Large-scale screening of rhizobacteria to enhance the chickpea-Mesorhizobium symbiosis using a plant-based strategy

Abstract

Rhizobacteria able to express multiple plant growth-promoting (PGP) traits has been frequently failed to enhance the chickpea-Mesorhizobium symbiosis possibly because a relatively large amount of organic acids exudated by chickpea root to form an acid rhizosphere reduced the survival and colonisation of plant growth-promoting rhizobacteria (PGPR). Therefore, this study was initiated to screen rhizobacteria that would enhance the chickpea-Mesorhizobium symbiosis using a plant-basedstrategyA total of 743 Bacillus-, Burkholderia- and Pseudomonas-like bacteria were isolated from 74 agricultural soil samples collected across Australia. Only 167 isolates were selected, based upon their individual performance in a seedling growth bioassay, to test their effect on nodulation and growth of chickpea in sterile growth pouches. Also, their ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase and indole acetic acid (IAA) was investigated in relation to their mode of action. The diversity of the 167 isolates was investigated using 16S rDNA sequencing. Of 743 isolates, 697 (83%) were able to produce IAA in the presence of L-tryptophan. ACC deaminase production was detected in 57 isolates (7.7%). Commercial inoculant strain Mesorhizobium ciceri CC1192 produced 7 μg mL−1 IAA and lacked ACC deaminase activity. In further investigation, the promotion of seedling root elongation by rhizobacteria predicted their effect on nodulation and growth of chickpea after dual inoculation in aseptic conditions. The effect of rhizobacteria on seedling root growth as well as shoot growth and nodulation was associated with the capacity of the isolates to produce IAA and ACC deaminase. These isolates were superior in plant growth promotion than those PGPR unable to produce PGP traits. The synthesis of IAA along with ACC deaminase activity by rhizobacteria gave an added advantage in promoting the ability of rhizobia to form an efficient chickpea-Mesorhizobium symbiosis as measured by biomass production and nodulation. Most of these rhizobacteria belonged to the genus Burkholderia. The use of a plant-based first-stage screening strategy in combination with assays for in vitro production of IAA and ACC deaminase enabled the identification of efficient PGPR that were able to enhance the legume-rhizobia symbiosis.