ABIOTIC AND BIOTIC STRESS IN PLANTS - RECENT ADVANCES AND FUTURE PERSPECTIVES

Abstract

The impact of global climate change on crop production has emerged as a major research priority during the past decade. Several forecasts for coming decades project increase in at‐ mospheric CO2 and temperature and changes in precipitation, resulting in more frequent droughts and floods. Therefore, programmes to develop climate ready varieties with abiotic stress tolerance traits such as temperature and drought tolerance and biotic stress tolerance traits such as insect pest and pathogen resistance in combination with high yield in various important crops should be initiated urgently. Successful adaptation to climate change re‐ quires long-term investments in strategic research and new policy initiatives that main‐ stream climate change adaptation into development planning. A very crucial and highly productive role is envisaged here for basic sciences in identifying metabolic alterations, stress signalling pathways, metabolites and the genes controlling tolerance responses to stresses and in engineering and breeding more efficient and better adapted new crop culti‐ vars. Plant traits that favour yield and also have a direct effect on the mechanism of toler‐ ance is one of the important characteristics that has to be considered when developing climate ready crops. Plant engineering strategies for abiotic stress tolerance have been fo‐ cused largely on the expression of genes that are involved in osmolyte production; genes encoding ROS scavenging enzymes, LEA proteins, and heterologous enzymes with different temperature optima; and genes for molecular chaperons and transcription factors and pro‐ teins involved in ion homeostasis and engineering of cell membranes. The practical value of any genes or pathways for stress tolerance in crop plants can only be useful if there is evi‐ dence of superior performance in the field, especially in terms of yield. Selection for yield and stress tolerance per se necessitates a “top-down” approach, starting from the dissection of the complex traits to components. Marker-assisted selection (MAS) for abiotic stress-relat‐ ed traits should preferably target ‘major’ QTLs characterized by a sizeable effect, consistent across germplasm and with a limited interaction with the environment. To achieve such a goal, an interdisciplinary and interinstitutional approach would be needed with well-de‐ fined targets on crops and problems prioritized at the national level. Climate change and agriculture are interconnected processes, both of which take place on a global scale. Agriculture is particularly vulnerable to climate change. Higher temperatures will tend to reduce yields of many crops; it may encourage weed and pest proliferation. Changes in precipitation patterns increase the probability of short-run crop failures and long-run production falloffs. Although there are increases in the yield of some crops in some regions of the world, the overall impacts of climate change on agriculture are expected to be negative, threatening global food security. In developing countries, climate change will cause decline in the yield of the most important crops; South Asia will be particularly hard hit. Climate change will have varying effects on irrigated yields across regions, but irrigated yields for all crops in South Asia will experience a large decline. Climate change will result in additional price increases for the most important agricultural crops – rice, wheat, maize, and soybeans. Higher feed prices will result in higher meat prices; as a result, climate change will reduce the growth in meat. It is important to assess the effects of global climate changes on agriculture; this will help to properly anticipate and adapt farming to maximize agricultural production in a sustainable way. The consequence of climate on agriculture is related to variabilities in local climates rather than in global climate patterns. The Earth’s average surface temperature has increased by 0.83°C since 1880. Therefore, it is important for any assessment to be done individually, considering each local area. It is important to note that not all effects of climate change are negative; favourable effects on yield can be seen by realization of the potentially beneficial effects of carbon dioxide on crop growth and increase of efficiency in water use. Agricultural productivity is sensitive to two broad classes of climate-induced effects: the di‐ rect effects because of changes in temperature, precipitation and carbon dioxide concentra‐ tions and the indirect effects through changes in soil moisture and the distribution and frequency of infestation by pests and diseases. The main direct effect is generally seen on the duration of the crop growth cycle. Duration of crop growth cycles is related to temperature. Therefore, an increase in temperature will speed up development. In the case of an annual crop, the duration between sowing and harvesting will shorten; for example, the duration in order to harvest a maize crop could shorten between 1 and 4 weeks. The shortening of such a cycle could have an adverse effect on productivity because senescence would occur soon‐ er. In India, an impact of 1–2°C increase in mean air temperature is expected to decrease rice yield by about 0.75 t ha -1 in efficient zones and 0.06 t ha -1 in coastal regions and the impact of 0.5°C increase