Rhizosphere indole-3-acetic acid as a mediator in the Sorghum bicolor–phenanthrene–Sinorhizobium meliloti interactions

We studied a model system consisting of Sorghum bicolor, phenanthrene, and an auxin-producing polycyclic aromatic hydrocarbon-degrading Sinorhizobium meliloti strain to clarify whether rhizosphere indole-3-acetic acid (IAA) takes part in the plant–pollutant–bacteria interactions. Phenanthrene and S. meliloti treatments of sorghum contributed to a decrease in the rhizosphere IAA concentration and to phytohormone accumulation, respectively. Regression analysis showed significant correlations between alteration in root-zone IAA content and alterations in the root-surface area, exudation, and rhizosphere effects for culturable heterotrophic bacteria, the S. meliloti strain, and other phenanthrene degraders. According to the data obtained, phenanthrene degraders get an advantage over nondegradative rhizobacteria from IAA for rhizosphere colonization. An IAA-dependent increase in the root-surface area leads to improved sorghum growth under pollutant stress. The carbon flux from the roots is corrected by the auxin because of its influence on the exuding-surface area and on the intensity of secretion by the root cells. On the other hand, the rhizosphere IAA pool may be plant-regulated by means of alteration in carboxylate exudation and its influence on bacterial auxin production. A scenario for the IAA-mediated S. bicolor–phenanthrene–S. meliloti interactions is proposed.

► Phenanthrene treatment leads to IAA and carbon limits in the sorghum rhizosphere.
► Sorghum inoculation with S. meliloti P221 favors IAA accumulation in the root zone.
► There is enhanced IAA production by the inoculum under carbon limitation in vitro.
► Alterations in the IAA pool correlate with those in the root surface and exudation.
► The greater inoculum-induced IAA, the greater the rhizosphere effect for degraders.