Receptor kinases with extracellular leucine-rich repeat domains (LRR-RKs) form the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecule, peptide, or protein ligands and may be activated by ligand-induced interaction with a shape complementary SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptor kinase. We have previously shown that SERKs can also form constitutive, ligand-independent complexes with the LRR ectodomains of BAK1-INTERACTING RECEPTOR-LIKE KINASE3 (BIR3) receptor pseudokinases, negative regulators of LRR-RK signaling. Here, we report that receptor chimera in which the extracellular LRR domain of BIR3 is fused to the cytoplasmic kinase domains of the SERK-dependent LRR-RKs BRASSINOSTEROID INSENSITIVE1, HAESA and ERECTA form tight complexes with endogenous SERK coreceptors in the absence of ligand stimulus. Expression of these chimeras under the control of the endogenous promoter of the respective LRR-RK leads to strong gain-of-function brassinosteroid, floral abscission, and stomatal patterning phenotypes, respectively. Importantly, a BIR3-GASSHO1 (GSO1)/SCHENGEN3 (SGN3) chimera can partially complement sgn3 Casparian strip formation phenotypes, suggesting that SERK proteins also mediate GSO1/SGN3 receptor activation. Collectively, our protein engineering approach may be used to elucidate the physiological functions of orphan LRR-RKs and to identify their receptor activation mechanism in single transgenic lines.

具有胞外富含亮氨酸重复结构域 (LRR-RK) 的受体激酶形成植物中最大的膜信号蛋白组。 LRR-RK 可以感知小分子、肽或蛋白质配体，并且可以通过配体诱导的与形状互补的体细胞胚胎发生受体样激酶 (SERK) 辅助受体激酶的相互作用来激活。我们之前已经证明，SERK 还可以与 BAK1 相互作用受体样激酶 3 (BIR3) 受体假激酶（LRR-RK 信号传导的负调节因子）的 LRR 胞外域形成组成型、配体独立的复合物。在这里，我们报道了受体嵌合体，其中 BIR3 的胞外 LRR 结构域与 SERK 依赖的 LRR-RKs BRASSINOSTEROID INSENSITIVE1、HAESA 和 ERECTA 的细胞质激酶结构域融合，在没有配体刺激的情况下与内源性 SERK 辅助受体形成紧密复合物。这些嵌合体在各自LRR-RK的内源启动子控制下的表达分别导致油菜素类固醇的功能获得性强、花脱落和气孔图案表型。重要的是，BIR3-GASSHO1 (GSO1)/SCHENGEN3 (SGN3) 嵌合体可以部分补充sgn3 Casparian strip 形成表型，表明 SERK 蛋白也介导 GSO1/SGN3 受体激活。总的来说，我们的蛋白质工程方法可用于阐明孤儿 LRR-RK 的生理功能，并确定其在单个转基因系中的受体激活机制。