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P.L. Steponkus, M. Uemura, R. A. Joseph, S. J. Gilmour and M. F. Thomashow*
Dept. of Soil, Crop and Atmospheric Sciences, Cornell University
*Dept. of Crop and Soil Sciences, Michigan State University
[Proc. Natl. Acad. Sci. USA 95 (1998)]

The ability to endure low temperatures and freezing is a major determinant of the geographical distribution and productivity of agricultural crops. Even in areas considered suitable for the cultivation of a given species, decreases in yields and crop failure frequently occur as a result of aberrant freezing temperatures. In spite of attempts to minimize damage to freezing-sensitive crops – primarily by using energy-costly practices to modify the microclimate – substantial economic losses resulting from freezing are incurred annually in a diverse array of agricultural crops.

This group has only recently provided the first direct evidence that expression of COR (cold-regulated) genes are functionally involved in cold acclimation. Simply stated, freezing injury is a consequence of freeze-induced cell dehydration, which results in several different lesions in the plasma membrane. In non-acclimated leaves of herbaceous species, the primary lesion is the result of lamellar-to-hexagonal II phase transitions in the molecular structure of the plasma membrane, most often near the chloroplast envelope, as a result of freeze-induced cell dehydration. Freeze-induced formation of the hexagonal II phase disrupts both the physical continuity and semipermeable characteristics of the plasma membrane such that the cells become leaky and flaccid.

Studies using NMR, freeze-fracture electron microscopy, and x-ray diffraction have shown that COR15a alters the intrinsic curvature of the inner membrane of the chloroplast envelope and thereby alters the phase transitions of the membranes. X-ray measurements were used to determine the structural phases and lattice constants of the membrane walls. These studies establish that expression of the COR15a gene decreases the propensity for freeze-induced lesions, and improves the freezing tolerance, of non-acclimated leaves of A. thaliana and winter cereals such as rye, wheat, barley, and oat.

High Magnification of Protoplasts
High magnification (65,200) of protoplasts showing ordered HII phase,
well-ordered plasma membrane striated (pm-st) regions,
and loosely ordered swirls (pm-sw). Bar represents 0.25 nanometers.