O antigen modulates insect vector acquisition of the bacterial plant pathogen Xylella fastidiosa

Peptides with Charged Amino Acids Mitigate nZnO-Induced Growth Inhibition of Lactobacillus rhamnosus LRa05

Growing concern is about the potential side effects of nanomaterials from food packaging, notably zinc oxide nanoparticles (nZnO). Previous research revealed that walnut-derived peptides could mitigate this inhibitory effect, but the mechanism involved is unclear. Here, we found that not all peptides have such an effect. Based on the growth inhibition model of Lactobacillus rhamnosus LRa05 induced by nZnO, we assessed the protective effects of various peptides. Notably, four peptides containing charged amino acids (PPKNW, WPPKN, ADIYTE, and WEREEQE) were found to effectively alleviate the growth inhibition phenomenon. We hypothesize that the peptide-nZnO interaction modifies this effect, as confirmed through infrared, Raman, and fluorescence spectroscopy. Our results highlight amide bonds, amino groups, carboxyl groups, and benzene rings as key peptide binding sites on nZnO, with static quenching primarily due to hydrogen bonds and van der Waals forces. This study elucidates peptide characteristics in nZnO interactions, facilitating a deeper exploration of food matrix-nanocomposite interactions.

Xylella fastidiosa modulates exopolysaccharide polymer length and the dynamics of biofilm development with a β-1,4-endoglucanase

IMPORTANCE

It is well established that exopolysaccharide (EPS) is an integral structural component of bacterial biofilms necessary for assembly and maintenance of the three-dimensional architecture of the biofilm. However, the process and role of EPS turnover within a developing biofilm is not fully understood. Here, we demonstrated that Xylella fastidiosa uses a self-produced endoglucanase to enzymatically process its own EPS to modulate EPS polymer length. This enzymatic processing of EPS dictates the early stages of X. fastidiosa's biofilm development, which, in turn, affects its behavior in planta. A deletion mutant that cannot produce the endoglucanase was hypervirulent, thereby linking enzymatic processing of EPS to attenuation of virulence in symptomatic hosts, which may be a vestige of X. fastidiosa's commensal behavior in many of its other non-symptomatic hosts.

Anatomy of an agricultural antagonist: Feeding complex structure and function of three xylem sap-feeding insects illuminated with synchrotron-based 3D imaging

Many insects feed on xylem or phloem sap of vascular plants. Although physical damage to the plant is minimal, the process of insect feeding can transmit lethal viruses and bacterial pathogens. Disparities between insect-mediated pathogen transmission efficiency have been identified among xylem sap-feeding insects; however, the mechanistic drivers of these trends are unclear. Identifying and understanding the structural factors and associated integrated functional components that may ultimately determine these disparities are critical for managing plant diseases. Here, we applied synchrotron-based X-ray microcomputed tomography to digitally reconstruct the morphology of three xylem sap-feeding insect vectors of plant pathogens: Graphocephala atropunctata (blue-green sharpshooter; Hemiptera, Cicadellidae) and Homalodisca vitripennis (glassy-winged sharpshooter; Hemiptera, Cicadellidae), and the spittlebug Philaenus spumarius (meadow spittlebug; Hemiptera, Aphrophoridae). The application of this technique revealed previously undescribed anatomical features of these organisms, such as key components of the salivary complex. The visualization of the 3D structure of the precibarial valve led to new insights into the mechanism of how this structure functions. Morphological disparities with functional implications between taxa were highlighted as well, including the morphology and volume of the cibarial dilator musculature responsible for extracting xylem sap, which has implications for force application capabilities. These morphological insights will be used to target analyses illuminating functional differences in feeding behavior.

