Effectiveness of halo-tolerant, auxin producing Pseudomonas and Rhizobium strains to improve osmotic stress tolerance in mung bean (Vigna radiata L.)

Halo-tolerant, auxin producing bacteria could be used to induce salt tolerance in plants. A number of Rhizobium and auxin producing rhizobacterial strains were assessed for their ability to tolerate salt stress by conducting osmoadaptation assay. The selected strains were further screened for their ability to induce osmotic stress tolerance in mung bean seedlings under salt-stressed axenic conditions in growth pouch/jar trials. Three most effective strains of Rhizobium and Pseudomonas containing ACC-deaminase were evaluated in combination, for their ability to induce osmotic stress tolerance in mung bean at original, 4, and 6 dS m-1 under axenic conditions. Results showed that sole inoculation of Rhizobium and Pseudomonas strains improved the total dry matter up to 1.4, and 1.9 fold, respectively, while the increase in salt tolerance index was improved up to 1.3 and 2.0 fold by the Rhizobium and Pseudomonas strains, respectively. However, up to 2.2 fold increase in total dry matter and salt tolerance index was observed due to combined inoculation of Rhizobium and Pseudomonas strains. So, combined application of Rhizobium and Pseudomonas strains could be explored as an effective strategy to induce osmotic stress tolerance in mung bean.

Genetic diversity of Rhizobia isolates from Amazon soils using cowpea (Vigna unguiculata) as trap plant

The aim of this work was to characterize rhizobia isolated from the root nodules of cowpea (Vigna unguiculata) plants cultivated in Amazon soils samples by means of ARDRA (Amplified rDNA Restriction Analysis) and sequencing analysis, to know their phylogenetic relationships. The 16S rRNA gene of rhizobia was amplified by PCR (polymerase chain reaction) using universal primers Y1 and Y3. The amplification products were analyzed by the restriction enzymes HinfI, MspI and DdeI and also sequenced with Y1, Y3 and six intermediate primers. The clustering analysis based on ARDRA profiles separated the Amazon isolates in three subgroups, which formed a group apart from the reference isolates of Bradyrhizobium japonicum and Bradyrhizobium elkanii. The clustering analysis of 16S rRNA gene sequences showed that the fast-growing isolates had similarity with Enterobacter, Rhizobium, Klebsiella and Bradyrhizobium and all the slow-growing clustered close to Bradyrhizobium.

Survival, nodulation and N2 fixation ability of root nodule bacteria under different nutritional regimes.

Eleven strains of Rhizobium and five strains of Bradyrhizobium were examined for their viability as well nodulation and nitrogen fixation ability after storage under different conditions for two years. The storage conditions comprised lateritic soil, lateritic soil plus 1% mannitol, lateritic soil plus 0.1% yeast extract, lateritic soil plus 1% mannitol and 0.1% yeast extract, organic soil, organic soil plus 1% mannitol, organic soil plus 0.1% yeast extract, organic soil plus 1% mannitol and 0.1% yeast extract, and sterile distilled water. All the slow growing strains showed better viability than the fast growing strains in any of these conditions. The survived strains maintained their nodulation ability about 50-60% after one year and 40-50% after two years of preservation as compared to control, but the nodulation ability in sterile distilled water was very poor. Acetylene reduction activity in the nodules was found to be 70-90% and 50-70% after 12 and 24 months of preservation, respectively. The strains retained their phenotypic characters like antibiotic resistance and salt tolerance up to their highest survivability in respective nutritional condition.

Growth behaviour and bioproduction of indole acetic acid by a Rhizobium sp. isolated from root nodules of a leguminous tree Dalbergia lanceolaria.

The Rhizobium sp. isolated from healthy and mature root nodules of a leguminous tree, Dalbergia lanceolaria Linn. f., preferred mannitol and KNO3 for growth as carbon and nitrogen sources, respectively. The bacterium produced a high amount (22.3 microg/ml) of indole acetic acid (IAA) from L-tryptophan supplemented basal medium. Growth and IAA production started simultaneously. IAA production was maximum at 20 hr when the bacteria reached the stationary phase of growth. Cultural requirements were optimized for maximum growth and IAA production. The IAA production by the Rhizobium sp. was increased by 270.8% over control when the medium was supplemented with mannitol (1%,w/v), SDS (1 microg/ml), L-asparagine (0.02%,w/v) and biotin (1 microg/ml) in addition to L-tryptophan (2.5 mg/ml). The possible role of IAA production in the symbiosis is discussed.

Exopolysaccharide production by some rhizobium strain in culture.

Eight Rhizobium strains were screened for exopolysaccharide production during growth in yeast extract mannitol medium. A strain of Rhizobium loti produced more than 250 mg of exopolysaccharide/100 ml of culture during the stationary phase of growth. Exopolysaccharide production was favoured by maltose and mannitol but N2 supplementation could not affect the production siginificantly. Preliminary studies indicate that the polymer contains 16% protein and 73.5% carbohydrate being composed of glucose, galactose and glucuronic acid.