The role of the plant specialized metabolites glucosinolates and isothiocyanates in microbial interactions and leaf colonization

Plant roots and leaves are colonized by microbes which form the plant microbiome. Plant specialized metabolites (PSMs) are known for their roles in defense against herbivores and pathogenic microbes. This work investigated how PSMs and PSM-mediated microbial interactions shape bacterial leaf colonization and leaf-associated microbiomes. Brassicaceae plants such as Arabidopsis thaliana produce glucosinolates (GLS) as defensive PSMs. Antimicrobial isothiocyanates (ITCs) are released from GLS when myrosinases come in contact with these PSMs, for example during pathogen attacks. However, ITCs can also inhibit non-pathogenic commensal bacteria and some bacteria evolved resistance mechanisms encoded by sax genes. These include efflux pumps and the ITC hydrolase SaxA, which detoxifies ITCs. To study the effect of GLS and ITCs on leaf colonization we first compared two A. thaliana genotypes: NG2, producing mainly allyl-GLS, and Col-0, producing mainly 4MSOB-GLS. 16S rRNA gene sequencing revealed different roles of GLS in the two genotypes. In the NG2 wildtype specific taxa were enriched compared to its aliphatic GLS-free mutant. In vitro enrichments on allyl-GLS as sole carbon source identified a bacterial strain that utilized this GLS, detoxified allyl-ITC, and co-enriched other bacteria by cross-feeding. The release of ITCs would be a serious threat for non-adapted co-colonizing microbes. Using 4MSOB-ITC and the SaxA-expressing pathogen Pseudomonas viridiflava 3D9, we next showed that SaxA-mediated ITC degradation rescued ITC-sensitive commensals. A mathematical model demonstrated that this rescue depended on ITC concentration, commensal:pathogen ratio, and bacterial traits. Nutrient competition dominated the bacterial interactions and might allow commensals to suppress Pseudomonas growth. Overall, this study highlights the role of GLS and ITCs in shaping leaf microbiomes and microbial interactions.