Summary
Background
Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot of stone fruits and almond, an important disease that may reduce the yield and vigour of the trees, as well as the ...marketability of affected fruits. Xap lies within the Xanthomonas genus, which has been intensively studied because of its strain specialization and host range complexity. Here, we summarize the recent advances in our understanding of the complexities of Xap, including studies of the molecular features that result after comparative phenotypic and genomic analyses, in order to obtain a clearer overview of the bacterial behaviour and infection mechanism in the context of the X. arboricola species.
Taxonomic status
Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; Species X. arboricola; Pathovar pruni.
Host range and symptoms
Xap infects most Prunus species, including apricot, peach, nectarine, plum and almond, and occasionally cherry. Symptoms are found on leaves, fruits, twigs and branches or trunks. In severe infections, defoliation and fruit dropping may occur.
Distribution
Bacterial spot of stone fruits and almond is worldwide in distribution, with Xap being isolated in Africa, North and South America, Asia, Europe and Oceania. It is a common disease in geographical areas in which stone fruits and almonds are grown. Xap is listed as a quarantine organism in several areas of the world.
Genome
The genomes of six isolates from Xap have been publicly released. The genome consists of a single chromosome of around 5 000 000 bp with 65 mol% GC content and an extrachromosomal plasmid element of around 41 000 bp with 62 mol% GC content. Genomic comparative studies in X. arboricola have allowed the identification of putative virulence components associated with the infection process of bacterial spot of stone fruits and almond.
Disease control
Management of bacterial spot of stone fruits and almond is based on an integrated approach that comprises essential measures to avoid Xap introduction in a production zone, as well as the use of tolerant or resistant plant material and chemical treatments, mainly based on copper compounds. Management programmes also include the use of appropriate cultivation practices when the disease is already established. Finally, for the effective control of the disease, appropriate detection and characterization methods are needed for use in symptomatic or asymptomatic samples as a first approach for pathogen exclusion.
Useful websites
https://gd.eppo.int/taxon/XANTPR; http://www.cost.eu/COST_Actions/ca/CA16107; http://www.xanthomonas.org
is a plant-associated bacterial species that causes diseases on several plant hosts. One of the most virulent pathovars within this species is
pv.
(
), the causal agent of bacterial spot disease of ...stone fruit trees and almond. Recently, a non-virulent
-look-a-like strain isolated from
was characterized and its genome compared to pathogenic strains of
, revealing differences in the profile of virulence factors, such as the genes related to the type III secretion system (T3SS) and type III effectors (T3Es). The existence of this atypical strain arouses several questions associated with the abundance, the pathogenicity, and the evolutionary context of
on
hosts. After an initial characterization of a collection of
strains isolated from
bacterial spot outbreaks in Spain during the past decade, six
-look-a-like strains, that did not clustered with the pathogenic strains of
according to a multi locus sequence analysis, were identified. Pathogenicity of these strains was analyzed and the genome sequences of two
-look-a-like strains, CITA 14 and CITA 124, non-virulent to
spp., were obtained and compared to those available genomes of
associated with this host plant. Differences were found among the genomes of the virulent and the
non-virulent strains in several characters related to the pathogenesis process. Additionally, a pan-genomic analysis that included the available genomes of
, revealed that the atypical strains associated with
were related to a group of non-virulent or low virulent strains isolated from a wide host range. The repertoire of the genes related to T3SS and T3Es varied among the strains of this cluster and those strains related to the most virulent pathovars of the species,
, and
. This variability provides information about the potential evolutionary process associated to the acquisition of pathogenicity and host specificity in
. Finally, based in the genomic differences observed between the virulent and the non-virulent strains isolated from
, a sensitive and specific real-time PCR protocol was designed to detect and identify
strains. This method avoids miss-identifications due to atypical strains of
that can cohabit
.
Plant diseases significantly impact food security and food safety. It was estimated that food production needs to increase by 50% to feed the projected 9.3 billion people by 2050. Yet, plant ...pathogens and pests are documented to cause up to 40% yield losses in major crops, including maize, rice, and wheat, resulting in annual worldwide economic losses of approximately US$220 billion. Yield losses due to plant diseases and pests are estimated to be 21.5% (10.1 to 28.1%) in wheat, 30.3% (24.6 to 40.9%) in rice, and 22.6% (19.5 to 41.4%) in maize. In March 2023, The American Phytopathological Society (APS) conducted a survey to identify and rank key challenges in plant pathology in the next decade. Phytopathology subsequently invited papers that address those key challenges in plant pathology, and these were published as a special issue. The key challenges identified include climate change effect on the disease triangle and outbreaks, plant disease resistance mechanisms and its applications, and specific diseases including those caused by
Liberibacter spp. and
. Additionally, disease detection, natural and man-made disasters, and plant disease control strategies were explored in issue articles. Finally, aspects of open access and how to publish articles to maximize the Findability, Accessibility, Interoperability, and Reuse of digital assets in plant pathology were described. Only by identifying the challenges and tracking progress in developing solutions for them will we be able to resolve the issues in plant pathology and ultimately ensure plant health, food security, and food safety.
