IAA-producing bacteria from the rhizosphere of chickpea (Cicer arietinum L.): Isolation, characterization, and their effects on plant growth performance

Abstract

Indole-3-acetic acid (IAA), a crucial plant hormone, regulates diverse physiological processes. This study aimed to isolate and characterize IAA-producing bacteria from the chickpea (Cicer arietinum L.) rhizosphere and evaluate their effects on plant growth. From 54 rhizosphere samples, 118 bacteria (designated as GAC) were isolated and screened for IAA production using a Salkowski colorimetric assay, and Bergey's manual was used for biochemical identification. Isolates were grown under various conditions and in vitro screened for their growth promotion traits. A PCR investigation was performed for IAA and nitrogen-fixing genes, and evaluated for greenhouse conditions. Among them, 27 isolates produced IAA, with eight high producers selected. Morphological and biochemical identification classified the six isolates as Pseudomonas and the other two as Bacillus. Optimal conditions for IAA production were observed at 500 μg/ml tryptophan, 35 °C, and pH 7.0. A 48-h incubation was ideal for IAA production, except for GAC-34 and GAC-73, which required 72 h. All the isolates achieved optimal IAA levels with tryptone and sucrose as nitrogen and carbon sources, respectively. Moreover, all isolates showed nitrogen fixation ability, and the six isolates exhibited phosphate solubilization. PCR confirmed the amplification of nifH (300 bp), nifK (360 bp), and ipdC (1170 bp) genes. Greenhouse experiments demonstrated that eight selected isolates significantly enhanced chickpea growth parameters (p < 0.001). These findings suggest that these IAA-producing bacteria have the potential to be used as biofertilizers to improve crop productivity, although further molecular identification and field studies are required.