The Control of Flowering by Vernalization

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Vernalization is the process whereby plants flower earlier following exposure of their seeds or juvenile organs to a specific period of temperatures just above freezing. In a review article, “The Control of Flowering”, published in the October, 2000 issue of “Current Opinion of Plant Biology, Dr. Jim Peacock and his associates at the CSIRO Lab, Australia, have reviewed the status of our knowledge in understanding the molecular basis of vernalization.

A breakthough to our understanding of the vernalization process is the recent cloning of the flowering locus gene, FLC. The dominant allele of FLC locus acts as a repressor of flowering and its level of expression determines the flowering time. It has also been shown that early flowering in the Arabidopsis ecotypes, Ler and C24, is conditioned by flc, the recessive allele of the FLC gene. Interestingly, the gene products of both dominant and recessive alleles are identical suggesting that the difference in their functioning lies in the activity of a regulatory gene. Another dominant gene designated FRI causes late flowering in Arabidopsis and both FLC and FRI interact influencing the same trait.

Exposure of germinating seeds to low temperatures reduces levels of transcripted mRNA of FLC and its protein product; furthermore, the longer the period of exposure to cold, the less is the production of FLC mRNA and the earlier is the flowering. These observations clearly indicate that the FLC gene is integrally involved in the process of vernalization.

Another important observation is that plants containing a MET1 anstisense construct with only 15% of the wild-type level of genomic methylation, flower early. Such early flowering trangenic plants were found to have a reduced level of FLC transcript. One proposed interpretation of this observation is that the expression of the FLC gene was suppressed because of the activity of a repressor binding to the FLC promoter.

Another of the hypotheses explaining the phenomenon of vernalization is activation of relevant genes by demethylation. The fact, that plant material grown at low temperatures shows reduced genomic DNA methylation, is consistent with this hypothesis. Induction of early flowering in verrnalization-responsive Arabidopsis ecotypes and mutants by treatment with 5-azacytidine or by a methyltransferase1 (MET1) antisense construct provides further proof of this hypothesis.

It has also been established that there are two more genes, VRN1 and VRN2, which diminish FLC gene activity, thereby hastening flowering time. Earlier studies have shown that the LEAFY (LFY) gene encodes a developmental switch which upon activation, converts all lateral shoots into solitary flowers. LEAFY mutants have a late flowering phenotype but this trait can be suppressed by insertion of a LFY::35S construct. Recent studies have futher shown that gibberellin and sucrose upregulate the LFY promoter synergistically; the LFY mutant individuals are unable to mobilize starch; and the late flowering behavior, under short short day conditions, is attributed to such a defect.

In summary, FLC is one of the key genes that plays a dominant role in vernalization. Also a major player in this process is DNA methylation. The other genes involved in vernalization are LFY, VRN1 and VRN2 which downregulate the activity of FLC.

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