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Additional file 1. Result S1. Analysis of rhizosheath sand composition. Result S2. Analysis of cultivable bacteria. Table S1. Climatic conditions throughout the year in Ksar Ghilane, Sahara Desert (Tunisia); data elaborated from https://www.worldweatheronline.com ; data as reported as monthly average measurement from January 2019 to now. Table S2. List of FISH probes used and conditions applied; probe sequence (5’-3’), attached fluorochrome (fluor.), bacterial target group, percentage of formaldehyde (FA) used during treatment, and references are also reported. Table S3. Number of KTUs and sequences used for each sample. RH: rhizosphere, RS: rhizosheath, RT: root tissue, BS: bulk sand. Table S4. Humidity (RH%) and temperature (T°C) measured in summer (June, 2016) and winter (November, 2016) for bulk sand and belowground speargrasses (i.e., plant rhizosheath–root system, RS). Table S5. Physico-chemical analyses conducted on bulk sand (BS) and rhizosheath matrix (RS) collected in summer (S) and winter (W). Table S6. PERMANOVA pair-wise comparison test of physico-chemical conditions (Table S5) in rhizosheath (RS) and bulk sand (BS) collected during summer (S) and winter (W) seasons. Table S7. Multivariate test (pairwise comparison) of beta-diversity associated with root tissue, rhizosheath, rhizosphere, and bulk sand. Table S8. Mean and standard error of multivariate dispersions from centroid calculated for each compartment (within-betadiversity). Table S9. Relative importance of different ecological processes in the assembly of bacterial community associated with the rhizosheath-root system compartments and bulk sand in two contrasting seasons. Table S10. Generalized linear model univariate test indicates the KTUs contributing to the difference in bulk soil among summer and winter. Table S11. Tukey’s honest significance difference (TukeyHSD) pairwise comparison tests for the degree, betweenness and keystone species detected across the four co-occurrence networks, namely bulk sand winter, bulk sand winter summer, rhizosheath–root system winter and rhizosheath–root system summer. Table S12. List of bacterial isolates, PGP activity, and abiotic resistance tested in vitro. Figure S1. Stipagrostis pungens rhizosheath-root system. Figure S2. Rarefaction curves of bacterial reads obtained by pair-ends MiSeq Illumina sequencing in bulk sand, rhizosheath, rhizosphere, and root tissues. Figure S3. Visualization of Stipagrostis pungens rhizosheath with scanning electron microscopy (SEM). Figure S4. SEM images and electron micrographs of the sand grains and root tissue within the rhizosheath of Stipagrostis pungens using environmental scanning electron microscopy (ESEM) to reveal chemical composition. Figure S5. Localization of bacteria in rhizosheath-root system by confocal laser-scanning microscopy (CLSM) and fluorescence in situ hybridization (FISH). Figure S6. Bacterial KTUs distribution across samples. Figure S7. Venn diagram shows the distribution of bacterial KTUs across compartment categories. Figure S8. Alpha diversity expressed as richness (number of KTUs) and Shannon diversity across the compartment categories. Figure S9. Quantification of betadiversity components in bulk sand bacterial communities across seasons (summer and winter). Figure S10. Alpha diversity expressed as richness (number of KTUs) and Shannon diversity across the compartments in summer and winter. Figure S11. Venn diagrams showing the number of KTUs present in summer and/or in winter in each compartment category. Figure S12. Analysis of 2-fold change was performed to evaluate the KTUs that had a significantly (p < 0.01) different relative abundance (2-fold change) over summer and winter. Figure S13. Taxonomy of co-occurrence network degrees in bulk sand and rhizosheath matrix across seasons. Figure S14. Microcosms to evaluate sand wettability in vitro. Evaluation of sand weight at 48 h and along the entire incubation (0, 24 and 48 h).

Background In hot deserts daily/seasonal fluctuations pose great challenges to the resident organisms. However, these extreme ecosystems host unique microenvironments, such as the rhizosheath–root system of desert speargrasses in which biological activities and interactions are facilitated by milder conditions and reduced fluctuations. Here, we examined the bacterial microbiota associated with this structure and its surrounding sand in the desert speargrass Stipagrostis pungens under the contrasting environmental conditions of summer and winter in the Sahara Desert. Results The belowground rhizosheath–root system has higher nutrient and humidity contents, and cooler temperatures than the surrounding sand. The plant responds to the harsh environmental conditions of the summer by increasing the abundance and diversity of extracellular polymeric substances (EPS) compared to the winter. On the contrary, the bacterial community associated with the rhizosheath–root system and its interactome remain stable and, unlike the bulk sand, are unaffected by the seasonal environmental variations. The rhizosheath–root system bacterial communities are consistently dominated by Actinobacteria and Alphaproteobacteria and form distinct bacteria communities from those of bulk sand in the two seasons. The microbiome-stabilization mediated by the plant host acts to consistently retain beneficial bacteria with multiple plant growth promoting functions, including those capable to produce EPS, which increase the sand water holding capacity ameliorating the rhizosheath micro-environment. Conclusions Our results reveal the capability of plants in desert ecosystems to stabilize their below ground microbial community under seasonal contrasting environmental conditions, minimizing the heterogeneity of the surrounding bulk sand and contributing to the overall holobiont resilience under poly-extreme conditions.