Towards The Development of Cold Tolerant PlantsC. S. Prakash
Center for Plant Biotechnology Research, College of Agricultural, Env and Nat Sci. , Tuskegee University, Tuskegee, AL 36088, USA
(Reprinted from January 1997 issue of ISB News Report (http://www.nbiap.vt.edu)
Any farmer or home gardener in the temperate region knows well one cold fact about the winter: freezing temperatures can kill most plants! Cold temperatures are a major environmental constraint in improving the crop productivity in these areas. Classical plant breeding has had limited success in imparting cold hardiness to crop plants. Little is understood as to why some plant species tolerate cold better than others. Biotechnology with its powerful tools however may provide answers through the isolation of cold-fighting genes and thus may help in the development of crop plants that can withstand frigid temperatures. This optimism is supported by a recent publication from Michael Thomashow's group at Michigan State University (1) which provides some valuable insights into how plants respond to cold temperatures. The research shows for the first time that freezing tolerance can be enhanced in plants through an insertion of a single gene involved in cold-acclimation.
Mike Thomashow's strategy has been to understand 'cold acclimation', a process where plants exposed to low, non-freezing temperatures somehow learn to put up with subsequent freezing temperatures. For instance, 'cold-acclimated' rye plants can survive temperatures of up to -30C! The underlying reasons for this phenomenon appears to be complex but it is known that cold acclimation is controlled by many genes and that cell membranes are particularly vulnerable to cold damage. Thomashow and colleagues have isolated many genes that are turned on during cold acclimation in Arabidopsis. One such gene is COR15a which is speculated to have a role in freezing tolerance.
The Michigan group developed transgenic Arabidopsis plants expressing the COR15a gene in a constitutive manner. In regular plants this gene is turned on only through cold acclimation while the transgenic plants showed COR15a gene product in both cold-acclimated and non acclimated plants. Chloroplasts in leaves of transgenic plants showed reduced damage to freezing temperatures when compared to those from the control plants. Further, in collaboration with Peter Steponkus and colleagues at Cornell University, it was found that protoplasts isolated from transgenic plants survived better at subzero temperatures (18% better survival at -6.5C) than the control. Although the COR15a gene produces a chloroplast-targeted protein, the results indicate that the expression of gene may also affect other cellular functions including improving the cryostability of the plasma membrane.
The COR15a gene enhanced the freezing tolerance of chloroplasts in engineered plants by almost 2C which was nearly one-third of the increase seen due to cold-acclimation. While this might not appear as a large increase, an improvement of freeze tolerance by 2C could potentially benefit certain crop plants, argue Artus et al. There are many more COR genes known to scientists and if one stacks them up together in a plant, even more dramatic tolerance to cold especially at the whole plant level may be achieved, according to Thomashow. Ongoing research in Michigan lab is striving to further understand the intricacies of plant gene expression in response to freezing temperatures and this knowledge may empower scientists to develop crop plants which can brave the cold. Clearly, the molecular biology has the potential to help plants and farmers beat the winter blues.
ReferenceArtus, N. N. et al. 1996. Constitutive expression of the cold-regulated Arabidopsis thaliana COR15a gene affects both chloroplast and protoplast freezing tolerance. Proc. Natl. Acad. Sci. 93: 13404-13409.