Occupancy of diverse bacterial species in mungbean nodules functioning as plant biostimulants under saline conditions

Abstract

Microorganisms hold the potential to revolutionize agriculture. The plant beneficial bacteria inhabit root zone and incentivize host by producing plant growth-promoting (PGP) substances. Their inherent PGP activities are severely affected under stressed environments such as salinity, drought, chilling, and high temperature. Salt-tolerant bacteria may serve as better biofertilizer candidates for the sustainable cultivation practices in salinity-affected soil. In this study, ten endophytic bacteria were isolated from the root nodules of mungbean and tested for salt-tolerance ability and PGP properties. Six efficient salt-tolerant bacteria exhibiting PGP potential were selected and taxonomically identified by 16S rRNA gene sequencing. TMN2, TMN4, TMN5, TMN6, TMN8 and TMN10 were identified as Enterobacter cloacae, Rhizobium leguminosarum, Klebsiella oxytoca, Rhizobium pusense, Pantoea agglomerans and Bradyrhizobium yuanmingense, respectively. This is the first study reporting a novel association of K. oxytoca with the mungbean root nodules. Under controlled conditions, the inoculation of P. agglomerans TMN8 and B. yuanmingense TMN10 demonstrated maximum potential to enhance plant growth parameters and biochemical contents of mungbean. Inoculation of TMN8 resulted in maximum enhancement of proline content (58%), glycine betaine (75%) and total soluble proteins (39%), compared to control. In field trials, the application of P. agglomerans TMN8 and B. yuanmingense TMN10 increased grain yield by 21% and 45% under normal soil conditions, whereas 35% and 36% under saline soil conditions, respectively. Irrefutably, the most efficient salt-tolerant bacteria, P. agglomerans TMN8 and B. yuanmingense TMN10, can be used as plant biostimulants for improved mungbean production in a sustainable manner.