Abstract
Resistance in tomato against race 1 strains of the fungal vascular wilt pathogens Verticillium dahliae and V. albo-atrum is mediated by the Ve locus. This locus comprises two closely linked inversely oriented genes, Ve1 and Ve2, which encode cell surface receptors of the extracellular leucine-rich repeat receptor-like protein (eLRR-RLP) type. While Ve1 mediates Verticillium resistance through monitoring the presence of the recently identified V. dahliae Ave1 effector, no functionality for Ve2 has been demonstrated in tomato. Ve1 and Ve2 contain 37 eLRRs and share 84% amino acid identity, facilitating investigation of Ve protein functionality through domain swapping. In this study it is shown that Ve chimeras in which the first thirty eLRRs of Ve1 were replaced by those of Ve2 remain able to induce HR and activate Verticillium resistance, and that deletion of these thirty eLRRs from Ve1 resulted in loss of functionality. Also the region between eLRR30 and eLRR35 is required for Ve1-mediated resistance, and cannot be replaced by the region between eLRR30 and eLRR35 of Ve2. We furthermore show that the cytoplasmic tail of Ve1 is required for functionality, as truncation of this tail results in loss of functionality. Moreover, the C-terminus of Ve2 fails to activate immune signaling as chimeras containing the C-terminus of Ve2 do not provide Verticillium resistance. Furthermore, Ve1 was found to interact through its C-terminus with the eLRR-containing receptor-like kinase (eLRR-RLK) interactor SOBIR1 that was recently identified as an interactor of eLRR-RLP (immune) receptors. Intriguingly, also Ve2 was found to interact with SOBIR1.
Highlights
Immunity in plants against pathogen attack is governed by immune receptors that detect appropriate ligands to activate defense
To screen for functionality of constructs encoding domain swaps between Ve1 and Ve2, the coding sequence (CDS) of V. dahliae Ave1 was cloned behind the cauliflower mosaic virus (CaMV) 35S promoter to generate expression construct Ave1
When tobacco leaves were co-infiltrated with a 1:1 mixture of A. tumefaciens cultures carrying Ave1 and Ve1HA respectively, hypersensitive response (HR) was observed (Figure 1B)
Summary
Immunity in plants against pathogen attack is governed by immune receptors that detect appropriate ligands to activate defense. These ligands can either be microbial structures or ligands that occur as a consequence of plant-manipulating activities of microbial effectors [1], [2]. From tomato (Solanum lycopersicum) a locus providing Verticillium resistance has been cloned [6]. This Ve locus controls V. dahliae and V. albo-atrum strains belonging to race 1, while strains that are not controlled are assigned to race 2 [7]. For other eLRR-containing receptors, the eLRRs have been implicated in recognition specificity [10], [11], [12], [13], [14], [15], [16]
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