Fire blight, caused by Erwinia amylovora, is a devastating bacterial disease threatening the worldwide production of pome fruit trees, including apple and pear. Within host xylem vessels, E. amylovora cells restrict water flow and cause wilting symptoms through formation of biofilms, that are matrix-enmeshed surface-attached microcolonies of bacterial cells. Biofilm matrix of E. amylovora is primarily composed of several exopolysaccharides (EPSs), including amylovora, levan, and, cellulose. The final step of biofilm development is dispersal, which allows dissemination of a subpopulation of biofilm cells to resume the planktonic mode of growth and consequentially cause systemic infection. In this work, we demonstrate that identified the Hfq-dependent small RNA (sRNA) RprA positively regulates amylovoran production, T3SS, and flagellar-dependent motility, and negatively affects levansucrase activity and cellulose production. We also identified the in vitro and in vivo conditions that activate RprA, and demonstrated that RprA activation leads to decreased formation of biofilms and promotes the dispersal movement of biofilm cells. This work supports the involvement of RprA in the systemic infection of E. amylovora during its pathogenesis.Bacterial toxin-antitoxin (TA) systems are small genetic loci composed of a proteinaceous toxin and a counteracting antitoxin. In this work, we identified and characterized a chromosomally encoded hok/sok-like type I TA system in Erwinia amylovora Ea1189. Ectopic overexpression of hok caused massive cell death in E. amylovora and its toxicity can be partially reversed through co-overexpression of the cognate sRNA sok. Phenotypic and transcriptomic examination of E. amylovora cells expressing hok at subtoxic levels demonstrated that hok causes membrane rupture and proton motive force dissipation, upregulates expression levels of ATP biosynthesis genes and consequently cellular ATP levels. Hok also positively affects phage shock protein genes to protect cells from further membrane damage. Taken together, the hok/sok TA system, besides being potentially self-toxic, appears to facilitate E. amylovora to manage various stress responses when the toxin gene is expressed in low levels.The succinate dehydrogenase inhibitor (SDHI) is a broad-spectrum fungicide class that has been widely utilized in agricultural fields. Blumeriella jaapii, the causative agent of cherry leaf spot (CLS), is the most important limiting factor for tart cherry production in the Midwestern United States. In the last decade, reduced efficacy in using SDHIs, i.e. boscalid, fluopyram, and fluxapyroxad, for management of CLS has been observed in many research and commercial orchard sites. Through whole-genome sequencing of B. jaapii using both PacBio long-reads and Illumina short-reads, we identified mutations in SdhB or SdhC that are correlative to resistance of this fungus to boscalid, fluopyram, and/or fluxapyroxad. SdhB and SdhC are components of the succinate dehydrogenase complex that contributes fungal respiration. In view of the widely conserved sequences of the SdhB and SdhC genes in phytopathogenic Ascomycete fungi, we expressed the wild-type or mutated alleles of the B. jaapii Sdh gene in the soybean white mold pathogen Sclerotinia sclerotiorum. We successfully validated the functions of the mutations in Sdh genes of B. jaapii in conferring resistance to several SDHIs examined in this study. The S. sclerotiorum heterologous expression system was also validated to be highly effective in characterizing the functions of other Sdh mutations of B. cinerea, C. homoeocarpa, and M. fructicola. The approach developed in this study can potentially be widely applied to interrogate SDHI fungicide resistance mechanisms in other phytopathogenic ascomycetes.
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