Azospirillum brasilense Az39 restricts cadmium entrance into wheat plants and mitigates cadmium stress

• High Cd 2+ contents impair plant growth due to oxidative damage to cell components. • Plant inoculation with PGPR can mitigate plant damages caused by different stresses. • Azospirillum brasilense Az39, a PGPR, was inoculated in wheat plants exposed to Cd. • Inoculation restricted Cd entrance into wheat roots and reduced Cd/Fe imbalance. • Inoculation alleviated Cd-induced adverse effects on plant growth and N metabolism. In this work, we tested if inoculation with the plant growth-promoting rhizobacteria Azospirillum brasilense strain Az39 alleviates Cd 2+ stress in wheat seedlings grown under controlled conditions. Growth, total N, N-related metabolites/enzymes, and oxidative stress parameters were measured. Additionally, the usefulness of a real-time PCR protocol to screen the preferred colonization site of the introduced microorganism was evaluated. Inoculated plants demonstrated mitigation of cadmium-induced adverse effects on plant growth and less reactive oxygen species accumulation in their roots by the end of the experiment, 28 days after sowing. Cd addition resulted in lower NO 3 − content in the leaves and higher NO 3 − content in the roots, and a significant rise in NH 4 + concentration in both organs in uninoculated plants; in inoculated plants, NH 4 + content in the roots did not vary. A. brasilense Az39 enhanced NO levels in wheat root tips, and more adventitious roots and root hairs were observed in inoculated plants. Despite having a more developed root system, inoculated plants showed lower Cd levels in their roots compared to non-inoculated plants. Inoculation with this PGPR favored ion homeostasis in the roots of metal-exposed plants, decreasing Cd/Fe ratio. We corroborated A. brasilense Az39 preference for wheat exorhizosphere using a real-time PCR-based method targeting the nifA gene.