Xylella fastidiosa inoculation behaviors (EPG X wave) are performed differently by blue-green sharpshooters based on infection status of prior probing host

Does Xylella fastidiosa, a bacterial plant pathogen with noncirculative foregut-borne transmission, manipulate behavior of its sharpshooter vector to facilitate its own inoculation? To answer this question, blue-green sharpshooters, Graphocephala atropunctata (Signoret), were reared on basil to clean their foreguts, then removed from the colony and given one of four pre-electropenetrography (EPG) treatments: i) old colony adults on basil, ii) young colony adults on basil, iii) young colony adults held on healthy grapevine for 4 days, and iv) young colony adults held on Xf-infected (symptomatic) grapevine for 4 days. After treatments, stylet probing behaviors were recorded on healthy grapevine via AC-DC electropenetrography. Waveforms representing putative Xf inoculation (XB1 [salivation and rinsing egestion] and XC1 [discharging egestion]) and other behaviors were statistically compared among treatments. Mean number of events per insect and 'total' duration per insect of XB1 and XC1 were highest for insects from healthy grape, lowest for basil (regardless of insect age), and intermediate for Xf-infected grape. The surprising results showed that prior exposure to healthy grapevines had a stronger effect on subsequent performance of inoculation behaviors on healthy grapevine than did prior exposure to Xf-infected grapevine. It is hypothesized that non-Xf microbes were acquired from healthy grapevine, causing greater clogging of the precibarium, leading to more performance of inoculation behaviors. This study shows for the first time that presence of noncirculative, foregut-borne microbes can directly manipulate a vector's behavior to increase inoculation. Also, EPG can uniquely visualize the dynamic interactions between vectors and the microbes they carry.

Association of Midgut Bacteria and Their Metabolic Pathways with Zika Infection and Insecticide Resistance in Colombian Aedes aegypti Populations

INTRODUCTION

Aedes aegypti is the vector of several arboviruses such as dengue, Zika, and chikungunya. In 2015-16, Zika virus (ZIKV) had an outbreak in South America associated with prenatal microcephaly and Guillain-Barré syndrome. This mosquito's viral transmission is influenced by microbiota abundance and diversity and its interactions with the vector. The conditions of cocirculation of these three arboviruses, failure in vector control due to insecticide resistance, limitations in dengue management during the COVID-19 pandemic, and lack of effective treatment or vaccines make it necessary to identify changes in mosquito midgut bacterial composition and predict its functions through the infection. Its study is fundamental because it generates knowledge for surveillance of transmission and the risk of outbreaks of these diseases at the local level.

METHODS

Midgut bacterial compositions of females of Colombian Ae. aegypti populations were analyzed using DADA2 Pipeline, and their functions were predicted with PICRUSt2 analysis. These analyses were done under the condition of natural ZIKV infection and resistance to lambda-cyhalothrin, alone and in combination. One-step RT-PCR determined the percentage of ZIKV-infected females. We also measured the susceptibility to the pyrethroid lambda-cyhalothrin and evaluated the presence of the V1016I mutation in the sodium channel gene.

RESULTS

We found high ZIKV infection rates in Ae. aegypti females from Colombian rural municipalities with deficient water supply, such as Honda with 63.6%. In the face of natural infection with an arbovirus such as Zika, the diversity between an infective and non-infective form was significantly different. Bacteria associated with a state of infection with ZIKV and lambda-cyhalothrin resistance were detected, such as the genus Bacteroides, which was related to functions of pathogenicity, antimicrobial resistance, and bioremediation of insecticides. We hypothesize that it is a vehicle for virus entry, as it is in human intestinal infections. On the other hand, Bello, the only mosquito population classified as susceptible to lambda-cyhalothrin, was associated with bacteria related to mucin degradation functions in the intestine, belonging to the Lachnospiraceae family, with the genus Dorea being increased in ZIKV-infected females. The Serratia genus presented significantly decreased functions related to phenazine production, potentially associated with infection control, and control mechanism functions for host defense and quorum sensing. Additionally, Pseudomonas was the genus principally associated with functions of the degradation of insecticides related to tryptophan metabolism, ABC transporters with a two-component system, efflux pumps, and alginate synthesis.

CONCLUSIONS

Microbiota composition may be modulated by ZIKV infection and insecticide resistance in Ae. aegypti Colombian populations. The condition of resistance to lambda-cyhalothrin could be inducing a phenome of dysbiosis in field Ae. aegypti affecting the transmission of arboviruses.

