Differential Colonization Dynamics of Cucurbit Hosts by Erwinia tracheiphila

Complementary roles of EPS, T3SS and Expansin for virulence of Erwinia tracheiphila, the causative agent of cucurbit wilt

Erwinia tracheiphila (Smith) is a recently emerged plant pathogen that causes severe economic losses in cucurbit crops in temperate Eastern North America. E. tracheiphila is xylem restricted, and virulence is thought to be related to Exopolysaccharides (EPS) and biofilm formation, which occlude the passage of sap in xylem vessels and causes systemic wilt. However, the role of EPS and biofilm formation, and their contribution to disease in relation to other virulence loci are unknown. Here, we use deletion mutants to explore the roles of EPS, Hrp Type III secretion system (Hrp T3SS) and Expansin in plant colonization and virulence. Then, we quantify the expression of the genes encoding these factors during infection. Our results show that Exopolysaccharides are essential for E. tracheiphila survival in host plants, while Hrp T3SS and Expansin are dispensable for survival but needed for systemic wilt symptom development. EPS and Hrp T3SS display contrasting expression patterns in the plant, reflecting their relevance in different stages of the infection. Finally, we show that expression of the eps and hrpT3SS operons is downregulated in mildly increased temperatures, suggesting a link between expression of these virulence factors and geographic restriction of E. tracheiphila to temperate regions. Our work highlights how E. tracheiphila virulence is a complex trait where several loci are coordinated during infection. These results further shed light into the relationship between virulence factors and the ecology of this pathosystem, which will be essential for developing sustainable management strategies for this emerging pathogen.

Expression and Functional Analysis of the Type III Secretion System Effector Repertoire of the Xylem Pathogen Erwinia tracheiphila on Cucurbits

The predicted repertoire of type III secretion system effectors (T3SEs) in Erwinia tracheiphila, causal agent of cucurbit bacterial wilt, is much larger than in xylem pathogens in the closely related genera Erwinia and Pantoea. The genomes of strains BHKY and SCR3, which represent distinct E. tracheiphila clades, encode at least 6 clade-specific and 12 shared T3SEs. The strains expressed the majority of the T3SE genes examined in planta. Among the shared T3SE genes, eop1 was expressed most highly in both strains in squash (Cucurbita pepo) and muskmelon (Cucumis melo) but the clade-specific gene avrRpm2 was expressed 40- to 900-fold more than eop1 in BHKY. The T3SEs AvrRpm2, Eop1, SrfC, and DspE contributed to BHKY virulence on squash and muskmelon, as shown using combinatorial mutants involving six T3SEs, whereas OspG and AvrB4 contributed to BHKY virulence only on muskmelon, demonstrating host-specific virulence functions. Moreover, Eop1 was functionally redundant with AvrRpm2, SrfC, OspG, and AvrB4 in BHKY, and BHKY mutants lacking up to five effector genes showed similar virulence to mutants lacking only two genes. The T3SEs OspG, AvrB4, and DspE contributed additively to SCR3 virulence on muskmelon and were not functionally redundant with Eop1. Rather, loss of eop1 and avrB4 restored wild-type virulence to the avrB4 mutant, suggesting that Eop1 suppresses a functionally redundant effector in SCR3. These results highlight functional differences in effector inventories between two E. tracheiphila clades, provide the first evidence of OspG as a phytopathogen effector, and suggest that Eop1 may be a metaeffector influencing virulence. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The Contributions to Virulence of the Effectors Eop1 and DspE Differ Between Two Clades of Erwinia tracheiphila Strains

Strains of Erwinia tracheiphila, causal agent of bacterial wilt of cucurbits, are divided into distinct clades. Et-melo clade strains wilt Cucumis spp. but not Cucurbita spp., thus exhibiting host specificity, whereas Et-C1 clade strains wilt Cucurbita spp. more rapidly than Cucumis melo, thus exhibiting a host preference. This study investigated the contribution of the effector proteins Eop1 and DspE to E. tracheiphila pathogenicity and host adaptation. Loss of eop1 did not enable Et-melo strains to infect squash (Cucurbita pepo) or an Et-C1 strain to induce a more rapid wilt of muskmelon (Cucumis melo), indicating that Eop1 did not function in host specificity or preference as in the related pathogen E. amylovora. However, overexpression of eop1 from Et-melo strain MDCuke but not from Et-C1 strain BHKY increased the virulence of a BHKY eop1 deletion mutant on muskmelon, demonstrating that the Eop1 variants in the two clades are distinct in their virulence functions. Loss of dspE from Et-melo strains reduced but did not eliminate virulence on hosts muskmelon and cucumber, whereas loss of dspE from an Et-C1 strain eliminated pathogenicity on hosts squash, muskmelon, and cucumber. Thus, the centrality of DspE to virulence differs in the two clades. Et-melo mutants lacking the chaperone DspF exhibited similar virulence to mutants lacking DspE, indicating that DspF is the sole chaperone for DspE in E. tracheiphila, unlike in E. amylovora. Collectively, these results provide the first functional evaluation of effectors in E. tracheiphila and demonstrate clade-specific differences in the roles of Eop1 and DspE.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Identification of Bacterial Wilt (Erwinia tracheiphila) Resistances in USDA Melon Collection

