P-216 - Bacillus zanthoxyli HS1 strain-derived volatiles modulate root exudates and rhizosphere microbiome to enhance disease resistance in cucumber against Colletotrichum orbiculare

As holobionts, plants live together with diverse microbes throughout their tissues. Among these interactions, the rhizosphere serves as a dynamic environment that can be favorable or adverse to plants. Plant growth-promoting rhizobacteria influence plant health by emitting volatile organic compounds (VOCs), while plants secrete root exudates to attract beneficial microbes. Therefore, we need to focus on the complex relationships between VOCs, root exudates and the rhizosphere microbiome. This study shows that VOCs from Bacillus zanthoxyli HS1 (BzaHS1) enhance plant growth and boost resistance against Colletotrichum orbiculare. A split-root approach revealed that under aerial fungal attack, bacterial VOCs modulated microbiome diversity and increased succinic acid levels in root exudates, while citric acid was dominant without pathogen. Exposure to synthetic VOCs showed that 2,3-dimethylpyrazine and dodecane promoted growth. However, combining each compound with citric acid or succinic acid suppressed growth accompanied by upregulation of genes important for salicylic acid-related events. Interestingly, treating crude VOCs with individual compounds dominated in root exudates reversed the suppression, restoring growth and enhancing resistance. These findings highlight the role of BzaHS1-derived VOCs in modulating root exudates, the microbiome, and hormonal defense pathways, underscoring the importance of microbial VOCs in shaping plant-microbe interactions and stress resilience.