在植物中,宿主-病原体共同进化通常表现为互惠,通过宿主核苷酸结合的富含亮氨酸的重复免疫受体(NLR)和促进毒力的病原体效应子的变化进行适应性遗传变化。在草白粉病(PM)真菌中,核糖核酸酶样效应子家族的极端扩张,叫做RALPH,主导效应器曲目,一些成员被谷物NLR受体识别为无毒力(AVR)效应子。我们报告了序列无关的大麦PM效应子AVRA6,AVRA7和等位基因AVRA10/AVRA22变体的结构,通过高度序列相关的大麦NLRMLA6,MLA7,MLA10和MLA22以及无关的小麦NLRPM2检测到的小麦PMAVRPM2。AVR效应器采用通用支架,它与RNaseT1/F1家族共有。我们发现数量有惊人的变化,position,和RALPHAVR之间的单个结构元素的长度,与RALPH效应子家族的分化有关。我们表明,所有测试的RALPHAVR都失去了RNaseT1/F1家族的核酸酶和合成酶活性,并且缺乏与RNA的显着结合,这意味着它们的毒力活动与新功能化事件有关。结构指导的诱变鉴定了六个AVRA6残基,这些残基足以将相同RALPH亚家族的序列发散成员转变为MLA6特异性检测的效应子。AVRA10和AVRA22的类似结构指导信息表明MLA受体检测到很大程度上不同的效应物表面斑块。因此,PMs的RALPH支架内的序列和结构多态性的偶联促进了NLR识别的逃逸和多种毒力功能的潜在获得。
In plants, host-pathogen coevolution often manifests in reciprocal, adaptive genetic changes through variations in host nucleotide-binding leucine-rich repeat immune receptors (NLRs) and virulence-promoting pathogen effectors. In grass powdery mildew (PM) fungi, an extreme expansion of a RNase-like effector family, termed RALPH, dominates the effector repertoire, with some members recognized as avirulence (AVR) effectors by cereal NLR receptors. We report the structures of the sequence-unrelated barley PM effectors AVRA6, AVRA7, and allelic AVRA10/AVRA22 variants, which are detected by highly sequence-related barley NLRs MLA6, MLA7, MLA10, and MLA22 and of wheat PM AVRPM2 detected by the unrelated wheat NLR PM2. The AVR effectors adopt a common scaffold, which is shared with the RNase T1/F1 family. We found striking variations in the number, position, and length of individual structural elements between RALPH AVRs, which is associated with a differentiation of RALPH effector subfamilies. We show that all RALPH AVRs tested have lost nuclease and synthetase activities of the RNase T1/F1 family and lack significant binding to RNA, implying that their virulence activities are associated with neo-functionalization events. Structure-guided mutagenesis identified six AVRA6 residues that are sufficient to turn a sequence-diverged member of the same RALPH subfamily into an effector specifically detected by MLA6. Similar structure-guided information for AVRA10 and AVRA22 indicates that MLA receptors detect largely distinct effector surface patches. Thus, coupling of sequence and structural polymorphisms within the RALPH scaffold of PMs facilitated escape from NLR recognition and potential acquisition of diverse virulence functions.