Search

Dickeya solani is a Gram-negative bacterium able to cause disease symptoms on a variety of crop and ornamental plants worldwide. Weeds including Solanum dulcamara (bittersweet nightshade) growing near agricultural fields have been reported to support populations of soft rot bacteria in natural settings. However, little is known about the specific interaction of D. solani with such weed plants that may contribute to its success as an agricultural pathogen. The aim of this work was to assess the interaction of D. solani with its crop plant (Solanum tuberosum) and an alternative (S. dulcamara) host plant. From a collection of 10,000 Tn5 transposon mutants of D. solani IPO2222 carrying an inducible, promotorless gusA reporter gene, 210 were identified that exhibited plant tissue-dependent expression of the gene/operon into which the Tn5 insertion had occurred. Thirteen Tn5 mutants exhibiting the greatest plant tissue induction of such transcriptional units in S. tuberosum or S. dulcamara as measured by qRT-PCR were assessed for plant host colonization, virulence, and ability to macerate plant tissue, as well as phenotypes likely to contribute to the ecological fitness of D. solani, including growth rate, carbon and nitrogen source utilization, motility, chemotaxis toward plant extracts, biofilm formation, growth under anaerobic conditions and quorum sensing. These 13 transcriptional units encode proteins involved in bacterial interactions with plants, with functions linked to cell envelope structure, chemotaxis and carbon metabolism. The selected 13 genes/operons were differentially expressed in, and thus contributed preferentially to D. solani fitness in potato and/or S. dulcamara stem, leaf, and root tissues.

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.