Heat Stress In Food Grain Crops: Plant Breeding and Omics Research E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Heat Stress In Food Grain Crops: Plant Breeding and Omics Research is a timely compilation of advanced research on heat stress affecting crop yield, plant growth & development of common food grain and cereal crops. Chapters in the book cover several aspects of crop science including the identification of potential gene donors for heat tolerance, physiological mechanisms of adaptation to heat stress, the use of conventional and modern tools of breeding for imparting tolerance against terminal temperature stress and precise mapping of heat tolerant QTLs through biparental and genome wide association mapping. The use of genomics and phenomics methods is focused on through chapters dedicated to important crops such as groundnut, pearl millet, maize, chickpea, cowpeas and wheat. Authors of the respective chapters explain the importance of harnessing a diverse crop genepool for sustaining crop production under conditions of increasing heat stress. Readers will be able to understand the relevance of functional genomics in elucidating candidate genes and their regulatory functions contributing to heat tolerance.
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Veröffentlichungsjahr: 2020
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PREFACE
High temperature during the growth period of most crop plants causes negative impact from crop germination, vegetative and reproductive stage to grain development, thus causing serious crop yield penalty. Exposure to high temperature can change different metabolic functions. Research on heat sensitive cultivars showed that heat stress for longer duration inhibits Rubisco activity and reproductive functions and thus produces an array of changes in plants. The insights on various mechanisms in plants for adaptation is crucial for developing resilience to high temperature.
The present book is an excellent review of recent advances in research on analyzing negative impacts of heat stress challenging crop yield and intervention of various studies to overcome the challenge of heat stress. To overcome the challenges of heat stress, the authors have elaborated on the various approaches, including conventional plant breeding, physiological trait-based breeding approach, various ‘omics’ based approaches covering genomics, transcriptomics, proteomics, metabolomics and ionomics. Efforts have been made to highlight the scope of emerging novel breeding schemes viz., genomic selection, and genome editing tools for improving genetic gain in crop plant.
The book contains chapters authored by scientists/researchers who are actively involved in improving the yield of agricultural crops by mitigating heat stress. Their contribution is enormous in presenting up-to-date information on the subject. The book will be beneficial to plant breeders, molecular biologist and plant physiologist as it gives insights into advanced breeding schemes, discovery of novel candidate gene(s)/QTLs related to heat stress tolerance and various adaptive mechanisms working at physiological and biochemical level mediating heat tolerance in plant. Thus, the information contained in this book will enrich our understanding of various pathways, genes rendering heat tolerance in plants and also helps us to develop various strategies to ensure global food security against heat stress.
We editors are thankful to our parent organization, Indian Council of Agricultural Research (ICAR), New Delhi for supporting our scientific pursuit in the form of a book “Heat Stress In Food Grain Crops: Plant Breeding and Omics Research”. We are highly thankful to Dr. T. Mohapatra, Director General, ICAR and Secretary, DARE, Ministry of Agriculture and Farmers’ Welfare, Government of India and Dr. T.R. Sharma, Deputy Director General (Crop Science), ICAR for their constant support and encouragement in this endeavor.
We thank our families for being patient and supportive in this long journey, without their moral support, it would not be possible. The entire team at Bentham, especially the Publishing Editor, and Production Editor who have always been cooperative to make this publication a reality. They have been very generous in accommodating even last minutes changes and deserve our genuine appreciation. We hope that this book will absolutely serve its purpose and will provide a latest and comprehensive treatise to the readers in furthering their academic and research pursuits.
Kanpur, the 16th August 2020
LIST OF CONTRIBUTORS
Mitigating Heat Stress in Wheat: Integrating Omics Tools With Plant Breeding
Karnam Venkatesh1,*,Vikas Gupta1,Senthilkumar K.M.2,Mamrutha H.M.1,Gyanendra Singh1,Gyanendra Pratap Singh1Abstract
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.
INTRODUCTION
Wheat being cultivated as a major staple crop from the prehistoric times, caters to the energy requirement of the human population in India and across the globe (Sharma et al. 2015). Wheat improvement efforts in the form of conventional breeding aimed at yield enhancement in the past have led to significant growth in productivity and production.
