Characterization of plant growth-promoting rhizobacterium Exiguobacterium NII-0906 for its growth promotion of cowpea (Vigna unguiculata)

Abstract

A phosphate solubilizing and antagonistic bacterial strain, isolated from a Western Ghat forest soil in Kerala province, India (designated as NII-0906), showed cold tolerance and grew from 10 to 37 degrees C (optimum temperature 30 degrees C). It was a Gram-positive, rod shaped, 0.8-1.6 mu m in size, and exhibited tolerance to a wide pH range (5-12; optimum 7.0) and salt concentration up to 7% (w/v). The isolate showed maximum similarity with Exiguobacterium marinum TF-80(T) based on 16S rRNA analysis. It solubilized tricalcium phosphate under in vitro conditions. The phosphate solubilization was estimated along a temperature range (5-40 degrees C), and maximum activity (84.7 mu g mL(-1) day(-1)) was recorded at 30 degrees C after 10 days of incubation. The phosphate solubilizing activity coincided with a concomitant decrease in pH of the medium. The isolate also exhibited antifungal activity against phytopathogenic fungi in Petri dish assays and produced siderophore and hydrogen cyanide. The strain's plant growth promotion properties were demonstrated through a cowpea-based bioassay under greenhouse conditions. The bacterial inoculation resulted in significant increment in plant root, stem and as well as in plant biomass. Further, scanning electron microscopic study revealed the root colonization in cowpea. These results could offer potential perspective for the strain to be used as plant growth-promoting rhizobacteria, which could be used as an inoculant for regional crops.