| Summary: | 碩士 === 國立中興大學 === 植物病理學系所 === 104 === Pseudomonas syringae van Hall 1902, originally isolated from lilac (Syringa vulgaris L.), is a complex species that infects many plant species under favorable conditions and is classified to different pathovars based on their compatibility with hosts. Generally, P. syringae pathovars harbor different virulence factors and elicit differential reactions on plants, which cannot be easily revealed by physiological and biochemical tests. The common virulence factors include the type III secretion system (T3SS)-secreted effector proteins and non-ribosomal protein synthetase (NRPS)-synthesized phytotoxins, both are involved in interfering plant metabolism and defense responses to promote bacterial parasitism. In Taiwan where the humid subtropical climate is generally considered unsuitable for P. syringae infection; a few disease incidences, e.g. bacterial spot of carambola and angular leaf spot of cucurbits, have been recorded in the orchards and nurseries of southern Taiwan. In 2014, Japan-imported pear scions developed typical blossom blast symptom after they were grafted to local pear stocks. Suspect bacterial pathogen was isolated from the symptomatic plant tissues, and the colony morphology on King’s B medium, LOPAT tests, and fatty acid methyl ester analysis (Agilent Technologies, Santa Clara, CA, USA) revealed the isolated bacteria shared similar characteristics of P. syringae. Koch’s postulates were fulfilled by prick-inoculating the isolated bacteria to pear leaves, and the bacteria showing similar characteristics can be re-isolated from the symptomatic tissues of the inoculated plants. To determine the pathovar identity, P. syringae pear strains were inoculated to the pods of common bean by syringe infiltration. The elicitation of brown necrosis in 3 days post inoculation was recorded as a typical feature of the pathovar syringae in comparison with water-soaked symptom induced by bean-pathogenic strains. Multilocus sequence typing (MLST) of 7 housekeeping genes (rpoD, gyrB, acnB, cts, gap, pgi, and pfk) revealed that the pear strains from the imported pear scions were phylogenetically separated from the other pear-pathogenic P. syringae pv. syringae (Psy) found in other countries. Genome-wide comparison of Psy from different sources also showed that these strains contained various virulence factors, suggesting that Psy strains may have co-evolved with their host plants to achieve optimum infection under diverse environments. For accurately and quickly identifying the suspect pathogen from infected plant tissues, the nucleotide sequences of syringomycin biosynthesis genes, a conserved 166-bp genomic locus, and genes coding for the type III secretion system were used to develop standard PCR detection protocol. The application of PCR detection and plant inoculation assay will ensure the correct identification of Psy.
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