Plants rely on cell-surface-localized pattern recognition receptors to detect pathogen- or host-derived danger signals and trigger an immune response^1,2,3,4,5,6. Receptor-like proteins (RLPs) with a leucine-rich repeat (LRR) ectodomain constitute a subgroup of pattern recognition receptors and play a critical role in plant immunity^1,2,3. Mechanisms underlying ligand recognition and activation of LRR-RLPs remain elusive. Here we report a crystal structure of the LRR-RLP RXEG1 from Nicotiana benthamiana that recognizes XEG1 xyloglucanase from the pathogen Phytophthora sojae. The structure reveals that specific XEG1 recognition is predominantly mediated by an amino-terminal and a carboxy-terminal loop-out region (RXEG1(ID)) of RXEG1. The two loops bind to the active-site groove of XEG1, inhibiting its enzymatic activity and suppressing Phytophthora infection of N. benthamiana. Binding of XEG1 promotes association of RXEG1(LRR) with the LRR-type co-receptor BAK1 through RXEG1(ID) and the last four conserved LRRs to trigger RXEG1-mediated immune responses. Comparison of the structures of apo-RXEG1(LRR), XEG1–RXEG1(LRR) and XEG1–BAK1–RXEG1(LRR) shows that binding of XEG1 induces conformational changes in the N-terminal region of RXEG1(ID) and enhances structural flexibility of the BAK1-associating regions of RXEG1(LRR). These changes allow fold switching of RXEG1(ID) for recruitment of BAK1(LRR). Our data reveal a conserved mechanism of ligand-induced heterodimerization of an LRR-RLP with BAK1 and suggest a dual function for the LRR-RLP in plant immunity.

植物依靠细胞表面定位的模式识别受体来检测病原体或宿主衍生的危险信号并触发免疫反应^1,2,3,4,5,6。具有富含亮氨酸重复 (LRR) 胞外域的受体样蛋白 (RLP) 构成模式识别受体的一个亚组，在植物免疫^1,2,3中发挥关键作用。LRR-RLPs 的配体识别和激活机制仍然难以捉摸。在这里，我们报告了来自本氏烟草的 LRR-RLP RXEG1 的晶体结构，它识别来自病原体大豆疫霉的 XEG1 木葡聚糖酶. 该结构显示特异性 XEG1 识别主要由 RXEG1 的氨基末端和羧基末端环出区域 (RXEG1(ID)) 介导。这两个环与 XEG1 的活性位点凹槽结合，抑制其酶活性并抑制本氏烟草的疫霉菌感染. XEG1 的结合通过 RXEG1(ID) 和最后四个保守的 LRR 促进 RXEG1(LRR) 与 LRR 型共同受体 BAK1 的结合，从而触发 RXEG1 介导的免疫反应。apo-RXEG1(LRR)、XEG1-RXEG1(LRR) 和 XEG1-BAK1-RXEG1(LRR) 的结构比较表明 XEG1 的结合诱导 RXEG1(ID) N 端区域的构象变化并增强结构灵活性RXEG1（LRR）的 BAK1 相关区域。这些变化允许 RXEG1(ID) 的折叠切换以招募 BAK1(LRR)。我们的数据揭示了配体诱导的 LRR-RLP 与 BAK1 异二聚化的保守机制，并表明 LRR-RLP 在植物免疫中的双重功能。