ISOLATION AND CHARACTERIZATION OF SOIL BACTERIA WITH ABILITY TO PRODUCE INDOLE ACETIC ACID

ABSTRACT Indole-3-acetic acid (IAA) is a fundamental phytohormone of the auxin class that plays a critical role in plant growth and development. Several rhizospheric bacteria have the capacity to produce IAA, which can directly stimulate plant growth, making them promising candidates for developing eco-friendly biofertilizers. This study aimed to isolate, screen, and characterize IAA-producing bacteria from soil samples collected from the rhizosphere of various healthy plants.A total of 35 morphologically distinct bacterial isolates were obtained from the soil samples using the serial dilution and spread plate technique on nutrient agar. These isolates were primarily screened for IAA production in a nutrient broth supplemented with L-tryptophan (0.1% w/v). The presence of IAA in the culture supernatant was qualitatively confirmed by the Salkowski reagent method, which produced a characteristic pink color. Out of the 35 isolates, 12 were found to be positive for IAA production. These 12 positive isolates were further subjected to quantitative analysis using spectrophotometric methods. The IAA production yields varied significantly among the isolates, ranging from 8.5 µg/mL to 28.4 µg/mL after 72 hours of incubation. The most potent producer, designated as isolate SBR-09, was selected for further characterization.Based on morphological, biochemical, and molecular analysis (16S rRNA gene sequencing), the promising isolate SBR-09 was identified as a strain of Bacillus subtilis. The isolate also exhibited additional plant growth-promoting traits, such as phosphate solubilization and siderophore production. The results conclusively demonstrate that the selected rhizospheric soil harbors diverse and potent IAA-producing bacteria. The characterized isolate, Bacillus subtilis SBR-09, shows significant potential for application as a microbial inoculant to enhance plant growth and reduce dependence on synthetic chemical fertilizers. Keywords: Indole-3-acetic acid (IAA), Plant Growth-Promoting Rhizobacteria (PGPR), Bacillus subtilis, Salkowski reagent, L-tryptophan, Biofertilizer.