PLANT ASSOCIATED BACTERIA: A SUSTAINABLE RESOURCE TO MINIMIZE WATER FOOTPRINT IN AGRICULTURE

Water scarcity is one of the major worldwide problems affecting in particular agriculture, an intensive water consumer activity. In the frame of the EU H2020 MADFORWATER project, aimed to develop integrated technological and management solutions to reduce water footprint in agriculture, this PhD thesis focused on plant-associated bacteria to improve the productivity of crops growing under drought conditions and wastewater phytodepuration efficiency to allow water reuse for irrigation purposes. A large collection of 681 bacterial strains was isolated from the root apparatus of six plants species adapted to dry soils or other unconventional environmental conditions. The isolates have been taxonomically identified and functionally characterized in order to select the most promising strains exploitable in in vivo experimentations. The isolate collection was compared with a literature-derived dataset of bacterial families identified in the plant microbiome by molecular methods. The comparison demonstrated that we were able to bring into culture members of 20% of the bacterial families detected by culture-independent approaches, partially confirming on the plant biome recent concepts emerged on others environmental microbiota. Moreover, the collection included ubiquitous and potentially beneficial core members of the plant microbiota and satellite taxa, which could have a role in sustaining plant growth under peculiar environmental conditions. The bacterial community associated to Argania spinosa, a drought tolerant tree relevant for the ecology and economy of Morocco, was described for the first time, by culture dependent and independent approaches. The Plant Growth Promoting (PGP) potential of isolates obtained from the root and soil environment of Argan revealed a particular abundance of PGP related traits in strains isolated from the residuesphere fraction, providing a putative explanation to the tradition of local farmers of using argan litter as soil amendment. A selection of strains was tested in in vivo experiments aimed to evaluate their potential to improve plant growth, water use efficiency and fruit productivity. Five PGP bacteria were applied on tomato plants, cultivated in greenhouse under optimal and deficit irrigation and plant physiology, biomass and fruit yield were compared with control, not bacterized plants. All the bacterial inocula showed at different extent to have the potential capacity to alleviate plant water stress, improving different plant and soil parameters, but no statistically significant effect was reported on plant productivity. Further experiments are needed to confirm the results, nevertheless it was demonstrated the importance of performing long-term experiments to obtain reliable and applicable data about the real efficacy of PGP crop application. A second selection of PGP strains significantly showed in microcosm constructed wetlands the ability to tolerate metal contaminated wastewaters and to enhance the azo-dye phytodepuration capacity of Juncus acutus plants. These bacteria have thus the potential to improve the quality of water treated in low cost systems, and will be selected in the project for experimentation at higher pilot scale. This work demonstrated the relevance of the plant microbiota in sustaining plant growth, and provided a collection of strains which need to be further evaluated but could potentially be exploited to mitigate water shortage effects in agriculture.