Figure 2. Phosphorylation of plant RKs involved in immunity, growth and development.

(A) Brassinosteroid signaling is activated by tyrosine autophosphorylation and transphosphorylation on BKI1, which triggers its dissociation from BRI1 (dashed arrow) to allow full activation of BRI1 kinase activity. This activates BSU1, dephosphorylating BIN2 pY200 in the activation loop and eventually promotes nuclear localization of the transcription factor BZR1 for transcriptional reprogramming. (B) In cell surface immunity, receptor kinases recognize their respective ligand, which leads to dimerization or oligomerization, activation of the cytosolic kinase domains, and phosphorylation on a key conserved Tyr residue (BAK1-Y403, EFR-Y836, CERK1- Y428 and LORE-Y600). RKs relay the signal to receptor-like cytoplasmic kinases that activate a battery of cellular responses including MAPK phosphorylation, Ca²⁺ influx and reactive oxygen species production. The signaling cascades are regulated by phosphorylation on Ser, Thr (orange dots) and Tyr (red dots). Receptor activation is regulated by plant (CIPP1) or microbial (HopAO1) phosphatases.