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In this article we will discuss about the physiology of cold stress on plants.
Plants produce number of proteins in response to cold and freezing temperature. More than 54 cold-inducible genes have been identified using cDNA microarray technology. In the process of various stress inducible gene expression, nearly 10% of the drought inducible genes are also induced by cold-stress indicating cross talk among abiotic stress relieving genes. The promoter analysis of cold-inducible genes exhibit atleast four independent regulatory systems of gene expression.
Cold responsive genes contain ds-element c-repeat or CRT, which is 5 bp core sequence CCGAC. The transcription factor for cold-responsive genes are c-repeat binding factor i.e., CBF1 (namely DREB1B) or DREB1A. Over expression of DREB1A enhances the expression of rd 29A, rd 17, kin 1, cor 6.6, cor 15a and erd 10 under cold and drought conditions. This suggests that DREB1A/CBF1 transcription factors of transgenic plants specifically interact with DRE element and induces expression of these stress tolerant genes.
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In addition, other cold inducible transcription factors are RAV2, RAV2, 6, RAV1, and ZAT12. Some of these exhibit CBF- independent expression. This shows that nearly 30% of cold-responsive genes are not directly regulated by CBF and some cold-inducible genes do not have CRT/DRE cis-element in their promoter region. Based on molecular dissection analysis CBF can be referred as main controlling switch, functions both in dicots and monocots.
Cold-responsive transcription factors CBF/DREB are categorized into three subgroups. The group CBF/DREB1 is specifically induced by cold whereas DREB2A can be induced by drought but not by cold. Plant chloroplast is targeted in the strongest attempt to increase cold tolerance; COR proteins have been transferred to cold sensitive plants.
Membrane Fluidity and Cold Tolerance:
One of the earliest signs of abiotic stress is injury to cell membrane. Chilling and freezing stress affects membrane fluidity. Transgene analysis has shown that plant deficient in unsaturated fatty acid exhibits strong tolerance to high temperature. Increased saturated fatty acid level in the membrane by silencing ω3-desaturase gene provides plants with more tolerance to high temperature.
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Dehydrins are the family of proteins associated with cell dehydration. Cold induced dehydrins have been reported in several species. In barley, for example, P-80 a cold induced 80 KDa dehydrin has been reported. Cryoprotection activity of a cold-induced dehydrin has been evidenced and it was purified from barley plant.
Ratio of saturated to unsaturated fatty acid level decides the fate of degree of cold tolerance, particularly in the plastid membrane of higher plants. Presence of cis-double bonds in the membrane lipid lowers phase-transition temperature to more or less approximately 0°C. In a transgenic strategy, transformed tobacco expressing ω-3 fatty acid desaturase had increased amount of dienoic and trienoic fatty acid, which in turn provide strong tolerance to chilling temperature.
A broad specific ∆9-desaturase gene from the cyanobacteria, Anacystis nidulans has been introduced into tobacco and the expressed enzyme inflict cis- double bonds in specific saturated fatty acid in several membrane lipids. High degree of unsaturated fatty acid in the membrane lipid shows enhanced tolerance to chilling temperature and vice-versa for resistance to higher temperature in plants.
