Genomics of Plant-Associated Bacteria: The Soft Rot Enterobacteriaceae

Abstract

The soft rot Enterobacteriaceae (SRE), which includes the two genera Pectobacterium and Dickeya, are broad host range pathogens that cause wilt and rot diseases on monocot and dicot plant hosts worldwide. These bacterial pathogens secrete high amounts of plant-cell-wall-degrading enzymes (PCWDE), such as pectinases and polygalacturonases, that digest plant cell walls and cause the soft rot symptoms characteristic of the SRE. The SRE can also colonize insects, and some strains are also insect pathogens. Genome sequences of a few dozen SRE strains are available, and these sequences have hastened discovery of SRE the role that many attributes play in virulence, including adhesins, toxins, volatile compounds, and contact-dependent secretion systems. Genomic and gene expression analyses have changed the view of the SRE from simple brute force pathogens to that of sophisticated pathogens that use intricately regulated gene clusters to colonize plants. Gene expression experiments also show that even though important regulatory proteins are conserved among the SRE, these proteins may not play the same regulatory roles in different SRE species. For example, only some SRE rely upon acyl-homoserine lactone (AHL)-based quorum sensing for regulation of pathogenesis genes, and SRE responses to oxygen limitation vary between Pectobacterium and Dickeya. Although there are many examples of resistance to the SRE, little effort has been made toward understanding how plants resist bacterial soft rot diseases. The realization that the SRE require more than pectinase production for disease may lead to increased interest in discovering plant mechanisms used to resist SRE pathogens.