Abstract
Pectobacterium species are enterobacterial plant-pathogens that cause soft rot disease in diverse plant species. Unlike hemi-biotrophic plant pathogenic bacteria, the type III secretion system (T3SS) of Pectobacterium carotovorum subsp. carotovorum (P. carotovorum) appears to secrete only one effector protein, DspE. Previously, we found that the T3SS regulator HrpL and the effector DspE are required for P. carotovorum pathogenesis on leaves. Here, we identified genes up-regulated by HrpL, visualized expression of dspE in leaves, and established that DspE causes host cell death. DspE required its full length and WxxxE-like motifs, which are characteristic of the AvrE-family effectors, for host cell death. We also examined expression in plant leaves and showed that hrpL is required for the expression of dspE and hrpN, and that the loss of a functional T3SS had unexpected effects on expression of other genes during leaf infection. These data support a model where P. carotovorum uses the T3SS early in leaf infection to initiate pathogenesis through elicitation of DspE-mediated host cell death.
Highlights
Pectobacterium carotovorum is a necrotrophic Enterobactericeae pathogen that causes soft rot disease on plant species from over 24 orders of angiosperms, including important crops in the Solanaceae and Brassicaceae
We found that DspE alone can cause cell death and demonstrated that WxxxD/E motifs are important for this function, which supports the hypothesis that P. carotovorum pathogenesis in leaf tissue is initiated by DspE-mediated host cell death
We found that hrpN expression was inducible via HrpL; its basal expression level was higher than 30% of the fluorescence of the library, suggesting the reason it was not captured during the screen
Summary
Pectobacterium carotovorum is a necrotrophic Enterobactericeae pathogen that causes soft rot disease on plant species from over 24 orders of angiosperms, including important crops in the Solanaceae and Brassicaceae. P. carotovorum attacks plant tissue by secreting an array of plant cell wall degrading enzymes through the type II secretion system [5,6,7,8,9,10]. It has a type III secretion system (T3SS) that is required for pathogenesis in leaves [11]. In plant pathogenic Enterobacteriaceae, such as P. carotovorum, and in the plant pathogen Pseudomonas syringae, the alternate sigma factor HrpL is the main regulator of genes in the hrp cluster and in these bacteria, HrpL regulates expression of all known T3-secreted effectors [14,15,16,17,18,19]
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