Sun Wenxian, Dunning F Mark, Pfund Christine, Weingarten Rebecca, Bent Andrew F
Department of Plant Pathology, University of Wisconsin, Madison, Wisconsin 53705, USA.
Plant Cell. 2006 Mar;18(3):764-79. doi: 10.1105/tpc.105.037648. Epub 2006 Feb 3.
Bacterial flagellins have been portrayed as a relatively invariant pathogen-associated molecular pattern. We have found within-species, within-pathovar variation for defense-eliciting activity of flagellins among Xanthomonas campestris pv campestris (Xcc) strains. Arabidopsis thaliana FLAGELLIN SENSING2 (FLS2), a transmembrane leucine-rich repeat kinase, confers flagellin responsiveness. The flg22 region was the only Xcc flagellin region responsible for detectable elicitation of Arabidopsis defense responses. A Val-43/Asp polymorphism determined the eliciting/noneliciting nature of Xcc flagellins (structural gene fliC). Arabidopsis detected flagellins carrying Asp-43 or Asn-43 but not Val-43 or Ala-43, and it responded minimally for Glu-43. Wild-type Xcc strains carrying nonrecognized flagellin were more virulent than those carrying a recognized flagellin when infiltrated into Arabidopsis leaf mesophyll, but this correlation was misleading. Isogenic Xcc fliC gene replacement strains expressing eliciting or noneliciting flagellins grew similarly, both in leaf mesophyll and in hydathode/vascular colonization assays. The plant FLS2 genotype also had no detectable effect on disease outcome when previously untreated plants were infected by Xcc. However, resistance against Xcc was enhanced if FLS2-dependent responses were elicited 1 d before Xcc infection. Prior immunization was not required for FLS2-dependent restriction of Pseudomonas syringae pv tomato. We conclude that plant immune systems do not uniformly detect all flagellins of a particular pathogen species and that Xcc can evade Arabidopsis FLS2-mediated defenses unless the FLS2 system has been activated by previous infections.
细菌鞭毛蛋白一直被描述为一种相对不变的病原体相关分子模式。我们发现，在野油菜黄单胞菌野油菜致病变种（Xcc）菌株中，鞭毛蛋白的防御诱导活性存在种内、致病型内变异。拟南芥的跨膜富含亮氨酸重复激酶鞭毛蛋白感应2（FLS2）赋予了对鞭毛蛋白的反应性。flg22区域是唯一负责引发拟南芥防御反应的Xcc鞭毛蛋白区域。缬氨酸43/天冬氨酸多态性决定了Xcc鞭毛蛋白（结构基因fliC）的诱导/非诱导性质。拟南芥能检测到携带天冬氨酸43或天冬酰胺43的鞭毛蛋白，但不能检测到携带缬氨酸43或丙氨酸43的鞭毛蛋白，对携带谷氨酸43的鞭毛蛋白反应微弱。当浸润到拟南芥叶肉中时，携带未被识别的鞭毛蛋白的野生型Xcc菌株比携带被识别的鞭毛蛋白的菌株更具毒性，但这种相关性具有误导性。在叶肉以及水孔/维管束定殖试验中，表达诱导性或非诱导性鞭毛蛋白的同基因Xcc fliC基因替换菌株生长情况相似。当先前未处理的植物被Xcc感染时，植物FLS2基因型对病害结果也没有可检测到的影响。然而，如果在Xcc感染前1天引发FLS2依赖性反应，则对Xcc的抗性会增强。对于丁香假单胞菌番茄致病变种，FLS2依赖性限制作用不需要预先免疫。我们得出结论，植物免疫系统并非能一致地检测到特定病原体物种的所有鞭毛蛋白，并且除非FLS2系统已被先前感染激活，否则Xcc可以逃避拟南芥FLS2介导的防御。
