Freezing tolerance is one of the most important traits that determine the productivity and distribution of plants. Even in global climate changes era, there are frequent occasions in both regional and/or local environments that temperature drops quite rapidly and stays low for a few days to weeks during crop growing seasons, especially in late fall and early spring. Thus, the increase of plant freezing tolerance is critical to maintain or improve crop productivity. Temperate plants including many crop species have an ability to increase their freezing tolerance when exposed to low but non-freezing temperatures, which is known as cold acclimation (for example, a winter rye can withstand freezing as low as -30℃ after cold acclimation). Many studies have tried to elucidate mechanism(s) of cold acclimation in plants but it still remains to be cleared in many issues due to diverse and seemingly unrelated changes occurring during cold acclimation. Because the plasma membrane (PM) is the primary site of freezing injury, the maintenance and rearrangement of the PM is thought to be the key issue for cold acclimation and survival at freezing temperatures. Thus, it is reasonable to hypothesize that diverse changes occurring during cold acclimation ultimately lead to the stabilization of the PM against various stresses imposed by freezing (such as dehydration, salinity, osmotic and mechanical stresses). This has been the standpoint of our research programs that are described in this article. First, this article overviews how plant cells respond to extracellular freezing and how different freeze-induced lesions occur at the PM before and after cold acclimation. Next, evidence of dynamic alterations in the PM compositions during cold acclimation is described. In addition, a recently proposed concept of the PM organization, microdomains that consist of specific lipid and protein components, is introduced. Microdomains in the PM are likely involved in functional changes of the PM, which have significant effects on the increase in freezing tolerance in plant cells. Lastly, experimental evidence showing the functional involvement of lipids and proteins in the PM is summarized. Apparently, PM components are actively involved in the reorganization of its functions and structure and, as a result, the PM determines the tolerance against freezing-induced stresses in plants.
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