Fig. 4.

SYMREM1 negatively regulates infection-triggered receptor endocytosis by stabilizing LYK3 in nanodomains. Shown are kymograph analyses of time-lapse image series taken from cell surfaces at the base of M. truncatula root hairs. LYK3 was mobile in the absence of rhizobia (A) and became immobilized at 16 h after rhizobial inoculation, i.e., at the onset of infection (B). The same effect was obtained when ectopically expressing SYMREM1 in the absence of rhizobia (i.e., in the absence of endogenous SYMREM1) (C). Analysis of uninoculated transgenic M. truncatula roots expressing LYK3-GFP under its native promoter revealed receptor mobility in WT R108 (D) and the symrem1-2 (E) mutant background. Inoculation of LYK3-GFP–expressing symrem1-2 mutant roots resulted in destabilization of the receptor (F) and its accumulation in endosome-like vesicles (G–G‴). (Scale bar in G‴: 10 μm.) (H) Frequency of LYK3 endocytosis in different genotypes (R108 WT, symrem1-2, and flot4-3) scored in independent root hairs under uninoculated conditions and at 2 dpi with. S. meliloti. (Scale bars: 10 μm.)