The protein kinase MLOP1 mediates pectin fragment signaling and feedback regulation of cell wall synthesis

Plants sense the breakdown products of cell wall components to trigger effective stress responses and to adjust wall synthesis during development. Oligogalacturonides (OGs), derived from pectin degradation, are known to serve as signals for cell wall remodeling and stress responses, while little is known about their perception by plant cells. Here, we characterized a malectin-like domain-containing leucine-rich repeat receptor-like protein kinase MLOP1, which is involved in pectin fragment signal sensing in Arabidopsis thaliana. Mutations of MLOP1 impaired cell wall synthesis and cell elongation in seedling hypocotyls and primary roots. MLOP1 and its homologs were tandem duplicated during evolution. It is expressed in the rapid-growth region of hypocotyl, root elongation zone, leaf and stem vasculature, and silique. MLOP1-GFP is localized to the plasma membrane. The MLOP1 ectodomain is capable to bind with pectic polysaccharide in vitro, and the kinase domain may mediate intracellular signal transduction. Tests on mlop1 mutants indicate its involvement in OG-induced cell defense responses, including phytoalexin synthesis, nitric oxide (NO) accumulation, callose deposition and cytosolic Ca²⁺ spike. Accordingly, mlop1 mutants showed a high susceptibility to Pseudomonas syringae infection. Furthermore, loss of MLOP1 function alleviated OG-induced inhibition of cellulose synthase complex (CSC) mobility and its density on the plasma membrane. These results indicate that MLOP1 plays a role in pectin fragment signaling and mediates their effects on stress responses and cell wall remodeling.