Heat Stress In Food Grain Crops: Plant Breeding and Omics Research

Wheat crop is adapted to cooler climatic conditions and has an optimal daytime growing temperature of 15 °C during the reproductive stage. Heat stress is becoming a major constraint to wheat production as it affects every stage of the crop but the anthesis and reproductive stages are more sensitive. The situation will be aggravated due to climate change as predicted by the Intergovernmental Panel on Climate Change, for every degree rise in temperature above this optimum leads to a 6% yield reduction. Being quantitative in nature, heat stress is a complex trait and is strongly influenced by genotype x environment interaction. The new omics approaches like transcriptomics, proteomics, metabolomics and ionomics will be useful in understanding the underlying mechanism of heat tolerance. In this chapter, we will summarize the impact of heat stress on wheat production, physiological traits contributing to heat tolerance and how to integrate new omics tools such as transcriptomics, proteomics, metabolomics and ionomics with plant breeding.

Globally, maize is an important crop and serves as a livelihood for millions of marginal farmers across South Asia and sub-Saharan Africa. However, heat stress has become a globally prominent and growing concern for maize farmers, owing to its adverse impact on maize growth and productivity. In addition, the mean maximum temperature may increase by 2.1–2.6°C in 2050, with significant temporal and spatial variations, across South Asia. Further, the heat-stressed areas would increase to 21% from the current baseline. Heat stress is known to induce a series of morphophysiological, anatomical and molecular changes in maize, thereby affecting growth and development, which ultimately leads to a drastic reduction in the grain yield. Regulation of osmoprotectants, detoxification of excess reactive oxygen species (ROS), expression of heat-responsive/shock genes, and change of plant phenology help in the development of heat tolerance. The molecular basis of heat stress tolerance mechanisms has been appreciably understood and updated using the innovative physiological and molecular tools. Further, functional genomics strategies resulted in the identification of genes and regulatory pathways involved in heat stress tolerance in maize. Several attempts have been made in breeding heat-tolerant maize cultivars. The availability of genomic resources, accessibility to sequence information and millions of SNP markers in maize facilitated the selection for heat tolerance at the genome-level. Genomics-assisted mapping revealed several QTL and interactions for heat stress functional adaptive traits. The new breeding approaches like doubled haploid inducers, genome editing tools and high throughput phenomics at the breeders' disposal are opening up a new era in maize breeding for development of heat resilient maize hybrids.

Pearl millet (Pennisetum glaucum L. R. Br.) is an important cereal crop grown by resource poor farmers of semi-arid and arid tropics. It is grown for food in Asia, Africa and Latin America and for fodder in the USA, Australia and Brazil. Due to its inherent ability for tolerance to temperature and drought, salinity and nutrient-poor soils, it is grown in harsh environments. Owing to the changing climatic conditions, the crop holds promise for food and nutrition security for the increasing world population. However, high temperature stress is one of the main reasons for low productivity in pearl millet under semi-arid and arid environments. Further improvement for thermo tolerance is needed for the economization of agriculture.

Heat stress (HS) is a complex function of intensity, duration, and rate of increase in temperature. Tolerance mechanisms to HS are exhibited in all stages of crops such as seedling emergence, vegetative stage, flowering/ reproductive, and grain filling stages. For surviving under HS, crop plants show short-term (avoidance) and long-term (adaptation) strategies. A wide range of plant developmental and physiological processes are negatively affected by HS. Heat tolerance (HT) has been linked to increased tolerance of the photosynthetic apparatus and correlated with increased capacity of scavenging and detoxifying of reactive oxygen species (ROS). Induction of thermotolerance may be ascribed to the maintenance of a better membrane thermostability (MTS) and low ROS accumulation due to improved antioxidant capacity, osmo-regulation of solutes and synthesis of heat shock proteins (HSPs). Heat tolerance can be evaluated by a field screening, lab cum field screening or laboratory screening protocols and testing under hotspot locations. In pearl millet, the studies on heat tolerance are limited and the few studies made to date suggest seedling thermotolerance index (STI), seed to seedling thermo-tolerance index (SSTI) in pearl millet, and heat tolerance index (HTI) are indicative of heat tolerance. However, these are not indicative of maturity stage traits wherein membrane thermo stability holds promise. For breeding for heat tolerance, information on genetic variability, gene action (additive and non-additive), heritability, stability and correlation are available. Landraces are adapted to their native environment and could be the potential sources of HT. Gene interaction on heat tolerance showed its complex nature of inheritance. Plants are relatively more sensitive to HT during reproductive than vegetative stages. Breeders should consider and devise tools for heat tolerance screening which directly links to the productivity of a crop. Different breeding strategies such as conventional breeding methods, physiological trait-based breeding, molecular or transgenic approach can be applied individually or in combination for genetic improvement for heat tolerance.

