Three New Pierce's Disease Pathogenicity Effectors Identified Using Xylella fastidiosa Biocontrol Strain EB92-1

Two Xylella fastidiosa subsp. multiplex Strains Isolated from Almond in Spain Differ in Plasmid Content and Virulence Traits

The plant-pathogenic bacterium Xylella fastidiosa is a major threat to agriculture and the environment worldwide. Recent devastating outbreaks in Europe highlight the potential of this pathogen to cause emergent diseases. X. fastidiosa subsp. multiplex ESVL and IVIA5901 strains that belong to sequence type 6 were isolated from almond orchards within the outbreak area in Alicante province (Spain). Both strains share more than 99% of the chromosomal sequences (average nucleotide identity), but the ESVL strain harbors two plasmids (pXF64-Hb_ESVL and pUCLA-ESVL). Here, virulence phenotypes and genome content were compared between both strains, using three strains from the United States as a reference for the phenotypic analyses. Experiments in microfluidic chambers, used as a simulation of xylem vessels, showed that twitching motility was absent in the IVIA5901 strain, whereas the ESVL strain had reduced twitching motility. In general, both Spanish strains had less biofilm formation, less cell aggregation, and lower virulence in tobacco compared with U.S. reference strains. Genome analysis of the two plasmids from ESVL revealed 51 unique coding sequences that were absent in the chromosome of IVIA5901. Comparison of the chromosomes of both strains showed some unique coding sequences and single-nucleotide polymorphisms in each strain, with potential deleterious mutations. Genomic differences found in genes previously associated with adhesion and motility might explain the differences in the phenotypic traits studied. Although additional studies are necessary to infer the potential role of X. fastidiosa plasmids, our results indicate that the presence of plasmids should be considered in the study of the mechanisms of pathogenicity and adaptation in X. fastidiosa to new environments. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pathogen Adaptation to the Xylem Environment

A group of aggressive pathogens have evolved to colonize the plant xylem. In this vascular tissue, where water and nutrients are transported from the roots to the rest of the plant, pathogens must be able to thrive under acropetal xylem sap flow and scarcity of nutrients while having direct contact only with predominantly dead cells. Nevertheless, a few bacteria have adapted to exclusively live in the xylem, and various pathogens may colonize other plant niches without causing symptoms unless they reach the xylem. Once established, the pathogens modulate its physicochemical conditions to enhance their growth and virulence. Adaptation to the restrictive lifestyle of the xylem leads to genome reduction in xylem-restricted bacteria, as they have a higher proportion of pseudogenes in their genome. The basis of xylem adaptation is not completely understood; therefore, a need still exists for model systems to advance the knowledge on this topic.

Comparative Genomics of Xylella fastidiosa Explores Candidate Host-Specificity Determinants and Expands the Known Repertoire of Mobile Genetic Elements and Immunity Systems

Xylella fastidiosa causes diseases in many plant species. Originally confined to the Americas, infecting mainly grapevine, citrus, and coffee, X. fastidiosa has spread to several plant species in Europe causing devastating diseases. Many pathogenicity and virulence factors have been identified, which enable the various X. fastidiosa strains to successfully colonize the xylem tissue and cause disease in specific plant hosts, but the mechanisms by which this happens have not been fully elucidated. Here we present thorough comparative analyses of 94 whole-genome sequences of X. fastidiosa strains from diverse plant hosts and geographic regions. Core-genome phylogeny revealed clades with members sharing mostly a geographic region rather than a host plant of origin. Phylogenetic trees for 1605 orthologous CDSs were explored for potential candidates related to host specificity using a score of mapping metrics. However, no candidate host-specificity determinants were strongly supported using this approach. We also show that X. fastidiosa accessory genome is represented by an abundant and heterogeneous mobilome, including a diversity of prophage regions. Our findings provide a better understanding of the diversity of phylogenetically close genomes and expand the knowledge of X. fastidiosa mobile genetic elements and immunity systems.