However, there is an increased demand for wheat due to changes in consumption patterns in the form of increased demand for wheat based end products such as biscuits, noodles, pasta, etc. According to FAO estimates, globally around 840 million tons of wheat must be produced by 2050 from the current levels. Further climate change scenarios in the form of increased heat and drought stress events would pose serious constraints for the achievement of 2050 targets (Reynolds et al. 2009). The global climate change in the form of elevated CO2 concentration, warming temperatures, and changes in rainfall patterns is becoming a major threat to crop production (IPCC 2007). The increased events of temperature rise in both the ocean and on the earth until 2012 has been reported (Team et al. 2014). The severe and more abrupt rise in temperatures in several parts of the world led to severely reduced crop yields (Kaushal et al. 2016). The adverse effect of increased temperatures on plant growth mechanisms is higher especially in the arid and semi-arid regions of the world (Cooper et al. 2009). The vulnerability of especially heading and grain filling period in wheat to high temperature stress has been reported (Liu et al. 2016; Yang et al. 2017; Priya et al. 2018).
The emergence and development of automated sequencing methods started the era of omics in the form of genomics and led to the sequencing of the whole genome of Arabidopsis thaliana in 2000 (Initiative and others 2000). Later on, several other organisms and crop plant genomes such as rice (Goff et al. 2002), soybean (Schmutz et al. 2010), maize (Schnable et al. 2009) and even the most complicated polyploidy species such as wheat (Consortium and others 2018) were sequenced and made the latest omics tools amenable to crop improvement. The word “omics” formally refers to a study related to genome, proteome, or metabolome, and aims at the characterization of a large family of cellular molecules and exploring their roles, and their interactive effects in an organism. These omics approaches are mainly performed through the application of several high-throughput technologies that mainly involve qualitative and/or quantitative detection of novel or previously identified genes, transcripts, proteins, and metabolites and other molecular species through genomics, transcriptomics, proteomics, and metabolomics, respectively (Ebeed 2019). Application of various omics approaches in understanding the abiotic stress responses in general (Kole et al. 2015; Meena et al. 2017; Lamaoui et al. 2018; Ebeed 2019; Wani 2019), drought stress (Hasanuzzaman et al. 2018; Ding et al. 2018) and heat stress (Xu et al. 2011; Jacob et al. 2017; Salman et al. 2019) particularly in crop plants and their mitigation has been reported by earlier researchers. It is therefore suggested that a multi-disciplinary and multi-pronged approach integrating the conventional plant breeding with the latest omics tools will be useful in mitigating the adverse effects of heat stress on wheat production. This chapter briefly deals with the latest reports of the application of omics approaches in improving wheat tolerance for heat stress.
IMPACT OF HEAT STRESS ON WHEAT PRODUCTION
Heat Stress, Extent of Damage/Threat to Wheat Area and Mechanisms Affected
The climate predictions by the Intergovernmental Panel on Climate Change (IPCC) indicated that the mean atmospheric temperatures are expected to increase between 1.8 to 5.8°C by the end of this century (IPCC 2007). The increase in the frequency of hot days and greater variability in temperatures in the future is also predicted as an effect of climate change (Pittock et al. 2003; Team et al. 2014). Extreme temperatures directly influence crop production by specifically affecting plant growth and yield realization posing a serious threat to food production (Team et al. 2014). Higher temperatures are likely to affect around seven million hectares of wheat area in developing countries and around 36 million hectares in temperate wheat production countries (Reynolds 2001). Warmer temperatures resulted in an annual wheat yield reduction to the tune of 19 million tons amounting to a monetary loss of $2.6 billion was observed between 1981-2002 (Lobell and Field 2007). In India, it has been predicted that with every rise in 1°C temperature, the wheat production will be decreased by 4–6 million tonnes (Ramadas et al. 2019). Approximately, 3 million ha wheat area in northeastern and northwest plain zones is exposed to terminal/reproductive heat stress (Gupta et al. 2013). Another report by Joshi et al., (2007) stated that around 13.5 million ha wheat area in India is vulnerable to heat stress. Temperatures above 34°C in northern Indian plains leading to significant yield loss was reported (Lobell et al. 2012).