Disrupting Irreversible Bacterial Adhesion and Biofilm Formation with an Engineered Enzyme

Biofilm formation is often attributed to postharvest bacterial persistence on fresh produce and food handling surfaces. In this study, a predicted glycosyl hydrolase enzyme was expressed, purified, and validated for the removal of microbial biofilms from biotic and abiotic surfaces under conditions used for chemical cleaning agents. Crystal violet biofilm staining assays revealed that 0.1 mg/ml of enzyme inhibited up to 41% of biofilm formation by Escherichia coli O157:H7, E. coli 25922, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes. Furthermore, the enzyme was effective at removing mature biofilms, providing a 35% improvement over rinsing with a saline solution alone. Additionally, a parallel-plate flow cell was used to directly observe and quantify the impact of enzyme rinses on E. coli O157:H7 cells adhering to spinach leaf surfaces. The presence of 1 mg/liter enzyme resulted in nearly 6-times-higher detachment rate coefficients than a deionized (DI) water rinse, while the total cells removed from the surface increased from 10% to 25% over the 30-min rinse time, reversing the initial phases of biofilm formation. Enzyme treatment of all 4 cell types resulted in significantly reduced cell surface hydrophobicity and collapse of negatively stained E. coli 25922 cells imaged by electron microscopy, suggesting potential polysaccharide surface modification of enzyme-treated bacteria. Collectively, these results point to the broad substrate specificity and robustness of the enzyme for different types of biofilm stages, solution conditions, and pathogen biofilm types and may be useful as a method for the removal or inhibition of bacterial biofilm formation. IMPORTANCE In this study, the ability of an engineered enzyme to reduce bacterial adhesion and biofilm formation of several foodborne pathogens was demonstrated, representing a promising option for enhancing or replacing chlorine and other chemical sanitizers in food processing applications. Specifically, significant reductions of biofilms of the pathogens Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes are observed, as are reductions in initial adhesion. Enzymes have the added benefits of being green, sustainable alternatives to chemical sanitizers, as well as having a minimal impact on food properties, in contrast to many alternative antimicrobial options such as bleach that aim to minimize food safety risks.

Functional foregut anatomy of the blue-green sharpshooter illustrated using a 3D model

Sharpshooter leafhoppers (Hemiptera: Cicadellidae: Cicadellinae) are important vectors of the plant pathogenic bacterium Xylella fastidiosa Wells et al. (Xanthomonadales: Xanthomonadaceae). This pathogen causes economically significant diseases in olive, citrus, and grapes on multiple continents. Bacterial acquisition and inoculation mechanisms are linked to X. fastidiosa biofilm formation and fluid dynamics in the functional foregut of sharpshooters, which together result in egestion (expulsion) of fluids likely carrying bacteria. One key X. fastidiosa vector is the blue–green sharpshooter, Graphocephala atropunctata (Signoret, 1854). Herein, a 3D model of the blue–green sharpshooter functional foregut is derived from a meta-analysis of published microscopy images. The model is used to illustrate preexisting and newly defined anatomical terminology that is relevant for investigating fluid dynamics in the functional foregut of sharpshooters. The vivid 3D illustrations herein and supplementary interactive 3D figures are suitable resources for multidisciplinary researchers who may be unfamiliar with insect anatomy. The 3D model can also be used in future fluid dynamic simulations to better understand acquisition, retention, and inoculation of X. fastidiosa. Improved understanding of these processes could lead to new targets for preventing diseases caused by X. fastidiosa.

Xylella fastidiosa and Glassy-Winged Sharpshooter Population Dynamics in the Southern San Joaquin Valley of California

Xylella fastidiosa is a vector-transmitted bacterial plant pathogen that affects a wide array of perennial crops, including grapevines (Pierce's disease). In the southern San Joaquin Valley of California, epidemics of Pierce's disease of grapevine were associated with the glassy-winged sharpshooter, Homalodisca vitripennis. During the growing season, rates of X. fastidiosa spread in vineyards are affected by changes in pathogen distribution within chronically infected grapevines and by vector population dynamics. Grapevines chronically infected with X. fastidiosa rarely tested positive for the pathogen prior to July, suggesting vector acquisition of X. fastidiosa from grapevines increases as the season progresses. This hypothesis was supported by an increase in number of X. fastidiosa-positive glassy-winged sharpshooters collected from vineyards during July through September. Analysis of insecticide records indicated that vineyards in the study area were typically treated with a systemic neonicotinoid in spring of each year. As a result, abundance of glassy-winged sharpshooters was typically low in late spring and early summer, with abundance of glassy-winged sharpshooter adults increasing in late June and early July of each year. Collectively, the results suggest that late summer is a crucial time for X. fastidiosa secondary spread in vineyards in the southern San Joaquin Valley, because glassy-winged sharpshooter abundance, number of glassy-winged sharpshooters testing positive for X. fastidiosa, and grapevines with detectable pathogen populations were all greatest during this period.

