The Conformation of the Plasma Membrane-Localized SlSERK1 - Mi-1.2 Complex is Altered by a Potato Aphid Derived Effector

Somatic embryogenesis receptor kinases (SERKs) are transmembrane receptors involved in plant immunity. Tomato (Solanum lycopersicum; Sl) 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. Co-immunoprecipitation experiments in these tomato plants, and in Nicotiana benthamiana transiently expressing Mi-1.2 and SlSERK1, showed that Mi-1.2 and SlSERK1 co-localize only in a microsomal complex. Interestingly, BiFC 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 signalling upon effector binding