High temperature stress when occurred at germination and early establishment stages is known to decrease germination and seedling emergence leading to abnormal seedlings, poor vigour, reduced overall growth of developing seedlings (Essemine et al., 2010; Kumar et al., 2011; Piramila et al., 2012). Further high temperature stress is found to severely impact dry matter partitioning, reproductive organ development and reproductive processes in crop plants (Prasad et al. 2011). Intermittent spells of temperature above 30 °C during the repro-ductive stage causes high temperature stress leading to decreased seed set and low grain number (Prasad and Djanaguiraman 2014; Sehgal et al. 2018; Qaseem et al. 2019). There are reports which also indicate deterioration of grain quality parameters under high temperature stress (Britz et al. 2007).
Grain filling is an essential growth stage involving mobilization and transport processes involving many biochemical processes regulating the synthesis of proteins, carbohydrates and lipids and their transport into the developing grains (Awasthi et al. 2014; Farooq et al. 2017). Processes leading to grain filling and the accumulation of reserves in the developing grains are highly sensitive to temperature changes (Yang and Zhang 2006). Heat stress affects enzymatic processes involved in the synthesis of starch and proteins and ultimately affecting the transport and accumulation of major components of grains primarily the starch and proteins (Asthir et al. 2012; Farooq et al. 2017).
Heat stress in wheat can lead to early senescence thereby reducing the time available for grain filling (Awasthi et al. 2014). The senescence effect is accelerated by heat disrupting chloroplasts and damaging chlorophyll and the leaf membranes and increase in ethylene production (Prasad and Djanaguiraman 2014) which further reduces photosynthetic efficiency, biomass accumulation and yield attainment. Under normal conditions, the photosynthetic assimilates accumulated during the pre-anthesis period in the form of stem reserves contribute to around 10-40% of final grain weight. (Gebbing and Schnyder 1999). Remobilization of these stem reserves to the grains is crucial to attain full grain size and yield (Asseng and van Herwaarden 2003). Heat stress led accelerated canopy senescence reduces photosynthetic area and hence source strength clubbed with reduced turgor in phloem cells due to water deficiency, thereby increasing the viscosity of sucrose inhibiting its transport through phloem toward the grains (sink)(Sevanto 2014). Heat causing a reduction in activities of PEP carboxylase and RuBP carboxylase leading to inhibition of carbon assimilation in maturing grains due to heat stress was also observed in wheat (Xu et al. 2004). Heat stress during grain filling markedly decreasing starch accumulation in wheat by altering the expression of starch-related genes leading to a reduction in seed size in wheat (Hurkman et al. 2003; Dupont and Altenbach 2003).
MECHANISM OF HEAT TOLERANCE ADOPTION BY PLANTS
Avoidance Mechanisms by Way of Phenotypic Adjustments
Crop plants as part of their adaptation mechanisms to higher temperatures display a greater level of phenotypic plasticity. Plants adapt to higher temperatures by way of certain morphological adjustments in its life cycle, increased pubescence (Maes et al. 2001; Banowetz et al. 2008), increased wax deposition of leaf, sheath and on the stem surface, changed leaf orientation, manipulation of membrane lipid fractions, etc. First and the foremost adaptation of the plant when heat is sensed is shortening its life cycle to escape the adverse effect of stress (Blum et al., 2001). Leaf rolling to reduce the excess loss of water through transpiration is also one of the adaptation mechanisms found in wheat (Sarieva et al. 2010). Further as the reproductive growth stages of wheat (flowering and grain development) are the most sensitive stages to heat stress and the plant is forced to quickly complete these stages thereby shortening the whole life cycle (Hall 1992; Hall 1993). Water conservation mechanisms such as increased wax deposition on the plant surfaces has been observed under high temperature conditions and is linked to several favourable effects on plant in the form of protection against excess radiation and also contributes to reflection of visible and infrared wavelengths of light thereby reducing evaporative water loss through plant surfaces (Shepherd and Wynne Griffiths 2006; Cossani and Reynolds 2012). The wax is also known to reduce the leaf temperatures thereby protecting the membranes and leaf structural components from heat damage (Mondal 2011). Larger xylem vessels enabling plants to compensate for increased water loss under high temperature is also an adaptive mechanism (Srivastava et al. 2012). Under well-watered conditions increased transpiration leading to reduced canopy temperature up to 10°C lower than ambient temperature was an adaptive ability.
