| Summary: | <i>Azospirillum brasilense</i> is a prolific grass-root colonizing bacteria well-known for its ability to promote plant growth in several cereal crops. Here we show that one of the mechanisms of action in boosting plant performance is through increased assimilation of the micronutrient manganese by the host. Using radioactive <sup>52</sup>Mn<sup>2+</sup> (t<sub>½</sub> 5.59 d), we examined the uptake kinetics of this micronutrient in young maize plants, comparing the performance of three functional mutants of <i>A. brasilense</i>, including HM053, a high auxin-producing and high N<sub>2</sub>-fixing strain; <i>ipdC</i>, a strain with a reduced auxin biosynthesis capacity; and FP10, a strain deficient in N<sub>2</sub>-fixation that still produces auxin. HM053 had the greatest effect on host <sup>52</sup>Mn<sup>2+</sup> uptake, with a significant increase seen in shoot radioactivity relative to non-inoculated controls. LA-ICP-MS analysis of root sections revealed higher manganese distributions in the endodermis of HM053-inoculated plants and overall higher manganese concentrations in leaves. Finally, increased leaf manganese concentration stimulated photosynthesis as determined by measuring leaf fixation of radioactive <sup>11</sup>CO<sub>2</sub> with commensurate increases in chlorophyll concentration.
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