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Receptor-like kinases (RLKs) represent the largest group of cell surface receptors in plants. The monophyletic leucine-rich repeat (LRR)-RLK subfamily II is considered to contain the somatic embryogenesis receptor kinases (SERKs) and NSP-interacting kinases known to be involved in developmental processes and cellular immunity in plants. There are only a few published studies on the phylogenetics of LRR-RLKII; unfortunately these suffer from poor taxon/gene sampling. Hence, it is not clear how many and what main clades this family contains, let alone what structure–function relationships exist. We used 1342 protein sequences annotated as ‘SERK’ and ‘SERK-like’ plus related sequences in order to estimate phylogeny within the LRR-RLKII clade, using the nematode protein kinase Pelle as an outgroup. We reconstruct five main clades (LRR-RLKII 1–5), in each of which the main pattern of land plant relationships re-occurs, confirming previous hypotheses that duplication events happened in this gene subfamily prior to divergence among land plant lineages. We show that domain structures and intron–exon boundaries within the five clades are well conserved in evolution. Furthermore, phylogenetic patterns based on the separate LRR and kinase parts of LRR-RLKs are incongruent: whereas the LRR part supports a LRR-RLKII 2/3 sister group relationship, the kinase part supports clades 1/2. We infer that the kinase part includes few ‘radical’ amino acid changes compared with the LRR part. Finally, our results confirm that amino acids involved in each LRR-RLKII–receptor complex interaction are located at N-capping residues, and that the short amino acid motifs of this interaction domain are highly conserved throughout evolution within the five LRR-RLKII clades.

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 in

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 in