Identification of new Dickeya dadantii virulence factors secreted by the type 2 secretion system

Abstract

ABSTRACTDickeya are plant pathogenic bacteria able to provoke disease on a wide range of plants. A type 2 secretion system (T2SS) named Out is necessary for bacterial virulence. Its study in D. dadantii showed that it secretes a wide range of plant cell wall degrading enzymes, including pectinases and a cellulase. However, the full repertoire of exoproteins it can secrete has probably not yet been identified. Secreted proteins are first addressed to the periplasm before their secretion by Out. No secretion signal present on the protein allows the identification of substrates of a T2SS. To identify new Out substrates, we analyzed D. dadantii transcriptome data obtained in plant infection condition and searched for genes strongly induced encoding a protein with a signal sequence. We identified four new Out-secreted proteins: the expansin YoaJ, the putative virulence factor VirK and two proteins of the DUF 4879 family, SvfA and SvfB. We showed that SvfA and SvfB are required for full virulence of D. dadantii and showed that svf genes are present in a variable number of copies in other Pectobacteriaceae, up to three in D. fanghzongdai. This work opens the way to the study of the role of non-pectinolytic proteins secreted by the Out pathway in Pectobacteriaceae.IMPORTANCEThe plant pathogen Dickeya rely on a type 2 secretion system named Out for their pathogenicity. Importance of plant cell wall degrading enzymes secreted by this system has been well studied. However, existence and role of other Out-secreted proteins has barely been investigated. By mining D. dadantii transcriptome data, we identified four new Out-secreted proteins. We showed that two of them, SvfA and SvfB, are necessary for the full virulence of the bacteria. These findings show that identification of all the proteins secreted by the Dickeya Out system is necessary for a better knowledge of the virulence of these bacteria.