In some species, flowering is induced only if the plant has experienced a prolonged period of cold temperatures, a process known as vernalization. A requirement for vernalization prevents long-day plants from flowering during their first summer, and instead forces them to wait until the following spring to flower, when the chances for successful pollination and seed development are greater (Fig. 31.26c). Even parts of the plant that form long after the cold stimulus is past seem to “remember” that they received the required cold treatment. This “memory” of winter is important because many plants that overwinter grow rapidly in the spring. What is the basis of this “memory”?

One common mechanism acts through chromatin remodeling. The DNA of all eukaryotic organisms is bound with proteins to form chromatin. We saw in Chapter 19 that modifications to chromatin, such as DNA methylation, can change gene expression. In Arabidopsis, for example, vernalization results in chromatin remodeling that turns off a gene whose protein product represses flowering. Chromatin remodeling is stable through mitotic divisions, explaining why newly formed parts of a plant “remember” winter. However, the slate is wiped clean during meiosis, and the requirement for vernalization is reinstated with each generation.