Cold, salinity and drought stresses: an overview.

World population is increasing at an alarming rate and is expected to reach about six billion by the end of year 2050. On the other hand food productivity is decreasing due to the effect of various abiotic stresses; therefore minimizing these losses is a major area of concern for all nations to cope with the increasing food requirements. Cold, salinity and drought are among the major stresses, which adversely affect plants growth and productivity; hence it is important to develop stress tolerant crops. In general, low temperature mainly results in mechanical constraint, whereas salinity and drought exerts its malicious effect mainly by disrupting the ionic and osmotic equilibrium of the cell. It is now well known that the stress signal is first perceived at the membrane level by the receptors and then transduced in the cell to switch on the stress responsive genes for mediating stress tolerance. Understanding the mechanism of stress tolerance along with a plethora of genes involved in stress signaling network is important for crop improvement. Recently, some genes of calcium-signaling and nucleic acid pathways have been reported to be up-regulated in response to both cold and salinity stresses indicating the presence of cross talk between these pathways. In this review we have emphasized on various aspects of cold, salinity and drought stresses. Various factors pertaining to cold acclimation, promoter elements, and role of transcription factors in stress signaling pathway have been described. The role of calcium as an important signaling molecule in response to various stress signals has also been covered. In each of these stresses we have tried to address the issues, which significantly affect the gene expression in relation to plant physiology.