Assessing the role of exogenous NO on plants and microbial communities in soil.

Nitric oxide (NO) is a reactive gas that functions as a signaling molecule regulating plant growth and stress responses, while also exerting various roles for microorganisms. In soil, NO is produced through microbial activity, plant metabolism, and physico-chemical processes. However, the impact of exogenous NO on plant physiology and the associated root microbiota remains unexplored. Here, we evaluated the effects of NO exposure on plant physiology, trace gas fluxes and N cycling, as well as the abundance, diversity, and composition of root-associated microbiota. We conducted two 37-day experiments with either Arabidopsis thaliana or tomato (Solanum lycopersicum) plants using innovative plant-soil mesocosms that allowed NO flushing while monitoring the CO2, N2O and NO fluxes. The mesocosms were subjected to four NO flushing periods (3-4 days each) at 0 ppbv or 400 ppbv. Our results revealed that exogenous NO400 exerted plant-specific effects. While flushing with NO400 had no effect on tomato plants or associated microbiota, it increased leaf area in Arabidopsis and modulated the expression of two genes involved in plant growth-defense balance compared to flushing with NO0. These changes in Arabidopsis physiology were concomitant with modest alterations in the fungal community and a decrease in the abundance of bacterial ammonia-oxidizers, 15N recovery as NO₃-, and cumulative CO₂ fluxes. However, it is still unclear how much of these effects were indirectly driven by plant-soil feedbacks. Our findings offer intriguing insights into the possible, though modest, effects of exogenous NO in shaping plant-microbe interactions.