Vernalizing plants show a characteristic delay between their exposure to cold and flowering, as if cells retained a memory of the cold spell and later acted on it by activating floral-promoting genes. By cloning and characterizing the VERNALIZATION 2 (
VRN2
) gene, Gendall et al. have come one step closer to finding out the mechanism behind this memory, which is stable over several cell divisions. Cold temperatures are thought to reduce FLOWERING LOCUS C (
FLC
) expression, and consequently repress floral-promoting genes. The VRN2 gene was previously recovered in a screen for mutants with a reduced sensitivity to vernalization, and encodes a nuclear protein similar to the Polycomb group of transcriptional repressors. Could there be a link between the function of VRN2, the delayed response to vernalization and FLC expression? The authors found that, although VRN2 does not affect the downregulation of FLC mRNA in response to the cold, it is required to maintain the repression of FLC after the plant returns to normal temperatures. Vernalization might therefore have an epigenetic basis — an idea that was further confirmed by the enhanced DNase sensitivity of the FLC promoter in a vrn2 mutant.
Plants at different latitudes respond differently to flower-inducing environmental stimuli, and several QTL for natural variation in flowering time have been identified. Now, El-Assal et al. have found that a single genetic lesion in a major-effect QTL is responsible for the difference in flowering response to photoperiod of two Arabidopsis strains — one from Northern Europe (Ler) and one from the Tropics (Cvi). Cvi plants flower earlier than most strains (including Ler) when days are short. The authors show that a QTL previously found to account for most of the difference in the photoperiod response for flowering time between Ler and Cvi corresponds to the
CRY2
gene, which encodes the blue-light photoreceptor cryptochrome 2. Remarkably, the difference in day-length sensitivity between Ler and Cvi is due to one amino-acid substitution in CRY2. The Cvi variant of CRY2 is a dominant allele that confers its characteristic short-day flowering when transformed into Ler plants, probably by preventing light-induced depletion of the CRY2 protein.
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