Expression of putative effectors of different Xylella fastidiosa strains triggers cell death-like responses in various Nicotiana model plants

The wide host range of Xylella fastidiosa (Xf) indicates the existence of yet uncharacterized virulence mechanisms that help pathogens to overcome host defences. Various bioinformatics tools combined with prediction of the functions of putative virulence proteins are valuable approaches to study microbial pathogenicity. We collected a number of putative effectors from three Xf strains belonging to different subspecies: Temecula-1 (subsp. fastidiosa), CoDiRO (subsp. pauca), and Ann-1 (subsp. sandyi). We designed an in planta Agrobacterium-based expression system that drives the expressed proteins to the cell apoplast, in order to investigate their ability to activate defence in Nicotiana model plants. Multiple Xf proteins differentially elicited cell death-like phenotypes in different Nicotiana species. These proteins are members of different enzymatic groups: (a) hydrolases/hydrolase inhibitors, (b) serine proteases, and (c) metal transferases. We also classified the Xf proteins according to their sequential and structural similarities via the I-TASSER online tool. Interestingly, we identified similar proteins that were able to differentially elicit cell death in different cultivars of the same species. Our findings provide a basis for further studies on the mechanisms that underlie both defence activation in Xf resistant hosts and pathogen adaptation in susceptible hosts.

Introduction and adaptation of an emerging pathogen to olive trees in Italy

The invasive plant pathogen Xylella fastidiosa currently threatens European flora through the loss of economically and culturally important host plants. This emerging vector-borne bacterium, native to the Americas, causes several important diseases in a wide range of plants including crops, ornamentals, and trees. Previously absent from Europe, and considered a quarantine pathogen, X. fastidiosa was first detected in Apulia, Italy in 2013 associated with a devastating disease of olive trees (Olive Quick Decline Syndrome, OQDS). OQDS has led to significant economic, environmental, cultural, as well as political crises. Although the biology of X. fastidiosa diseases have been studied for over a century, there is still no information on the determinants of specificity between bacterial genotypes and host plant species, which is particularly relevant today as X. fastidiosa is expanding in the naive European landscape. We analysed the genomes of 79 X. fastidiosa samples from diseased olive trees across the affected area in Italy as well as genomes of the most genetically closely related strains from Central America. We provided insights into the ecological and evolutionary emergence of this pathogen in Italy. We first showed that the outbreak in Apulia is due to a single introduction from Central America that we estimated to have occurred in 2008 [95 % HPD: 1930–2016]. By using a combination of population genomic approaches and evolutionary genomics methods, we further identified a short list of genes that could play a major role in the adaptation of X. fastidiosa to this new environment. We finally provided experimental evidence for the adaptation of the strain to this new environment.

Complete Genome Sequence of Xylella taiwanensis and Comparative Analysis of Virulence Gene Content With Xylella fastidiosa

The bacterial genus Xylella contains plant pathogens that are major threats to agriculture in America and Europe. Although extensive research was conducted to characterize different subspecies of Xylella fastidiosa (Xf), comparative analysis at above-species levels was lacking due to the unavailability of appropriate data sets. Recently, a bacterium that causes pear leaf scorch (PLS) in Taiwan was described as the second Xylella species (i.e., Xylella taiwanensis; Xt). In this work, we report the complete genome sequence of Xt type strain PLS229^T. The genome-scale phylogeny provided strong support that Xf subspecies pauca (Xfp) is the basal lineage of this species and Xylella was derived from the paraphyletic genus Xanthomonas. Quantification of genomic divergence indicated that different Xf subspecies share ∼87–95% of their chromosomal segments, while the two Xylella species share only ∼66–70%. Analysis of overall gene content suggested that Xt is most similar to Xf subspecies sandyi (Xfs). Based on the existing knowledge of Xf virulence genes, the homolog distribution among 28 Xylella representatives was examined. Among the 11 functional categories, those involved in secretion and metabolism are the most conserved ones with no copy number variation. In contrast, several genes related to adhesins, hydrolytic enzymes, and toxin-antitoxin systems are highly variable in their copy numbers. Those virulence genes with high levels of conservation or variation may be promising candidates for future studies. In summary, the new genome sequence and analysis reported in this work contributed to the study of several important pathogens in the family Xanthomonadaceae.

