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

O_LIRoot exudates are thought to play an important role in plant-microbial interactions. In return for nutrition, soil bacteria can increase the bioavailability of soil minerals. However, root exudates typically decrease in situations such as drought, calling into question the efficacy of bacteria-dependent mineral uptake in such stress. C_LIO_LIHere we tested the hypothesis of exudate-driven microbial priming on Cupressus saplings grown in forest soil in custom-made rhizotron boxes. A 1-month imposed drought and concomitant inoculations with Bacillus subtilis and Pseudomonas stutzeri, bacteria species isolated from the forest soil, were applied using factorial design. C_LIO_LIDirect bacteria counts and visualization by confocal microscopy showed that both bacteria associated with Cupressus roots. Interestingly, root exudation rates increased with bacteria under drought. Forty four metabolites in exudates were significantly different in concentration between irrigated and drought trees, including phenolic acid compounds and quinate, that were shown to be used as carbon and nitrogen sources by both bacterial species. Importantly, soil phosphorous bioavailability was maintained only in inoculated trees, mitigating drought-induced decrease in leaf phosphorus and iron. C_LIO_LIOur observations of increased root exudation rate when drought and inoculation regimes were combined, support the idea of root recruitment of beneficial bacteria. C_LI