A lectin receptor kinase as a potential sensor for extracellular nicotinamide adenine dinucleotide in Arabidopsis thaliana

Nicotinamide adenine dinucleotide (NAD ) participates in intracellular and extracellular signaling events unrelated to metabolism. In animals, purinergic receptors are required for extracellular NAD (eNAD ) to evoke biological responses, indicating that eNAD may be sensed by cell-surface receptors. However, the identity of eNAD -binding receptors still remains elusive. Here, we identify a lectin receptor kinase (LecRK), LecRK-I.8, as a potential eNAD receptor in . The extracellular lectin domain of LecRK-I.8 binds NAD with a dissociation constant of 436.5 ± 104.8 nM, although much higher concentrations are needed to trigger in vivo responses. Mutations in inhibit NAD -induced immune responses, whereas overexpression of enhances the response to NAD . Furthermore, is required for basal resistance against bacterial pathogens, substantiating a role for eNAD in plant immunity. Our results demonstrate that lectin receptors can potentially function as eNAD -binding receptors and provide direct evidence for eNAD being an endogenous signaling molecule in plants.