Nodulation Efficiency, Morphological, Biochemical Properties, and Genetic Diversity of Rhizobium Isolates from Black Gram (Vigna mungo L.)

Black gram (Vigna mungo L.) is an agronomically important pulse crop whose productivity is strongly influenced by its symbiotic association with Rhizobium and the efficiency of biological nitrogen fixation (BNF). This review synthesizes current knowledge on the nodulation efficiency, morphological and biochemical characteristics, and genetic diversity of Rhizobium isolates associated with black gram. Indigenous rhizobial strains exhibit considerable variation in colony morphology, stress tolerance, and plant growth-promoting traits such as indole-3-acetic acid production, phosphate solubilization, siderophore release, and ACC deaminase activity, all of which contribute to enhanced nodulation and plant performance. Molecular studies using markers such as 16S rRNA, recA, atpD, nodC, and nifH reveal substantial phylogenetic and functional diversity, reflecting strain-specific differences in host compatibility and nitrogen-fixing potential. Despite notable progress, significant knowledge gaps remain regarding region-specific rhizobial diversity, ecological adaptation, and the interactions between rhizobia and the broader soil microbiome. Future research integrating genomics, transcriptomics, and long-term field evaluations is critical for identifying elite indigenous strains that can be developed into effective, locally adapted biofertilizers. Harnessing the diversity and functional potential of Rhizobium offers a nature-based strategy to enhance black gram productivity, reduce reliance on synthetic nitrogen fertilizers, and support sustainable legume-based agricultural systems.