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AY-WB Phytoplasma Secretes a Protein That Targets Plant Cell Nuclei

¹Department of Entomology and ²Department of Plant Pathology, The Ohio State University--OARDC, Wooster 44691, U.S.A.; ³Sainsbury Laboratory Norwich Research Park, Norwich, NR4 7UH, U.K.; ⁴Department of Disease and Stress Biology, The John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, U.K.

The fully sequenced genome of aster yellows phytoplasma strain witches' broom (AY-WB; Candidatus Phytoplasma asteris) was mined for the presence of genes encoding secreted proteins based on the presence of N-terminal signal peptides (SP). We identified 56 secreted AY-WB proteins (SAP). These SAP are candidate effector proteins potentially involved in interaction with plant and insect cell components. One of these SAP, SAP11, contains an N-terminal SP sequence and a eukaryotic bipartite nuclear localization signal (NLS). Transcripts for SAP11 were detected in AY-WB-infected plants. Yellow fluorescence protein (YFP)-tagged SAP11 accumulated in Nicotiana benthamiana cell nuclei, whereas the nuclear targeting of YFP-tagged SAP11 mutants with disrupted NLS was inhibited. The nuclear transport of YFP-SAP11 was also inhibited in N. benthamiana plants in which the expression of importin α was knocked down using virus-induced gene silencing (VIGS). Furthermore, SAP11 was detected by immunocytology in nuclei of young sink tissues of China aster plants infected with AY-WB. In summary, this work shows that AY-WB phytoplasma produces a protein that targets the nuclei of plant host cells; this protein is a potential phytoplasma effector that may alter plant cell physiology.

Additional keywords:insect vector, leafhopper, mollicute, sec-dependent secretion, virulence protein.