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Title: Lilac (Syringa sp.) hosts a new ‘Ca. Phytoplasma pruni’-related strain in Utah: ribosomal RNA and secY genes distinguish the phytoplasma from previously described lineages

Author
item Davis, Robert
item Dally, Ellen
item Zhao, Yan
item THOMSON, S - Utah State University
item NISCHWITZ, C - Utah State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2015
Publication Date: 1/1/2016
Citation: Davis, R.E., Dally, E.L., Zhao, Y., Thomson, S.V., Nischwitz, C. 2016. Lilac (Syringa sp.) hosts a new ‘Ca. Phytoplasma pruni’-related strain in Utah: ribosomal RNA and secY genes distinguish the phytoplasma from previously described lineages. Plant Disease. vol. 100: page 207. doi.org/10.1094/PDIS-06-15-0679-PDN.

Interpretive Summary: Minute, specialized bacteria called phytoplasmas cause diseases in crop plants around the world. Phytoplasmas live in the food conducting tissues (phloem) of plant veins, and they are spread from plant to plant by insects that feed in this kind of tissue. In order to design methods to reduce the spread of crop diseases caused by phytoplasmas, it is important to know whether or not a given diseased plant is infected by a phytoplasma, and to learn what type of phytoplasma is causing the disease. In this work, we investigated lilac plants that were diseased and dying in the state of Utah. In previous years, extensive work had failed to find the cause of the disease. No virus or bacterium or fungus could be found in the plants. We suspected that the disease, lilac decline (LlcDec), might be caused by a phytoplasma. In our experiments, we discovered that the diseased plants contained DNA from a phytoplasma. By analyzing the DNA from two different kinds of genes, we learned that the phytoplasma was related to a phytoplasma that causes disease in peach and cherry trees, but that the phytoplasma in the diseased lilac plants was a new type of phytoplasma that had not been seen previously. We identified specific regions in the two genes that can be used to specifically identify this new phytoplasma. We also found that a protein coded by one of the genes also distinguished this new phytoplasmas from the related peach- and cherry-infecting phytoplasma. The results of this work will make it possible for scientists to figure out how this disease causing phytoplasma is spread, learn whether it threatens food or other crop plants, and to design methods to reduce or prevent its spread.

Technical Abstract: A decline in lilac (Syringa sp.) has been observed in Utah for over 25 years. Disease symptoms including leaf chlorosis, mosaic and rolling are initially present, but plants eventually die over a period of 2-3 years. Attempts over several years did not determine any consistent isolations or detection of bacterial, fungal or virus pathogens. Polymerase chain reactions (PCRs) were used to assess possible association of a phytoplasma with the disease, lilac decline (LlcDec). DNA for use as PCR template was separately extracted from excised veins of symptomatic leaves collected from two diseased plants. Amplicons of ribosomal (r) RNA gene sequences (rDNA) and of secY genomic regions were of predicted sizes, indicating possible phytoplasmal infection in both plants. Nucleotide sequencing confirmed that the amplicons were derived from a phytoplasma; the nucleotide sequences determined for the amplified 1.5 kbp rDNA and 1.7 kbp secY genomic regions were deposited in the GenBank database. The LlcDec phytoplasma was classified, by using iPhyClassifier, as a member of group 16SrIII. The 16S rDNA virtual RFLP patterns were most similar to those of subgroup 16SrIII-A (‘Ca. Phytoplasma pruni’), but the MseI RFLP pattern differed, indicating strain LlcDec represents a variant subgroup lineage, 16SrIII-A*. LlcDec phytoplasma also differed from ‘Ca. Phytoplasma pruni’ by a single base in each of three 16S rRNA gene regions corresponding to unique sequences in the original description of ‘Ca. Phytoplasma pruni’. Accordingly, strain LlcDec is referred to as a ‘Ca. Phytoplasma pruni’-related strain. The deduced amino acid sequence of strain LlcDec SecY protein translocase differed from that of ‘Ca. Phytoplasma pruni’ by two residues (at positions 90, P [proline in LlcDec]'S [serine in ‘Ca. Phytoplasma pruni’]; and 167, I [isoleucine in LlcDec]'V [valine in ‘Ca. Phytoplasma pruni’]). Phylogenetic analyses of 16S rDNA and SecY amino acid sequences indicated that strain LlcDec represents a distinct lineage closely related to ‘Ca. Phytoplasma pruni’. Although available data now indicate that lilac can be infected by a ‘Ca. Phytoplasma pruni’-related strain, ‘Ca. Phytoplasma fraxini’, ‘Ca. Phytoplasma pruni’, and a ‘Ca. Phytoplasma asteris’-related strain, it is noteworthy that the symptoms associated with infection by a ‘Ca. Phytoplasma pruni’-related strain differ significantly from those associated with infection of lilac by the other phytoplasmas.