Impact of zinc-tolerant plant growth-promoting rhizobacteria on lentil grown in zinc-amended soil

Abstract - Though zinc is a plant nutrient at low levels, Zn ions can be highly phytotoxic at higher concentrations found in contaminated soils. Plant growth-promoting rhizobacteria can be used to decrease this toxicity. Indeed, in addition to their role in plant-growth promotion, rhizobacteria also reduce the toxicity of heavy metals. In turn, they can be effective for crops grown in metal-contaminated soils. Here, we isolated a zinc-tolerant plant growth-promoting rhizobacterium, Rhizobium species RL9, from a zinc-contaminated soil and assayed its plant growth-promoting activities in vitro. We found that the rhizobacterium strain RL9 tolerated zinc up to a concentration of 400 g mL^-1 on yeast extract mannitol agar medium. It produced 33 g mL^-1 of indole acetic acid in Luria Bertani broth at 100 g mL^-1 of tryptophan and was positive for siderophore, hydrogen cyanide and ammonia. Such phytohormones released by this strain could help in promoting the growth of legumes. We further tested the effect of rhizobacterium strain RL9 on lentils grown in zinc-amended soil. We found that when the rhizobacterium strain RL9 was added to soil contaminated with Zn at 4890 mg/kg, lentil dry matter increased by 150%, nodule numbers by 15%, nodule dry mass by 27%, leghaemogloblin by 30%, seed yield by 10% and grain protein by 8%, compared with uninoculated plants. We also found that the concentration of zinc was higher in uninoculated plant organs than in the inoculated counterpart. Our findings thus suggest that rhizobacterium strain RL9 could be exploited for bacteria-assisted reduction of zinc toxicity in zinc-contaminated soils due to its intrinsic abilities of expressing growth-promoting substances and reduction of the toxic effects of zinc.