Functional auxin signaling and phosphorus acquisition induced in planta by the extremophile bacterium Pseudomonas extremaustralis

Abstract

Environmental fluctuations strongly influence beneficial plant–microbe interactions, yet the specific effects of oxygen availability on plant-beneficial bacterial functions remain poorly understood. Here, we investigated plant growth–promoting traits of Pseudomonas extremaustralis 14-3b, an Antarctic isolate, under different oxygen conditions. Genome analysis identified genes associated with phosphorus solubilization and indole-3-acetic acid (IAA) biosynthesis.In vitro assays confirmed its ability to solubilize tricalcium phosphate and produce gluconic acid. IAA production was higher in microaerobic than in aerobic cultures (Unpaired Student´s t-test; p = 0.0003). Using Arabidopsis thaliana DII-VENUS reporter lines, we detected increased auxin signaling in roots, with reduced VENUS fluorescence upon bacterial inoculation, regardless of prior oxygen conditions. Confocal microscopy revealed rapid bacterial recruitment to the root surface and further colonization in both roots and cotyledons, with the presence also in the sub-stomatal cavity.In A. thaliana, inoculation reduced primary root length and increased root hair length. In wheat, P. extremaustralis was capable of colonizing root tissues. After 30 days in pot experiments, inoculated wheat plants exhibited significantly greater shoot dry weight and higher leaf phosphorus content than controls. These results suggest that P. extremaustralis promotes plant growth through multiple mechanisms, supporting its potential as a bioinoculant capable of maintaining key plant-beneficial traits under low oxygen availability.