Grapevine phenolic compounds influence cell surface adhesion of Xylella fastidiosa and bind to lipopolysaccharide

Bacterial phytopathogen Xylella fastidiosa specifically colonizes the plant vascular tissue through a complex process of cell adhesion, biofilm formation, and dispersive movement. Adaptation to the chemical environment of the xylem is essential for bacterial growth and progression of infection. Grapevine xylem sap contains a range of plant secondary metabolites such as phenolics, which fluctuate in response to pathogen infection and plant physiological state. Phenolic compounds are often involved in host-pathogen interactions and influence infection dynamics through signaling activity, antimicrobial properties, and alteration of bacterial phenotypes. The effect of biologically relevant concentrations of phenolic compounds coumaric acid, gallic acid, epicatechin, and resveratrol on growth of X. fastidiosa was assessed in vitro. None of these compounds inhibited bacterial growth, but epicatechin and gallic acid reduced cell-surface adhesion. Cell-cell aggregation decreased with resveratrol treatment, but the other phenolic compounds tested had minimal effect on aggregation. Expression of attachment (xadA) and aggregation (fimA) related genes were altered by presence of the phenolic compounds, consistent with observed phenotypes. All four of the phenolic compounds bound to purified X. fastidiosa lipopolysaccharide (LPS), a major cell-surface component. Information regarding the impact of chemical environment on pathogen colonization in plants is important for understanding the infection process and factors associated with host susceptibility.

Lipopolysaccharide O-antigens-bacterial glycans made to measure

Lipopolysaccharides are critical components of bacterial outer membranes. The more conserved lipid A part of the lipopolysaccharide molecule is a major element in the permeability barrier imposed by the outer membrane and offers a pathogen-associated molecular pattern recognized by innate immune systems. In contrast, the long-chain O-antigen polysaccharide (O-PS) shows remarkable structural diversity and fulfills a range of functions, depending on bacterial lifestyles. O-PS production is vital for the success of clinically important Gram-negative pathogens. The biological properties and functions of O-PSs are mostly independent of specific structures, but the size distribution of O-PS chains is particularly important in many contexts. Despite the vast O-PS chemical diversity, most are produced in bacterial cells by two assembly strategies, and the different mechanisms employed in these pathways to regulate chain-length distribution are emerging. Here, we review our current understanding of the mechanisms involved in regulating O-PS chain-length distribution and discuss their impact on microbial cell biology.

Targeted Mutations in Xylella fastidiosa Affect Acquisition and Retention by the Glassy-Winged Sharpshooter, Homalodisca vitripennis (Hemiptera: Cicadellidae)

Xylella fastidiosa (Wells) is a xylem-limited bacterium that causes Pierce's disease of grapevines. The bacterium is transmitted by insect vectors such as the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis (Germar). Experiments were conducted to compare the role of selected X. fastidiosa genes on 1) bacterial acquisition and retention in GWSS foreguts, and 2) transmission to grapevines by GWSS. Bacterial genotypes used were: mutants Xf-ΔpilG, Xf-ΔpilH, Xf-ΔgacA, and Xf-ΔpopP; plus wild type (WT) as control. Results showed that Xf-ΔpilG had enhanced colonization rate and larger numbers in GWSS compared with WT. Yet, Xf-ΔpilG exhibited the same transmission efficiency as WT. The Xf-ΔpilH exhibited poor acquisition and retention. Although initial adhesion, multiplication, and retention of Xf-ΔpilH in GWSS were almost eliminated compared with WT, the mutation did not reduce transmission success in grapevines. Overall, Xf-ΔgacA showed colonization rates and numbers in foreguts similar to WT. The Xf-ΔgacA mutation did not affect initial adhesion, multiplication, and long-term retention compared with WT, and was not significantly diminished in transmission efficiency. In contrast, numbers of Xf-ΔpopP were variable over time, displaying greatest fluctuation from highest to lowest levels. Thus, Xf-ΔpopP had a strong, negative effect on initial adhesion, but adhered and slowly multiplied in the foregut. Again, transmission was not diminished compared to WT. Despite reductions in acquisition and retention by GWSS, transmission efficiency of genotypes to grapevines was not affected. Therefore, in order to stop the spread of X. fastidiosa by GWSS using gene-level targets, complete disruption of bacterial colonization mechanisms is required.

