Optimization of Indole-3-Acetic Acid (IAA) Biosynthesis by Enterobacter Cloacae 38 in Defatted Soybean Meal Via Response Surface Methodology

Abstract

Plant growth promoting srhizosphere (PGPR) research has gained significant interest for improving crop productivity and promoting sustainable agriculture. Among various plant hormones, indole-3-acetic acid (IAA) is a key auxin typically produced by PGPR that supports the growth and development of plants. In this study, Enterobacter cloacae 38 was identified as an effective IAA-producing bacterium using defatted soybean meal (DSM), a soybean by-product, as an inexpensive substitute for the commonly used Luria–Bertani (LB) medium. This study reports the optimization of indole-3-acetic acid (IAA) biosynthesis by Enterobacter cloacae 38 using defatted soybean meal (DSM) as a low-cost alternative medium through response surface methodology (RSM). Unlike conventional soybean-based media for microbial cultivation or biofertilizer production, this study demonstrates DSM as a cost-effective medium for enhancing microbial IAA biosynthesis compared with commercial LB medium. Key culture conditions, including L-tryptophan concentration, pH, and different carbon and nitrogen sources, were screened and optimized to enhance IAA biosynthesis. One-factor-at-a-time (OFAT) analysis showed that E. cloacae 38 produced the highest IAA titer of 529.71 µg/mL after 120 h of fermentation. Among the tested parameters, DSM as a nitrogen source and L-tryptophan concentration had the strongest influence on IAA production, whereas carbon sources had no significant effect. Further optimization using response surface methodology (RSM) increased IAA biosynthesis to 627 µg/mL under the optimal conditions of 20 mg/mL DSM, 5 mg/mL L-tryptophan, pH 9, 37 °C, and 120 h incubation. Post-validation experiments confirmed that the DSM-optimized medium produced 25.6% more IAA than commercial LB medium.