vernalization
What is vernalization?
What temperature range is most effective for vernalization?
What are obligate and facultative vernalization?
How does vernalization affect flowering at the genetic level?
What is the role of vernalization in agriculture?
vernalization, the process by which plants or seeds are exposed to low temperatures in order to stimulate flowering or enhance seed germination. Many plant species need cold exposure to transition from the vegetative growth stage to the reproductive stage so that flowers bloom during spring, when conditions are favorable for seed production. An above-freezing temperature, usually between 5 and 8 °C (41 and 46 °F), lasting four to six weeks is the most effective for inducing flowering in a large number of plants. Some plants—such as winter wheat, winter barley, and cabbage—have an obligate vernalization requirement, meaning that they do not flower until going through a period of chilling. Others—such as certain varieties of field mustard, thale cress, and corydalis—display facultative vernalization, meaning that cold exposure is not essential to induce flowering but may hasten the process.
Genetic control
Vernalization prepares a plant for blooming, but flowering occurs only when other requirements are met, such as an inductive photoperiod—that is, the specific day length that stimulates a plant to flower. Vernalized plants do not flower under noninductive photoperiods but initiate flowering under inductive conditions. This suggests that vernalization creates a lasting cellular memory in plants that remains through cell division. Epigenetic mechanisms, which regulate gene activity without changing the DNA sequence, control this memory as well as the expression of genes involved in flowering.
In thale cress (Arabidopsis thaliana), which is the most widely used model organism to study plant biology, vernalization is controlled by the gene FLOWERING LOCUS C (FLC). In autumn, epigenetic modifications such as acetylation and methylation of the FLC’s chromatin (the DNA and protein complex that forms chromosomes) keep the gene active, and its activity is further increased by the dominant allele (version) of another gene, FRIGIDA (FRI). The protein encoded by the active FLC suppresses flowering in thale cress in winter. In spring, proteins encoded by the vernalization-related genes VRN1, VRN2, and VIN3 alter the epigenetic modifications of FLC, thereby suppressing its actions and removing the block to flowering. Some theories suggest that prolonged cold lowers the expression of FLC, which permits these FLC suppressors to act. FLC suppression triggers several floral activators (genes that promote flowering) such as FT and SOC1. The protein products of these genes in turn activate LEAFY (LFY) and APETALA 1 (AP1), which stimulate flower formation in the meristem (the region of active growth in the plant body).
" height="20" alt="HEW Logo">
In cereals such as wheat, the gene VRN2 suppresses flowering (like FLC in Arabidopsis), while VRN1 promotes flowering (like AP1 in Arabidopsis).
During vernalization the tip of the shoot (aerial parts) of a plant senses the cold stimulus. This has been shown through grafting experiments. Most species flower when a vernalized shoot tip is grafted onto nonvernalized stock (lower parts) but not when a nonvernalized tip is grafted onto vernalized stock.
Applications
Vernalization prevents plants from flowering too early and is important for many fruit trees, which need a certain amount of winter cold before breaking dormancy and producing flowers. Flowering when the conditions are optimal—typically in spring—is necessary for enhanced fruit and seed production. Artificial exposure to cold can induce flowering to occur earlier than normal or in warm climates that lack the requisite seasonal chilling. The process also enables uniform and simultaneous flowering in all plants of the same species, which is beneficial for crop management, particularly for ornamental crops such as tulips and lilies. Knowledge of vernalization has been used to eliminate the normal two-year growth cycle required of winter wheat. By partially germinating the seed and then chilling it to 0 °C (32 °F) until spring, it is possible to cause winter wheat to produce a crop in the same year.
Devernalization
Devernalization can be brought about by exposing previously vernalized plants or seeds to high temperatures, causing a reversion to the original nonflowering condition. Onion sets that are commercially stored at near-freezing temperatures to slow spoilage are thereby automatically vernalized and ready to flower as soon as they are planted. Exposure to temperatures above 26.7 °C (80 °F) for two to three weeks before planting, however, shifts the sets to the desired bulb-forming phase.