Isolation and characterization of symbiotically defective pyrimidine and amino acid auxotroph of Mesorhizobium ciceri in chickpea.

Transposon Tn5 induced, four Mesorhizobium ciceri auxotroph were isolated and characterized. Unlike its wild type parent (TL 620), all four mutants were found defective for amino acid and pyramiding biosynthesis. The auxotroph mutants were characterized and found TL130 as cytosine and uracil, TL 196 for guanine, cytocine, uracil and riboflavin, TL 141 as serine and TL 38 as argentine defective. Symbiotic characterization of these mutants revealed phenotypic deformities and deficiencies in biological nitrogen fixation. All the four auxotrophic mutants were characterized as nod+/fix+ nature with reduced nitrogenase activity of 42.2, 26.3 and 17.13% respectively as compared to the wild type which is further supported by sub cellular examination of the nodules section by TEM study.

Phenotypic and molecular characterization of indigenous rhizobia nodulating chickpea in India.

In a combined approach of phenotypic and genotypic characterization, 28 indigenous rhizobial isolates obtained from different chickpea growing regions in peninsular and northern India were analyzed for diversity. The field isolates were compared to two reference strains TAL620 and UPM-Ca142 representing M. ciceri and M. mediterraneum respectively. Phenotypic markers such as resistance to antibiotics, tolerance to salinity, temperature, pH, phosphate solubilization ability, growth rate and also symbiotic efficiency showed considerable diversity among rhizobial isolates. Their phenotypic patterns showed adaptations of rhizobial isolates to abiotic stresses such as heat and salinity. Two salt tolerant strains (1.5% NaCl by T1 and T4) with relatively high symbiotic efficiency and two P-solubilising strains (66.7 and 71 mg/ml by T2 and T5) were identified as potential bioinoculants. Molecular profiling by 16S ribosomal DNA Restriction Fragment Length Polymorphism (RFLP) revealed three clusters at 67% similarity level. Further, the isolates were differentiated at intraspecific level by 16S rRNA gene phylogeny. Results assigned all the chickpea rhizobial field isolates to belong to three different species of Mesorhizobium genus. 46% of the isolates grouped with Mesorhizobium loti and the rest were identified as M. ciceri and M. mediterraneum, the two species which have been formerly described as specific chickpea symbionts. This is the first report on characterization of chickpea nodulating rhizobia covering soils of both northern and peninsular India. The collection of isolates, diverse in terms of species and symbiotic effectiveness holds a vast pool of genetic material which can be effectively used to yield superior inoculant strains.

Characterization of intrinsic variability of Mesorhizobium ciceri isolates of cultivated fields.

A large number of putative rhizobial isolates were obtained from the root nodules of various chickpea cultivars growing in agricultural research fields. Of these, thirty were selected and characterized for traits, such as, generation time, intrinsic azide resistance and several symbiotic characters.

Pleiotropic effects of purine auxotrophy in Rhizobium meliloti on cell surface molecules.

Rhizobial purine auxotrophs have earlier been shown to be defective in symbiosis, though the exact reason for this failure is not clear. Using various dyes that specifically bind different cell surface molecules, we show that there are multiple changes in the cell surface molecules associated with different purine auxotrophs. Affected molecules in different purine auxotrophs that were tested include (i) acidic exopolysaccharides, (ii) cellulose fibrils, and (iii) beta (1–3) glucans. Our results show that the symbiotic deficiency of purine auxotrophs is likely to be a result of these associated changes on the cell surface.