Structural divergence of stress-inducible HMGR isoforms underpins abiotic stress tolerance in chickpea

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the committed step of the mevalonate pathway and is encoded by multigene families in plants. However, the functional specialization of individual isoforms in abiotic stress remains poorly understood in legumes. Here, we characterize three HMGR isoforms from chickpea (Cicer arietinum) and demonstrate that CaHMGR2 and CaHMGR3 function as stress-inducible isoforms with structural and regulatory divergence from the constitutively expressed CaHMGR1. Quantitative expression profiling revealed strong induction of CaHMGR2 and CaHMGR3 under drought, salinity, and cold stress. Promoter analysis identified enrichment of abiotic stress-responsive cis-elements exclusively in these isoforms. Structural comparison uncovered conserved substitutions within the dimerization motif, transmembrane regions, and residues proximal to the regulatory serine, suggesting altered regulatory properties. Subcellular localization further distinguished CaHMGR2 and CaHMGR3, which associate with both endoplasmic reticulum and nuclear envelope. Functional validation using overexpression lines demonstrated enhanced germination, root growth, reduced water loss, and improved recovery under stress. Metabolic profiling and KEGG pathway enrichment revealed selective metabolic re-programming associated with branched chain amino acid and fatty acid metabolism. Together, these findings establish functional specialization of HMGR isoforms and their role in abiotic stress resilience in chickpea.