Adaptive Response of Plants to Freezing Stress: a Physiological Perspective

Freezing is a major environmental stress during the annual cycle of temperate zone perennials. Freeze- injury can occur due to mid-winter temperatures that are colder than the tolerance threshold of a tissue / plant or due to untimely freezing temperatures before cold acclimation (development of freezing tolerance) in the fall or after deacclimation (loss of acquired freezing tolerance) in the spring. Therefore, the timing and extent of seasonal cold acclimation and deacclimation are of critical importance for winter survival, particularly in view of the climate change, i.e. unpredictable extreme weather occurrences. For example, plants may acclimate less completely to cold if exposed to milder autumn climate, and thus may be damaged by even mild sudden frosts. Alternatively, they may deacclimate prematurely due to unseasonable, midwinter warm spells and be damaged by the cold that follows. Efficient cold acclimation ability, high deacclimation resistance, and efficient reacclimation capacity are, therefore, important components of the winter survival in overwintering perennials. Understanding the fundamental mechanisms of cold acclimation is of key importance in efforts to develop cold-hardy plants. Spatial redistribution of cellular water during a natural freeze-thaw episode imposes numerous physical and biochemical stresses at the cellular level. Cold acclimation is, therefore, a complex quantitative trait involving a sizable suite of genes and the product of integrated processes that coordinate the induction of freezing tolerance. Transcriptomic studies, in combination with mutational and transgenic plant analyses, have revealed a complex transcriptional network operating during cold stress. Some of these gene products likely function in stress tolerance directly (dehydrins, aquaporins, enzymes for fatty acid biosynthesis, osmolytes etc.), while others are involved in signal transduction and the regulation of gene expression (transcription factors, protein kinases, phosphatases etc.). Also, depending upon how plants are cold-acclimated by the experimenters, some of these genes might, conceivably, be associated with cold shock and not true acclimation. For example, unrealistic artificial acclimation regimes, such as a sudden exposure to cold in growth chambers, can result in ‘shock' rather than gradual ‘acclimation'. Current status of cold acclimation research will be discussed in light of results obtained by various research groups including our own recent work with Rhododendron and Vaccinium.