Identification of RNA silencing suppressor encoded by wheat blue dwarf (WBD) phytoplasma

Plants possess an innate immune system to defend plant pathogens. In turn, pathogens have various strategies to overcome complex defenses of plants. Among diverse pathogens, phytoplasmas are associated with serious diseases in a range of plants species. RNA silencing serves as an efficient defensive system to pathogens in eukaryotes and can be interrupted by RNA silencing suppressors (RSSs) encoded by pathogens. Nowadays, many RNA silencing suppressors have been identified from virus, bacteria, oomycete, and fungi. Phytoplasmas are pathogens to several hundred plant species. In this research, 37 candidate effectors of wheat blue dwarf (WBD) phytoplasma were screened for RSS. Agro-infiltrating assay, yeast expression system, floral-dip method for constructing transgenic A. thaliana, western blotting, and RT-qPCR were used for identification of RNA silencing suppressor. As results, SWP16 encoded by WBD phytoplasma was proved to be a secretory protein which inhibited accumulation of GFP siRNA and led to the accumulation of GPF mRNA in systemic leaves of N. benthamiana 16c. Furthermore, in A. thaliana SWP16 inhibited the production of miRNAs which are components of RNA silencing. SWP16 also promoted infection of potato virus X. It concluded that SWP16 encoded by WBD phytoplasma was an RNA silencing suppressor suppressing systemic RNA silencing. This is the first evidence that phytoplasma encodes RNA silencing suppressor, which laid theoretical basis for researches on interaction mechanism between pathogens and plants.