Isolation, characterization, and plasmid pUPI126-mediated indole-3-acetic acid production in Acinetobacter strains from rhizosphere of wheat

Thirty-seven strains of Acinetobacter isolated and characterized from rhizosphere of wheat were screened for indole-3-acetic acid (IAA) production. Only eight Acinetobacter strains showed IAA production. The genus Acinetobacter was confirmed by chromosomal DNA transformation assay. Biotyping of eight strains was carried out and they were found to be genospecies of A. junii, A. baumannii, A. genospecies 3, and A. haemolyticus. Five of eight strains produced IAA at the early stationary phase: A. haemolyticus (A19), A. baumannii (A18, A16, A13), and Acinetobacter genospecies 3 (A15). A. junii A6 showed maximum IAA production at log phase and A. genospecies 3 and A. baumannii (A28, A30) at late stationary phase. IAA was extracted by ethyl acetate and purified by preparative thin-layer chromatography. Purified IAA was confirmed by H-1-nuclear magnetic resonance and infrared spectrum analysis. Pot experiments showed a significant increase in plant growth inoculated with eight Acinetobacter genospecies as compared to control plants. IAA production was found to be encoded by plasmid pUPI126. All eight strains of Acinetobacter contain a plasmid pUPI126 with a molecular weight of 40 kb. Plasmid pUPI126 was transformed into Escherichia coli HB101 at a frequency of 5 x 10(-5), and E. coli HB101 (pUPI126) transformants also showed IAA activity. PUPI126 also encoded resistance to selenium, tellurium, and lead. This is the first report of plasmid-encoded IAA production in the genus Acinetobacter.