Spatiotemporal Dynamics of the BRI1 Receptor and its Regulation by Membrane Microdomains in Living Arabidopsis Cells

The major brassinosteroid （BR） receptor of Arabidopsis BRASSINOSTEROID INSENSITIVE1 （BRI1） plays fundamental roles in BR signaling, but the molecular mechanisms underlying the effects of BR on BRI1 internalization and assembly state remain unclear. Here, we applied variable angle total internal reflection fluorescence microscopy and fluorescence cross-correlation spectroscopy to analyze the dynamics of GFP-tagged BRII. We found that, in response to BR, the degree of co-localization of BRI1-GFP with AtFIotl-mCherry increased, and especially BR stimulated the membrane microdomain-associated pathway of BRI1 internalization. We also verified these observations in endocytosis-defective chc2-1 mutants and the AtFIotl amiRNA 15-5 lines. Furthermore, examination of the phosphorylation status of bril-EMS-suppressor 1 and measurement of BR-responsive gene expression revealed that membrane microdomains affect BR signaling. These results suggest that BR promotes the partitioning of BRI1 into functional membrane microdomains to activate BR signaling.