The Conformation of a Plasma Membrane-Localized Somatic Embryogenesis Receptor Kinase Complex Is Altered by a Potato Aphid-Derived Effector

Somatic embryogenesis receptor kinases (SERKs) are transmembrane receptors involved in plant immunity. Tomato (Solanum lycopersicum) carries three SERK members. One of these, SlSERK1, is required for Mi-1.2-mediated resistance to potato aphids (Macrosiphum euphorbiae). Mi-1.2 encodes a coiled-coil nucleotide-binding leucine-rich repeat protein that in addition to potato aphids confers resistance to two additional phloem-feeding insects and to root-knot nematodes (Meloidogyne spp.). How SlSERK1 participates in Mi-1.2-mediated resistance is unknown, and no Mi-1.2 cognate pest effectors have been identified. Here, we study the mechanistic involvement of SlSERK1 in Mi-1.2-mediated resistance. We show that potato aphid saliva and protein extracts induce the Mi-1.2 defense marker gene SlWRKY72b, indicating that both saliva and extracts contain a Mi-1.2 recognized effector. Resistant tomato cultivar Motelle (Mi-1.2/Mi-1.2) plants overexpressing SlSERK1 were found to display enhanced resistance to potato aphids. Confocal microscopy revealed that Mi-1.2 localizes at three distinct subcellular compartments: the plasma membrane, cytoplasm, and nucleus. Coimmunoprecipitation experiments in these tomato plants and in Nicotiana benthamiana transiently expressing Mi-1.2 and SlSERK1 showed that Mi-1.2 and SlSERK1 colocalize only in a microsomal complex. Interestingly, bimolecular fluorescence complementation analysis showed that the interaction of Mi-1.2 and SlSERK1 at the plasma membrane distinctively changes in the presence of potato aphid saliva, suggesting a model in which a constitutive complex at the plasma membrane participates in defense signaling upon effector binding.