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Open Access Publications from the University of California

Characterization of an RTX-Like Toxin and an Alpha-2-Macroglobulin in Pantoea stewartii subsp. stewartii, Causal Agent of Stewart's Wilt of Sweet Corn

  • Author(s): Williams, Kayla Marie
  • Advisor(s): Roper, Caroline
  • et al.

Pantoea stewartii subsp. stewartii DC283, the causal agent of Stewart's wilt, is an important bacterial pathogen of sweet corn. P. stewartii colonizes the apoplastic space and xylem tissue, resulting in characteristic water-soaked (WS) lesions and wilting. A gene encoding a putative RTX-like toxin, rtx2, has been identified in P. stewartii. RTX toxins belong to the pore-forming toxin family and have lytic properties in animal systems. Little is known about the role of RTX toxins in plant pathogenesis. Interestingly, a P. stewartii Δrtx2 mutant was noticeably unable to incite WS lesions in planta implicating it in the cell lysis associated with lesion formation. In addition, rtx2 is genetically linked to components of the Rcs phosphorelay; a global regulatory system of critical virulence factors in P. stewartii. This suggests a possible functional relationship, particularly because they are transcribed from a single promoter. The RTX2 domain structure is consistent with an autotransported protein and contains conserved transmembrane domains suggesting it translocates itself to the cell surface. The signal relay pathway that stimulates the Rcs phosphorelay is unknown, but membrane perturbation plays a key role in stimulating the Rcs system. We hypothesize that RTX2 contributes to the membrane dynamics that are being sensed by the Rcs system, thus modulating activation of signal transduction through the system.

Pantoea stewartii is a bacterial plant pathogen that causes Stewart's Wilt of sweet corn. P. stewartii is transmitted by the corn flea beetle, which creates wounds in the plant and allows the bacterium to enter the xylem and leaf apoplast resulting in characteristic water-soaked lesions and wilting. The P. stewartii genome has been fully sequenced and two putative alpha-2-macroglobulin (α2M) genes have been identified. The α2M protein is a known serine protease inhibitor in eukaryotic organisms that binds non-self proteases and degrades them. In prokaryotes, the α2M gene can be found juxtaposed next to a gene that encodes a penicillin-binding protein (PBP) associated with cell wall repair. It has been suggested that the α2M protein protects the bacteria from non-self proteases and the PBP repairs damaged peptidoglycan that may result from a damage cell envelope. Proposed functions of the α2M protein include subverting host proteases. The α2M protein may also function in defense against other bacterial proteases present in multicellular microbial communities. The goal of this study was to characterize the role of α2MI during the host-pathogen interaction between P. stewartii and sweet corn.

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