Abstract. In (hyper)arid-desert ecosystems resources are limited, and xerophytic plants evolved morphological and physiological adaptation to optimize water/nutrient uptake and storage, such as the formation of a rhizosheath root system. The water and nutrients enriched in and by the rhizosheath represent important resources for the plant, but also for the desert edaphic microorganisms (bacteria, archaea, and fungi) that are attracted by this “resource island” in an otherwise oligotrophic sandy-rocky soil. However, by adapting the Darwinian “Survival of the Fittest” theory to the microbial world, we expect that desert soil microorganisms strongly compete to colonize the favorable rhizosheath–root system niches, and only those functionally equipped (i) to support the host (and their own) survival through biopromotion and biofertilization activities, and (ii) to outcompete potential rivals, can succeed in this. By combining metabarcoding and shotgun metagenomics, we demonstrated that edaphic microbial community diversity, stability and biomass increased from the non-vegetated soils to the rhizosheath–root system. Non-vegetated soil communities promoted autotrophy lifestyle, while those associated with the plant-niches were mainly heterotrophs and enriched in microbial plant-growth promoting capacities, as well as in antibiotic resistance genes and CRISPR-Cas motifs. These results revealed how the colonization of the rhizosheath zone is trigged by an intense microbial “Arms Race” aimed at the control of microbial biomass and by the plant-selection of beneficial microorganisms able to improve the fitness and survival of the host in a win-win interaction. Our results support that such density/competitive niches may also represent evolutionary hotspots that can enhance the resilience and success of the rhizosheath–root microbial communities and their host during environmental stresses and fluctuations, such as those predicted by climate change.

Bio: Ramona Marasco received her BSc degree in Agriculture Biotechnology (2007) and her MSc degree in Plant, food, and environmental Biotechnology (2008) from the University of Milan (Italy). Afterwards she obtained her PhD in Chemistry, Biochemistry and Ecology of Pesticides (2011). During this period, she focused her research on the study of the plant-microbe interaction in arid and saline environments, including hot and cold desert, salty system, and agricultural area in temperate zones. She participated in several scientific expeditions in North Africa deserts, arid and saline environments during her postdoctoral fellow at the Department of Food, Environmental and Nutritional Science at the University of Milan (2012-2014). She moved to KAUST in 2014 as a postdoctoral fellow in the Extreme Systems Microbiology Lab lead by prof. Daniele Daffonchio, and she became Research Scientist in 2017. The main topic of her research is to understand the ecological role of microbial communities naturally associated to plant under stress conditions, such as drought and salinity, and to use this knowledge to gain information about the functional role of microbes in plant fitness, and their possible use to counteract the effects induced by the global climate change.