Increase in ring width, vessel number and δ18O in olive trees infected with Xylella fastidiosa

Xylella fastidiosa (Xf) Wells, Raju et al., 1986 is a bacterium that causes plant diseases in the Americas. In Europe, it was first detected on the Salento Peninsula (Italy), where it was found to be associated with the olive quick decline syndrome. Here, we present the results of the first tree-ring study of infected and uninfected olive trees (Olea europaea L.) of two different cultivars, one resistant and one susceptible, to establish the effects induced by the spread of the pathogen inside the tree. Changes in wood anatomical characteristics, such as an increase in the number of vessels and in ring width, were observed in the infected plants of both the cultivars Cellina di Nardò (susceptible to Xf infection) and Leccino (resistant to Xf infection). Thus, whether infection affects the mortality of the tree or not, the tree shows a reaction to it. The presence of occlusions was detected in the wood of both 4-year-old branches and the tree stem core. As expected, the percentage of occluded vessels in the Xf-susceptible cultivar Cellina di Nardò was significantly higher than in the Xf-resistant cultivar Leccino. The δ 18O of the 4-year-old branches was significantly higher in infected trees of both cultivars than in noninfected trees, while no variations in δ 13C were observed. This suggests a reduction in leaf transpiration rates during infection and seems to be related to the occlusions observed in rings of the 4-year-old branches. Such occlusions can determine effects at leaf level that could influence stomatal activity. On the other hand, the significant increase in the number of vessels in infected trees could be related to the tree's attempt to enhance water conductivity in response to the pathogen-induced vessel occlusions.

Complete Genome Sequence Reveals Evolutionary and Comparative Genomic Features of Xanthomonas albilineans Causing Sugarcane Leaf Scald

Leaf scald (caused by Xanthomonas albilineans) is an important bacterial disease affecting sugarcane in most sugarcane growing countries, including China. High genetic diversity exists among strains of X. albilineans from diverse geographic regions. To highlight the genomic features associated with X. albilineans from China, we sequenced the complete genome of a representative strain (Xa-FJ1) of this pathogen using the PacBio and Illumina platforms. The complete genome of strain Xa-FJ1 consists of a circular chromosome of 3,724,581 bp and a plasmid of 31,536 bp. Average nucleotide identity analysis revealed that Xa-FJ1 was closest to five strains from the French West Indies and the USA, particularly to the strain GPE PC73 from Guadeloupe. Comparative genomic analysis between Xa-FJ1 and GPE PC73 revealed prophage integration, homologous recombination, transposable elements, and a clustered regulatory interspaced short palindromic repeats (CRISPR) system that were linked with 16 insertions/deletions (InDels). Ten and 82 specific genes were found in Xa-FJ1 and GPE PC73, respectively, and some of these genes were subjected to phage-related proteins, zona occludens toxin, and DNA methyltransferases. Our findings highlight intra-species genetic variability of the leaf scald pathogen and provide additional genomic resources to investigate its fitness and virulence.

Lessons from One Fastidious Bacterium to Another: What Can We Learn about Liberibacter Species from Xylella fastidiosa

Huanglongbing is causing economic devastation to the citrus industry in Florida, and threatens the industry everywhere the bacterial pathogens in the Candidatus Liberibacter genus and their insect vectors are found. Bacteria in the genus cannot be cultured and no durable strategy is available for growers to control plant infection or pathogen transmission. However, scientists and grape growers were once in a comparable situation after the emergence of Pierce’s disease, which is caused by Xylella fastidiosa and spread by its hemipteran insect vector. Proactive quarantine and vector control measures coupled with interdisciplinary data-driven science established control of this devastating disease and pushed the frontiers of knowledge in the plant pathology and vector biology fields. Our review highlights the successful strategies used to understand and control X. fastidiosa and their potential applicability to the liberibacters associated with citrus greening, with a focus on the interactions between bacterial pathogen and insect vector. By placing the study of Candidatus Liberibacter spp. within the current and historical context of another fastidious emergent plant pathogen, future basic and applied research to develop control strategies can be prioritized.

