Engineering temperature tolerance in agricultural crops

Temperature stress due to high temperature is a major problem in agriculture in many areas. Continuously increasing temperature cause an disorder of physiological and biochemical changes in crop plants, which affects many growth process i.e. different phenological stages of the crops and development that may lead to a sharp reduction in grain yield. The detrimental effect of temperature stress can be mitigated by developing crop plants with thermotolerance using various approaches. For this, purpose a complete understanding of physiological responses of crop plants to increased temperature, and possible approaches for improving crop thermotolerance is important. Temperature stress affects plants development throughout its ontogeny. The threshold levels of temperature differ considerably at different phenological stages of the crops plants that is during seed germination, the high temperature the adversely affect photosynthesis, respiration, water relations and membrane stability, and also changes in hormones and primary and secondary metabolites. Furthermore, enhanced expression of various heat shock proteins other stress related proteins and production of reactive oxygen species contribute towards major plants responses to high temperature throughout plants ontogeny. In order to cope with thermotolerance, various mechanisms including membrane thermal stability, production of antioxidants, accumulation and adjustment of osmolytes and calcium dependent proteins kinase, and most importantly transcriptional activation. All these mechanism which are regulated at the molecular level enable the plants to improve under temperature stress. Based on thorough understanding of all these mechanisms, potential genetic approaches to improve plant thermotolerance include molecular breeding and transgenic strategy. There are several examples of plants with improved thermotolerance through the use of breeding programme. The genetic transformation approach has been far restricted. This is due to incomplete knowledge and availability of genes with known effects on thermotolerance plants. There, is an other approach by which heat tolerance can be increased by preconditioning of plants under various environmental stress or the osmoprotectants exogenous application that is glycinebetaine and proline. Acquired heat tolerance is an active process by which considerable amount of plants resources are diverted to structural and functional maintance to avoid detrimental effect caused by increased temperature. The physiological and biochemical basis of heat tolerance in crop plants are well known, further studies on assimilate partitioning under temperature stress and traits controlling crop thermotolerance are needed. These studies combined with genetic strategies to identify and map genes conferring thermotolerance will not enhance markers-assisted breeding forthermotolerance but also provided the way for cloning and characterization of various factors which could be advantageous for engineering crop plants with improved thermotolerance.