dean (2003). Copyright 2003 AAAS.

The second regulatory input on FLC from the autonomous pathway comes from the floral-promotion genes FCA and FY. The way in which these genes regulate FLC expression represents another complicated but beautiful example of gene regulation underlying life-history traits (Eckardt 2002; Macknight et al. 2002; Amasino 2003; MacDonald and McMahon 2003; Quesada et al. 2003; Simpson et al. 2003). The FCA gene includes 20 introns, which are spliced out during mRNA assembly; however, introns 3 and 13 are spliced alternatively, leading to four different transcripts, designated as a, b, g, and 8. Only the g transcript functions in the control of flowering time; it encodes a protein that, together with the FY protein, suppresses FLC mRNA and thus stimulates flowering. In addition, FY and FCA proteins feed back upon the activity of the FCA gene because they promote the formation of the inactive b transcript, by polyadenylation in intron 3. This negative-feedback loop is essential to maintain a low concentration of active transcripts of FCA. When the autoregulation breaks down, active FCA is formed, causing inhibition of FLC mRNA and induction of flowering (Fig. 5.17). The alternative splicing of FCA transcripts provides a mechanism

FCA pre-mRNA

FCA pre-mRNA

T feedback loop

Repression of

FLC mRNA levels

T feedback loop

Repression of

FLC mRNA levels

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