BAK-up:the receptor kinase BAK-TO-LIFE 2 enhances immunity when BAK1 is lacking

BRI1-ASSOCIATED KINASE 1(BAK1/SERK3)and its closest homolog BAK1-LIKE 1(BKK1/SERK4)are leucine-rich repeat receptor kinases(LRR-RKs)belonging to the SOMATIC EMBRYOGENESIS RECEPTOR KINASE(SERK)family.They act as co-receptors of various other LRR-RKs and participate in multiple signaling events by complexing and transphosphorylating ligand-binding receptors.Initially identified as the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1(BRI1)co-receptor,BAK1 also functions in plant immunity by interacting with pattern recognition receptors.Mutations in BAK1 and BKK1 cause severely stunted growth and cell death,characterized as autoimmune cell death.Several factors play a role in this type of cell death,including RKs and components of effector-triggered immunity(ETI)signaling pathways,glycosylation factors,ER quality control components,nuclear trafficking components,ion channels,and Nod-like receptors(NLRs).The Shan lab has recently discovered a novel RK BAK-TO-LIFE 2(BTL2)that interacts with BAK1 and triggers cell death in the absence of BAK1 and BKK1.This RK compensates for the loss of BAK1-mediated pattern-triggered immunity(PTI)by activating phytocytokine-mediated immune and cell death responses.