Beneficial plant-associated microbiota, particularly those inhabiting the rhizosphere, contribute substantially to plant health through pathogen suppression, stimulation of plant defence responses, and enhanced nutrient acquisition. Plants actively influence the composition of their rhizosphere microbiome, especially under stress conditions, by releasing chemical signals that recruit beneficial microorganisms, a process commonly referred to as the “cry for help.” While previous studies have predominantly focused on water-soluble root exudates, the role of root-emitted volatile organic compounds (rVOCs) in belowground signalling remains less well understood. This thesis investigates how aboveground biotic stress affects rVOC emissions in tomato plants and how stress-induced changes in rVOC profiles are associated with alterations in the rhizosphere microbial community. Specifically, the effects of foliar infection by the fungal pathogen Botrytis cinerea and herbivory by the insect Spodoptera exigua on rVOC emission profiles were examined. In addition, this research evaluates whether rVOCs released under stress conditions function as cues that influence the recruitment of beneficial rhizosphere bacteria. An integrated experimental approach combining bioassays, metabolomic analyses, high-throughput amplicon sequencing, and targeted isolation of rhizosphere bacteria was employed to investigate rVOC-mediated microbial recruitment in plants exposed to biotic stress. The results demonstrate that aboveground biotic stresses induce distinct and stress-specific alterations in rVOC emissions, which are correlated with shifts in the composition of recruited microbial taxa, including the enrichment of bacterial groups associated with plant-beneficial functions that contribute to resistance against aboveground biotic stress. Moreover, a stress-induced rVOC was found to modulate bacterial behaviours linked to recruitment, such as increased motility, enhanced biofilm formation, and promoted root colonization, traits that are important for rhizosphere competence.
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