| Summary: | 碩士 === 國立臺灣大學 === 植物科學研究所 === 99 === Ralstonia solanacearum is a soil-borne bacterium causing devastating wilting diseases in many economically important crops. Gaining insight into pathogenesis mechanism of this pathogen is thus important. The aim of this work was to study function of R. solanacearum tol-pal gene cluster and RSc0727 in bacterial fitness and pathogenesis. Previously, our group found that transposon (Tn) insertional mutants of tol-pal gene cluster conferred normal virulence on tomato plant, but significantly reduced virulence on Arabidopsis. In addition, these mutants displayed reduced motility, defective cell integrity and altered capability in carbons oxidation. This study, further revealed that tol-pal mutants had normal function in Type III secretion system and displayed normal virulence on additional solanaceous plants. However, stress response and complementation assays indicated that tol-pal gene cluster was essential for cell integrity maintenance and stress tolerance. All of the defects together contributed to the reduced fitness of the tol-pal mutants in the non-host plant Arabidopsis and thus led to severely reduced virulence. Furthermore, this study showed that R. solanacearum tol-pal gene cluster could be organized as five transcription units, which were differentially modulated throughout the growth phase, by the global virulence transcriptional regulalor PhcA, or by stress factors. The multifaceted modulation in tol-pal gene cluster may contribute to R. solanacearum fitness in various environments. By analyzing response of various Arabidopsis Col-0 mutants defective in hormone signaling pathways to infection of a local highly aggressive R. solanacearum Pss190, this study also reveals that the jasmonic acid (JA)/ethylene (ET) signaling pathways play a negative role in Arabidopsis tolerance , while salicylic acid (SA) and camalexin related pathways play positive roles. In the second part of this study, RSc0727, which was a putative orthologue of pilV gene which was suggested to be involved in Type IV pilus (Tfp) biogenesis, was studied. The organization of RSc0727-related gene cluster was mostly conserved among representative gram-negative bacteria alalyzed. Therefore, RSc0727 was designated as pilV. Our previous study showed that pilV is involved in twitching motility and biofilm formation. In this study, quantitative assays further confirmed the reduced biofilm formation and altered cell permeability of pilV mutant. In addition, pathogenesis and complementation assays further evidenced that pilV contributes to full spectrum of virulence. Furthermore, pilV was negatively regulated by PhcA, SA and JA, suggesting that the function of Tfp might be controlled or interfered by PhcA and plant defense mechanisms in the course of R. solanacearum infection. The results of this study together further demonstrate that differentially regulation and sophisticated coordination of various R. solanacearum pathogenesis mechanisms and the involved factors contribute greatly to versatile fitness of R. solanacearum in various abiotic and host environments, leading to its success in infecting a broad range of plant species in wide geographic distribution.
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