These investigators found that overall DNA methylation levels were reduced by some 15% when plants were vernalized, recovering rapidly when plants were returned to normal growth temperatures. Experimental reduction of methylation by treatment with azaC, introduction of a methylase antisense gene or the ddm1 mutation all hastened flowering without a cold treatment. A gene encoding a repressor of flowering, designated FLC and FLF in different labs, may be either the or a key methylation-sensitive gene. FLF overexpression delays flowering, while reduced expression shortens the time to flowering. Vernalization down-regulates the FLF gene (and reduces methylation generally). Hypomethylation results in a reduction in the level of FLF transcripts. Thus FLF may be a central repressor of flowering and when the gene is hypomethylated, the transcript level is reduced, allowing earlier flowering.
The next question is what controls the methylation level, of course, and that remains to be discovered. Nonetheless, there is growing evidence that methylation is a component of the complex mechanism that links environmental signals to the initiation of flowering.
What other aspects of development might involve epigenetic mechanisms? I’d venture a guess that there will be many more, although not all will involve DNA methylation. The recent discovery that plants have genes resembling the Drosophila polycomb group genes and the emerging evidence that these are involved in determining developmental patterns of gene expression suggests that we are at just at the beginning of understanding developmental epigenetics.
In closing, I hope I have presented a reasonable case that epigenetic mechanisms are central to the survival, development, and evolution of plants. That related or overlapping mechanisms are involved should be no surprise. That’s pretty much how evolution works: duplication and diversification.