Patterns of inter- and intrasubspecific homologous recombination inform eco-evolutionary dynamics of Xylella fastidiosa

High rates of homologous recombination (HR) in the bacterial plant pathogen Xylella fastidiosa have been previously detected. This study aimed to determine the extent and explore the ecological significance of HR in the genomes of recombinants experimentally generated by natural transformation and wild-type isolates. Both sets of strains displayed widespread HR and similar average size of recombined fragments consisting of random events (2-10 kb) of inter- and intrasubspecific recombination. A significantly higher proportion and greater lengths (>10 kb, maximum 31.5 kb) of recombined fragments were observed in subsp. morus and in strains isolated in Europe from intercepted coffee plants shipped from the Americas. Such highly recombinant strains pose a serious risk of emergence of novel variants, as genetically distinct and formerly geographically isolated genotypes are brought in close proximity by global trade. Recently recombined regions in wild-type strains included genes involved in regulation and signaling, host colonization, nutrient acquisition, and host evasion, all fundamental traits for X. fastidiosa ecology. Identification of four recombinant loci shared between wild-type and experimentally generated recombinants suggests potential hotspots of recombination in this naturally competent pathogen. These findings provide insights into evolutionary forces possibly affecting the adaptive potential to colonize the host environments of X. fastidiosa.

Trans-Graft Protection Against Pierce's Disease Mediated by Transgenic Grapevine Rootstocks

A field study showed that transgenic grapevine rootstocks can provide trans-graft-mediated protection to a wild type scion against Pierce's disease (PD) development. We individually field-tested two distinct strategies. The first expressed a chimeric antimicrobial protein (CAP) that targeted the functionality of the lipopolysaccharide (LPS) surface of Xylella fastidiosa (Xf), the causative agent of PD. The second expressed a plant polygalacturonase inhibitory protein (PGIP) that prevents PD by inhibiting breakdown of pectin present in primary cell walls. Both proteins are secreted to the apoplast and then into the xylem, where they migrate past the graft union, transiting into the xylem of the grafted scion. Transgenic Vitis vinifera cv. Thompson Seedless (TS) expressing ether CAP or PGIP were tested in the greenhouse and those lines that showed resistance to PD were grafted with wild type TS scions. Grafted grapevines were introduced into the field and tested over 7 years. Here we present data on the field evaluation of trans-graft protection using four CAP and four PGIP independent rootstock lines, compared to an untransformed rootstock. There was 30 to 95% reduction in vine mortality among CAP- and PGIP-expressing lines after three successive yearly infections with virulent Xf. Shoot tissues grafted to either CAP or PGIP transgenic rootstocks supported lower pathogen titers and showed fewer disease symptoms. Grafted plants on transgenic rootstocks also had more spring bud break following infection, more shoots, and more vigorous growth compared to those grafted to wild type rootstocks. No yield penalty was observed in the transgenic lines and some PGIP-expressing vines had enhanced yield potential. Trans-graft protection is an efficient way to protect grape scions against PD while preserving their valuable varietal genotypes and clonal properties.

Identification and Characterization of Menadione and Benzethonium Chloride as Potential Treatments of Pierce's Disease of Grapevines

Xylella fastidiosa infects a wide range of plant hosts and causes Pierce's disease (PD) of grapevines. The type 1 multidrug resistance (MDR) efflux system is essential for pathogenicity and survival of bacterial pathogens in planta. X. fastidiosa, with a single MDR system, is significantly more vulnerable to inhibition by small-molecule treatments than most bacterial pathogens that typically carry redundant MDR systems. A high-throughput cell viability assay using a green fluorescent protein-marked strain of X. fastidiosa Temecula 1 was developed to screen two Prestwick combinatorial small-molecule libraries of drugs and phytochemicals (1,600 chemicals in total) approved by the Food and Drug Administration and European Medicines Agency for cell growth inhibition. The screens revealed 215 chemicals that inhibited bacterial growth by >50% at 50 µM concentrations. Seven chemicals proved to lyse X. fastidiosa cells at 25 µM, including four phytochemicals. Menadione (2-methyl-1,4-naphthoquinone, vitamin K) from the phytochemical library and benzethonium chloride (a topical disinfectant) from the chemical library both showed significant bactericidal activity against X. fastidiosa. Both menadione and benzethonium chloride foliar spray (15 and 5 mM, respectively) and soil drench (5 and 25 mM, respectively) treatments were equally effective in reducing PD symptoms by 54 to 59% and revealed that the effects of both chemical treatments became systemic. However, menadione was phytotoxic when applied as a foliar spray at effective concentrations, causing significant loss of photosynthetic capacity.

The Xylella fastidosa RTX operons: evidence for the evolution of protein mosaics through novel genetic exchanges

BACKGROUND

Xylella fastidiosa (Xf) is a gram negative bacterium inhabiting the plant vascular system. In most species this bacterium lives as a benign symbiote, but in several agriculturally important plants (e.g. coffee, citrus, grapevine) Xf is pathogenic. Xf has four loci encoding homologues to hemolysin RTX proteins, virulence factors involved in a wide range of plant pathogen interactions.

