We had assembled a synthetic bacterial community (SynComH) from the rhizosphere microbiota of Haloxylon ammodendron, which enhance drought tolerance in Arabidopsis thaliana under laboratory conditions. Here we assessed the efficacy of SynComH, alone or in combination with SynComS, a consortium of previously identified growth-promoting bacteria from Crocus sativus, in increasing maize plants’ fitness under optimal and water-deficit conditions. We were also interested in unveiling the mechanisms by which the bacteria confer drought tolerance. We first performed detailed greenhouse experiments on maize plants that were artificially inoculated with SynComS, SynComH, and SynComH+S, and measured 22 different plant morphological, physiological, biochemical, and histological traits under optimal and water-deficit conditions. Data were analyzed by different univariate and multivariate statistical methods. Lastly, we tested the effects of the SynComs on maize in field in a split-plot experiment with two irrigation regimes as main plots and SynComs as subplots. Results from both greenhouse and field experiments showed that all three SynComs had growth-promoting properties and improved growth-related traits, but only SynComH and SynComH+S were effective in ameliorating the drought stress. SynComH substantially enhanced drought tolerance by increasing stomatal conductance, photosynthesis rate, and proline content, while decreasing transpiration rate, leading to up to 700% increase of water use efficiency. Morphological and histological investigations suggested that these positive effects are probably attributed to modulation of plant stomatal density and xylem structure by SynComH. Besides the significance for practical agronomic use, our results suggest unprecedented capabilities of non-pathogenic bacteria in modulation of plant stomatal density.

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