The Phytopathogen Dickeya dadantii ( Erwinia chrysanthemi 3937) Is a Pathogen of the Pea Aphid

Abstract

Dickeya dadantii (Erwinia chrysanthemi) is a phytopathogenic bacterium causing soft rot diseases on many crops. The sequencing of its genome identified four genes encoding homologues of the Cyt family of insecticidal toxins from Bacillus thuringiensis, which are not present in the close relative Pectobacterium carotovorum subsp. atrosepticum. The pathogenicity of D. dadantii was tested on the pea aphid Acyrthosiphon pisum, and the bacterium was shown to be highly virulent for this insect, either by septic injury or by oral infection. The lethal inoculum dose was calculated to be as low as 10 ingested bacterial cells. A D. dadantii mutant with the four cytotoxin genes deleted showed a reduced per os virulence for A. pisum, highlighting the potential role of at least one of these genes in pathogenicity. Since only one bacterial pathogen of aphids has been previously described (Erwinia aphidicola), other species from the same bacterial group were tested. The pathogenic trait for aphids was shown to be widespread, albeit variable, within the phytopathogens, with no link to phylogenetic positioning in the Enterobacteriaceae. Previously characterized gut symbionts from thrips (Erwinia/Pantoea group) were also highly pathogenic to the aphid, whereas the potent entomopathogen Photorhabdus luminescens was not. D. dadantii is not a generalist insect pathogen, since it has low pathogenicity for three other insect species (Drosophila melanogaster, Sitophilus oryzae, and Spodoptera littoralis). D. dadantii was one of the most virulent aphid pathogens in our screening, and it was active on most aphid instars, except for the first one, probably due to anatomical filtering. The observed difference in virulence toward apterous and winged aphids may have an ecological impact, and this deserves specific attention in future research.Dickeya dadantii (syn. Erwinia chrysanthemi, Pectobacterium chrysanthemi) (53), as well as many other erwinia species, is one of the phytopathogenic Enterobacteriaceae that can cause soft rot diseases in a wide range of economically important crops. These bacteria can survive in soils, from where they are transmitted to plants by water, miscellaneous insects, or cultural techniques. There is no recorded specific vector organism for such pathogens, although Drosophila melanogaster was claimed to be a potential vector for some Erwinia species (3). The common symptoms for D. dadantii infection, as well as other so-called pectinolytic soft rot Erwinia, are characterized by the rapid disorganization of parenchymatous tissues, mainly driven by pectic enzymes (38). Nevertheless, plant colonization by soft rot Erwinia is a multifactorial process requiring numerous additional factors, such as cellulases, iron assimilation, an Hrp type III secretion system, exopolysaccharides, motility, and proteins involved in resistance against plant defense mechanisms (58). The recent deciphering of the complete genome sequence of D. dadantii, strain 3937 (28), after that of Pectobacterium carotovorum subsp. atrosepticum (syn....

Bacterial strategies to overcome insect defences

“…ability to colonize and persist in insect species might have resulted from horizontal gene transfer (HGT) of plasmid-borne persistence genes in S. entomophila (afp-encoding genes) and in M. ulcerans (mycolactone biosynthetic genes). A role for HGT in the evolution of entomopathogens is also revealed by the distribution of genes that encode similar toxins in unrelated genera 50,77,78 , which suggests that the insect gut constitutes an environment that is favourable for gene transfer 79 .…”

The semiochemically mediated interactions between bacteria and insects

“…Oliver et al 2003Oliver et al , 2005Oliver et al , 2008Oliver et al , 2010Scarborough et al 2005;Douglas 2006;Nakabachi et al 2006;Thao and Baumann 2004;Baumann et al 2002;Kaltenpoth 2009;Schoenian et al 2011;Oh et al 2009aOh et al , b, 2011Brachmann et al 2006;Piel et al 2004;Barke et al 2010;Haeder et al 2009;Scott et al 2008;Leroy et al 2011;Sabri et al 2010) and several examples illustrate pathogenic interactions (e.g. Grenier et al 2006;Ffrench-Constant et al 2007;Herbert and Goodrich-Blair 2007;Lecadet et al 1999;Schnepf et al 1998;de Maagd et al 2003) while the semiochemically mediated interactions are more rarely described. However, plenty of bacteria have been shown to play an important role in the production of volatile compounds which ones may act as semiochemicals.…”

Bacterial Communities of Two Parthenogenetic Aphid Species Cocolonizing Two Host Plants across the Hawaiian Islands

“…This group of insects has been extensively studied with respect to their association with bacteria, which display diverse phenotypic effects on the aphid hosts and range from detrimental to beneficial. While a few bacteria, typically from the family Enterobacteriaceae, have been shown to be pathogenic (11,12), the majority of characterized bacteria of aphids are mutualists and maternally inherited (i.e., vertically transmitted) symbionts (6,13,27). For instance, nearly all aphids are infected with Buchnera aphidicola (5), an obligate endosymbiont localized within specific cells derived from the host, the bacteriocytes (5).…”