Isolation, characterization, and optimization of indole acetic acid–producing Providencia species (7MM11) and their effect on tomato (Lycopersicon esculentum) seedlings

Indole-3-acetic acid (IAA) is the most prevalent plant hormone of the auxin family that occurs naturally in the environment and regulates the plant growth and development. The bacteria were isolated from the tomato rhizosphere soils and screened for IAA production. Among the all isolates, the isolate designated as 7MM11 showed maximum amount of IAA (89.22 μg/mL) production in short incubation period (24 h) by HPLC. The isolate was identified as Providencia sp. by its morphological characteristics and 16S rRNA sequence study. Further, optimal conditions such as l-tryptophan concentration, incubation time, agitation, pH, carbon sources, and nitrogen sources, were identified for IAA production. The optimization results suggested that the IAA production increased with medium supplemented with sucrose (0.1%), urea (0.1%), and tryptophan (0.1%) with pH 8.0 at 30 ± 2 °C on a shaker at 150 rpm after 24 h of incubation. The production of IAA by Providencia sp. was further confirmed and characterized with help of thin-layer chromatography and high-performance liquid chromatography. In an in vivo experiment, the tomato seedlings treated with Providencia sp. showed improved germination, seedling vigor index, and enhanced growth promotion compared to un-inoculated control. Thus, the results of this study suggest that Providencia sp. strain 7MM11 is a potent isolate for the maximum production of IAA in short period, which may help in achieving the plant growth promotion.