Arabidopsis Lectin Receptor Kinase P2K2 Is a Second Plant Receptor for Extracellular ATP and Contributes to Innate Immunity

In animals, extracellular ATP is a well-studied signaling molecule that is recognized by plasma membrane-localized P2-type purinergic receptors. However, in contrast, much less is known about purinergic signaling in plants. P2 receptors play critical roles in a variety of animal biological processes, including immune system regulation. The first plant purinergic receptor, Arabidopsis ( Arabidopsis thaliana ) P2K1 (L-type lectin receptor kinase-I.9), was shown to contribute to plant defense against bacterial, oomycete, and fungal pathogens. Here, we demonstrate the isolation of a second purinergic receptor, P2K2, by complementation of an Arabidopsis p2k1 mutant. P2K2 (LecRK-I.5) has 74% amino acid similarity to P2K1. The P2K2 extracellular lectin domain binds to ATP with higher affinity than P2K1 (dissociation constant [ K d ] = 44.47 ± 15.73 nm). Interestingly, p2k2 and p2k1 p2k2 mutant plants showed increased susceptibility to the pathogen Pseudomonas syringae, with the double mutant showing a stronger phenotype. In vitro and in planta studies demonstrate that P2K2 and P2K1 interact and cross-phosphorylate upon extracellular ATP treatment. Thus, similar to animals, plants possess multiple purinergic receptors.