ABIOTIC STRESS RESPONSE IN PLANTS PHYSIOLOGICAL, BIOCHEMICAL AND GENETIC PERSPECTIVES

Abstract

Plants, unlike animals, are sessile. This demands that adverse changes in their environment are quickly recognized, distinguished and responded to with suitable reactions. Drought, heat, cold and salinity are among the major abiotic stresses that adversely affect plant growth and productivity. Abiotic stress is the principal cause of crop yield loss worldwide, reducing normal yields of major food and cash crops by more than 50 percent and thereby causing enormous economic loss as well. Water availability and water use efficiency are among the important abiotic factors that have had and continue to have a decisive influence on plant evolution. Water stress in its broadest sense encompasses both drought and flooding stress. Salinity usually accompanies water stress and may occur concurrently. Drought and salinity are becoming particularly widespread in many regions, and may cause serious salinization of more than 50% of all arable lands by the year 2050. In general, abiotic stress often causes a series of morphological, physiological, biochemical and molecular changes that unfavorably affect plant growth, development and productivity. Drought, salinity, extreme temperatures (cold and heat) and oxidative stress are often interrelated; these conditions singularly or in combination induce cellular damage. These stress stimuli are complex in nature and may induce responses that are equally, if not more, complex in nature. For example severe drought during critical growth phases may directly result in mechanical damage, changes in the synthesis of macromolecules, and low osmotic potential in the cellular settings. In addition it should be noted that almost all of these abiotic stresses lead to oxidative stress and involve the formation of reactive oxygen species (ROS) in plant cells. Usually, plants have mechanisms to reduce their oxidative damage by the activation of antioxidant enzymes and the accumulation of compatible solutes that effectively scavenge ROS. However, if the production of activated oxygen exceeds the plant’s capacity to detoxify it, deleterious degenerative reactions do occur, the typical symptoms being loss of osmotic responsiveness, wilting and necrosis. Therefore, it is the balance between the production and the scavenging of activated oxygen that is critical to the maintenance of active growth and metabolism of the plant and overall environmental stress tolerance. There has been considerable progress in the area of abiotic stress research, especially in the direction of producing improved crop varieties that counter these stresses X Preface effectively. Plant engineering strategies for abiotic stress tolerance has been focused largely on the expression of genes that are involved in osmolyte biosynthesis (glycine betaine, mannitol, proline, trehalose etc.); genes encoding enzymes for scavenging ROS (super oxide dismutase (SOD), glutathione S- transferase, Glutathione reductase, glyoxylases etc); genes encoding late embryogenesis protein (LEA) (LEA, HVA1, LE25, Dehydrin etc); genes encoding heterologous enzymes with different temperature optima; genes for molecular chaperons (Heat Shock Proteins (HSPs));genes encoding transcription factors (DREB 1A,CBF 1, Alfin 1); engineering of cell membranes; proteins involved in ion homeostasis. These aspects have undoubtedly opened up the avenue to produce transgenics with improved tolerance. To cope with abiotic stresses it is of paramount significance to understand plant responses to abiotic stresses that disturb the homeostatic equilibrium at cellular and molecular level in order to identify a common mechanism for multiple stress tolerance. A very crucial and highly productive role is envisaged here for biotechnology in identifying metabolic alterations and stress signaling pathways, metabolites and the genes controlling these tolerance responses to stresses and in engineering and breeding more efficient and better adapted new crop cultivars. This book is broadly divided into sections on signaling in abiotic stress, nucleic acids, proteins and enzymes, genes and genomes and adaptation and tolerance. It focuses on in depth molecular mechanism of abiotic stress effects on plants. In addition, insights from the genomics area are highlighted in one of the chapters of the book. Of special significance in the book is the comprehensive state of the art understanding of stress and its relationship with cyclic nucleotides in plants. This multi authored edited compilation attempts to put forth an all-inclusive biochemical and molecular picture in a systems approach wherein mechanism and adaptation aspects of abiotic stress will be dealt with. The chief objective of the book hence is to deliver state of the art information for comprehending the effects of abiotic stress in plants at the cellular level. Our attempt here was to put forth a thoughtful mixture of viewpoints which would be useful to workers in all areas of plant sciences. We trust that the material covered in this book will be valuable in building strategies to counter abiotic stress in plants.