SaxA-mediated degradation of ITC plant toxin functions as a public good in a bacterial community

Plant leaves host diverse microbial communities of beneficial commensals and opportunistic pathogens, whose balance is crucial for plant health. Brassicaceae plants like Arabidopsis thaliana defend against pathogens by producing glucosinolates (GLS), which are enzymatically converted into antimicrobial isothiocyanates (ITCs) upon tissue damage by bacteria. ITCs inhibit bacterial growth and are a key element of the plant defense. However, pathogenic Pseudomonas spp. can detoxify ITCs via the hydrolase SaxA, while commensals are typically unable to detoxify ITCs. Understanding the effects of ITC degradation dynamics on the leaf microbiome is essential for advancing knowledge of disease resistance in ecologically relevant plants.

In this study, we test whether Pseudomonas viridiflava 3D9 (Ps 3D9)"s ability to degrade 4-methylsulfinylbutyl (4MSOB -ITC) can shield the entire bacterial leaf community from plant defenses, even when other commensals cannot. We explored this by constructing an artificial community of five commensals which were isolated along with Ps 3D9 from healthy wild A. thaliana leaves. We either added the wildtype Ps 3D9 or Ps 3D9ΔsaxA which does not degrade ITCs.

We first examined the effects of 4MSOB-ITC on the growth rates of axenic cultures of Pseudomonas 3D9, its ΔsaxA mutant, and commensals. To assess the impact of detoxification, we further developed a mathematical model based on extended Lotka-Volterra equations, incorporating ITC toxicity and the ability of Pseudomonas to degrade it. We then extended the model to include interactions between ITC-degrading Pseudomonas and non-degrading commensals, predicting community composition at varying degrader concentrations. Our results identify the threshold where the degrader provides communal protection from ITC toxicity. Model predictions align well with experimental data, confirming the protective role of the degrader strain.

Studying the GLS defense system in A. thaliana enhances our understanding of plant-microbe interactions and offers valuable insights for modeling microbial communities.