IAA-PRODUCING BACTERIA FROM THE RHIZOSPHERE OF CHICKPEA PLANT (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. Plant growth-promoting rhizobacteria naturally present in soil can enhance plant growth by producing IAA. This study aimed to isolate and characterize IAA-producing bacteria from the chickpea (Cicer arietinum L.) rhizosphere and evaluate their effects on plant growth. A total of 118 bacteria were isolated from 54 chickpea rhizosphere samples collected from Gurage Zone, Ethiopia. Purified isolates were designated as GAC; a Salkowski colorimetric was used for IAA production; and Bergey's systematic bacteriology manual was used for biochemical examination. The highest IAA-producing isolates were grown under various conditions and in vitro screened for their growth promotion traits. A PCR investigation was performed to determine the presence of IAA and nitrogen-fixing genes, and the isolates were evaluated for greenhouse conditions. Out of 118, 27 isolates produced IAA and eight isolates with the highest IAA-production (22.88-26.47 µg/ml) were selected. Morphological and biochemical identification classified six isolates as Pseudomonas and two as Bacillus. Optimal conditions for IAA-production were observed at 500 µg/ml tryptophan, 35 °C, and pH 7.0. A 48-hour incubation was ideal, except for GAC-34 and GAC-73, which required 72 hours. GAC-2 isolate achieved optimal IAA production with sucrose (45.28 µg/ml) and lowest by GAC-92 with fructose (7.72 µg/ml),and GAC-91 isolate produced the optimum IAA level with tryptone (9.62 µg/ml) and lowest by GAC-34 with peptone (2.81 µg/ml). Selected isolates demonstrated nitrogen fixation by producing ammonia, changing the medium to a dark blue/yellow color. The GAC-118 isolate exhibited maximum phosphate solubilization (11.00 mm). GAC-118 isolate confirmed the presence of nifH, nifK, and ipdC genes. Greenhouse experiments revealed that these isolates significantly enhanced chickpea growth parameters (P≤0.05) compared to the control. Thus, uses of IAA-producing bacteria from chickpea rhizosphere could enhance Ethiopian crop productivity; further molecular identification and field studies needed.