Deciphering the Potential of Rhizobium pusense MB-17a, a Plant Growth-Promoting Root Endophyte, and Functional Annotation of the Genes Involved in the Metabolic Pathway.

Twinkle Chaudhary,Rajesh Gera,Pratyoosh Shukla

doi:10.3389/fbioe.2020.617034

Abstract

Plant growth-promoting rhizobacteria (PGPR) are root endophytic bacteria used for growth promotion, and they have broader applications in enhancing specific crop yield as a whole. In the present study, we have explored the potential of Rhizobium pusense MB-17a as an endophytic bacterium isolated from the roots of the mung bean (Vigna radiata) plant. Furthermore, this bacterium was sequenced and assembled to reveal its genomic potential associated with plant growth-promoting traits. Interestingly, the root endophyte R. pusense MB-17a showed all essential PGPR traits which were determined by biochemical and PGPR tests. It was noted that this root endophytic bacterium significantly produced siderophores, indole acetic acid (IAA), ammonia, and ACC deaminase and efficiently solubilized phosphate. The maximum IAA and ammonia produced were observed to be 110.5 and 81 μg/ml, respectively. Moreover, the PGPR potential of this endophytic bacterium was also confirmed by a pot experiment for mung bean (V. radiata), whose results show a substantial increase in the plant's fresh weight by 76.1% and dry weight by 76.5% on the 60th day after inoculation of R. pusense MB-17a. Also, there is a significant enhancement in the nodule number by 66.1% and nodule fresh weight by 162% at 45th day after inoculation with 100% field capacity after the inoculation of R. pusense MB-17a. Besides this, the functional genomic annotation of R. pusense MB-17a determined the presence of different proteins and transporters that are responsible for its stress tolerance and its plant growth-promoting properties. It was concluded that the unique presence of genes like rpoH, otsAB, and clpB enhances the symbiosis process during adverse conditions in this endophyte. Through Rapid Annotation using Subsystem Technology (RAST) analysis, the key genes involved in the production of siderophores, volatile compounds, indoles, nitrogenases, and amino acids were also predicted. In conclusion, the strain described in this study gives a novel idea of using such type of endophytes for improving plant growth-promoting traits under different stress conditions for sustainable agriculture.

Highlights

The soil bacteria Rhizobia of the family Rhizobiaceae cover an array of different bacterial genera, including Sinorhizobium, Rhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium, and Allorhizobium (Manasa et al, 2017)
Qualitative Screening for PSB The bacterial isolate was screened for phosphate solubilization activity by a qualitative method on a Pikovskaya (1948) agar medium plate containing Ca3 (PO4)2
The halo zone formed around the colony was determined by the phosphate solubilization index (PSI)

Summary

Introduction

The soil bacteria Rhizobia of the family Rhizobiaceae cover an array of different bacterial genera, including Sinorhizobium, Rhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium, and Allorhizobium (Manasa et al, 2017). Rhizobia is employed as plant growth-promoting rhizobacteria (PGPR) to increase plant growth through phosphate solubilization, fixation of nitrogen, development of ACC deaminase, siderophore development, and production of indole-3-acetic acid (IAA) (Pravin et al, 2016). This symbiosis relationship between the rhizobium and leguminous crop is essential for better productivity in agricultural systems. Bradyrhizobium japonicum is reported as the best one for siderophore production and Rhizobium leguminosarum trifolii for phosphate solubilization and IAA production on a large scale (Garcia-Fraile et al, 2012). The isolation, selection, screening, and exploitation of efficient endophytic bacteria in agricultural practices have immense significance in maintaining the sustainability of agroecosystems

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Results

Conclusion