Marginal chlorosis of strawberry: transmission of 'Candidatus Phlomobacter fragariae' to periwinkle by the planthopper Cixius wagneri (China) and characterization of the isolate.

摘要

The uncultured phloem-restricted gamma 3-proteobacterium, Candidatus Phlomobacter fragariae, is associated with marginal chlorosis of strawberry in French production fields. Strawberry isolates could not be maintained under greenhouse conditions, since the infected plants died and graft transmission could not be achieved. The recent identification of the planthopper Cixius wagneri as vector of this proteobacterium allowed the transmission of Ca. P. fragariae to Catharanthus roseus, providing the first Ca. P. fragariae reference isolate. Electron microscopy, 16S-rDNA and spoT PCR amplification confirmed the presence of Ca. P. fragariae in diseased periwinkles. Graft transmission of infected shoots to healthy periwinkle seedlings led to the stable maintenance of isolate LG2001 and reproducible observations of symptom expression. Recent description of 16S-rDNA sequences from other proteobacteria revealed that Ca. P. fragariae was closely related to a proteobacterium associated with a sugarbeet disease and transmitted by another planthopper and to secondary symbionts of whiteflies and psyllids. A remarkable feature in Ca. P. fragariae 16S-rDNA sequence was a unique 36 bp deletion at the beginning of the gene. To better characterize our reference isolate, we further studied its 16S-rDNA. Because approximately 11 different Ca. P. fragariae 16S-rDNA copies could be found, we concluded that isolate LG 2001 is either a mixed infection of 2 different Ca. P. fragariae strains or possesses more than eleven 16S-rDNA copies, and only some have the 36 bp deletion at the beginning of the gene.