RESULTS

We show that all four genes are expressed during pathogenesis in grapevine. The sequences from these four genes have a complex repetitive structure. At the C-termini, sequence diversity between strains is what would be expected from orthologous genes. However, within strains there is no N-terminal homology, indicating these loci encode RTXs of different functions and/or specificities. More striking is that many of the orthologous loci between strains share this extreme variation at the N-termini. Thus these RTX orthologues are most easily visualized as fusions between the orthologous C-termini and different N-termini. Further, the four genes are found in operons having a peculiar structure with an extensively duplicated module encoding a small protein with homology to the N-terminal region of the full length RTX. Surprisingly, some of these small peptides are most similar not to their corresponding full length RTX, but to the N-termini of RTXs from other Xf strains, and even other remotely related species.

CONCLUSIONS

These results demonstrate that these genes are expressed in planta during pathogenesis. Their structure suggests extensive evolutionary restructuring through horizontal gene transfers and heterologous recombination mechanisms. The sum of the evidence suggests these repetitive modules are a novel kind of mobile genetic element.

Xylella fastidiosa: an examination of a re-emerging plant pathogen

Xylella fastidiosa is a Gram-negative bacterial plant pathogen with an extremely wide host range. This species has recently been resolved into subspecies that correlate with host specificity. This review focuses on the status of X. fastidiosa pathogenic associations in plant hosts in which the bacterium is either endemic or has been recently introduced. Plant diseases associated with X. fastidiosa have been documented for over a century, and much about what is known in the context of host-pathogen interactions is based on these hosts, such as grape and citrus, in which this pathogen has been well described. Recent attention has focused on newly emerging X. fastidiosa diseases, such as in olives.

TAXONOMY

Bacteria; Gammaproteobacteria; family Xanthomonadaceae; genus Xylella; species fastidiosa.

MICROBIOLOGICAL PROPERTIES

Gram-negative rod (0.25-0.35 × 0.9-3.5 μm), non-flagellate, motile via Type IV pili-mediated twitching, fastidious.

HOST RANGE

Xylella fastidiosa has a broad host range that includes ornamental, ecological and agricultural plants belonging to over 300 different species in 63 different families. To date, X. fastidiosa has been found to be pathogenic in over 100 plant species. In addition, it can establish non-symptomatic associations with many plants as a commensal endophyte. Here, we list the four distinct subspecies of X. fastidiosa and some of the agriculturally relevant diseases caused by them: X. fastidiosa ssp. fastidiosa causes Pierce's disease (PD) of grapevine (Vitis vinifera); X. fastidiosa ssp. multiplex causes almond leaf scorch (ALS) and diseases on other nut and shade tree crops; X. fastidiosa ssp. pauca causes citrus variegated chlorosis (CVC) (Citrus spp.), coffee leaf scorch and olive quick decline syndrome (OQDS) (Olea europaea); X. fastidiosa ssp. sandyi causes oleander leaf scorch (OLS) (Nerium oleander). Significant host specificity seemingly exists for some of the subspecies, although this could be a result of technical biases based on the limited number of plants tested, whereas some subspecies are not as stringent in their host range and can infect several plant hosts.

DISEASE SYMPTOMS

Most X. fastidiosa-related diseases appear as marginal leaf necrosis and scorching of the leaves. In the case of PD, X. fastidiosa can also cause desiccation of berries (termed 'raisining'), irregular periderm development and abnormal abscission of petioles. In olive trees affected with OQDS, leaves exhibit marginal necrosis and defoliation, and overall tree decline occurs. Plants with ALS and OLS also exhibit the characteristic leaf scorch symptoms. Not all X. fastidiosa-related diseases exhibit the typical leaf scorch symptoms. These include CVC and Phony Peach disease, amongst others. In the case of CVC, symptoms include foliar wilt and interveinal chlorosis on the upper surfaces of the leaves (similar to zinc deficiency), which correspond to necrotic, gum-like regions on the undersides of the leaves. Additional symptoms of CVC include defoliation, dieback and hardening of fruits. Plants infected with Phony Peach disease exhibit a denser, more compact canopy (as a result of shortened internodes, darker green leaves and delayed leaf senescence), premature bloom and reduced fruit size. Some occlusions occur in the xylem vessels, but there are no foliar wilting, chlorosis or necrosis symptoms . USEFUL WEBSITES: http://www.piercesdisease.org/; https://pubmlst.org/xfastidiosa/; http://www.xylella.lncc.br/; https://nature.berkeley.edu/xylella/; https://ec.europa.eu/food/plant/plant_health_biosecurity/legislation/emergency_measures/xylella-fastidiosa_en.

