A dynamic rhizosphere interplay between tree roots and soil bacteria under drought stress

Root exudates are thought to play an important role in plant-microbial interactions. In return, soil bacteria can increase the bioavailability of soil minerals, which is typically decreasing in situations such as drought. Here we describe an exudate-driven microbial priming on Cupressus saplings grown outside in forest soil in custom-made rhizotron boxes. A 1-month imposed drought and inoculations with Bacillus subtilis and Pseudomonas stutzeri, bacteria species forest soil isolation, were applied in a factorial design. We revealed that both bacteria associated with Cupressus roots and were more abundant in rhizosphere than in bulk soil. Moreover, root exudation rate increased in inoculated trees under drought with >100 first identified metabolites from Cupressus roots. Among these metabolites, phenolic acid compounds, quinate, and others, were used as carbon and nitrogen sources by both bacterial species. Furthermore, soil phosphorous bioavailability was maintained only in inoculated trees, where a drought-induced decrease in leaf phosphorus and iron was prevented. We provide evidence that changes in exudation rate and composition under drought and bacteria inoculation, support the idea of root recruitment of beneficial bacteria. In turn, trees secreted further carbon source to the rhizosphere and hosted more bacteria, benefited from improved nutrition.