A membrane localized RTX-like protein mediates physiochemical properties of the Pantoea stewartii subsp. stewartii cell envelope that impact surface adhesion, cell surface hydrophobicity and plant colonization

Abstract

Abstract Pantoea stewartii subsp. stewartii (Pnss), is the bacterial causal agent of Stewart’s wilt of sweet corn. Disease symptoms include systemic wilting and foliar, water-soaked lesions. A Repeat-in-toxin (RTX)-like protein, RTX2, causes cell leakage and collapse in the leaf apoplast of susceptible corn varieties and is a primary mediator of water-soaked lesion formation in the P. stewartii-sweet corn pathosystem. RTX toxins comprise a large family of proteins, which are widely distributed among Gram-negative bacteria. These proteins are generally categorized as cellulolysins, but the Biofilm-Associated Proteins (Bap) subfamily of RTX toxins are implicated in surface adhesion and other biofilm behaviors. The Pnss RTX2 is most phylogenetically related to other Bap proteins suggesting that Pnss RTX2 plays a dual role in adhesion to host surfaces in addition to mediating the host cell lysis that leads to water-soaked lesion formation. Here we demonstrated that RTX2 localizes to the bacterial cell envelope and influences physiochemical properties of the bacterial cell envelope that impact bacterial cell length, cell envelope integrity and overall cellular hydrophobicity. Interestingly, the role of RTX2 as an adhesin was only evident in absence of exopolysaccharide (EPS) production suggesting that RTX2 plays a role as an adhesin early in biofilm development before EPS production is fully induced. However, deletion of rtx2 severely impacted Pnss’ colonization of the xylem suggesting that the dual role of RTX2 as a cytolysin and adhesin is a mechanism that links the apoplastic water-soaked lesion phase of infection to the wilting phase of the infection in the xylem.