High temperature stress effect can also be minimised by manipulating agronomic practices such as using proper sowing methods, proper seed rate, selection of suitable cultivar, increased irrigation frequency, crop mulching etc. (Meena et al. 2015; Meena et al. 2019a; Meena et al. 2019b). Examples of managing high temperature stress in wheat by deliberately choosing heat tolerant cultivars (Glennson 81) over heat sensitive (Pavon 76) resulted in higher yields under stress (Badaruddin et al. 1999). They further demonstrated that application of animal manure, straw mulch along with increased doses of inorganic nutrients and irrigation frequency achieved enhanced wheat yields under high temperature stress. Foliar spray of potassium orthophosphate (KH2PO4), calcium, Mg and Zn were reported to enhance high temperature tolerance of wheat (Dias and Lidon 2010; Waraich et al. 2011). Practice of integrated approach combining above options could help in minimizing high temperature effects.
Tolerance Mechanisms
Ability of the plant to achieve normal growth and produce economic yield under higher temperatures is called as heat tolerance. The plants have evolved various tolerance mechanisms such as altering ion transporter systems, production of late embryogenesis abundant (LEA) proteins, accumulation of osmoprotectant molecules, ion transporters, free-radical scavengers and manipulating systems involving factors like ubiquitin and dehydrin through signaling cascades and transcriptional control (Wang et al. 2004; Rodríguez et al. 2005). Stomatal closure leading to reduced evaporative water loss to sustain the water dependent plant processes under heat stress (Woodward et al. 2002).
Enhanced root development under abiotic stress conditions to reach the deeper layers of soil in order to absorb more water has been reported as an tolerance mechanism (Lehman and Engelke 1993). Extended grain filling duration was also observed as a tolerance mechanism and positive association of grain filling duration with higher yield under heat stress has been reported (Yang et al. 2002).
Adjustments in the photosynthetic mechanisms and the enzymes involved by the plants have been found to be an alternate tolerance mechanism adapted by plants. Increased affinity of Rubisco the main enzyme responsible for fixation of carbon to CO2 under higher temperature conditions has been reported in some plants like Limonium gibertii (Parry et al. 2010). At very high temperatures above optimal, higher activity in the photosynthetic apparatus (Ristic et al. 2007; Allakhverdiev et al. 2008) and higher carbon allocation and nitrogen uptake rates were seen as tolerance mechanisms (Xu et al. 2006).
TRAITS OF IMPORTANCE FOR HEAT TOLERANCE AND THEIR PHENOTYPING TECHNIQUES
Canopy Temperature
Reduction in the temperature of the crop canopy under high temperature stress as a result of increased transpiration has been found to be an important trait to be associated with heat tolerance (Cossani and Reynolds 2012). The ability of the plant to maintain a cooler canopy was found to be genetically controlled and therefore amenable for selection of germplasm lines with cooler canopies (Pinto et al. 2010). Canopy temperature (CT) can be easily measured for germplasm screening using an infrared thermometer. The infrared thermometer senses this radiation and converts into electrical signal and is displayed as temperature. CT measurement by infrared thermometer being a non-destructive method can be used under field conditions and can covers large number of genotypes and selection for this trait indirectly allows for the selection of genotypes with better water use, deep root and stomatal conductance under stress.
Leaf Chlorophyll Content
The crop canopy greenness contributed mainly by the photosynthetic pigment chlorophyll is another trait of importance to screen germplasm for heat tolerance. The chlorophyll pigment reflects only the green fraction of the light after absorbing all other colour fractions and hence it is green in colour. The canopy greenness is directly related to photosynthetic efficiency of the plants. The chlorophyll content of the leaf can be estimated by a destructive lab based DMSO: acetone extraction method and by using an instrument called chlorophyll meter which is non-destructive and optical method. The measurement by optical method using different types of chlorophyll meters is found to be more relevant than DMSO method under field conditions (Dwyer et al. 1991). The chlorophyll content measured through chlorophyll meters is in the form of an index called chlorophyll content index (CCI). The CCI ranges from 0 to 99.9 and with the increase in the level of heat stress the CCI decreases and CCI of healthy plant ranges from 40 to 60. As optical method is based on leaf reflectance, it is influenced by time of day in terms of light (Mamrutha et al. 2017). Care should be taken to measure chlorophyll content at uniform time and in specific leaf across the genotypes under field (Mamrutha et al. 2017).
