Screening and Optimization of Indole-3-Acetic Acid Production by Cowpea (<i>Vigna unguiculata</i> L. Walp.) Rhizobia Isolates

The production of indole-3-acetic acid (IAA) is a key trait for rhizobacteria to improve plant growth. For this, 14 rhizobia isolated from root nodules of Vigna unguiculata were first screened for their ability to produce IAA. Then, purified IAA samples were compared to standard IAA using thin-layer chromatography (TLC). The four best-producing isolates were selected for optimization of IAA production under different cultural conditions. Results showed that all of tested isolates were capable of producing IAA, with the highest concentration of 59.93 µg/ml recorded for VuChEl3 isolate, followed by VuJaEl3, VuJaEl14 and VuPlEl2 isolates recording concentrations of 29.1 µg/ml, 22.3 µg/ml and 12.4 µg/ml respectively. TLC analysis confirmed that IAA matched authentic IAA, showing the same retention factor (Rf) value of 0.9. Mannitol and yeast extract were found to be the best carbon and nitrogen sources for IAA production. An incubation temperature of 30˚C, a pH of 6.5, an incubation time of 72 h, and respective tryptophan (Trp) and NaCl concentrations of 1 mg/ml and 0% were found to be optimal parameters resulting in a maximum IAA yield of 118.43 µg/ml.