Infection of Blueberry Cultivar 'Emerald' with a California Pierce's Disease Strain of Xylella fastidiosa and Acquisition by Glassy-Winged Sharpshooter

Bacterial leaf scorch disease caused by Xylella fastidiosa occurs in southern highbush blueberry varieties in the southeastern United States. Susceptibility to X. fastidiosa varies by blueberry cultivar, and these interactions are often strain-specific. Xylella fastidiosa subsp. fastidiosa is the causal agent of Pierce's disease in grapevines, and it has been problematic in the San Joaquin Valley of California since the introduction of the glassy-winged sharpshooter (Homalodisca vitripennis). The glassy-winged sharpshooter is known to feed on blueberry, a crop that is expanding in the San Joaquin Valley. Currently, little is known about the potential for the spread of X. fastidiosa between grape and blueberry in this region. The ability of a Pierce's disease strain of X. fastidiosa from the San Joaquin Valley to cause disease in southern highbush blueberry and the potential for the glassy-winged sharpshooter to transmit X. fastidiosa between blueberry and grapevine were investigated. Experimental inoculations showed that the X. fastidiosa subsp. fastidiosa strain Bakersfield-1 can cause disease in blueberry cv. Emerald, and that the glassy-winged sharpshooter can acquire X. fastidiosa from artificially inoculated blueberry plants under laboratory conditions. Understanding the possibility for X. fastidiosa strains from the San Joaquin Valley to infect multiple crops grown in proximity is important for area-wide pest and disease management.

Fly foregut and transmission of microbes

Two areas of research that have greatly increased in attention are: dipterans as vectors and the microbes they are capable of vectoring. Because it is the front-end of the fly that first encounters these microbes, this review focuses on the legs, mouthparts, and foregut, which includes the crop as major structures involved in dipteran vectoring ability. The legs and mouthparts are generally involved in mechanical transmission of microbes. However, the crop is involved in more than just mechanical transmission, for it is within the lumen of the crop that microbes are taken up with the meal of the fly, stored, and it is within the lumen that horizontal transmission of bacterial resistance has been demonstrated. In addition to storage of microbes, the crop is also involved in depositing the microbes via a process known as regurgitation. Various aspects of crop regulation are discussed and specific examples of crop involvement with microorganisms are discussed. The importance of biofilm and biofilm formation are presented, as well as, some physical parameters of the crop that might either facilitate or inhibit biofilm formation. Finally, there is a brief discussion of dipteran model systems for studying crop microbe interactions.

Xylella fastidiosa: bacterial parasitism with hallmarks of commensalism

All organisms evolve in the presence of other organisms and these intimate associations are major drivers of evolution. Broadly speaking, these interactions are considered symbioses and can take on a full range of positive, negative or seemingly neutral interactions. Just two examples of these symbiotic interactions are parasitism and commensalism. Parasitism results in one partner benefitting while one partner suffers adverse consequences. Commensalism is a form of symbiosis where one partner benefits and the other partner is neutrally affected. Research efforts are more often focused on understanding parasitic symbioses related to disease, hence, much research is performed on identifying virulence factors to understand the fundamentals of pathogenesis. In turn, much less is understood about the fundamentals of commensal relationships. Here, we will take an introspective look at the plant-associated bacterium, Xylella fastidiosa. In some of its many plant hosts, this bacterium participates in seemingly commensal relationships while in other hosts, it causes devastating diseases that result in epidemics, making it a good model for exploring the determinants of where bacteria fall on the spectrum of parasitic and commensal relationships from both the microbial and the plant host perspective. Recent discoveries in how pathogenic X. fastidiosa imposes self-limiting behaviors upon itself indicate that even in its parasitic form, X. fastidiosa displays hallmarks of a commensal lifestyle. Understanding how commensalism can 'go wrong' and manifest into pathologies in specific hosts is a useful vantage point from which to study the determinants of virulence and pathogenicity.

