Recognition And Activation Mechanism Of Plant Pattern Recognition Receptor Kinase FLS2

Plants become the targets of pathogens for their abundent nutrients and water. Contrasting to animals, plants are sessile organisms that can’t escape from attack due to lack of specialized immune cells, the nervous system and the circulation system. Plants instead have evolved the cell to cell system that controls responses and communications in order to adapt to the environment. Plants exploit cell surface-localized pattern-recognition receptors(PRR) as the first line of sensing the pathogen conserved molecules(PAMPs/DAMPs) to induce defense response. Thus far most of the PRRs identified belong to leucine-rich repeat receptor kinases(LRR-RKs).FLS2 is a typical LRR-RK and the first PRR identified in Arabidopsis, playing a critical role in plant immunity by recognizing the well-conserved N-terminal region of flagellin(flg22). Flg22 binding to FLS2 recruits the LRR-RK BAK1 to initiate immune responses. Many studies on FLS2-flg22 interaction have been reported, but the structural information was lacking. More importantly, how BAK1 was involved in flg22-induced FLS2 activation remains less well understood.Here we have successfully reconstituted flg22-induced FLS2-BAK1 heteromerization in vitro by pull down and the gel filtration assays with purified protein expressed in insect cells. We then solved the crystal structure of flg22 in complex with the ectodomains of FLS2 and BAK1, illustrating the molecular mechanism of flg22-induced FLS2-BAK1 heterodimerization. Recognition of flg22 is through an elongated groove at the inner surface of the FLS2 solenoid, whereas BAK1 makes extensive and direct contacts with the C-terminus of FLS2. Structural comparison indicated that flg22 binding induces no conformational changes and oligomerization in FLS2. Importantly, BAK1 specifically recognizes the C-terminal side of flg22 in the structure. Biochemical and cell-based assays supported an important role of this interaction in flg22-induced signaling. Together, these results indicate that BAK1 functions as a co-receptor with FLS2 rather than signaling enhance as previously thought. Based on our data, we suggest that flg22-induced FLS2-BAK1 heterodimerization acts as a closing switch for receptor activation and transmembrane signaling.BAK1 forms ligand-dependent heterodimers with several PRR or RKs. Our work provides a template for understanding the activation mechanisms of the PRR and RKs. Finally, we proposed that changes in extracellular pH have role in plant growth-defense trade-offs.