The effect of Sinorhizobium meliloti volatilomes and synthetic long-chain methylketones on soil and Medicago truncatula microbiomes

Bacterial volatile compounds play important roles in intra- and interkingdom interactions but little is known about their effects on soil and plant microbiomes. The legume symbiont Sinorhizobium meliloti (Sm) releases volatile methylketones (MKs), one of which acts as an infochemical among bacteria and hampers plant-bacteria interactions. Inactivation of the fatty acyl-CoA ligase FadD in Sm moderately enhances MK production. To further explore the ecological role of MKs on soil and plant bacterial communities, we aimed at obtaining an MK-overproducing Sm strain by deleting the 3-oxoacyl-CoA thiolase-encoding fadA gene. Analyses of the Sm wild-type (WT) and fad mutant volatilomes identified seventeen compounds, primarily consisting of MKs and fatty acid methyl esters (FAMEs). The fadA mutant released more MKs than the fadD mutant, and substantially more than the WT, whereas FAME emission was increased in the fadD mutant. Exposure of natural soil and the Medicago truncatula rhizosphere to WT and fadA volatilomes or synthetic volatile MKs did not significantly alter bacterial alpha or beta diversity but certain genera responded differentially to each condition. Interestingly, Sm volatilomes significantly affected root endosphere Ensifer/Sinorhizobium populations by maintaining their abundance over time, in contrast to control conditions or exposure to synthetic volatile MKs. This study provides new insights on the synthesis of rhizobial volatile compounds and represents the first exploration of the effects of rhizobial volatilomes on soil and plant bacterial communities, contributing to a deeper understanding of the complex molecular bases underlying plant-bacteria interactions.