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 Arabidopsis. 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 LecRK-I.8 inhibit NAD(+)-induced immune responses, whereas overexpression of LecRK-I.8 enhances the Arabidopsis response to NAD(+). Furthermore, LecRK-I.8 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.