Canopy Greenness/Stay Green Canopy
Prolonged maintenance of canopy greenness also referred to as stay-green nature is a physiological adaptation mechanism by plants under heat stress. Lim et al., (2007) described stay greenness as “leaf senescence is characterized initially by structural changes in the chloroplast, followed by a controlled vacuolar collapse, and a final loss of integrity of plasma membrane and disruption of cellular homeostasis”. Stay green trait in tolerant genotypes help in withstanding chlorophyll loss and maintain photosynthesis levels under high temperature stress. Association of stay green habit with sustained yield levels under heat stress has been earlier reported and QTL regions regulating this have been identified (Vijayalakshmi et al. 2010). There are mainly two types of stay green types. One is productive type, where in the stay green plant parts actually contribute for sink/ grain filling. Another is cosmetic stay green type, where in greenness in these plants will not contribute for grain filling. Hence, identification of true and productive stay green types are also a challenge and can be done by considering other traits like water soluble carbohydrates in stem, peduncle etc. (Mamrutha et al., 2019).
The canopy greenness can be measured by an instrument known Normalized difference vegetation index (NDVI) sensor. Spectral reflectance based NDVI values (range between 0 to 1) are highly correlated with yield under temperature stress (Lopes and Reynolds 2012). Zero represents no greenness and one represents maximum greenness (Mamrutha et al. 2017). stay green habit can also be measured by other instruments such as canopy analyser (Licor) or porometer which measures leaf area index and green area index (GAI). Many other techniques like the digital photography of the canopy can also be taken from same height from the ground level and pictures can be analysed with different softwares (Adobe photoshop CS3 extended or later version) to assess the early ground cover (Mullan and Reynolds 2010).
Earliness Per se in Wheat
Earliness (earliness per se) in wheat is an adaptation strategy characterized by early heading followed by early maturity of genotypes under high temperature stress environments. Earliness helps genotypes to complete the essential plant growth stages such as seed setting and grain filling under favourable temperatures thereby avoiding the occurrence of terminal/late heat stress. Mondal et al. (2013) reported that the early heading entries performed well in areas affected from terminal heat stress as earliness helps them to escape high temperatures during grain filling stages. In addition to helping them escape the terminal heat stress, earliness also resulted in achieving >10% higher yield compared to the local check varieties under high temperature stress environments. High grain filling rate in early maturing gentotypes was also reported to be promoting heat stress tolerance in durum wheat (Al-Karaki 2012). Tewolde et al. (2006) reported that earliness helped cultivars adopt to high temperature stress as they had longer post-heading period resulting in longer grain filling duration. Therefore, earliness was also suggested as a key trait in breeding for high temperature stress tolerance (Joshi et al. 2007b).
Photosynthetic Efficiency
The differential rate of photosynthesis expressed as photosynthetic efficiency is again a very essential component trait contributing to tolerance under high temperature stress. Stable photosynthetic rates over longer duration in heat tolerant genotypes contributed to higher grain weight, higher harvest index under stress showing the positive association of rate of photosynthesis with yield parameters under heat (Al-Khatib and Paulsen 1990). Looking at the major role played by photosynthesis in determining yield under heat stress, it is also pertinent to have phenotyping techniques to help breeders to select for genotypes with higher photosynthetic efficiency. The relative photosynthetic efficiency can be indirectly predicted using the chlorophyll content index, however there are instruments available which can measure the photosynthesis exactly. Infra-red gas analyser (IRGA) is used to measure the photosynthesis on a real time basis when stress period is available or stress is imposed under experimental conditions. IRGA measures the amount of CO2 fixed during photosynthesis by estimating the difference in amount of CO2 pumped in and moving out of closed leaf chamber (Nataraja and Jacob 1999). Photosynthesis using IRGA should be measured at noon or prior or after noon to get maximum photosynthesis and to avoid error and it should be recorded at uniform positions in the leaf.
