The microbiome associated with plants used in phytodepuration systems can boost plant growth and services, especially in ecosystems dealing with recalcitrant compounds, hardly removed via traditional wastewater (WW) treatments, such as azo-dyes used in textile industry. In this context, we aimed to study the cultivable microbiome selected by Phragmites australis plants in a Constructed Wetland (CW) in Morocco, in order to obtain candidate inoculants for the phytodepuration of azo-dye contaminated WW. A collection of 152 rhizospheric and endophytic bacteria was established. The strains were phylogenetically identified and characterized for traits of interest in the phytodepuration context. All strains showed Plant Growth Promotion potential in vitro and 67% of them significantly improved the growth of a model plant in vivo compared to the non bacterized control plants. Moreover, most of the isolates were able to grow in presence of several model micropollutants typically found in WW, indicating their potential use in phytodepuration of a wide spectrum of effluents. The six most promising strains of the collection were tested in CW microcosms alone or as consortium: the consortium and two single inocula demonstrated to significantly increase the removal of the model azo-dye Reactive Black 5 compared to the non bacterized controls.

Antibiotic resistance spread must be considered in a holistic framework which comprises the agri-food ecosystems, where plants can be considered a bridge connecting water and soil habitats with the human microbiome. However, the study of horizontal gene transfer events within the plant microbiome is still overlooked. Here, the environmental strain Acinetobacter baylyi BD413 was used to study the acquisition of extracellular DNA (exDNA) carrying an antibiotic resistance gene (ARG) on lettuce phylloplane, performing experiments at conditions (i.e., plasmid quantities) mimicking those that can be found in a water reuse scenario. Moreover, we assessed how the presence of a surfactant, a co-formulant widely used in agriculture, affected exDNA entry in bacteria and plant tissues, besides the penetration and survival of bacteria into the leaf endosphere. Natural transformation frequency in planta was comparable to that occurring under optimal conditions (i.e., temperature, nutrient provision, and absence of microbial competitors), representing an entrance pathway of ARGs into an epiphytic bacterium able to penetrate the endosphere of a leafy vegetable. The presence of the surfactant determined a higher presence of culturable transformant cells in the leaf tissues but did not significantly increase exDNA entry in A. baylyi BD413 cells and lettuce leaves. More research on HGT (Horizontal Gene Transfer) mechanisms in planta should be performed to obtain experimental data on produce safety in terms of antibiotic resistance.

The plant growth promoting bacteria (PGPB) harbored in the rhizosphere develop specialized mechanisms that may have a key role to ameliorate soil properties and plant growth under prolonged dry conditions. Accordingly, this study aimed to assess the effects of bacterial growth on the soil hydraulic properties and the root response under water stress conditions induced by drip irrigated tomato. At pot scale, a silty soil was inoculated with two PGPB strains (Micrococcus yunnanensis M1 and Pseudomonas stutzeri SR7-77) to cultivate tomato plants under three different water regimes: full irrigation (100% of Pot Capacity- PC), moderate and severe water stress levels (75 and 50% of PC, respectively). Bacterized soil altered the pore size distribution of the rhizosphere compared to no-bacterized soil, increasing root zone plant-available water holding capacity. On the contrary, PGPB occupying the pores reduced the saturated hydraulic conductivity near-saturated soil conditions compared to the uninoculated trial. PGPB shown root surface density (RSD) equal to 0.540 % and 0.355 % to inoculated SR7-77 and M1 tests, respectively and under 50% PC, compared to 0.097% to the uninoculated soil test. Soil water potential values, retrieved through soil water retention parameters, were more negative to M1 and SR7-77 strains tests, corresponding to water hold in the pores with smaller radii conferring resistance to the plant following contrasting stresses. The results demonstrated that PGPB elongated continuous transmission pores and bridged with air-filled spaces in stressed periods.