Abstract
Flavescence dorée (FD) is a severe epidemic disease of grapevines caused by FD phytoplasma (FDP) transmitted by the leafhopper vector Scaphoideus titanus. The recent sequencing of the 647-kbp FDP genome highlighted an unusual number of genes encoding ATP-dependent zinc proteases FtsH, which have been linked to variations in the virulence of “Candidatus Phytoplasma mali” strains. The aims of the present study were to predict the FtsH repertoire of FDP, to predict the functional domains and topologies of the encoded proteins in the phytoplasma membrane and to measure the expression profiles in different hosts. Eight complete ftsH genes have been identified in the FDP genome. In addition to ftsH6, which appeared to be the original bacterial ortholog, the other seven gene copies were clustered on a common distinct phylogenetic branch, suggesting intra-genome duplication of ftsH. The expression of these proteins, quantified in plants and insect vectors in natural and experimental pathosystems, appeared to be modulated in a host-dependent manner. Two of the eight FtsH C-tails were predicted by Phobius software to be extracellular and, therefore, in direct contact with the host cellular content. As phytoplasmas cannot synthesize amino acids, our data raised questions regarding the involvement of FtsH in the adaptation to hosts via potentially enhanced recycling of phytoplasma cellular proteins and host protein degradation.
Highlights
In Europe, Flavescence dorée (FD) threatens viticulture [1,2]
In Mycoplasmas, Spiroplasmas, and Acholeplasmas, the three genera belonging to the class Mollicutes along with Phytoplasmas, the ftsH gene is present as a single copy, with Acholeplasmas being the cultivable mollicute genus that is closely related to the “Candidatus Phytoplasma” genus [20,21]
The eight ftsH genes identified in the genome of the FD phytoplasma (FDP) strain FD92 had sizes ranging from 1587 bp to 2025 bp, encoding proteins ranging in size from 528 to 674 amino acids and containing all conserved motifs
Summary
In Europe, Flavescence dorée (FD) threatens viticulture [1,2] This epidemic disease is caused by a phytoplasma (FDP) of taxonomic subgroups 16SrV-C and 16SrV-D transmitted by Scaphoideus titanus, a leafhopper vector of North American origin [3,4]. The draft genome of the FDP strain FD92 highlighted the extended gene set for protein degradation, especially genes encoding the ATP-dependent zinc-binding membrane proteases FtsH/HflB [17]. The present study aims to describe the gene repertoire for FtsH in the FDP genome, to predict the functional domains and topologies of the encoded proteins, and to establish the expression profile of these genes in different plant hosts and vector insects. The natural pathosystem consisting of grapevine and S. titanus, and an experimental pathosystem in which the phytoplasma strain had been maintained since its transmission to broadbean
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