Abstract

Rhizobia, Gram-negative bacilli are capable of fixing atmospheric nitrogen in symbiotic association with leguminous roots. These were classified earlier on the basis of morphology, physiology and biochemical characters. But with the dawn of new era of molecular approaches, various evolutionary chronometers have led to re-classification of these soil bacteria. Chickpea is the most important leguminous crop of arid and semi-arid regions of world. The rhizobia strains which nodulate chickpea exhibit genomic heterogeneity. The various molecular techniques such as DNA-DNA hybridization, RAPD-PCR, REP (repetitive extragenic palindrome) PCR, ERIC (enterobacterial repetitive intergeneric consensus) and BOX element or the finger printing of rhizobia genomes using ribosomal genes (16S, 23S or 16S-23S inter generic spacer - IGS), DAPD (direct amplified polymorphic DNA) are regularly used to identify, differentiate and characterize rhizobia infecting chickpea. Though, the ribosomal RNA is conserved but it exhibits variability in some of the domains and this makes ribosomal gene sequence (16S and 23S DNA) an optimum choice for deducing phylogenetic and evolutionary relationship among chickpea rhizobia. But 16S r RNA sequence is not sufficient enough to distinguish the strains of one species or even closely related species. The additional genes like nod genes, nif genes, glutamine synthetase and other housekeeping genes have also been used in combination to 16S rRNA to study the evolutionary relationship among these rhizobia. This has led to proper taxonomy and systematics of the rhizobia group, with the description of new genera up to the level of species and sub species. The chickpea rhizobia have been grouped under genus Mesorhizobium with several species such as M. ciceri, M. mediterraneum, M amorphae, M. tianshanense, M. loti, M. huakuii, M. opportunistum and M. muleiense.

Authors and Affiliations