B-151. The Role of RTX Toxins in Virulence of Pantoea stewartii subsp. stewartii, the Causal Agent of Stewart’s Wilt of Corn

C. Roper, S. von Bodman;
Univ. of Connecticut, Storrs, CT.

Pantoea stewartii subsp. stewartii is a serious pathogen affecting sweet corn and maize. This bacterium colonizes the xylem tissue and produces copious amounts of stewartan EPS, the major virulence factor contributing to plant wilt. The P. stewartii genome possesses two genes with high homology to large repetitive RTX toxins. RTX toxins comprise a large family of pore-forming proteins that are widely distributed among gram-negative bacteria. These cytotoxins usually lyse their target host cells, thus causing significant tissue damage. RTX toxins are well described for mammalian pathogens, but little is known about their role in plant pathogenesis. During the initial stages of infection, P. stewartii creates water-soaked lesions. This is due, in part, to WtsE, a type III effector secreted by the Hrp type III secretion system. We hypothesize that RTX toxins also play a role in the water-soaking phase of infection because of their lytic properties. Moreover, the promoters for rtx1 and rtx2 contain potential binding sites for ArcA and OxyR, transcription factors that are activated under microaerobic conditions and oxidative stress, respectively. These are conditions that P. stewartii likely encounters during the initial phase of plant infection. We hypothesize that RTX1 and RTX2 are important virulence factors when P. stewartii initially enters the plant tissue. In this study we constructed deletion mutants in the two P. stewartii rtx genes by marker exchange mutagenesis and tested them for virulence and water-soaking in planta. These data indicate that RTX toxins contribute to the characteristic water-soaked symptom and are important virulence factors for P. stewartii. Therefore, RTX toxins are one strategy that P. stewartii employs during the initial infection of sweet corn.