Chlorophyll Fluorescence (CFL), is also one of the traits used extensively to indirectly measure the photosynthetic efficiency of the genotypes. It is used, mainly as indicator of Photo system II (PSII) function. Applicability of CFL in screening wheat genotypes for heat tolerance and its use in accumulating genes favouring heat tolerance is well-known (Moffatt et al. 1990; Dash and Mohanty 2001). CFL meters are used to measure the CFL and they measure Fv/Fm ratio i.e. immediately after dark adaptation when leaf is exposed to light. The maximum amount of photons used for photochemistry is estimated as ratio of Fv/Fm where in Fv-Variable fluorescence and Fm-Maximal fluorescence. When photons fall on the leaf surface, it is being dissipated mainly into two processes i.e. photochemical quenching in the form of photosynthesis and non-photochemical quenching in the form of heat and fluorescence. When the plant is stressed, the PSII efficiency will be reduced and hence will get less value of the ratio compared to tolerant genotypes (Maxwell and Johnson 2000).
Cell Membrane Thermal Stability
Under high temperature conditions the cell membrane becomes weak and tends to rupture leading to leakage of electrolytes. Membrane thermal stability is being repeatedly used as a measure of electrolyte diffusion resulting from heat induced cell membrane leakage. Increased level of electrolyte leachates diffused from cells is measured here. Heat tolerant genotypes are identified by measuring electrical conductivity as an index to indirectly measure membrane thermal stability (Blum and Ebercon 1981; Saadalla et al. 1990; Blum 2018). Greater amount of electrical conductivity said to be indicating better heat-stress tolerance (Saadalla et al. 1990). Presence of high genetic heritability of membrane stability in wheat was seen to be an advantage for its use in breeding for heat tolerance (Fokar et al. 1998; Reynolds 2001). An electrical conductivity meter can be used to measure the membrane thermal stability with a reference standardizing solution of 0.005N KCl (Ibrahim and Quick 2001; Bala and Sikder 2017; ElBasyoni et al. 2017).
WHEAT IMPROVEMENT FOR HEAT TOLERANCE, INTEGRATING PLANT BREEDING AND OMICS TOOLS
Breeding for Heat Tolerance in Wheat
Heat tolerance of wheat can be improved by following conventional plant breeding approaches like identification of superior germplasm followed by hybridization and pedigree selection. Molecular marker assisted selection wherein conventional breeding clubbed with indirect selection of superior genotypes using linked DNA-based markers.
Germplasm Identification for Heat Tolerance
The differential performance of genotypes for yield traits under heat stress has been widely used as a selection criterion to identify tolerant wheat genotypes. The genetic variability available in the wheat gene pool in the form of wild progenitor species and synthetics can be utilized for continued improvement of wheat adaptation to abiotic stress. Sareen et al., (2012) identified four heat tolerant lines which from screening of synthetic wheat derivative germplasm. Wild relatives of wheat also present a rich source of diversity. Triticum dicoccoides and T. mono-coccum have been reported as potential sources of germplasm that can be used to enhance heat tolerance in bread wheat. Additionally, variable degrees of heat tolerance were observed in Aegilops speltoides, Ae. longissima Ae. taushii and Ae. Searsii (Ehdaie and Waines 1992; Waines 1994; Zaharieva et al. 2001; Pradhan et al. 2012; Awlachew et al. 2016). Awlachew et al., (2016) developed backcross introgressed lines (BILs) using heat-tolerant accession of Ae. speltoides pau3809 as one of the parent, these BILs showed improvement over heat tolerance component traits compared to recurrent parent. One of the most widely used wheat relatives is rye (Secale cereale L.), which is well-documented as a rich source of biotic and abiotic resistance/tolerance (Ehdaie and Waines 1992; Mondal et al. 2016).
Heat Tolerant Varieties by Conventional Selection
Heat tolerance is composed of several component traits and influenced by physiological parameters discussed in the above sections. Presence of greater genotypic variation among the wheat germplasm and involvement of mainly genetic control in the inheritance of these traits gives way for improving heat tolerance via plant breeding. The traits regulating yield and yield contributing parameters under high temperature stress are good candidates for targeted breeding for heat tolerance. For example, cooler canopy temperature is shown to be associated with deeper roots as well as higher yield under stress (Pinto et al. 2010; Lopes and Reynolds 2012).
