Phenotypic and Genetic Characterization of Novel Strains of Xanthomonas axonopodis pv. citri Which Induce Atypical Symptoms on Citrus Leaves in Taiwan

Abstract

A total of 46 strains of Xanthomonas axonopodis pv. citri isolated from Taiwan were examined for their pathogenicity on leaves of Mexican lime (Citrus aurantifolia), grapefruit (C. paradisi ), Liucheng (C. sinensis) and lemon (C. limon ). These strains were grouped into three types (A, Af and Ar) based on symptoms induced on leaves of the four citrus species. There were 43 (93.5%) strains belonging to type A which induced typical erumpent canker lesions with watersoaked margin on leaves of all four citrus species. Whereas strains in the other two types Af and Ar caused atypical symptoms on citrus leaves. Strain XW47 in type Af which induced typical erumpent canker lesions with watersoaked margin on Mexican lime, but induced flat necrotic lesions with watersoaked margin on grapefruit, Liucheng, and lemon; Strains XW16 and XW121 in type Ar induced restricted and raised corky lesions with no watersoaked margin on leaves of all the four citrus species. Based on physiological, biochemical and genetic characterizations including NaCl tolerance, hydrolysis of gelatin, oxidation of carbon sources, polymerase chain reactions with primers specific to X. axonopodis pv. citri (Xac), rep-PCR and DNA sequence of leucine-responsive regulatory protein (lrp), strains in types Af and Ar were identified as Xac. These two types of atypical symptoms-inducing Xac strains could be differentiated from two atypical Xac strains A* and Aw isolated from southwest Asia and Florida by lrp sequence assay and amplified DNA profiles of PCR with primer pairs pthAP7/pthAR2, 2/3, 4/7 or ERIC1R/ ERIC2. They appeared to be novel strains of Xac. We designated these two types of atypical symptoms-inducing strains as Xac-Af type and Xac-Ar type, respectively. 將台灣分離之46株十附商潰瘍病菌(Xanthomonas campestris pv. citri)菌株，在墨西哥萊姆(Citrus aurantifolia)、葡萄柚(C. paradisi)、柳橙(C. sinensis)及檸檬(C. limon)等四種不同品種之柑橘植株葉片上，分別測試其病原性。依其在四種柑橘植株葉片上所早現之病徵，可將其分成A、Af及AI等三種類型之菌株，其中有43(93.5%)株菌株屬於A型菌株，可在測試之四種柑橘植株葉片上，引起裂開突起並具有水浸狀邊緣之典型潰瘍病斑。而其他菌株則分別屬於引起非典型病徵之Af及Ar型菌株；其中Af類型菌株XW47，雖然在墨西哥萊姆上可引起裂開突起且具水浸狀邊緣之典型潰瘍病斑，然而在葡萄柚、柳橙與擰檬葉片上，則引起具水浸狀邊緣之扁平壞疽病斑；而Ar類型菌株XW121及XW16，在測試之四種柑橘葉片上，則會引起不具水浸狀邊緣之木栓化突起小病斑。依據耐鹽性、明膠水解作用、碳素源利用、應用柑橘潰瘍病菌專一性引子對之聚自酵素連鎖反應、東複性序列聚合酵素連鎖反應及lrp基因序列分析等生理、生化及基因特性分析，顯示Af及Ar兩類型菌株係屬於Xanthomonas axonopodis pv. Citri。利用lrp基因序列分析及pthAP7/pthAR2、2/3、4/7或ERIC1R/ERIC2等引子對之聚合酵素連鎖反應的DNA圖譜，可將Af及Ar菌株與西南亞及美國佛羅里達州所分離到，可引起非典型病徵之A＊與Aw菌株區分閉。因此由台灣分離到可引起非典型病徵之Af與Ar菌株，為X. axonopodis pv. citri之新特殊菌株，並將其分別命名為Xac-Af及Xac-Ar。