Lawson Aaron W, Flores-Ibarra Andrea, Cao Yu, An Chunpeng, Neumann Ulla, Gunkel Monika, Saur Isabel M L, Chai Jijie, Behrmann Elmar, Schulze-Lefert Paul
Department of Plant Microbe Interactions, Max Planck Institute for Plant Breeding Research, 50829, Cologne, Germany.
University of Cologne, Faculty of Mathematics and Natural Sciences, Institute of Biochemistry, 50674, Cologne, Germany.
EMBO J. 2025 Feb 13. doi: 10.1038/s44318-025-00373-9.
Co-evolution between cereals and pathogenic grass powdery mildew fungi is exemplified by sequence diversification of an allelic series of barley resistance genes encoding Mildew Locus A (MLA) nucleotide-binding leucine-rich repeat (NLR) immunoreceptors with an N-terminal coiled-coil domain (CNLs). Each immunoreceptor recognises a matching, strain-specific powdery mildew effector encoded by an avirulence gene (AVR). We present here the cryo-EM structure of barley MLA13 in complex with its cognate effector AVR-1. The effector adopts an RNase-like fold when bound to MLA13 in planta, similar to crystal structures of other RNase-like AVR effectors unbound to receptors. AVR-1 interacts via its basal loops with MLA13 C-terminal leucine-rich repeats (LRRs) and the central winged helix domain (WHD). Co-expression of structure-guided MLA13 and AVR-1 substitution variants show that the receptor-effector interface plays an essential role in mediating immunity-associated plant cell death. Furthermore, by combining structural information from the MLA13-AVR-1 heterocomplex with sequence alignments of other MLA receptors, we engineered a single amino acid substitution in MLA7 that enables expanded effector detection of AVR-1 and the virulent variant AVR-V2. In contrast to the pentameric conformation of previously reported effector-activated CNL resistosomes, MLA13 was purified and resolved as a stable heterodimer from an in planta expression system. Our study suggests a common structural principle for RNase-like effector binding to MLAs and highlights the utility of structure-guided engineering of plant immune receptors for broadening their pathogen effector recognition capabilities.
谷类作物与致病禾本科白粉菌之间的协同进化，以大麦抗性基因等位基因系列的序列多样化为例，这些基因编码具有N端卷曲螺旋结构域（CNL）的霉病位点A（MLA）核苷酸结合富含亮氨酸重复序列（NLR）免疫受体。每个免疫受体识别由无毒基因（AVR）编码的匹配的、菌株特异性的白粉菌效应子。我们在此展示了大麦MLA13与其同源效应子AVR-1复合物的冷冻电镜结构。该效应子在植物中与MLA13结合时采用类似核糖核酸酶的折叠结构，类似于其他未与受体结合的类似核糖核酸酶的AVR效应子的晶体结构。AVR-1通过其基部环与MLA13的C端富含亮氨酸重复序列（LRR）和中央翼状螺旋结构域（WHD）相互作用。结构导向的MLA13和AVR-1替代变体的共表达表明，受体-效应子界面在介导免疫相关的植物细胞死亡中起关键作用。此外，通过将MLA13-AVR-1异源复合物的结构信息与其他MLA受体的序列比对相结合，我们在MLA7中设计了一个单氨基酸取代，能够扩大对AVR-1和毒性变体AVR-V2的效应子检测。与先前报道的效应子激活的CNL抗性小体的五聚体构象不同，MLA13是从植物表达系统中纯化并解析为稳定的异二聚体。我们的研究提出了类似核糖核酸酶的效应子与MLA结合的共同结构原理，并强调了结构导向的植物免疫受体工程在拓宽其病原体效应子识别能力方面的实用性。
