Among legumes, chickpea, pigeonpea, and peanut are the primary crop legumes of the semi-arid tropics. Drought is the severe abiotic stress affecting the global production of these legume crops. The NAM, ATAF1/2, and CUC2 (NAC) family genes are involved in a majority of abiotic stresses, especially drought. Due to the large size of this plant-specific NAC gene family and its relevance in enhancing stress tolerance, the extensive research on the use of NAC proteins in developing superior cultivars of these legume crops for drought tolerance still remains interesting. Keeping this in view, we have identified 72, 96, and 166 NAC proteins from the genome database of chickpea, pigeonpea and peanut, respectively, using phylogenetic analysis. The identified proteins were grouped into 10 clusters in chickpea and pigeonpea, while 12 clusters in peanut. Phylogenetic analysis with well-known stress-related Arabidopsis, Oryza, Medicago, and Glycine max NACs enabled us to predict 22, 31, and 33 putative stress- responsive NAC genes in chickpea, pigeonpea, and peanut. Furthermore, on exploring available transcriptome data of these putative stress-related NACs at various developmental stages revealed differential expression patterns in different tissues. Also, real-time quantitative PCR was carried out on drought-stressed and unstressed root tissues from two contrasting cultivars of these legumes. This was performed to access the gene expression of selected drought-responsive Ca_NAC (14), CcL_NAC (15), and Ah_NAC (14) genes. Upon analysis, it was observed that majority of the NAC genes examined (12/14) in chickpea showed up-regulation with a maximum of 4.3 folds (Ca_18090) in ICC 4958 (tolerant) genotype. In pigeonpea, genes such as, CcL_26125, CcL_43030, CcL_43785, CcL_43786, CcL_22429, and CcL_22430 were up- regulated in ICPL 227 (more drought-tolerant) genotype with a maximum of 10 folds (CcL_43030). The relative fold expression was up-regulated in 10 genes for CSMG 84-1 genotype with a maximum of 1.6 folds (Ah_ann1.MI72XM.2) in peanut. Hence, based on expression analysis, it was concluded that 10 of 14 Ca_NACs (Ca_06899, Ca_18090, Ca_22941, Ca_04337, Ca_04069, Ca_04233, Ca_12660, Ca_16379, Ca_16946, and Ca_21186) ; 6 of 15 CcL_NACs (CcL_26125, CcL_43030, CcL_43785, CcL_43786, CcL_22429, and CcL_22430); 5 of 14 Ah_NACs (Ah_ann1.G1V3KR.2, Ah_ann1.MI72XM.2, Ah_ann1.V0X4SV.1,2 Ah_ann1.FU1JML.2, and Ah_ann1.8AKD3R.1) genes examined may be considered as potential candidates for developing drought-tolerant legume varieties with improved productivity.
Keywords: Legumes, Chickpea, Pigeonpea, Peanut, Drought-responsive, NAC, Phylogenetic, cis-acting regulatory elements (CARE)