Indoleacetic acid production by the rhizosphere bacterium Azospirillum brasilense Cd under in vitro conditions

Factors influencing indoleacetic acid release from Azospirillum brasilense Cd (ATCC 29729) were investigated under batch culture conditions. In the presence of ammonium in the liquid culture solution, indoleacetic acid release from bacterial cells was considerably higher than in the corresponding N-free medium that was slightly solidified with agar to provide conditions for N2 fixation. In the solution culture, the concentration of indoleacetic acid was low during the logarithmic growth phase and increased rapidly with the beginning of the stationary phase. A similar time course was found for the indoleacetic acid content (dry weight basis) of the bacterial cells. Addition of tryptophan strongly stimulated the release of indoleacetic acid, which again showed a steep rise in the early stationary phase. The pH of the nutrient solution increased from 6.8 to 9.3 during incubation but this was not responsible for the steep rise in indoleacetic acid levels. The sole carbon source in the substrate (DL-malic acid) was identified as the limiting growth factor. This suggests that the high increase in indoleacetic acid production in the stationary phase is the expression of an overall change in cell metabolism when the carbon source is exhausted. At this developmental stage the high production of indoleacetic acid seems to have the character of a secondary product. Press sap from wheat roots did not promote indoleacetic acid production by bacterial cells.