Xanthomonas arboricola comprises a number of economically important fruit tree pathogens classified within different pathovars. Dozens of non-pathogenic and taxonomically un-validated strains are ...also designated as X. arboricola leading to a complicated taxonomic status in the species. In this study, we have evaluated the whole genome resources of all available Xanthomonas spp. strains designated as X. arboricola in the public databases to refine the members of the species based on DNA similarity indexes and core genome-based phylogeny. Our results show that out of the nine validly described pathovars within X. arboricola, pathotype strains of seven pathovars are taxonomically genuine belonging to the core clade of the species regardless of the argue of their pathogenicity on the host of isolation (thus the validity of pathovar status). However, strains of X. arboricola pv. guizotiae and X. arboricola pv. populi do not belong to X. arboricola due to the low DNA similarities between the type strain of the species and the pathotype strains of these two pathovars. Thus, we propose to elevate the two pathovars to the rank of a species as X. guizotiae sp. nov. with the type strain CFBP 7408T, and X. populina sp. nov. with the type strain CFBP 3123T. In addition, other mislabeled strains of X. arboricola were scattered within Xanthomonas spp. either belonging to previously described species or representing novel species which await formal description.
Three isolates obtained from symptomatic nectarine trees (Prunus persica var. nectarina) cultivated in Murcia, Spain, which showed yellow and mucoid colonies similar to Xanthomonas arboricola pv. ...pruni, were negative after serological and real-time PCR analyses for this pathogen. For that reason, these isolates were characterized following a polyphasic approach that included both phenotypic and genomic methods. By sequence analysis of the 16S rRNA gene, these novel strains were identified as members of the genus Xanthomonas, and by multilocus sequence analysis (MLSA) they were clustered together in a distinct group that showed similarity values below 95 % with the rest of the species of this genus. Whole-genome comparisons of the average nucleotide identity (ANI) of genomes of the strains showed less than 91 % average nucleotide identity with all other species of the genus Xanthomonas. Additionally, phenotypic characterization based on API 20 NE, API 50 CH and BIOLOG tests differentiated the strains from the species of the genus Xanthomonas described previously. Moreover, the three strains were confirmed to be pathogenic on peach (Prunus persica), causing necrotic lesions on leaves. On the basis of these results, the novel strains represent a novel species of the genus Xanthomonas, for which the name Xanthomonas prunicola is proposed. The type strain is CFBP 8353 (=CECT 9404=IVIA 3287.1).
Fluorescent proteins have been used to track plant pathogens to understand their host interactions. To be useful, the transgenic pathogens must present similar behaviour than the wild-type isolates. ...Herein, a GFP marker was used to transform two plant pathogenic bacteria,
Agrobacterium
and
Xanthomonas
, to localize and track the bacteria during infection. The transgenic bacteria were evaluated to determine whether they showed the same fitness than the wild-type strains or whether the expression of the GFP protein interfered in the bacterial activity. In
Agrobacterium
, the plasmid used for transformation was stable in the bacteria and the strain kept the virulence, while
Xanthomonas
was not able to conserve the plasmid and transformed strains showed virulence variations compared to wild-type strains. Although marking bacteria with GFP to track infection in plants is a common issue, works to validate the transgenic strains and corroborate their fitness are not usual. Results, presented here, confirm the importance of proper fitness tests on the marked strains before performing localization assays, to avoid underestimation of the microbe population or possible artificial effects in its interaction with the plant.
Here, we report the draft genome sequence of Xanthomonas arboricola pv. pruni strain PVCT 262.1, isolated from almond (Prunus dulcis) leaves affected by bacterial spots in Italy in 2020. Genome size ...is 5,076,418 bp and G+C content is 65.44%. A total of 4,096 protein-coding genes and 92 RNAs are predicted.
Summary
Biological control of plant diseases has gained acceptance in recent years. Bacillus subtilis UMAF6639 is an antagonistic strain specifically selected for the efficient control of the ...cucurbit powdery mildew fungus Podosphaera fusca, which is a major threat to cucurbits worldwide. The antagonistic activity relies on the production of the antifungal compounds iturin and fengycin. In a previous study, we found that UMAF6639 was able to induce systemic resistance (ISR) in melon and provide additional protection against powdery mildew. In the present work, we further investigated in detail this second mechanism of biocontrol by UMAF6639. First, we examined the signalling pathways elicited by UMAF6639 in melon plants, as well as the defence mechanisms activated in response to P. fusca. Second, we analysed the role of the lipopeptides produced by UMAF6639 as potential determinants for ISR activation. Our results demonstrated that UMAF6639 confers protection against cucurbit powdery mildew by activation of jasmonate‐ and salicylic acid‐dependent defence responses, which include the production of reactive oxygen species and cell wall reinforcement. We also showed that surfactin lipopeptide is a major determinant for stimulation of the immune response. These results reinforce the biotechnological potential of UMAF6639 as a biological control agent.
In the present work we investigated in detail the mechanisms by which the antagonistic strain Bacillus subtilis UMAF6639 is also able to induce systemic resistance in melon and provide protection against powdery mildew. Our results demonstrated that UMAF6639 combats disease by activation of jasmonate‐ and salicylic acid‐dependent defence responses, which include the production of reactive oxygen species and cell wall reinforcement. We also showed that surfactin lipopeptide is a major determinant for stimulation of the plant immune response. These results reinforce the biotechnological potential of UMAF6639 as a biological control agent.
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