Abstract
Pantoea stewartii subsp. stewartii is a Gram-negative proteobacterium that causes leaf blight and Stewart’s wilt disease in corn. Quorum sensing (QS) controls bacterial exopolysaccharide production that blocks water transport in the plant xylem at high bacterial densities during the later stage of the infection, resulting in wilt. At low cell density the key master QS regulator in P. stewartii, EsaR, directly represses rcsA, encoding an activator of capsule biosynthesis genes, but activates lrhA, encoding a transcription factor that regulates surface motility. Both RcsA and LrhA have been shown to play a role in plant virulence. In this study, additional information about the downstream targets of LrhA and its interaction with RcsA was determined. A transcriptional fusion assay revealed autorepression of LrhA in P. stewartii and electrophoretic mobility shift assays (EMSA) using purified LrhA confirmed that LrhA binds to its own promoter. In addition, LrhA binds to the promoter for the RcsA gene, as well as those for putative fimbrial subunits and biosurfactant production enzymes in P. stewartii, but not to the flhDC promoter, which is the main direct target of LrhA in Escherichia coli. This work led to a reexamination of the physiological function of RcsA in P. stewartii and the discovery that it also plays a role in surface motility. These findings are broadening our understanding of the coordinated regulatory cascades utilized in the phytopathogen P. stewartii.
Highlights
Pantoea stewartii subsp. stewartii, a Gram-negative rod-shaped, gamma-proteobacterium, is the causal agent of leaf blight and Stewart’s wilt in susceptible varieties of Zea mays
E. coli S17-1 was transformed with this plasmid construct containing PlrhA-gfp, which was moved into the wild-type P. stewartii DC283, lrhA and lrhA/lrhA+ strains (Table 1) via conjugation
Expression levels of Green fluorescent protein fusion (GFP) in the lrhA strain were significantly higher than the wild-type strain (Fig. 1, p = 0.00001) indicating that LrhA normally represses its own expression in the wild-type strain
Summary
Pantoea stewartii subsp. stewartii, a Gram-negative rod-shaped, gamma-proteobacterium, is the causal agent of leaf blight and Stewart’s wilt in susceptible varieties of Zea mays. How to cite this article Duong and Stevens (2017), Integrated downstream regulation by the quorum-sensing controlled transcription factors LrhA and RcsA impacts phenotypic outputs associated with virulence in the phytopathogen Pantoea stewartii subsp. In a second phase of the disease, the bacteria migrate to the xylem, where they grow to high cell density and form a biofilm that blocks water flow within the plant. This results in wilt disease and even death, if the plants were infected at the seedling phase (Braun, 1982). Quorum sensing (QS), a mechanism of bacterial cell density-dependent communication, controls the virulence, capsule production and surface motility of this pathogen (Roper, 2011; Von Bodman, Bauer & Coplin, 2003)
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