Characterization and crop production efficiency of diazotrophic isolates from the rhizosphere of semi-arid tropical grasses of India

Abstract The search for diverse plant growth-promoting diazotrophic bacteria is gaining momentum as efforts are made to exploit them as bioinoculants for various crops. In particular, the use of strains with multiple plant growth promoting properties would help to increase crop productivity on a sustainable basis. This study investigated the effects of plant growth promoting potential of diazotrophs isolated from rhizosphere of semi-arid tropical grasses and evaluated their inoculation effects on the growth of rice plants under in vitro and in vivo conditions. The diazotrophic isolates from grass species were characterized for nitrogenase activity by acetylene reduction assay (ARA) and 16S rRNA gene sequencing. The ARA activity of the isolates ranged from 50.83 to 172.25 nmol ethylene/mg protein/h and the putative diazotrophs from rhizosphere of grass species were identified by nifH gene amplification. The 16S rRNA gene sequence analysis identified the isolates as belonging to class of alpha Proteobacteria and Firmicutes. Plant growth promoting traits of all the selected diazotrophic isolates were analysed and results revealed that the diazotrophs were found to produce phytohormone, siderophores, HCN, solubilized minerals such as P, K and Zn. Diazotrophs also produced enzyme such as ACC deaminase that can modulate plant growth and development. Based on the presence of multiple plant growth promoting traits, the isolates were selected for inoculation studies. In gnotobiotic experiment, inoculation of diazotrophic isolates significantly improves the growth of rice. In the field experiment, Serratia sp. (CB2) and K. pneumoniae (CR3) treated plots, grain yields were recorded more by 31 and 28%, respectively, over yield obtained using full doses of fertilizers. This trait of improving growth parameters and yield of rice indicates that the diazotrophs isolated from grass species can be utilized as bioinoculant for rice.