Escherichia coli O157:H7 and Salmonella Typhimurium adhesion to spinach leaf surfaces: Sensitivity to water chemistry and nutrient availability

This study investigated the effects of solution chemistry and growth conditions on bacterial deposition on spinach leaf surfaces using a parallel plate flow cell. Two food safety pathogens of concern and two non-pathogen bacterial surrogates (environmental E. coli isolates) were grown in ideal (LB media) and nutrient-restricted (M9 media) conditions. Bacterial attachment was quantified as mass transfer rate coefficients for cells suspended in 10 mM KCl, CaCl₂ and artificial groundwater, and cell and leaf surfaces were extensively characterized (zeta potential, hydrophobicity, extracellular polymer (EPS) composition). Between the pathogens, E. coli O157:H7 attachment was greater than that of Salmonella Typhimurium, attributed to measurable variability in cell surface charge and hydrophobicity. When grown in M9 media, both pathogens were significantly more adhesive to spinach surfaces (p < 0.01) than when grown in LB media. Surrogates did not follow this trend and showed minimal changes in adhesion kinetics and surface properties between growth conditions. EPS sugar/protein ratios were reduced in some of the highest attachment scenarios, suggesting that changes in EPS composition in favor of proteins may play a role. These results show the importance of growth conditions and solution complexities in understanding mechanisms of aqueous bacterial adhesion to food surfaces.

The multifaceted functions of lipopolysaccharide in plant-bacteria interactions

In Gram-negative bacteria, the cell envelope largely consists of lipopolysaccharide (LPS), a class of heterogeneous glycolipids. As a fundamental component of the outer membrane, LPS provides stability to the bacterial cell and forms a protective cover shielding it from hostile environments. LPS is not only fundamental to bacterial viability, but also makes a substantial contribution both directly and indirectly to multiple aspects of inter-organismic interactions. During infection of animal and plant hosts, LPS promotes bacterial virulence but simultaneously betrays bacteria to the host immune system. Moreover, dynamic remodulation of LPS structures allows bacteria to fine-tune OM properties and quickly adapt to diverse and often hostile environments, such as those encountered in host tissues. Here, we summarize recent insights into the multiple functions of LPS in plant-bacteria interactions and discuss what we can learn from the latest advances in the field of animal immunity. We further pinpoint open questions and future challenges to unravel the different roles of LPS in the dynamic interplay between bacteria and plant hosts at the mechanistic level.

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.

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.

Functional and Evolutionary Characterization of a UDP-Xylose Synthase Gene from the Plant Pathogen Xylella fastidiosa, Involved in the Synthesis of Bacterial Lipopolysaccharide

Xylella fastidiosa is a plant-infecting bacillus, responsible for many important crop diseases, such as Pierce's disease of vineyards, citrus variegated chlorosis, and coffee leaf scorch (CLS), among others. Recent genomic comparisons involving two CLS-related strains, belonging to X. fastidiosa subsp. pauca, revealed that one of them carries a frameshift mutation that inactivates a gene encoding an oxidoreductase of the short-chain dehydrogenase/reductase (SDR) superfamily, which may play important roles in determining structural variations in bacterial glycans and glycoconjugates. However, the exact nature of this SDR has been a matter of controversy, as different annotations of X. fastidiosa genomes have implicated it in distinct reactions. To confirm the nature of this mutated SDR, a comparative analysis was initially performed, suggesting that it belongs to a subgroup of SDR decarboxylases, representing a UDP-xylose synthase (Uxs). Functional assays, using a recombinant derivative of this enzyme, confirmed its nature as XfUxs, and carbohydrate composition analyses, performed with lipopolysaccharide (LPS) molecules obtained from different strains, indicate that inactivation of the X. fastidiosa uxs gene affects the LPS structure among CLS-related X. fastidiosa strains. Finally, a comparative sequence analysis suggests that this mutation is likely to result in a morphological and evolutionary hallmark that differentiates two subgroups of CLS-related strains, which may influence interactions between these bacteria and their plant and/or insect hosts.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.