High temperature stress is one of the important abiotic stresses hindering in achieving potential yield in crop plants, particularly cool-season grain legumes. Chickpea is one of the important cool-season grain legume crops. It experiences high temperature stress at different growth stages. Prevalence of heat stress during the reproductive stage reduces the crop yield drastically. Although genetic resource is available for heat stress tolerance in chickpea, studies on inheritance and its utilization in breeding program remain very limited. Research efforts through conventional breeding have been targeted to identify the traits for indirect selection. Advancement of molecular breeding approaches has led to the identification of markers linked to traits contributing to heat stress tolerance. Despite the availability of large scale genomic resources, most of the studies were limited to identify the molecular markers linked to quantitative trait loci (QTL). The functional genomics provides better insight into the molecular pathways and functions of the genes involved in heat stress tolerance. Limited information is available on the genes and pathways of gene activation controlling effective stress resistance in chickpea. Genome-wide analysis of Hsfs gene family resulted in the identification of Hsf genes which belong to four major groups with several paralogous and orthologous genes, and are unevenly distributed across all of the eight chromosomes. The next-generation sequencing and genome-editing techniques will greatly contribute in designing abiotic stress tolerant crop plants including chickpea.

Groundnut or peanut (Arachis hypogaea L.), an annual legume, is an important oil, food, fodder and feed crop grown in more than 100 countries. Heat and drought stress, and their combination are important abiotic constraints of groundnut production in Asia and Africa, which together accounts over 90% of global groundnut area. An increase in mean air temperature of 2-3 °C is predicted to reduce groundnut yields in India by 23-36% as heat stress during critical stages affects the pod yield. Moreover, heat stress worsens the burden of moisture stress aggravating the pod yield losses. Although groundnut genotypes continue to produce photosynthates under heat stress, only tolerant genotypes possibly have coping mechanisms to partition photosynthates to pods. Understanding the physiological, biochemical, molecular and genetic mechanism of heat-stress tolerance in groundnut is useful to devise breeding strategies to improve adaptation to heat stress. Intense phenotyping of plants grown in the field and glasshouses distinguishes sensitive and tolerant genotypes for heat stress, and to study the associated physiological and morphological differences between such genotypes. This chapter elaborates on the effects of heat stress on different life stages in groundnut, mechanisms contributing to adaptation to heat stress and recent developments in phenotyping, genetics and genomic tools to improve adaptation to heat stress.

Considering the current scenario of global climate change, high-temperature stress is becoming a major threat limiting crop yield and productivity of crops including mungbean (Vigna radiata L. Wilczeck), globally. Significant yield reduction in mungbean due to high-temperature stress, especially during the reproductive stage, has been observed by various researchers. Therefore, identification of heat-tolerant mungbean lines by using different selection criteria, based on field trials evaluating various yield traits, is urgently needed. An overview of different morpho-physiological responses of mungbean under heat stress may help in formulating appropriate strategies for improving its yield potential. In addition, identification and incorporation of appropriate management strategies may enhance the productivity and sustainability of mungbean worldwide. The key findings of this chapter include the effects of heat stress on growth, reproduction and physiology of mungbean growing at different agroclimatic zones. Further, effective approaches for managing heat stress such as selection and screening of available germplasm under field trials, application of exogenous thermo-protectants and well-integrated genetic and agronomic management methods, are also discussed to improve mungbean performance under heat stress. However, the implications of the above-mentioned techniques for heat stress management require deep insight into heat tolerance mechanisms, molecular breeding, and gene characterization methods.