Plant pathogenic bacteria can evolve tolerance to antimicrobial plant allelochemicals during a model biocontrol experiment. Plant allelochemical tolerance in plant pathogenic bacteria

Crop losses to plant pathogens are a growing threat to global food security, and hence, more effective control strategies are urgently required. Biofumigation, an agricultural technique involving the mulching of Brassica plant tissues into soils to release antimicrobial plant allelochemicals called isothiocyanates (ITCs), has been proposed as an environmentally sustainable alternative to agrochemicals. While biofumigation has been shown to be efficient against a range of plant pathogens, its effects on plant pathogenic bacteria remain largely unexplored. Here we used a laboratory model system to compare the efficacy of different types of ITCs against the bacterial plant pathogen Ralstonia solanacearum and further evaluated the potential of ITC-tolerance evolution under high (1-day), intermediate (2-day) and low (3-day) serial transfer frequency treatments in the absence and presence of ITCs. We found that only allyl-ITC was efficient at suppressing R. solanacearum growth and its efficacy was not improved in the presence of other types of ITCs. While allyl-ITC was effective at suppressing pathogen growth in all treatments, ITC tolerance evolution was only observed in the low transfer frequency treatment, and mechanistically, this was associated with mutations in genes affecting toxin breakdown (dehydrogenases) and cell wall structure (serine/threonine kinase). Control populations also evolved increased ITC tolerance in the absence of allyl-ITC in the low transfer frequency treatment. However, the underlying cause for this was adaptation to the growth media, and at the genetic level, it was associated with a distinct set of mutations linked with DNA replication, recombination and repair (deoxyribonucleases). Together, these results suggest that R. solanacearum has the potential to rapidly evolve tolerance to antimicrobial plant allelochemicals, which could potentially affect its invasiveness in the rhizosphere and weaken the long-term efficiency of biofumigation-based biocontrol.