Environmental Factors Modulating Indole-3-Acetic Acid Biosynthesis by Four Nitrogen Fixing Bacteria in a Liquid Culture Medium

Abstract

This study evaluated the effects of some environmental conditions on IAA biosynthesizing capacity of four nitrogen fixing bacteria, namely Paenibacillus cineris TP-1.4, Bacillus megaterium MQ-2.5, Klebsiella pneumoniae OM-17.2, and Pseudomonas boreopolis CP-18.2. Carbon source, pH, NaCl, and tryptophan supplement treatments were set to investigate the effects of those environmental factors on IAA synthesis. The IAA synthesizing capacity of bacterial strains in liquid medium was measured spectroscopically following incubation by Salkowski's reagent method. The results showed that, under the sucrose amendment, the IAA concentrations produced by all four bacterial strains were significantly higher than those of the other four carbon source added treatments. Two of the four bacterial strains produced the highest yield of IAA in liquid medium at pH 7 (TP-1.4 and OM-17.2), whereas pH 8 was optimum for the other two strains (MQ-2.5 and CP-18.2). The MQ-2.5 strain could synthesize IAA fairly well in up to 5% NaCl and produced the highest amount of IAA with 1% NaCl. Furthermore, IAA synthesizing capability of tested bacterial strains increased sharply along with increasing tryptophan content in culture medium except for the TP-1.4 strain. From the current study, these isolates emerged as possible alternatives for future IAA production for plant growth and yield enhancement. Hence, they have a great potential to be used as bio-inoculants for plant growth promotion in eco-friendly and sustainable agriculture.