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  • 學位論文

百慕達草白葉病植物菌質體的基因體比較分析以及其效應因子之研究

Comparative Genomics Analysis and Effector Characterization of the Bermudagrass White Leaf (BGWL) Phytoplasma

指導教授 : 林長平
共同指導教授 : 郭志鴻(Chih-Horng Kuo)
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摘要


植物菌質體‘Candidatus Phytoplasma cynodontis’ 為百慕達草白葉病(bermudagrass white leaf; BGWL)的病原菌,此草皮病害於1972年在台灣初次被發現。該細菌性病原菌可於寄主植物的篩管中及媒介昆蟲體內增殖,致使植物產生葉片白化與植株矮化等病徵,進而影響受感染之運動場草皮與牧草的品質,並使草皮水土保持的功能降低。即便已經過多年的努力,目前仍然無法在實驗室中離體培養植物菌質體,因此相關的研究一直面臨很大的困難。為了突破此限制,本論文針對於桃園觀音採集到的病株GY2015進行全基因體定序,藉以瞭解其基因組成。經過基因體組裝目前得到5條連續序列,總長為502,218 bp,共含有425個蛋白編碼基因(protein-coding gene)。為了能對不同植物菌質體間基因組成演化有更多的了解,我們挑選了另外六株已發表且寄主不同的植物菌質體基因體序列進行分子演化及比較基因體學分析,結果顯示不同菌株間的遺傳歧異度有著相當大的差異。此外,BGWL植物菌質體GY2015擁有一組尚未在其他植物菌質體中發現過的新型potential mobile unit (PMU)。由於已知PMU可能與基因體的穩定性及效應因子基因水平轉移有關,此發現拓展了我們於植物菌質體遺傳多樣性的了解。另外我們也發現,前人從其他植物菌質體物種中找到的效應因子基因SAP11、SAP54/PHYL1和TENGU在BGWL植物菌質體的基因體中皆不存在,意味著BGWL植物菌質體可能擁有不同的效應因子。我們透過生物資訊分析預測出45個候選效應因子基因,且從基因轉殖植物的表現型初步篩選出一個可能為效應因子基因的候選基因,表現該基因的轉殖植物出現葉片白化、植株矮化、叢生及不孕花等多項類似植物菌質體感染的性狀。未來針對這些候選效應因子進行功能研究,將有助於我們對此病原菌之深入認識。本篇針對一株與現有較完整基因體學研究的植物菌質體在演化上距離較遠的物種進行研究,透過比較基因體學分析,可增進我們對植物菌質體於演化、遺傳學及病理學上的認識。

並列摘要


‘Candidatus Phytoplasma cynodontis’ is a causative agent of a turf disease—bermudagrass white leaf (BGWL), which was first reported in Taiwan in 1972. This phytopathogenic bacterium is transmitted by insects and is restricted to sieve tubes of infected plants. The symptoms include chlorosis and dwarfism, thus decreasing the quality of sports fields and pasture, as well as lowering the protection against soil erosion. Despite extensive effort, phytoplasmas have remained uncultivable outside of their hosts, making the investigation of their biology extremely difficult. To circumvent this difficulty, we conducted whole genome sequencing for a diseased plant collected in Guanyin (Taoyuan, Taiwan) to investigate the gene content of this bacterium (GY2015). The draft genome assembly contains five contigs with a combined size of 502,218 bp and 425 protein-coding genes. To better understand the evolution of gene content within this genus, we selected six other phytoplasma genomes for molecular phylogenetics and comparative genomics analysis. The results indicated that the gene content is highly diversified across different phytoplasma species. Moreover, the BGWL phytoplasma harbors a new type of potential mobile unit (PMU) that has not been described in other phytoplasmas. Because PMUs could promote genome instability and horizontal transfer of effector genes, this finding expands our understanding of phytoplasma genetic diversity. Finally, none of the three characterized phytoplasma effectors (i.e., SAP11, SAP54/PHYL1, and TENGU) was found in this BGWL phytoplasma genome, suggesting that this bacterium harbors novel effectors. Our bioinformatics prediction identified 45 putative effector genes and our preliminary screening through expression in transgenic Arabidopsis identified one candidate that induced morphological phenotypes similar to phytoplasma infection (e.g., chlorosis, dwarfism, witches’-broom, and sterile flower). Future functional characterization of these putative effectors could further improve our knowledge of this plant pathogen. In this study, we focus on a phytoplasma species that represents a highly diverged lineage from all of the other phytoplasmas with genomic studies. Through comparative genomic analysis, this study promoted our understanding on the evolution, genetics, and pathology of phytoplasmas.

參考文獻


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