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.

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