Vernalization (from Latin vernus, "of the spring") is the acquisition of a plant's ability to flower in the spring by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower. Vernalization can also refer to herbal (non-woody) plants requiring a cold dormancy to produce new shoots and leaves. 
Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn. The needed cold is often expressed in chill hours. Typical vernalization temperatures are between 5 and 10 degrees Celsius (40 and 50 degrees Fahrenheit).
For many perennial plants, such as fruit tree species, a period of cold is needed first to induce dormancy and then later, after the requisite period of time, re-emerge from that dormancy prior to flowering. Many monocarpic annuals and biennials, including some ecotypes of Arabidopsis thaliana and winter cereals such as wheat, must go through a prolonged period of cold before flowering occurs.
History of vernalization research
In the history of agriculture, farmers observed a traditional distinction between "winter cereals," whose seeds require chilling, and "spring cereals," whose seeds can be sown in spring and flower soon thereafter. The word "vernalization" is a translation of "яровизация" ("jarovization") a word coined by Trofim Lysenko to describe a chilling process he used to make the seeds of winter cereals behave like spring cereals ("Jarovoe" in Russian). Scientists had also discussed how some plants needed cold temperatures to flower, as early as the 18th century, with the German plant physiologist Gustav Gassner often mentioned for his 1918 paper.
Lysenko's 1928 paper on vernalization and plant physiology drew wide attention due to its practical consequences for Russian agriculture. Severe cold and lack of winter snow had destroyed many early winter wheat seedlings. By treating wheat seeds with moisture as well as cold, Lysenko induced them to bear a crop when planted in spring. Later however, Lysenko inaccurately asserted that the vernalized state could be inherited - i.e., that the offspring of a vernalized plant would behave as if they themselves had also been vernalized and would not require vernalization in order to flower quickly.
Early research on vernalization focused on plant physiology; the increasing availability of molecular biology has made it possible to unravel its underlying mechanisms. For example, longer days as well as cold temperatures are required for winter wheat plants to go from the vegetative to the reproductive state; the three interacting genes are called VRN1, VRN2, and FT (VRN3).
In Arabidopsis thaliana
Arabidopsis thaliana, also known as "thale cress," is a much-studied model species. In 2000, the entire genome of its five chromosomes was completely sequenced. Some variants, called "winter annuals", require vernalization to flower; others ("summer annuals") do not. The genes that underlie this difference in plant physiology have been intensively studied.
The reproductive phase change of A. thalliana occurs by a sequence of two related events: first, the bolting transition (flower stalk elongates), then the floral transition (first flower appears). Bolting is a robust predictor of flower formation, and hence a good indicator for vernalization research.
In arabidopsis winter annuals, the apical meristem is the part of the plant that needs to be chilled. Vernalization of the meristem appears to confer competence to respond to floral inductive signals on the meristem. A vernalized meristem retains competence for as long as 300 days in the absence of an inductive signal.
Before vernalization, flowering is repressed by the action of a gene called Flowering Locus C (FLC). Prolonged exposure to cold induces expression of VERNALIZATION INSENSTIVE3, which interacts with the VERNALIZATION2 polycomb-like complex to reduce FLC expression through chromatin remodeling. Since vernalization also occurs in flc mutants (lacking FLC), vernalization must also activate a non-FLC pathway. A day-length mechanism is also important.
It is possible to devernalize a plant by exposure to high temperatures subsequent to vernalization. For example, commercial onion growers store seeds at low temperatures, but devernalize them before planting, because they want the plant's energy to go into enlarging its bulb (underground stem), not making flowers.
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