Non-host Resistance Induced by the Xanthomonas Effector XopQ Is Widespread within the Genus Nicotiana and Functionally Depends on EDS1

Abstract

Most Gram-negative plant pathogenic bacteria translocate effector proteins (T3Es) directly into plant cells via a conserved type III secretion system, which is essential for pathogenicity in susceptible plants. In resistant plants, recognition of some T3Es is mediated by corresponding resistance (<i>R</i>) genes or R proteins and induces effector triggered immunity (ETI) that often results in programmed cell death reactions. The identification of <i>R</i> genes and understanding their evolution/distribution bears great potential for the generation of resistant crop plants. We focus on T3Es from <i>Xanthomonas campestris</i> pv. <i>vesicatoria</i> (<i>Xcv</i>), the causal agent of bacterial spot disease on pepper and tomato plants. Here, 86 <i>Solanaceae</i> lines mainly of the genus <i>Nicotiana</i> were screened for phenotypical reactions after <i>Agrobacterium tumefaciens</i>-mediated transient expression of 21 different <i>Xcv</i> effectors to (i) identify new plant lines for T3E characterization, (ii) analyze conservation/evolution of putative <i>R</i> genes and (iii) identify promising plant lines as repertoire for <i>R</i> gene isolation. The effectors provoked different reactions on closely related plant lines indicative of a high variability and evolution rate of potential <i>R</i> genes. In some cases, putative <i>R</i> genes were conserved within a plant species but not within superordinate phylogenetical units. Interestingly, the effector XopQ was recognized by several <i>Nicotiana</i> spp. lines, and <i>Xcv</i> infection assays revealed that XopQ is a host range determinant in many <i>Nicotiana</i> species. Non-host resistance against <i>Xcv</i> and XopQ recognition in <i>N. benthamiana</i> required <i>EDS1</i>, strongly suggesting the presence of a TIR domain-containing XopQ-specific R protein in these plant lines. XopQ is a conserved effector among most xanthomonads, pointing out the XopQ-recognizing R_xopQ as candidate for targeted crop improvement.