A pair of GPI-anchored proteins regulate soybean immunity and disease resistance in coordination with GmLMM1 and GmRALFs.

Plant cell-surface-localized pattern recognition receptors and co-receptors recognize conserved microbial patterns to initiate pattern-triggered immunity (PTI), which confers moderate and broad-spectrum resistance to most pathogens. For instance, the FLS2 receptor recognizes bacterial flagellin (or epitope flg22) by forming a complex with its co-receptor BAK1. Malectin-like receptor-like kinases (RLKs) interact with several GPI-anchored proteins (GPI-APs) to recognize rapid alkalinization factor (RALF) peptides, thereby regulating multiple physiological processes. We have previously shown that the soybean malectin-like RLK protein GmLMM1 negatively regulates immunity by suppressing the GmFLS2-GmBAK1 interaction. However, there is currently no research on soybean GPI-APs, particularly with regard to their role in immunity. Here, we identified a pair of GPI-APs, GmLLG1 and GmLLG2, which are negative regulators of plant PTI and disease resistance, and these functions differed from those of their Arabidopsis homologues. The Gmllg1 Gmllg2 mutants exhibited enhanced resistance to bacterial and oomycete pathogens, as well as an elevated ROS burst and MAPK activation upon flg22 treatment. The GmLLG1 and GmLLG2 overexpression lines exhibited reduced flg22-induced immunity and enhanced susceptibility to pathogen infection. Transcriptome analysis showed that GmLLG1/2 and GmLMM1 coordinately regulate plant defence gene expression. GmLLG1/2 interacted with and regulated the plasma membrane localization of GmLMM1. We further identified two GmRALFs, GmRALF1 and GmRALF18, which negatively regulated plant immunity and exhibited strong binding affinity with GmLLG1/2. Notably, Gmllg1 Gmllg2 mutants showed almost normal yield-related traits, including stem height and 100-grain weight. Thus, this study identified GmRALF1/18-GmLLG1/2-GmLMM1 as an immune regulatory molecular module in soybeans that could aid breeding practices.