Xylella fastidiosa Requires the Type II Secretion System for Pathogenicity and Survival in Grapevine

Xylella fastidiosa is a xylem-limited bacterial pathogen that causes Pierce's disease (PD) of grapevine. In host plants, this bacterium exclusively colonizes the xylem, which is primarily non-living at maturity. Understanding how X. fastidiosa interfaces with this specialized conductive tissue is at the forefront of investigation for this pathosystem. Unlike many bacterial plant pathogens, X. fastidiosa lacks a type III secretion system and cognate effectors that aid in host colonization. Instead, X. fastidiosa utilizes plant cell-wall hydrolytic enzymes and lipases as part of its xylem colonization strategy. Several of these virulence factors are predicted to be secreted via the type II secretion system (T2SS), the main terminal branch of the Sec-dependent general secretory pathway. In this study, we constructed null mutants in xpsE and xpsG, which encode for the ATPase that drives the T2SS and the major structural pseudopilin of the T2SS, respectively. Both mutants were non-pathogenic and unable to effectively colonize Vitis vinifera grapevines, demonstrating that the T2SS is required for X. fastidiosa infection processes. Furthermore, we utilized mass spectrometry to identify type II-dependent proteins in the X. fastidiosa secretome. In vitro, we identified six type II-dependent proteins in the secretome that included three lipases, a β-1,4-cellobiohydrolase, a protease, and a conserved hypothetical protein. [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.

Xylella fastidiosa causes transcriptional shifts that precede tylose formation and starch depletion in xylem

Pierce's disease (PD) in grapevine (Vitis vinifera) is caused by the bacterial pathogen Xylella fastidiosa. X. fastidiosa is limited to the xylem tissue and following infection induces extensive plant-derived xylem blockages, primarily in the form of tyloses. Tylose-mediated vessel occlusions are a hallmark of PD, particularly in susceptible V. vinifera. We temporally monitored tylose development over the course of the disease to link symptom severity to the level of tylose occlusion and the presence/absence of the bacterial pathogen at fine-scale resolution. The majority of vessels containing tyloses were devoid of bacterial cells, indicating that direct, localized perception of X. fastidiosa was not a primary cause of tylose formation. In addition, we used X-ray computed microtomography and machine-learning to determine that X. fastidiosa induces significant starch depletion in xylem ray parenchyma cells. This suggests that a signalling mechanism emanating from the vessels colonized by bacteria enables a systemic response to X. fastidiosa infection. To understand the transcriptional changes underlying these phenotypes, we integrated global transcriptomics into the phenotypes we tracked over the disease spectrum. Differential gene expression analysis revealed that considerable transcriptomic reprogramming occurred during early PD before symptom appearance. Specifically, we determined that many genes associated with tylose formation (ethylene signalling and cell wall biogenesis) and drought stress were up-regulated during both Phase I and Phase II of PD. On the contrary, several genes related to photosynthesis and carbon fixation were down-regulated during both phases. These responses correlate with significant starch depletion observed in ray cells and tylose synthesis in vessels.

Xylella fastidiosa Endoglucanases Mediate the Rate of Pierce's Disease Development in Vitis vinifera in a Cultivar-Dependent Manner

Xylella fastidiosa is a gram-negative bacterium that causes Pierce's disease (PD) in grapevine. X. fastidiosa is xylem-limited and interfaces primarily with pit membranes (PMs) that separate xylem vessels from one another and from adjacent xylem parenchyma cells. PMs are composed of both pectic and cellulosic substrates, and dissolution of PMs is facilitated by X. fastidiosa cell wall-degrading enzymes. A polygalacturonase, which hydrolyzes the pectin component of PMs, is required for both movement and pathogenicity in grapevines. Here, we demonstrate that two X. fastidiosa β-1,4-endoglucanases (EGases), EngXCA1 and EngXCA2, also play a role in how X. fastidiosa interfaces with grapevine PMs. The loss of EngXCA1 and EngXCA2 in tandem reduces both X. fastidiosa virulence and population size and slows the rate of PD symptom development and progression. Moreover, we demonstrate that single and double EGases mutants alter the rate of PD progression differently in two grapevine cultivars, Cabernet Sauvignon and Chardonnay, and that Chardonnay is significantly more susceptible to PD than Cabernet Sauvignon. Interestingly, we determined that there are quantitative differences in the amount of fucosylated xyloglucans that make up the surface of PMs in these cultivars. Fucosylated xyloglucans are targets of the X. fastidiosa EGases, and xyloglucan abundance could impact PM dissolution and affect PD symptom development. Taken together, these results indicate that X. fastidiosa EGases and the PM carbohydrate composition of different grape cultivars are important factors that influence PD symptom development and progression.

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.

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.

Polydiacetylene-Coated Sensor Strip for Immunochromatic Detection of Xylella fastidiosa subsp. fastidiosa

This study presents a sensor strip for user-friendly, naked-eye detection of Xylella fasitdiosa, the bacterial causal agent of Pierce's disease in grapevine. This sensor uses anti- X. fastidiosa antibodies conjugated to a polydiacetylene layer on a polyvinylidene fluoride strip to generate specific color transitions and discriminate levels of the pathogen. The detection limit of the sensor is 0.8 × 10⁸ cells/mL, which is similar to bacterial load in grapevine 18 days following bacterial inoculation. This sensor enables equipment-free detection that is highly desirable for in-field diagnostic tools in resource-limited settings.

Bacterial filtration efficiency of green soy protein based nanofiber air filter

High bacterial filtration efficiency (BFE) filters, based on nanofibers derived from blends of grain proteins and poly-ethylene-oxide (PEO), were produced by an electrospinning process. Specifically, polymer blends consisting of purified soy flour/PEO with a ratio of 7/3 were spun into nanofibers and characterized. A new laboratory based experimental apparatus for testing BFE was designed and used to test BFE of bacterial aerosols consisting of Escherichia coli (E. coli). Performances of soy protein based nanofiber filters with nanofiber mass varying from 1 to 5 g/m2 as well as a nanofiber filters prepared from pure PEO were compared. The results showed that BFE values for filters containing 5 g/m2 protein based nanofibers and PEO nanofiber filter were 100 and 81.5%, respectively. The results also indicated that the BFE increased as the protein content in the nanofiber filter increased. These novel protein based nanofiber filters have demonstrated a clear potential for effective removal and retention of E. coli bacteria during air-filtration. These filters can be effectively deployed in environments such as hospitals and senior residential areas to reduce bacterial infections.