**4.2 Genetic resources for thermo tolerance**

Inter-mating among closely related individuals for improvement of economic traits resulted in decline of genetic variability in a crop species [33]. Characterization of gene pool including land races and wild relatives would offer several tolerant genes for abiotic tolerance. Extensive efforts were made in screening of heat tolerant genotypes which can be directly introduced as a cultivar or utilized to introgress gene into new genetic background [34]. Thermo-tolerant lines were successfully isolated from wild gene pool in wheat [35]. High magnitude of variation was observed in wild progenitor "*Aegilops tauschii*" of wheat for cell viability and membrane stability [36]. Similarly, a heat tolerant source for reproductive stage was identified in *A. geniculata* and *A. speltoides* Tausch which would pave way in development of thermo-tolerant hexaploid wheat cultivars in near future [37]. A higher growth rate and improved photosynthetic efficiency was observed in wild relative "*Oryza meridionalis*" of rice at high temperature [38]. Indirect selection on pollen viability led to identification of thermo-tolerant accessions in soybean (DG 5630RR) [39], chickpea (ICC15614 & ICC1205) [40], maize (AZ100) [41], and several other crop species. Direct selection based on yield under target environment (heat stress) resulted in development of tolerant lines in many tropical grain legumes. Four tolerant genotypes/accessions *viz*., SRC-1-12- 1-48, SRC-1-12-1-182, 98012-3-1-2-1 and 98020-3-1-7-2 were isolated in common bean by employing stress tolerant indices [42]. Nine thermo-tolerant wild accessions were delineated in USDA upland cotton germplasm by employing chlorophyll fluorescence technique [43].

### **4.3 Conventional breeding approaches**

Evolving thermo-tolerance through conventional breeding approach proves promising in many crop species. Breeding for early maturing genotype in broccoli had improved head quality by avoiding heat stress at flowering stage [44]. In general, breeding programmes are carried out in hotter regions which promote selection of thermo-tolerant traits. Physiological based trait breeding was practiced at International Maize and Wheat Improvement Center (CIMMYT) for development *Breeding Mechanisms for High Temperature Tolerance in Crop Plants DOI: http://dx.doi.org/10.5772/intechopen.94693*

of heat tolerant cultivars in wheat. The parental genotypes were characterized through various crossing schemes and appropriate breeding programme was framed for improvement of thermo related traits [45]. A wild ancestor "*T. tauschii*" was utilized as a gene donor for achieving increased grain size and filling percent under high temperature through recurrent selection [46]. Similarly, three cycles of recurrent selection had led to improved yield under heat stress regimes in potato [47]. Thermo tolerant alleles were introgressed into heat sensitive cultivar "Paymaster 404" from a donor accession "7456" of *G*. *barbadense* through backcross breeding [48]. A significant improvement in yield was realized under heat stress environment by adoption of gametic selection in maize [41]. A deep rooted cultivar "Nagina 22 (N22)" of aus rice exhibited high pollen viability and spikelet fertility (64–86%) under heat stress [49]. The thermo-tolerance of N22 was successfully introgressed into Xieqingzao B line through backcross method [50]. Dissecting out the genetic and physiological basis of thermo-tolerance will hasten up the development of resilient cultivars suited to hotter regions.

#### **4.4 Advanced breeding approaches for thermo tolerance**

The genetic basis of thermo-tolerance is not clearly understood because of complex trait inheritance. Advances in molecular approaches such as DNA marker identification and genotyping assay had paved way in determination of several QTL's associated with high temperature tolerance [51]. In wheat, QTL's were identified for canopy temperature, and chlorophyll fluorescence imparting tolerance to heat stress [52]. A major QTL "Htg 6.1" in lettuce was involved in enhancement of seed germination capacity at high temperature [53]. A recessive QTL for increased spikelet fertility under high temperature was dissected out in rice at chromosome 4. The identified QTL were found in several populations of heat tolerant rice cultivars [54]. Six QTL's were involved to enhance fruit set at high temperature in tomato [55]. Five thermo tolerant QTL's were identified in *Brassica campestris* by employing random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) markers [56]. In maize, eleven major QTL's for increased pollen germination and pollen tube growth under high temperature was mapped using restriction fragment length polymorphism (RFLP) markers [57]. Identification of candidate QTL's would pave way in precise introgression of heat tolerant genes into superior cultivars through marker assisted breeding approach.

The closely associated markers with targeted QTL will hasten the recovery of superior genotypes with heat tolerant traits in a population. A marker assisted breeding approach was employed in rice to derive heat tolerant line with superior grain quality. Two flanking markers *viz*., ktIndel001 and RFT1 enclosing 1.5 Mb chromosomal region was transferred from tolerant cultivar "Kokoromachi" to Tohoku 168. Significant improvement in grain quality under high temperature was observed in the derived NIL's compared to susceptible cultivar "Tohoku 168" [58]. Fourteen SSR markers linked to heat susceptibility index of grain filling per cent and single kernel weight was identified in bread wheat which was employed in marker assisted selection (MAS) to screen genotypes for thermo tolerance [59]. Utilization of MAS approach for heat tolerance remains less efficient because of high gene x environment and epistatic interactions. The low breeding efficiency can be resolved by genomic selection (GS) approach which involves wide number of molecular markers exhibiting high genome coverage. High genetic gain is realized in GS approach due to close association between predicted and true breeding value over generations [60].

At present, transgenic approach also proves to be desirable tool for designing thermo tolerant lines *via* introgression of genes from diverse gene pools [61]. The genetic transformation was focused primarily on transcription factors, induction of heat shock proteins, molecular chaperones, osmolytes, antioxidant components and growth regulators [62]. Heat shock proteins play a primary role in imparting thermo tolerance in crop species. It is functionally associated with diverse group of molecular chaperones that is involved in restoration of degraded proteins to their native structure under high temperature. Induction of heat shock proteins through genetic manipulation was achieved in *arabidopsis* [63], maize [64], rice [65], soybean [66], and pepper [67]. The DREB gene family was also reported to impart heat tolerant response in many crop species. Over expression of ZmDREB2A in maize [68] and GmDREB2A in soybean [69] was associated with increased survival and adaptation under high temperature. Transgenic techniques were employed to alter membrane lipid properties for thermo-tolerance in crop species. High proportion of saturated fatty acid in membrane had increased tolerance under heat stress. Suppression of omega-3 fatty acid desaturase gene in chloroplast had reduced the accumulation of trieonic fatty acid in transgenic tobacco [70] and tomato [71] leading to thermotolerance. A significant accumulation of glycine betaine (osmolyte) was achieved in *arabidopsis* through transfer of "cod gene" from *Arthrobacter globiformis* [72]. High proportion of glycine betaine protects the PSII component by inhibiting the ROS activities under heat stress. Implementation of transgenic approaches in other crop species will accelerate the development of resilient genotypes suited to high temperature regimes.