The lower canopy temperature was found to be contributed by a cascade of plant processes such as increased stomatal conductance and the presence of genetic variation for this trait is an added advantage (Reynolds et al. 2007) and therefore selection for lower canopy temperature can lead to higher yield levels under stress. Additionally, selection of superior parents with better germination (Cargnin et al. 2006), early establishment and vigour can also help in developing heat tolerant cultivars (Richards and Lukacs 2002; Mullan and Reynolds 2010). Indian wheat program has released few varieties possessing moderate level of heat tolerance like Raj 3765, UP 2425, DBW16, HW2045, HD2643, DBW 14 and DBW 90 (ICAR-IIWBR 2018). There are a few more popular cultivars adapted to high temperature regions such as C 306 and Lok-1 which are characterised by tall stature and good early vigour with higher shoot and root biomass at seedling stage (Nagarajan and Rane 2000). Recently released cultivars viz., Zakia and Akasha have raised the yield levels in Sudan where the temperatures usually are above 40°C.
QTLs for Heat Tolerance
Heat tolerance is a quantitative trait, controlled by a number of genes/QTL. Over the last decade’s efforts have been made to reveal the genetic basis of heat tolerance. Grain filling duration and yield under heat stress are also important traits to breed for heat tolerance. QTL regions controlling grain filling duration and yield under heat stress have been identified (Pinto et al. 2010; Vijayalakshmi et al. 2010) which can be used to select heat tolerant progeny in breeding programmes through marker aided selection. Heat susceptibility index (HSI) was used as an indicator of yield stability and QTL controlling HSI were identified (Mason et al. 2010) which could be used as a selection criterion in breeding for heat tolerance.
Langdon chromosome substitution lines were firstly used in mapping heat tolerance genes and associated genes were found on chromosomes 3A, 3B, 4A, 4B, and 6A in 1991 (Sun and Quick 1991). Xu et al., (1996) later reported that chromosomes 3A, 3B and 3D were associated with heat tolerance in wheat cultivar (cv) Hope. Using chromosome substitution lines between Chinese Spring and Hope, chromosomes 2A, 3A, 2B, 3B, and 4B of Hope significantly enhanced heat tolerance (Chen et al. 2007). Two key QTL on chromosome 3B for canopy temperature and grain yield were detected by Bennett et al., (2012).
Marker Assisted Recurrent Selection for Improving Heat Stress Tolerance
All the favourable traits governing heat stress tolerance such as early vigour, chlorophyll content, canopy temperature, NDVI, chlorophyll fluorescence, leaf area, grain filling duration, thousand kernels weight, grain yield were accumulated into progeny lines through marker assisted recurrent selection. Following a meticulous inter-mating of F5 progenies carrying different alleles led to accumulation of 4-8 favourable QTLs per progeny and progenies were superior in performance to the parents (Jain et al. 2014).
OMICS APPROACHES FOR IDENTIFICATION OF HIGH TEMPERATURE STRESS TOLERANCE GENES
Recent advances in the next generation sequencing (NGS) based genomics technologies enable to generate huge high-quality genomic data in less time with cost-effective for plant research. Several genes have been identified and functionally validated for high temperature stress tolerance in plants (Yeh et al. 2012; Muthusamy et al. 2017; Lenka et al. 2019). Several groups have employed various omics tools to decipher the molecular mechanism of heat stress tolerance in wheat (Dwivedi et al. 2018; Ni et al. 2018). Transcriptomics, proteomics, and metabolomics tools were widely used to understand the function of the high-temperature stress responsive genes in wheat (Peng et al. 2006; Chauhan et al. 2011; Yang et al. 2011; de Leonardis et al. 2015; Ni et al. 2018). Combining omics tools with phenomics would help in understanding the molecular, biochemical and physiological adaptive response of the plants during high temperature stress conditions (Großkinsky et al. 2018).
Transcriptomics
Transcriptome profiling is an extensively used approach to systematically investigate and understand the gene expression during abiotic stress conditions (Katiyar et al. 2015; Wang et al. 2019). Several groups have deciphered the transcriptome profiles of different tissues of wheat in order to understand the role of genes/QTLs in growth and development (Guan et al. 2019). Databases like WheatExp (Pearce et al. 2015) and exVIP (Borrill et al. 2016; Ramírez-González et al. 2018) are developed to study the expression of a gene across different tissues at different developmental stages of wheat. Transcriptome studies revealed the altered expression of ~6000 genes with diverse functions under heat stress in wheat (Qin et al. 2008; Vishwakarma et al. 2018; Nandha et al. 2019; Rangan et al. 2020). Heat shock proteins, transcription factors and ribosomal proteins expression were highly altered under heat stress (Vishwakarma et al. 2018; Rangan et al. 2020). Interestingly, the expression of many genes was modulated by miRNAs during heat stress tolerance, suggesting their role in gene transcription (Ravichandran et al. 2019).