The Major Outer Membrane Protein MopB Is Required for Twitching Movement and Affects Biofilm Formation and Virulence in Two Xylella fastidiosa strains

MopB is a major outer membrane protein (OMP) in Xylella fastidiosa, a bacterial plant pathogen that causes losses on many economically important crops. Based on in silico analysis, the uncharacterized MopB protein of X. fastidiosa contains a β-barrel structure with an OmpA-like domain and a predicted calcium-binding motif. Here, MopB function was studied by mutational analysis taking advantage of the natural competence of X. fastidiosa. Mutants of mopB were constructed in two different X. fastidiosa strains, the type strain Temecula and the more virulent WM1-1. Deletion of the mopB gene impaired cell-to-cell aggregation, surface attachment, and biofilm formation in both strains. Interestingly, mopB deletion completely abolished twitching motility. Electron microscopy of the bacterial cell surface revealed that mopB deletion eliminated type IV and type I pili formation, potentially caused by destabilization of the outer membrane. Both mopB mutants showed reduced virulence using tobacco (Nicotiana tabacum) as a host under greenhouse conditions. These results suggest that MopB has pleiotropic functions in biofilm formation and twitching motility and is important for virulence of X. fastidiosa.

Characterization of the Xylella fastidiosa PD1311 gene mutant and its suppression of Pierce's disease on grapevines

Xylella fastidiosa causes Pierce's disease (PD) on grapevines, leading to significant economic losses in grape and wine production. To further our understanding of X. fastidiosa virulence on grapevines, we examined the PD1311 gene, which encodes a putative acyl-coenzyme A (acyl-CoA) synthetase, and is highly conserved across Xylella species. It was determined that PD1311 is required for virulence, as the deletion mutant, ΔPD1311, was unable to cause disease on grapevines. The ΔPD1311 strain was impaired in behaviours known to be associated with PD development, including motility, aggregation and biofilm formation. ΔPD1311 also expressed enhanced sensitivity to H₂ O₂ and polymyxin B, and showed reduced survival in grapevine sap, when compared with wild-type X. fastidiosa Temecula 1 (TM1). Following inoculation, ΔPD1311 could not be detected in grape shoots, which may be related to its altered growth and sensitivity phenotypes. Inoculation with ΔPD1311 2 weeks prior to TM1 prevented the development of PD in a significant fraction of vines and eliminated detectable levels of TM1. In contrast, vines inoculated simultaneously with TM1 and ΔPD1311 developed disease at the same level as TM1 alone. In these vines, TM1 populations were distributed similarly to populations in TM1-only inoculated plants. These findings suggest that, through an indirect mechanism, pretreatment of vines with ΔPD1311 suppresses pathogen population and disease.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Grape Cultivar and Sap Culture Conditions Affect the Development of Xylella fastidiosa Phenotypes Associated with Pierce's Disease

Xylella fastidiosa is a xylem-limited bacterium in plant hosts and causes Pierce's disease (PD) of grapevines, which differ in susceptibility according to the Vitis species (spp.). In this work we compared X. fastidiosa biofilm formation and population dynamics when cultured in xylem saps from PD-susceptible and -resistant Vitis spp. under different conditions. Behaviors in a closed-culture system were compared to those in different sap-renewal cultures that would more closely mimic the physicochemical environment encountered in planta. Significant differences in biofilm formation and growth in saps from PD-susceptible and -resistant spp. were only observed using sap renewal culture. Compared to saps from susceptible V. vinifera, those from PD-resistant V. aestivalis supported lower titers of X. fastidiosa and less biofilm and V. champinii suppressed both growth and biofilm formation, behaviors which are correlated with disease susceptibility. Furthermore, in microfluidic chambers X. fastidiosa formed thick mature biofilm with three-dimensional (3-D) structures, such as pillars and mounds, in saps from all susceptible spp. In contrast, only small aggregates of various shapes were formed in saps from four out of five of the resistant spp.; sap from the resistant spp. V. mustangensis was an exception in that it also supported thick lawns of biofilm but not the above described 3-D structures typically seen in a mature biofilm from the susceptible saps. Our findings provide not only critical technical information for future bioassays, but also suggest further understanding of PD susceptibility.

