Heat tolerance in plants: An overview

Heat stress due to increased temperature is an agricultural problem in many areas in the world. Transitory or constantly high temperatures ause an array of morpho-anatomical, physiological and biochemical changes in plants, which affect plant growth and development and may ead to a drastic reduction in economic yield. The adverse effects of heat stress can be mitigated by developing crop plants with improved hermotolerance using various genetic approaches. For this purpose, however, a thorough understanding of physiological responses of plants to igh temperature, mechanisms of heat tolerance and possible strategies for improving crop thermotolerance is imperative. Heat stress affects lant growth throughout its ontogeny, though heat-threshold level varies considerably at different developmental stages. For instance, during eed germination, high temperature may slow down or totally inhibit germination, depending on plant species and the intensity of the stress. At ater stages, high temperature may adversely affect photosynthesis, respiration, water relations and membrane stability, and also modulate levels f hormones and primary and secondary metabolites. Furthermore, throughout plant ontogeny, enhanced expression of a variety of heat shock roteins, other stress-related proteins, and production of reactive oxygen species (ROS) constitute major plant responses to heat stress. In order o cope with heat stress, plants implement various mechanisms, including maintenance of membrane stability, scavenging of ROS, production of ntioxidants, accumulation and adjustment of compatible solutes, induction of mitogen-activated protein kinase (MAPK) and calcium-dependent rotein kinase (CDPK) cascades, and, most importantly, chaperone signaling and transcriptional activation. All these mechanisms, which are egulated at the molecular level, enable plants to thrive under heat stress. Based on a complete understanding of such mechanisms, potential genetic trategies to improve plant heat-stress tolerance include traditional and contemporary molecular breeding protocols and transgenic approaches. hile there are a few examples of plants with improved heat tolerance through the use of traditional breeding protocols, the success of genetic ransformation approach has been thus far limited. The latter is due to limited knowledge and availability of genes with known effects on plant eat-stress tolerance, though these may not be insurmountable in future. In addition to genetic approaches, crop heat tolerance can be enhanced y preconditioning of plants under different environmental stresses or exogenous application of osmoprotectants such as glycinebetaine and roline. Acquiring thermotolerance is an active process by which considerable amounts of plant resources are diverted to structural and functional aintenance to escape damages caused by heat stress. Although biochemical and molecular aspects of thermotolerance in plants are relatively ell understood, further studies focused on phenotypic flexibility and assimilate partitioning under heat stress and factors modulating crop heat olerance are imperative. Such studies combined with genetic approaches to identify and map genes (or QTLs) conferring thermotolerance will ot only facilitate marker-assisted breeding for heat tolerance but also pave the way for cloning and characterization of underlying genetic factors hich could be useful for engineering plants with improved heat tolerance. 2007 Published by Elsevier B.V.