Sterols regulate endocytic pathways during flg22-induced defense responses in Arabidopsis

The plant transmembrane receptor kinase FLAGELLIN SENSING 2 (FLS2) is critical for innate immunity. Although previous studies have reported FLS2-mediated signal transduction and endocytosis via the clathrin-mediated pathway, whether additional endocytic pathways affect FLS2-mediated defense responses remains unclear. Here, we showed that the Arabidopsis thaliana sterol-deficient mutant steroid methyltransferase 1 displays defects in immune responses induced by the flagellin-derived peptide flg22. Variable-angle total internal reflection fluorescence microscopy (VA-TIRFM) coupled with single-particle tracking showed that the spatiotemporal dynamics of FLS2-GFP changed on the millisecond time scale and the FLS2-GFP dwell time at the plasma membrane increased in cells treated with a sterol-extracting reagent, compared with untreated counterparts. We further demonstrated that flg22-induced FLS2 clustering and endocytosis involves the sterol-associated endocytic pathway, which is distinct from the clathrin-mediated pathway. Moreover, flg22 enhanced the co-localization of FLS2-GFP with the membrane microdomain marker Flot 1-mCherry and FLS2 endocytosis via the sterol-associated pathway. This indicates that plants may respond to pathogen attacks by regulating two different endocytic pathways. Taken together, our results suggest the key role of sterol homeostasis in flg22-induced plant defense responses.