Comparative Genomic Analysis of the Lettuce Bacterial Leaf Spot Pathogen, Xanthomonas hortorum pv. vitians, to Investigate Race Specificity

Novel PCR-Based Detection Methods for the Lettuce Bacterial Leaf Spot Pathogen, Xanthomonas hortorum pv. vitians Morinière et al., 2020

Bacterial leaf spot in lettuce is a sporadic but devastating disease that threatens lettuce production worldwide. Severe outbreaks have resulted in up to 100% crop loss, and even smaller outbreaks can cause a significant yield loss, as the affected tissue must be removed from lettuce heads prior to their sale. The pathogen, Xanthomonas hortorum pv. vitians (Xhv), has at least three races, with each defined by the disease or resistance phenotype it elicits in lettuce cultivars and accessions. Specific molecular detection of Xhv would facilitate the work of clinicians, growers, seed companies, and researchers in the lettuce industry. We present an Xhv-specific touchdown PCR method and progress toward race-specific methods. We used an alignment of 18 Xhv strains and 9 closely related, non-target strains to identify pathovar- and race-specific gene clusters as targets for PCR primers. We evaluated the specificity first using in silico methods and then empirically using a collection of Xanthomonas strains. Our protocol demonstrated Xhv-specific detection from two sample types, including genomic DNA extracts and bacterial suspensions. Additional research is required to refine the race-specific protocols.

Differential Sources of Resistance from Lactuca serriola Against Three Races of Xanthomonas hortorum pv. vitians Causing Bacterial Leaf Spot of Lettuce

Bacterial leaf spot (BLS) of lettuce (Lactuca sativa L.) is caused by the bacterium Xanthomonas hortorum pv. vitians that is hypothesized to have at least three races of the pathogen present in North America as defined by their differential resistance phenotypes in lettuce cultivars/accessions. Although resistance to X. hortorum pv. vitians race 1 has been identified in cultivated lettuce, numerous other X. hortorum pv. vitians strains cause disease on cultivars carrying this resistance locus. Thus far, resistance to these "additional" X. hortorum pv. vitians strains has not been adequately described in L. sativa or in any other wild Lactuca species sexually compatible with cultivated lettuce. We have performed an extensive screening of approximately 500 Lactuca accessions from L. sativa, L. serriola, L. saligna, L. virosa, L. aculeata, L. altaica, and L. perennis species to identify accessions resistant to these additional X. hortorum pv. vitians races. Following the initial screenings, greenhouse tests confirmed that X. hortorum pv. vitians race 2 and race 3 could be defined using L. serriola accessions. Race 2 strain BS3127 had an incompatible response (hypersensitive response) on 10 L. serriola accessions, including PI 491114 and PI 491108, whereas race 1 (BS0347) and race 3 (BS2861) strains of X. hortorum pv. vitians showed a compatible response (disease) on these genotypes. L. serriola accession ARM09-161 (and selections derived from it) was the only genotype resistant to the race 3 strain BS2861. L. serriola accessions identified in this study to be resistant to race 2 and race 3 of X. hortorum pv. vitians, together with race 1-resistant cultivars, can be used for pyramiding resistance loci against the three races of the BLS-causing pathogen.

Multiple Acquisitions of XopJ2 Effectors in Populations of Xanthomonas perforans

Type III effectors (T3Es) are major determinants of Xanthomonas virulence and targets for resistance breeding. XopJ2 (synonym AvrBsT) is a highly conserved YopJ-family T3E acquired by X. perforans, the pathogen responsible for bacterial spot disease of tomato. In this study, we characterized a new variant (XopJ2b) of XopJ2, which is predicted to have a similar three-dimensional (3D) structure as the canonical XopJ2 (XopJ2a) despite sharing only 70% sequence identity. XopJ2b carries an acetyltransferase domain and the critical residues required for its activity, and the positions of these residues are predicted to be conserved in the 3D structure of the proteins. We demonstrated that XopJ2b is a functional T3E and triggers a hypersensitive response (HR) when translocated into pepper cells. Like XopJ2a, XopJ2b triggers HR in Arabidopsis that is suppressed by the deacetylase, SOBER1. We found xopJ2b in genome sequences of X. euvesicatoria, X. citri, X. guizotiae, and X. vasicola strains, suggesting widespread horizontal transfer. In X. perforans, xopJ2b was present in strains collected in North America, Africa, Asia, Australia, and Europe, whereas xopJ2a had a narrower geographic distribution. This study expands the Xanthomonas T3E repertoire, demonstrates functional conservation in T3E evolution, and further supports the importance of XopJ2 in X. perforans fitness on tomato. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Host-Driven Selection, Revealed by Comparative Analysis of Xanthomonas Type III Secretion Effectoromes, Unveils Novel Recognized Effectors

Xanthomonas species are specialized plant pathogens, often exhibiting a narrow host range. They rely on the translocation of effector proteins through the type III secretion system to colonize their respective hosts. The effector arsenal varies among Xanthomonas spp., typically displaying species-specific compositions. This species-specific effector composition, collectively termed the effectorome, is thought to influence host specialization. We determined the plant host-derived effectoromes of more than 300 deposited genomes of Xanthomonas species associated with either Solanaceae or Brassicaceae hosts. Comparative analyses revealed clear species-specific effectorome signatures. However, Solanaceae or Brassicaceae host-associated effectorome signatures were not detected. Nevertheless, host biases in the presence or absence of specific effector classes were observed. To assess whether host-associated effector absence results from selective pressures, we introduced effectors unique to Solanaceae pathogens to X. campestris pv. campestris and effectors unique to Brassicaceae pathogens to X. euvesicatoria pv. euvesicatoria (Xeue) and evaluated if these introductions hindered virulence on their respective hosts. Introducing the effector XopI into X. campestris pv. campestris reduced virulence on white cabbage leaves without affecting localized or systemic colonization. Introducing the XopAC or XopJ5 effectors into Xeue reduced virulence and colonization on tomato but not on pepper. Additionally, XopAC and XopJ5 induced a hypersensitive response on tomato leaves when delivered by Xeue or through Agrobacterium-mediated transient expression, confirming recognition in tomato. This study demonstrates the role of host-derived selection in establishing species-specific effectoromes, identifying XopAC and XopJ5 as recognized effectors in tomato.