Indole-3-acetic acid (IAA) protects <i>Azospirillum brasilense</i> from indole-induced stress

<i>Azospirillum brasilense</i> is plant-growth promoting rhizobacteria that produces the phytohormone indole-3-acetic acid (IAA) to induce changes in plant root architecture. The major pathway for IAA biosynthesis in <i>A. brasilense</i> converts tryptophan into indole-3-pyruvic acid (I3P) and then, through the rate-limiting enzyme, indole-3-pyruvate decarboxylase (IpdC), into IAA. Here, we characterize the potential role for IAA biosynthesis in the physiology of these bacteria by characterizing the expression pattern of the <i>ipdC</i> promoter, analyzing an <i>A. brasilense ipdC</i> mutant using multiple physiological assays and characterizing the effect of I3P, which likely accumulates in the absence of <i>ipdC</i> and affects bacterial physiology. We found that the <i>ipdC</i> mutant derivative has a reduced growth rate and an altered physiology, including reduced translation activity as well as a more depolarized membrane potential compared to the parent strain. Similar effects could be recapitulated in the parent strain by exposing these cells to increasing concentrations of I3P, as well as other indole intermediates of IAA biosynthesis. Our results also indicate a protective role for IAA against the harmful effects of indole derivatives, with exogenous IAA restoring the membrane potential of cells exposed to indole derivatives for prolonged periods. These protective effects appeared to restore cell physiology, including in the wheat rhizosphere. Together, our data suggest that the IAA biosynthesis pathway plays a major role in <i>A. brasilense</i> physiology by maintaining membrane potential homeostasis and regulating translation, likely to mitigate the potential membrane-damaging effects of indoles that accumulate during growth under stressful conditions.IMPORTANCEIAA is widely synthesized in bacteria, particularly in soil and rhizosphere bacteria, where it functions as a phytohormone to modulate plant root architecture. IAA as a secondary metabolite has been shown to serve as a signaling molecule in several bacterial species, but the role of IAA biosynthesis in the physiology of the producing bacterium remains seldom explored. Results obtained here suggest that IAA serves to protect <i>A. brasilense</i> from the toxic effect of indoles, including metabolite biosynthetic precursors of IAA, on membrane potential homeostasis. Given the widespread production of IAA in soil bacteria, this protective effect of IAA may be conserved in diverse soil bacteria.