Role of cold-responsive genes in plant freezing tolerance.

Plants vary greatly in their ability to survive freezing temperatures. At one extreme are plants from tropical regions that have virtually no capacity to survive even the slightest freeze. In contrast, herbaceous plants from temperate regions generally survive freezing temperatures ranging from 25°C to 230°C, depending on the species, whereas perennials in the boreal forests routinely survive winter temperatures below 230°C. Significantly, the maximum freezing tolerance of plants is not “constitutive” but is induced in response to low, nonfreezing temperatures (below approximately 10°C), a phenomenon known as “cold acclimation.” Wheat plants grown at normal warm temperatures, for instance, are killed by freezing at about 25°C, but after cold acclimation, they can survive freezing temperatures as low as 220°C. What accounts for the differences in freezing tolerance among plant species and the increase in freezing tolerance that occurs with cold acclimation? Determining the answers to these questions is not only of basic scientific interest, but also has potential practical applications. Freezing temperatures periodically cause significant losses in plant productivity and limit the geographical locations where crop and horticultural plant species can be grown. Despite considerable effort, traditional breeding approaches have resulted in only modest improvements of freezing tolerance. For example, the freezing tolerance of the most hardy wheat varieties today is essentially the same as that of varieties developed in the early part of this century (Thomashow, 1990). Knowledge of the molecular basis of freezing tolerance and the cold acclimation process could potentially lead to the development of new strategies to improve plant freezing tolerance and result in increased plant productivity and expanded areas of agricultural production. In 1985, Guy et al. established that changes in gene expression occur during cold acclimation. Since then, a major goal in cold acclimation research has been to identify cold-responsive genes and to determine whether they have roles in freezing tolerance. The thought has been that many cold-responsive genes probably mediate biochemical and physiological changes required for growth and development at low temperature. Other genes, however, might have roles in freezing tolerance. The primary purpose of this Update is to highlight recent developments indicating that cold-responsive genes do indeed contribute to freezing tolerance. To begin, however, I present background information concerning the nature of freezing injury and the general mechanisms thought to be important in freezing tolerance. Additional information about the coldacclimation response can be found in other recent reviews (Steponkus and Webb, 1992; Thomashow, 1994; Hughes and Dunn, 1996).