The Secreted Protease PrtA Controls Cell Growth, Biofilm Formation and Pathogenicity in Xylella fastidiosa

Pierce's disease (PD) is a deadly disease of grapevines caused by the Gram-negative bacterium Xylella fastidiosa. Though disease symptoms were formerly attributed to bacteria blocking the plant xylem, this hypothesis is at best overly simplistic. Recently, we used a proteomic approach to characterize the secretome of X. fastidiosa, both in vitro and in planta, and identified LesA as one of the pathogenicity factors of X. fastidiosa in grapevines that leads to leaf scorching and chlorosis. Herein, we characterize another such factor encoded by PD0956, designated as an antivirulence secreted protease "PrtA" that displays a central role in controlling in vitro cell proliferation, length, motility, biofilm formation, and in planta virulence. The mutant in X. fastidiosa exhibited reduced cell length, hypermotility (and subsequent lack of biofilm formation) and hypervirulence in grapevines. These findings are supported by transcriptomic and proteomic analyses with corresponding plant infection data. Of particular interest, is the hypervirulent response in grapevines observed when X. fastidiosa is disrupted for production of PrtA, and that PD-model tobacco plants transformed to express PrtA exhibited decreased symptoms after infection by X. fastidiosa.

Calcium transcriptionally regulates the biofilm machinery of Xylella fastidiosa to promote continued biofilm development in batch cultures

The functions of calcium (Ca) in bacteria are less characterized than in eukaryotes, where its role has been studied extensively. The plant-pathogenic bacterium Xylella fastidiosa has several virulence features that are enhanced by increased Ca concentrations, including biofilm formation. However, the specific mechanisms driving modulation of this feature are unclear. Characterization of biofilm formation over time showed that 4 mM Ca supplementation produced denser biofilms that were still developing at 96 h, while biofilm in non-supplemented media had reached the dispersal stage by 72 h. To identify changes in global gene expression in X. fastidiosa grown in supplemental Ca, RNA-Seq of batch culture biofilm cells was conducted at three 24-h time intervals. Results indicate that a variety of genes are differentially expressed in response to Ca, including genes related to attachment, motility, exopolysaccharide synthesis, biofilm formation, peptidoglycan synthesis, regulatory functions, iron homeostasis, and phages. Collectively, results demonstrate that Ca supplementation induces a transcriptional response that promotes continued biofilm development, while biofilm cells in nonsupplemented media are driven towards dispersion of cells from the biofilm structure. These results have important implications for disease progression in planta, where xylem sap is the source of Ca and other nutrients for X. fastidiosa.

The Type II Secreted Lipase/Esterase LesA is a Key Virulence Factor Required for Xylella fastidiosa Pathogenesis in Grapevines

Pierce's disease (PD) of grapevines is caused by Xylella fastidiosa (Xf), a xylem-limited gamma-proteobacterium that is responsible for several economically important crop diseases. The occlusion of xylem elements and interference with water transport by Xf and its associated biofilm have been posited as the main cause of PD symptom development; however, Xf virulence mechanisms have not been described. Analysis of the Xf secretome revealed a putative lipase/esterase (LesA) that was abundantly secreted in bacterial culture supernatant and was characterized as a protein ortholog of the cell wall-degrading enzyme LipA of Xanthomonas strains. LesA was secreted by Xf and associated with a biofilm filamentous network. Additional proteomic analysis revealed its abundant presence in outer membrane vesicles (OMVs). Accumulation of LesA in leaf regions associated positively with PD symptoms and inversely with bacterial titer. The lipase/esterase also elicited a hypersensitive response in grapevine. Xf lesA mutants were significantly deficient for virulence when mechanically inoculated into grapevines. We propose that Xf pathogenesis is caused by LesA secretion mediated by OMV cargos and that its release and accumulation in leaf margins leads to early stages of observed PD symptoms.