The soil harbors billions of Microbes supporting the growth of several plants, resulting in the constant symbiotic or non-symbiotic interactions between the plants and these microorganisms known as plant-microbe interactions. Plants need nutrients (available or available but inaccessible) in the soil to grow while microbes need shelter and nutrients supplied by plants and also promote plant-growth. Microbes are constantly trading with plants either buying or selling nutrients (the nutrients are considered the main currencies for trading and the product being bought in the soil rhizosphere). Plants including legumes inoculated with Rhizobia and microbes in experimental studies show that they are able to solubilizing phosphate and metals, fixing nitrogen fixers, producing IAA, cytokinins, Gibberellins and Ethylene. Microbiomes are therefore essential for plant growth and health as they govern most soil functions affecting plant-growth. For higher crop-yields and increased soil- fertility using ecofriendly manner, researches focused on the new concepts of exotic biomolecules, hormones, enzymes and metabolites which create a suitable environment for the interaction between plant and microbes using molecular and biotechnological approaches are necessary to increase our knowledge of rhizosphere biology and to achieve an integrated management of soil microbial populations that can ultimately enhance the health of plants.

Heavy-metal pollution from many industrial processes is a major threat to human health and the environment resulting in loss of farming and grazing land. Many bacterial strains have been reported to tolerate metals but there is a dearth of research on the metal-tolerating ability of rhizobia strains, hence the need for this study to screen selected Rhizobial strains for their ability to tolerate varying concentrations of selected heavy metals. Ten Rhizobia strains including Bradhyrhizobium japonicum strains FA3, UDSA136, USDA 9032, USDA110, RANI 22, USDA 4675, RAUG and Bradhyrhizobium sp. strains B574, R25B and USDA 3541 obtained from the culture collection of the International Institute of Tropical Agriculture (IITA) were used for this study. They were screened for their ability to tolerate varying concentrations (10-150 μg/mL) of six selected metals (Copper, Cobalt, Cadmium, Lead, Zinc and Iron) on metal-incorporated Congo-red medium. FA3 (Bradhyrhizobium japonicum) showed the highest tolerance to iron (100 μg/mL), while FA3, USDA110 and USDA 4675 showed highest resistance to zinc with Minimum Inhibitory Concentration (MIC) of 150 μg/mL. Strains USDA 3451, USDA 4675, B547 were unable to grow on the cobalt-incorporated medium, while strains RAUG 1, USDA 4675, USDA136 and R25B had the highest MIC of 150 μg/mL for lead. Copper was the most toxic to the Rhizobial strains as the MIC recorded was between 10-20 μg/mL, while all the strains were able to tolerate 150 μg/mL concentration of Cadmium. Rhizobial strains could find a use in the bioremediation and recovery of soils contaminated with heavy metals as shown by their potentials to tolerate certain degree of metal concentration in this study.

Bambara groundnut (BG) (Vigna subterranean L. verdc) is a highly nutritious and economically important leguminous crop of African origin that forms symbiotic relationships nitrogen fixing rhizobia, but it has a low yield. Currently, there is paucity of information on effective rhizobium inoculant, (to improve the yield), for BG which are cheaper and safer alternatives to chemical fertilizers. Indigenous rhizobia strains were trapped from 54 farm soils collected at three Nigerian states (Niger, Kaduna and Kano). Six selected rhizobium strains (Bradhyrhizobium spp) were identified using 16S rDNA sequencing, applied as inoculant in field experiment using completely randomized block design and compared with USDA110 strain (Bradhyrhizobim japonicum), urea fertilizer and un-inoculated plants on 2 selected BG varieties (TVSU1248 and TVSU 631). Nutrient utilization and nitrogen fixation in plants were determined at 6 weeks while leaf chlorophyll was determined forth-nightly. Seed yield was determined at maturity. The strains increased the %nitrogen (1.33±0.10 - ≥1.37±0.05), and %phosphorous (0.48±0.02 - ≥0.51±0.02), %nitrogen fixed (21.07±2.38 - ≥61.30±6.13), nodulation (72.50±21.075 - 154.00±23.79) and leaf chlorophyll were ≥49.74±1.47mg/L, ≥51.82±1.86mg/L and ≥49.65±1.56mg/L at 2nd, 4th and 6th week after planting respectively and the yield of BG (from average of 398.6 Kgha-1 to 1454.725 Kgha-1). Seed yield was highest (1869.85±273.68 Kgha-1) in BG variety TVSU1248 using BN5 strain. Indigenous Bambara-symbiotic strains significantly increased the seed yields, and plant nutrient, showing their potential as use for inoculant to improve the yield of Bambara groundnut.