Abstract
Dickeya solani is a Gram-negative necrotrophic, plant pathogenic bacterium able to cause symptoms in a variety of plant species worldwide. As a facultative anaerobe, D. solani is able to infect hosts under a broad range of oxygen concentrations found in plant environments. However, little is known about oxygen-dependent gene expression in Dickeya spp. that might contribute to its success as a pathogen. Using a Tn5 transposon, harboring a promoterless gusA reporter gene, 146 mutants of D. solani IPO2222 were identified that exhibited oxygen-regulated expression of the gene into which the insertion had occurred. Of these mutants 114 exhibited higher expression under normal oxygen conditions than hypoxic conditions while 32 were more highly expressed under hypoxic conditions. The plant host colonization potential and pathogenicity as well as phenotypes likely to contribute to the ecological fitness of D. solani, including growth rate, carbon and nitrogen source utilization, production of pectinolytic enzymes, proteases, cellulases and siderophores, swimming and swarming motility and the ability to form biofilm were assessed for 37 strains exhibiting the greatest oxygen-dependent change in gene expression. Eight mutants expressed decreased ability to cause disease symptoms when inoculated into potato tubers or chicory leaves and three of these also exhibited delayed colonization of potato plants and exhibited tissue specific differences in gene expression in these various host tissues. The genes interrupted in these eight mutants encoded proteins involved in fundamental bacterial metabolism, virulence, bacteriocin and proline transport, while three encoded hypothetical or unknown proteins. The implications of environmental oxygen concentration on the ability of D. solani to cause disease symptoms in potato are discussed.
Highlights
Soft Rot Enterobacteriaceae (SRE: Pectobacterium spp. and Dickeya spp.) are important phytopathogenic, pectinolytic bacteria that cause large and increasing economic losses in agricultural crops worldwide (Pérombelon, 2002; Kado et al, 2006)
It is widely considered that these traits are central to the pathogenic success of other SRE such as Pectobacterium spp. and Dickeya spp. where they experience a broad range of oxygen concentrations depending on the host plant tissue they invade (Lund and Wyatt, 1972; De Boer and Kelman, 1978; Morris et al, 2009)
In this study we used a random Tn5- based reporter transposons to identify those genes whose transcription were responsive to oxygen concentrations and test the hypothesis that such oxygen-responsive genes would commonly be required for virulence in potato tubers and aboveground plant tissues that would be expected to vary in oxygen availability
Summary
Soft Rot Enterobacteriaceae (SRE: Pectobacterium spp. and Dickeya spp.) are important phytopathogenic, pectinolytic bacteria that cause large and increasing economic losses in agricultural crops worldwide (Pérombelon, 2002; Kado et al, 2006). In potato (Solanum tuberosum L.), Pectobacterium spp. and Dickeya spp. are responsible for tuber soft rot in transit and storage as well as potato blackleg in plants in the field (Pérombelon, 2002; Charkowski, 2006). While tuber contamination with SRE can occur during plant growth in the field, harvesting and grading are considered the most important stages at which healthy potato tubers acquire bacteria. This process most commonly occurs when rotten tubers harboring high densities of inoculum are present (Van Der Wolf and De Boer, 2007). Rotting in storage and transit is almost exclusively linked to poor ventilation and high humidity which together lead to the creation of a water film on tuber surface, which restricts the diffusion of oxygen, causing at least local oxygen depletion (hypoxia) (Kotoujansky, 1987)
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