Al-Saadi Abdulwahid, Reddy Joseph D, Duan Yong P, Brunings Asha M, Yuan Qiaoping, Gabriel Dean W
Plant Molecular and Cellular Biology Program, Plant Pathology Dept., University of Florida, Gainesville, FL 32611, USA.
Mol Plant Microbe Interact. 2007 Aug;20(8):934-43. doi: 10.1094/MPMI-20-8-0934.
Citrus canker disease is caused by five groups of Xanthomonas citri strains that are distinguished primarily by host range: three from Asia (A, A*, and A(w)) and two that form a phylogenetically distinct clade and originated in South America (B and C). Every X. citri strain carries multiple DNA fragments that hybridize with pthA, which is essential for the pathogenicity of wide-host-range X. citri group A strain 3213. DNA fragments that hybridized with pthA were cloned from a representative strain from all five groups. Each strain carried one and only one pthA homolog that functionally complemented a knockout mutation of pthA in 3213. Every complementing homolog was of identical size to pthA and carried 17.5 nearly identical, direct tandem repeats, including three new genes from narrow-host-range groups C (pthC), A(w) (pthAW), and A* (pthA*). Every noncomplementing paralog was of a different size; one of these was sequenced from group A* (pthA*-2) and was found to have an intact promoter and full-length reading frame but with 15.5 repeats. None of the complementing homologs nor any of the noncomplementing paralogs conferred avirulence to 3213 on grapefruit or suppressed avirulence of a group A* strain on grapefruit. A knockout mutation of pthC in a group C strain resulted in loss of pathogenicity on lime, but the strain was unaffected in ability to elicit an HR on grapefruit. This pthC- mutant was fully complemented by pthA, pthB, or pthC. Analysis of the predicted amino-acid sequences of all functional pthA homologs and nonfunctional paralogs indicated that the specific sequence of the 17th repeat may be essential for pathogenicity of X. citri on citrus.
柑橘溃疡病由柑橘黄单胞菌的五组菌株引起，这些菌株主要根据寄主范围来区分：三组来自亚洲（A、A和A(w)），两组在系统发育上形成一个独特的分支，起源于南美洲（B和C）。每个柑橘黄单胞菌菌株都携带多个与pthA杂交的DNA片段，pthA对于广寄主范围的柑橘黄单胞菌A组菌株3213的致病性至关重要。从所有五组的代表性菌株中克隆了与pthA杂交的DNA片段。每个菌株携带一个且仅一个pthA同源物，该同源物在功能上互补3213中pthA的敲除突变。每个互补同源物的大小与pthA相同，并携带17.5个几乎相同的直接串联重复序列，包括来自窄寄主范围组C（pthC）、A(w)（pthAW）和A（pthA*）的三个新基因。每个非互补旁系同源物的大小不同；其中一个来自A组（pthA-2）并进行了测序，发现其具有完整的启动子和全长阅读框，但有15.5个重复序列。互补同源物和非互补旁系同源物均未赋予3213对葡萄柚的无毒力，也未抑制A*组菌株对葡萄柚的无毒力。C组菌株中pthC的敲除突变导致对酸橙的致病性丧失，但该菌株在引发葡萄柚过敏反应的能力上未受影响。这个pthC突变体被pthA、pthB或pthC完全互补。对所有功能性pthA同源物和非功能性旁系同源物的预测氨基酸序列分析表明，第17个重复序列的特定序列可能对柑橘黄单胞菌在柑橘上的致病性至关重要。
