Vernalization May Involve Epigenetic Changes in Gene Expression

It is important to note that for vernalization to occur, active metabolism is required during the cold treatment. Sources of energy (sugars) and oxygen are required, and temperatures below freezing at which metabolic activity is suppressed are not effective for vernalization. Furthermore, cell division and DNA replication also appear to be required.

One model for how vernalization affects competence is that there are stable changes in the pattern of gene expression in the meristem after cold treatment. Changes in gene expression that are stable even after the signal that induced the change (in this case cold) is removed are known as epigenetic regulation. Epigenetic changes of gene expression in many organisms, from yeast to mammals, often require cell division and DNA replication, as is the case for vernalization.

The involvement of epigenetic regulation in the vernalization process has been confirmed in the LDP Arabidopsis. In winter-annual ecotypes of Arabidopsis that require both vernalization and long days to flower, a gene that acts as a repressor of flowering has been identified: FLOWERING LOCUS C (FLC). FLC is highly expressed in nonvernalized shoot apical meristems (Michaels and Amasino 2000). After vernalization, this gene is epigenetically switched off by an unknown mechanism for the remainder of the plant's life cycle, permitting flowering in response to long days to occur (Figure 24.27). In the next generation, however, the gene is switched on again, restoring the requirement for

FIGURE 24.26 The duration of exposure to low temperature increases the stability of the vernalization effect. The longer that winter rye (Secale cereale) is exposed to a cold treatment, the greater the number of plants that remain vernalized when the cold treatment is followed by a devernalizing treatment. In this experiment, seeds of rye that had imbibed water were exposed to 5°C for different lengths of time, then immediately given a dever-nalizing treatment of 3 days at 35°C. (Data from Purvis and Gregory 1952.)

FIGURE 24.27 (Left) Vernalization blocks the expression of the gene FLOWERING LOCUS C (FLC) in cold-requiring winter annual ecotypes of Arabidopsis. (Right) A winter annual with an FLC mutation exhibits early flowering without cold treatment. (Photo courtesy of R. Amasino.)

FIGURE 24.27 (Left) Vernalization blocks the expression of the gene FLOWERING LOCUS C (FLC) in cold-requiring winter annual ecotypes of Arabidopsis. (Right) A winter annual with an FLC mutation exhibits early flowering without cold treatment. (Photo courtesy of R. Amasino.)

Winter annual after 40 days cold

Winter annual without cold, but with an FLC mutation

Winter annual after 40 days cold

FLC mRNA

Winter annual without cold, but with an FLC mutation

FLC mRNA

cold. Thus in Arabidopsis, the state of expression of the FLC gene represents a major determinant of meristem competence (Michaels and Amasino 2000).

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