Proteomics
Proteomics approaches harness the robust high throughput technologies for identification and quantification of the proteins in the given tissues (Aslam et al. 2017). Proteome analysis helps to decode the structural and functions of different proteins including protein-protein interactions and post-translational modi-fications (Aslam et al. 2017). 309 proteins comprising molecular chaperones (HSP70s, HSP40s, and HSP20s, redox regulatory proteins, proteins involved in metabolic pathways displayed differential expression during heat stress (Wang et al. 2018). Zhang et al., (2018) studied the protein profiles of developing wheat kernel under heat stress and identified 78 differentially expressed proteins mainly involved in the signalling and abiotic stress response. These proteins are enriched in the 51 KEGG pathways including protein synthesis, starch and sucrose metabolism. Interestingly, protein profiling of wheat genotypes contrasting in thermotolerance showed the high expression of heat stable proteins and in thermotolerant genotype (Kumar et al. 2013). Generation of more high stress responsive proteome profiles from various tissues and development stages would help in deciphering the protein-protein interactions and post-translational modi-fications in wheat (Kumar et al. 2013; Muthusamy et al. 2017).
Metabolomics
High temperature stress severely affects the nutrients including vitamins, minerals, carbohydrates and fats in wheat grains. Thus, studying the metabolite profiles of thermo-tolerant and thermo-sensitive cultivars would help in identification polar molecules including water-soluble organic acids to non-polar lipids (Kumar et al. 2017). Metabolite profiling helped to decipher the role of chitosan, a natural linear polysaccharide, in increasing the growth of wheat seedlings through enhanced the metabolism of Carbon and Nitrogen assimilation (Zhang et al. 2017). Allwood et al., (2015) studied the metabolite profiles and showed the effect of nitrate deprivation on the composition of amino acids, organic acids and carbohydrates in wheat. Sixty-four metabolites display differential expression in the heat-stressed pollen of wheat (Thomason et al. 2018). Enzymes involved in glycolysis, TCA cycle, and lipid metabolism reduced in the wheat grains exposed to heat stress (Wang et al. 2018). Combing both metabolomic and protein profiling of wheat grains showed the insights on high temperature stress adaption through channelling of photosynthates towards synthesis of heat responsive proteins in wheat (Wang et al. 2018).
Ionomics
Ionomics harness the potential of the high-throughput technologies for profiling the elements in order to study the molecular mechanism underlying nutrient and trace element composition in plant tissues (Huang and Salt 2016). Fatiukha et al., (2020) studied the association of ionome comprising 11 elements viz., Aluminum, (Al), Calcium, (Ca), Copper, (Cu), Iron, (Fe), Potassium, (K), Magnesium, (Mg), Manganese, (Mn), Phosphorus, (P), Sulfur, (S) and Zinc, (Zn) linked to grain development in wheat. Interestingly, 617 QTL effects distributed among 105 QT loci were known to regulate the grain ionome in wheat. Three QTLs viz., 3A.3, 5B.4 and 7B.1 display strong effects with Ca concentration codes for the genes TRIDC3AG040050, a calcium channel protein, TRIDC5BG045550 (Ca-transporting ATPase), and TRIDC7BG001820, Ca+ exchanger protein, respectively, whereas 2B.6, 5A.1 and 7A.6 loci linked to Cu contains three Cu-transporting ATPase genes (TRIDC2BG062440, TRIDC5AG004320 and TRIDC7AG058420). The QTL 6A.3 region codes for three genes related to the K transporter family (TRIDC6AG029740, TRIDC6AG044480 and TRIDC6AG044820) (Fatiukha et al. 2020).
Functional Genomics
Decoding of the genomes of important plants including the model plants rice and Arabidopsis have helped in identification and characterisation of many agronomically important genes through functional genomics approach (Bevan 2005; Li et al. 2018b; Lenka et al. 2018; Muthusamy et al. 2019