in winter temperature is projected to reduce wheat yields by 0.45 t ha -1. Furthermore, crops may experience both low and high weather extremes such as drought and flood and heat and chilling in a single cropping season, and such changes will have varying and complex impacts on agricultural production. Reductions in yields as a result of climate change are predicted to be more pronounced for rain-fed crops than in irrigated crop and under limited water supply situations because there are no coping mechanisms for rainfall variability. Crop growth and yield can be impaired in diverse ways by either high day or high night temperatures and, in addition, soil temperatures also play an important role in the response of crops to heat stress. Additional challenge to temperature increase stems from the fact that higher temperatures will increase the rate at which plants lose mois‐ ture, resulting in increased transpiration and water loss. Temperature affects the stages of development of crops during its progress to physiological maturity; the main stages in de‐ velopment of food grain crops that are sensitive to temperature are (i) germination, (ii) cano‐ py and leaf area development, (iii) flowering and reproductive development and (iv) grain development – anthesis to maturity. Further, climate change will indirectly affect crop pro‐ ductivity by changing pest and disease dynamics. Direct effects of pathogens or other organ‐ isms can be the induction of resistance or susceptibility and its associated cost or benefit to the host plant. The likelihood of most damaging impacts of diseases and pest can be expect‐ ed especially in cereals such as wheat and rice. These are likely to have a large impact in terms of food security under climate change scenarios as seen in the case of wheat. Changes in the levels of CO2, ozone and UV-B will have an influence on diseases by modifying host physiology and resistance mechanisms. Furthermore, changes in temperature, precipitation and the frequency of extreme events will influence disease epidemiology. An acute change that may arise in the host as an outcome of climate change and the subsequent indirect ef‐ XII Preface fects on the pathogen is a possible outcome. Changes in geographical distribution will possi‐ bly alter the comparative importance and range of diseases and may give rise to new disease complexes. Evolution of pathogen populations may hasten from enhanced UV-B radiation and increased fecundity under elevated CO2. Consequently, host resistances may be over‐ whelmed more swiftly; specifically, increases in leaf waxes and epidermal thickness as a re‐ sult of increased CO2 atmospheres can result in the host exhibiting higher physical resistance to some pathogens. Carbon dioxide–induced alterations in the architecture of a crop could lead to increased humidity inside the canopy and can create additional favoura‐ ble condition for pathogen survival. In addition, high speed winds and cyclones can contrib‐ ute to increased dispersal of airborne plant pathogens such as rusts, splash-borne pathogens such as bacteria and windborne insects and vectors such as aphids and psyllids. Soil and soil water will be adversely affected by climate change, and this in turn will lead to reduction in the yield of many crops. First-generation climate–carbon cycle models suggest that climate change will suppress carbon accumulation in soils and could even lead to a net loss of global soil carbon over the next century. Changes in soil carbon status are also a mat‐ ter of concern under changing temperature and changing rainfall regimes; soil carbon is not only important for growth and development of the crop but also for retention of water and nutrients and as an energy source for decomposition process in the soil. The risk of in‐ creased erosion is imminent in soils of dry agro ecosystems. High and extreme precipitation will increase runoff primarily due to the inability of the soils to absorb and hold water. Ex‐ tended dry periods will reduce vegetation cover, which again will result in substantial run‐ off. Such erosion events occurring frequently will lead to ecosystem change and also loss of soil nutrients. In addition, aridity can hinder surface decomposition and nutrient recycling, thereby affecting crop productivity. A comprehensive understanding of biotic and abiotic stress, especially the mechanism and tolerance aspects and strategies for adaptation, across the full range of warming scenarios and regions would go a long way in preparing for climate change. A multipronged strategy of using indigenous coping mechanisms, wider adoption of the existing technologies and concerted research and development efforts for evolving new technologies are needed coun‐ tering biotic and abiotic stress. This multi-authored edited compilation attempts to put forth an all-inclusive picture wherein most aspects of stress will be dealt with. The main purpose of the book, therefore, is to synthesize and present information for developing strategies to combat plant stress. Our effort here is to present a judicious mixture of basic as well as ap‐ plied research outlooks so as to interest workers in all areas of plant science. We trust that the information covered in this book would bridge the much researched area of stress in plants with the much needed information for evolving climate ready crop cultivars to en‐ sure food security in the future.