**7. Genome-wide association studies (GWAS)**

The identification of candidate gene(s)/QTLs for complex characteristics is significantly assisted by GWAS. GWAS methods have been used to find small and minor genetic changes linked to a variety of biotic and abiotic stresses as well as crop agronomic traits [88–90]. GWAS analyzes the entire genome for QTLs and requires for genome-wide markers. Through the GWAS method, different QTLs were also discovered for several abiotic stress-tolerant genes. The genetic resources and gene(s)/ QTLs for morphological, quality, and biotic and abiotic stressors have recently been enriched in pigeonpea [89, 90]. Through MAS, the yield traits as well as the detected QTLs/gene(s), such as pod borer and Phytophthora stem blight resistance genes, have been successfully introgressed into the cultivated varieties of pigeonpea [91]. To speed up genetic gain, two high-density Affymetrix Axiom genotyping chips have recently been created. 103 lines were studied using a 56 K *Cajanus* SNP chip to examine genetic diversity. The SNPs lack haplotype information and are distributed at random [40].

A 62 K genic-SNP chip called "CcSNPnks" has recently been created using the resequencing of 45 different genotypes. Additionally, the 'CcSNPnks' chip array will be helpful for gene-based association studies and high-resolution mapping of yield-related QTLs. With the use of these high throughput genotyping arrays, many samples may be genotyped quickly, and the analysis of the primary genotyping data is also relatively simple [92]. In pigeonpea from diverse sets of wild and cultivated genetic backgrounds, this led to the discovery of the most effective genomic loci (genes) associated with abiotic and biotic stress related genes [4, 83].

### **8. Transcriptomics profiling**

Transcriptomic tools scan, provide gene-expression and protein expression levels in real time, making them important in plant improvement in this advanced era. The development of next-generation sequencing technology has made it possible and reliable to sequence plant species [93]. Furthermore, transcriptomics technologies help to understand gene and protein levels. According to the findings of several research, not all genes are turned on or off at the same time; hence, the metabolism adopts a complex phenotype that cannot be determined by genotype [38, 94]. As of December *Perspective Chapter: An Insight into Abiotic Stresses in Pigeonpea – Effects and Tolerance DOI: http://dx.doi.org/10.5772/intechopen.110368*

26th, 2014, 25,577 ESTs for pigeonpea were discovered at NCBI (National Centre for Biotechnology Information). CcTAv1 transcriptome assembly contigs were created with 1, 27, 754 TUS (Tentative Unique Sequences) and were then upgraded with Illumina GAIIX by 454 platforms to construct CcTav2 transcriptome assembly contigs with four data groups and 21, 434 transcriptome assembly contigs (TAC's) [95–97]. The expression of WRKY genes in two different genotypes was examined in leaf and root tissue in response to drought and salt stress [98].

Furthermore, Comparative transcriptome analysis and biochemical tests showed that *Cajanus* species' responses to heat stress varied widely. The most thermotolerant of the examined species was *C. scarabaeoides*, followed by *C. cajanifolius*, *C. cajan*, and *C. acutifolius*. When under heat stress, a significant number of genes have been studied that undergo alternative splicing in a species-specific pattern. Chlorophyll content, electrolyte leakage assay, histochemical assay, and gene expression profiling analysis all demonstrated that *C. scarabaeoides* possesses adaptive traits for heat stress tolerance [61]. It would help breeders find promising candidate genes and appropriate features for creating and boosting legume crop productivity under abiotic challenges [24]. In depth analysis of the transcriptomics would be definitely fascinating for better perception of pigeonpea.

### **9. Conclusion and future prospects**

Food production will face severe hurdles in the near future due to a gradual drop in soil water and an increase in temperature. Drought and high temperature tolerant crops, such as pigeonpea, may be a viable option for ensuring food security. Efforts should be made to define the genetic resources of pigeonpea at both the phenotypic and molecular levels in order to uncover genetic variations that can be leveraged to generate improved cultivars. To achieve a consistent rise in pigeonpea productivity, existing breeding efficiency must be improved.

In order to focus on trait associated marker study, new methodologies such as transcriptome assembly, MAGIC, and NAM populations were developed. It is feasible to introduce genes from wild species to commercially farmed types using cutting-edge advanced backcross-QTL techniques. In the future, efforts should be made to concentrate on phenotypic approaches that are affordable, high throughput, and effective. Innovative breeding designs that are supported by relevant genomic technology will be critical in modernizing breeding programmes. Current genetic advances in pigeonpea for resistance to abiotic stress will also considerably benefit hybrid breeding. Furthermore, intense attempts are being made using in vitro techniques to find complicated abiotic stress features, foreign gene introgression facilitated by embryo rescue, and quick fixation of stress tolerant recombinants via doubled haploid breeding. These procedures, together with more efficient screening methods, demand special attention in the coming days to make pigeonpea farming an attractive, profitable, and feasible option for the world's pulse farmers.

### **Acknowledgements**

NS acknowledges the Department of Science and Technology; Government of India for the DST INSPIRE Faculty Award (DST/INSPIRE/04/2018/003674).
