Citrus Canker and Its Pathogenic Bacterium

Abstract

柑橘潰瘍病，分佈廣泛，是國際柑橘栽培上最具破壞性的一種細菌性病害，由Xonthomonos compestris pv. citri所引起。X. compestris pv. citri是一種具有一條極生鞭毛的短桿狀細菌，含有不同大小的質體及一、二種的轉位子。柑橘潰瘍病菌A病原型菌株的病原性基因，經接合交配至x. compestris pv. citri的E病原型菌株、X. compestris pv. alfol fae及X. compestris pv. cyomopsidis等細菌，使其在葡萄抽葉片上能誘導產出類似潰瘍病的病斑。柑橘潰瘍病菌的CPI和CP122噬菌體為球形頭部，帶有一條長尾巴，而CPZ和CPlls為多邊球型，具有一條短尾。Cf和Cfl6為絲狀噬菌體，皆含有環狀單股DNA。Cf DNA可利用電孔法成功地經轉染而進入柑橘潰瘍病菌。cfl6侵入X. compestris pv. citri後，其DNA先複製大量複製型(RF) DNA，其中一組RF DNA插入寄主染色體，其他RF DNA仍可和染色體同時存在並自行複製，細胞不斷進行分裂時，RF DNA會由細胞中逐漸丟失，最後僅剩有插入於寄主染色體中的RF DNA狀態噬菌體基因體，此種潛溶過程不同於其他潛溶現象，故稱為新潛溶過程（Neolysogenizotion）。柑橘潰瘍病菌xw47菌株，被cfl6潛溶後分別誘導半胱氨酸及組氨酸合成的突變，而且從柑橘潰瘍病菌所分離的Cf29、CBZ、Cf99、Cfl17、Cfl34等另一群絲狀噬菌體，也可引起寄主細菌的營養缺陷突變，也可轉導細菌鞭毛的運動性。依病原性之差異，X. compestris pv. citri可分五個病原型；A病原型，分佈於世界各地，對葡萄抽、甜橙、檸檬、墨西哥萊姆危害最強；B病原型，分佈於阿根廷、巴拉圭、烏拉圭，對檸檬、墨西哥萊姆具強病原性；C病原型分佈於巴西，僅對墨西哥萊姆有病原性；D病原型分佈於墨西哥，僅感染墨西哥萊姆的葉片；上述四病原型皆可使植株產生凸起、木栓化、外圍具有黃暈的病斑；E病原型僅發生於美國佛羅里達州柑橘苗圃，主要危害根殼和葡萄ha的Swingle citrumelo雜交種，可使植株產生扁平、水浸狀的病斑，因其病斑形態不同於前者，少數學者稱此病原型所引起的柑橘病害為細菌性斑點病，而其病原菌為compestris pv. citrumelo。二病害雖有病理學及生化、DNA同源性的差異，目前多數學者仍稱此五病原型所引起的柑橘病害為潰瘍病，而其病原菌為X. compestris pv. citri 。柑橘潰瘍病菌的鑑定或偵測，有接種、生理生化特性、噬菌體、酵素連結抗體法、聚合碳酸膜固定免疫螢光法及單元抗體等測試法；隨著分子生物技術的精進，利用基因體指紋分析、DNA-DNA雜合及RFLP分析更可準確地鑑定或區別柑橘潰瘍病菌各病原型。柑橘潰瘍病菌細胞內含有質體，以其限制嗨片段做成的核酸探針，並配合PCR技術，可偵測到25pg純化的DNA或103cells/ml濃度的病菌。 Citrus canker caused by Xanthomonas campestris pv. citri has occurred widely in the world. X. campestris pv. citri is a short rod with a polar flagellum and harbors several plasmids and transposable elements. Pathogenicity gene pthA of this bacterium belonging to pathotype A, after conjugal transfer through the plasmid pSS1O.35 of Escherichia coil, enables strains of X. campestris pv. citri pathotype E and several pathovars of X. campestris to elicit cankerlike lesions on grapefruit leaves. Phages CP1 and CP122 of this bacterium have spherical heads each with a long tail,whereas CP2 and CP115 are polyhedrial cubics each with a short tail. Phages Cf and Cf 16 are filamentous,containing circular, single-stranded DNA. The replicative form (RF) DNA of Cf is able to transfect X.campestris pv. citri XW47 by electroporation. Cf 16, after infection, neolysogenizes its host by integra tion of its RE DNA into the host chromosome. Phages Cf 16, as well as Cf29,Cf32, Cf99, Cf 117, and Cf 134, after lysogenization, induce several auxotrophic mutations of X. campestris pv. citri XW47; the latters also mediate transduction of mobility of the bacterium. Five pathotypes of X. campestris pv. citri have been distinguished: pathotype A occurs widely in the world and is most pathogenic on grapefruit, sweet oranges, and Mexican lime; pathotype B occurs in Agrentina, Paraguay, and Uruguay and is most pathogenic on lemon and Mexican lime; pathotype C occurs in Brazil and pathogenic on Mexican lime; pathotype D distributes in Mexico and pathogenic on the Mexican lime leaves; all of these produce corky and raised lesions; pathotype E occurs only in Florida, USA and produces lesions of flat,water-soaked, and surrounded chiorosis on grapefruit, trifoliate orange, and citrumelo. Symptoms produced between pathotypes A-D and E are so different that some plant pathologists have named the disease and the causal agent belonging to pathotype E as citrus bacterial spot and X. campestris pv. citrumelo, respectively. Although some degrees of pathological, biochemical, and molecular differences exist, the diseases on citrus caused by the 5 pathotypes of this bacterium have still called canker and, hence, the causal bacterium X. campestris pv. citri This bacterium and citrus canker can be identified or detected by pathological inoculation, physiological and biochemical tests, phage,enzyme-linked immunofluorescence, monoclonal antibody, genomic fingerprinting, DNA-DNA hybridization, restriction-fragment length polymorphism, and polymerase chain reaction te chniques