Evaluation of plant growth-promoting traits of Burkholderia and Rhizobium strains isolated from Amazon soils for their co-inoculation in common bean

Abstract

Several processes that promote plant growth were investigated in diazotrophic bacteria isolated from soils of various land-use systems in the western Amazon region using siratro and bean as the trap species. The processes observed in the 17 studied strains were as follows: Inhibition of phytopathogenic fungal growth; free-living nitrogen fixation in semi-solid nitrogen-free LO medium; solubilisation of insoluble calcium and aluminium inorganic phosphates in solid media and auxin production based on a colorimetric assay with or without the addition of L-tryptophan. The ability to use phenol as the sole carbon source in solid media and antibiotic resistance, based on the disk diffusion method, was also evaluated. Nearly all of the selected strains, including Rhizobium strains, were able to fix nitrogen as free-living bacteria. Some Burkholderia fungorum strains had a strong ability to solubilise calcium phosphate, whereas strains UFLA 04-229 and UFLA 04-217 displayed high levels of indole-3-acetic acid synthesis in the absence (12.59 µg mL-1) or presence (29.08 µg mL-1) of L-tryptophan, respectively. Four of the studied strains potentially biodegrade the pollutant compound phenol. Rhizobium strains were more tolerant to antibiotics than Burkholderia strains. The Rhizobium strains, which are highly efficient at fixing nitrogen in symbiosis with the common bean plant, and the CIAT 899Tstrain demonstrated the ability to perform other processes that promote plant growth. Co-inoculation of CIAT 899T and UFLA 04-155 (B. fungorum) enhanced significantly the dry matter of nodules and shoot and P contents in relation to single inoculation with CIAT 899T.   Key words: Biological nitrogen fixation, solubilisation of inorganic phosphates, plant growth hormones, phenol biodegradation, antibiotic resistance