Abstract

The compatibility between rhizobia and legumes for nitrogen-fixing nodules and the stages of root hair curling, formation of infection thread, and nodulation initiation have been vitally studied, but the factors for the sustainable root surface colonization and efficient symbiosis within chickpea and rhizobia have been poorly investigated. Hence, we aimed to analyze phenotypic properties and phylogenetic relationships of root-nodule bacteria associated with chickpea (Cicer arietinum) in the north-west Indo Gangetic Plains (NW-IGP) region of Uttar Pradesh, India. In this study, 54 isolates were recovered from five agricultural locations. Strains exhibited high exopolysaccharide production and were capable of survival at 15–42 °C. Assays for phosphate solubilization, catalase, oxidase, Indole acetic acid (IAA) production, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity revealed that all the tested isolates possessed plant growth-promoting potential. Metabolic profiling using Biolog plates indicated that patterns of substrate utilization differed considerably among isolates. A biofilm formation assay showed that isolates displayed a nearly four-fold range in their capacity for biofilm development. Inoculation experiments indicated that all isolates formed nodules on chickpea, but they exhibited more than a two-fold range in symbiotic efficiency. No nodules were observed on four other legumes (Phaseolus vulgaris, Pisum sativum, Lens culinaris, and Vigna mungo). Concatenated sequences from six loci (gap, edD, glnD, gnD, rpoB, and nodC) supported the assignment of all isolates to the species Mesorhizobium ciceri, with strain M. ciceri Ca181 as their closest relative.

Highlights

  • Plant-associated bacteria have been speculated to be the organisms capable of promoting growth and/or suppressing diseases when present in the rhizosphere and as endophytes within healthy plant tissues [1,2]

  • Biofilm provides the space for growth, a protective degree of homeostasis, and allows the bacteria to overcome biotic and abiotic stresses—this is done by a complex extrapolymeric substance (EPS) matrix sheath [7,8,9]

  • Most of the isolates were moderately fast growing with a generation time (GT) between 4 and 6 h, CPN3 and CPN16 were fast growing with GT lesser than 3 h

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Summary

Introduction

Plant-associated bacteria have been speculated to be the organisms capable of promoting growth and/or suppressing diseases when present in the rhizosphere and as endophytes within healthy plant tissues [1,2]. Bacteria have typically been associated with or adhere around the legumes plant roots and act as a plant growth promoter by several secretory or substrate capabilities, such as phosphate, ACC deaminase, siderophore, IAA, catalase, oxidase, and NH3 production [3,4]. They act as endophytic or nodule-forming bacteria (found within the tissues of the plant) as well as saprophytic bacteria (found free-living in the soil). The first criterion for a Rhizobium strain to be used in legume inocula is that it must be highly effective when fixing nitrogen [15]

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