Cascading effects of elevated ozone on wheat rhizosphere microbial communities depend on temperature and cultivar...

Tropospheric ozone (O₃) concentrations have now reached levels that can potentially affect crop production in several regions of the world. The interacting effects of the elevated O₃ and temperature on plants are still unclear and their consequences on the rhizosphere microbial communities never studied yet. Here, we conducted a 3-week fumigation experiment on two cultivars of wheat with different tolerance to O₃ (Premio and Soissons) at two temperatures (20 °C and 30 °C). The impacts of O₃ were measured on plants physiology, rhizosphere chemical environment and microbial communities. Globally, most of the results showed that elevated O₃ effects were more pronounced at 20 °C than 30 °C, especially on the most O_3-sensitive cultivar (Soissons). Elevated O₃ reduced significantly plant root biomass (up to -37% for Soissons) compared to non-fumigated plants. A decrease in the dissolved organic matter with a relative increase of aromatic compounds concentration was also observed under elevated O₃, suggesting quantitative and qualitative impacts on roots exudation. While bacterial abundance was negatively affected by O₃ plant stress, fungal abundance was found to be stimulated (up to 12 fold compared to non-fumigated plants for Soissons at 20 °C). These changes were accompanied by modifications of the genetic structures and metabolic profiles, with a relative increase of amino acids catabolism. This fully controlled laboratory experiment showed that the effects of elevated O₃ on soil microbial communities i) are plant-mediated and depend on the cultivar sensitivity, ii) decrease in warming condition, iii) increase the fungi to bacteria ratio and iv) alter both the genetic structure and the metabolic activities. This study highlights the importance of considering interactive effects between pollutants and climate changes on plant-microbe relationship to better inform models and improve predictions of future states of agroecosystems.