The Arabidopsis (Arabidopsis thaliana) SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) genes belong to a small family of five plant receptor kinases that are involved in at least five different signaling pathways. One member of this family, BRASSINOSTEROID INSENSITIVE1 (BRI1)-ASSOCIATED KINASE1 (BAK1), also known as SERK3, is the coreceptor of the brassinolide (BR)-perceiving receptor BRI1, a function that is BR dependent and partially redundant with a second member SERK1. SERK3/BAK1 alone controls plant innate immunity, is the coreceptor of the flagellin receptor Flagellin Sensing 2 (FLS2), and, together with SERK4, can mediate cell death control, all three functions are BR-independent. In tomato (Solanum lycopersicum), three SlSERK members were identified with homologies to Arabidopsis SERK1 or SERK3/BAK1, and were named SlSERK1, SlSERK3A and SlSERK3B. A previous study showed that SlSERK1 is required for Mi-1.2-mediated resistance against potato aphids (Macrosiphum euphorbiae) but not against root-knot nematodes (RKN; Meloidogyne spp.), while the roles for SlSERK3A and SlSERK3B remain unknowns. Mi-1.2 encodes a coiled-coil nucleotide-binding leucine-rich repeat protein. In chapter one, I focus on the two members that exhibit particularly high levels of sequence similarity to BAK1/SERK3 the SlSERK3A and SlSERK3B. To characterize a role for SlSERK3A and SlSERK3B in defense, we suppressed each gene individually or co-silenced both using virus-induced gene silencing (VIGS) in the tomato cv. Moneymaker. Co-silencing SlSERK3A and SlSERK3B resulted in spontaneous necrotic lesions and reduced sensitivity to exogenous BR treatment. Silencing either SlSERK3A or SlSERK3B resulted in enhanced susceptibility to RKN and to the non-pathogenic Pseudomonas syringae pv. tomato (Pst) DC3000 hrcC indicating that both SlSERK3s are positive regulators of defense. Interestingly, silencing SlSERK3B, but not SlSERK3A, resulted in enhanced susceptibility to the pathogenic strain Pst DC3000 indicating distinct roles for these two SlSERK3 paralogs. SlSERK3A and SlSERK3B are active kinases, localized to the plasma membrane, and interact in vivo with the SlFLS2 receptor in a flg22-dependent manner. Complementation of the Atserk3/bak1-4 mutant with either SlSERK3A or SlSERK3B partially rescued the mutant phenotype. Thus, SlSERK3A and SlSERK3B are likely to constitute tomato orthologs of BAK1. Chapter two addresses the mechanism by which SlSERK1 contributes to Mi-1.2-mediated aphid defense and the dynamic of the interaction between SlSERK1 and Mi-1.2. We show that SlSERK1 is the rate limiting factor in Mi-1.2-mediated resistance. Transgenic 35S-SlSERK1-HA tomato lines in the resistant cv. Motelle displayed enhanced resistance to potato aphids compared to wild-type tomato cv. Motelle. Moreover, we showed that potato aphid saliva and total protein could induce Mi-1.2-dependent defense marker SlWRKY72b, indicating that aphid total protein can be used as the Mi-1.2 elicitor. Furthermore, co-immunoprecipitation experiments in both Nicotiana benthamiana, transiently expressing Mi-1.2 and SlSERK1, and in 35S-SlSERK1-HA Motelle tomato showed that Mi-1.2 and SlSERK1 are present in a complex in the microsomal fractions. In addition, by confocal microscopy and biochemical fractionation I showed that Mi-1.2 is localized to three subcellular pools including the plasma membrane, cytoplasm and the nucleus. Furthermore, BiFC analysis showed that Mi-1.2 does not directly interact with SlSERK1 in the absence of the aphid elicitor but does so soon after aphid treatment. Moreover, Mi-1.2 levels on the microsomal fractions decreased in the presence of the potato aphid elicitor suggesting Mi-1.2 dynamic at the plasma membrane is the trigger for the defense responses.