Bacterial wilt (BW) caused by the Gram-negative bacterium, Erwinia tracheiphila (Et.), is an important disease in melon (Cucumis melo L.). BW-resistant commercial melon varieties are not widely available. There are also no effective pathogen-based disease management strategies as BW-infected plants ultimately die. The purpose of this study is to identify BW-resistant melon accessions in the United States Department of Agriculture (USDA) collection. We tested 118 melon accessions in two inoculation trials under controlled environments. Four-week-old seedlings of test materials were mechanically inoculated with the fluorescently (GFP) labeled or unlabeled E. tracheiphila strain, Hca1-5N. We recorded the number of days to wilting of inoculated leaf (DWIL), days to wilting of whole plant (DWWP) and days to death of the plant (DDP). We identified four melon lines with high resistance to BW inoculation based on all three parameters. Fluorescent microscopy was used to visualize the host colonization dynamics of labeled bacteria from the point of inoculation into petioles, stem and roots in resistant and susceptible melon accessions, which provides an insight into possible mechanisms of BW resistance in melon. The resistant melon lines identified from this study could be valuable resistance sources for breeding of BW resistance as well as the study of cucurbit—E. tracheiphila interactions.

Bioluminescent Xanthomonas hortorum pv. gardneri as a Tool to Quantify Bacteria in Planta, Screen Germplasm, and Identify Infection Routes on Leaf Surfaces

Imaging technology can provide insight into biological processes governing plant-pathogen interactions. We created and used a bioluminescent strain of Xanthomonas hortorum pv. gardneri (Xgb) to quantify infection processes in plants using tomato as a model. An X. hortorum pv. gardneri is one of the four Xanthomonas species that causes bacterial spots in tomatoes. We used Xgb to quantify bacterial growth in planta, to assess disease severity in resistant and susceptible tomato lines, and to observe infection routes in leaves. A positive and significant linear correlation r (67) = 0.57, p ≤ 0.0001 was observed between bioluminescence signals emitted by Xgb in planta and bacterial populations determined through dilution plating. Based on bioluminescence imaging, resistant and susceptible tomato lines had significantly different average radiances. In addition, there was a positive and significant correlation r = 0.45, p = 0.024 between X. hortorum pv. gardneri-inoculated tomato lines evaluated by bioluminescence imaging and tomatoes rated in the field using the Horsfall-Barrat Scale. Heritability was calculated to compare the genetic variance for disease severity using bioluminescence imaging and classical field ratings. The genetic variances were 25 and 63% for bioluminescence imaging and field ratings, respectively. The disadvantage of lower heritability attained by bioluminescence imaging may be offset by the ability to complete germplasm evaluation experiments within 30 days rather than 90-120 days in field trials. We further explored X. hortorum pv. gardneri infection routes on leaves using spray and dip inoculation techniques. Patterns of bioluminescence demonstrated that the inoculation technique affected the distribution of bacteria, an observation verified using scanning electron microscopy (SEM). We found significant non-random distributions of X. hortorum pv. gardneri on leaf surfaces with the method of inoculation affecting bacterial distribution on leaf surfaces at 4 h postinoculation (hpi). At 18 hpi, regardless of inoculation method, X. hortorum pv. gardneri localized on leaf edges near hydathodes based on bioluminescence imaging and confirmed by electron microscopy. These findings demonstrated the utility of bioluminescent X. hortorum pv. gardneri to estimate bacterial populations in planta, to select for resistant germplasm, and to detect likely points of infection.

Novel Small Molecule Growth Inhibitors of Xanthomonas spp. Causing Bacterial Spot of Tomato

Bacterial spot (BS) of tomato, caused by Xanthomonas gardneri, X. perforans, X. vesicatoria, and X. euvesicatoria, is difficult to control because of the high prevalence of copper- and streptomycin-resistant strains and the lack of resistance cultivars and effective bactericides. The objective of this study was to identify novel growth inhibitors of BS-causing Xanthomonas (BS-X) species by using small molecules (SM; n = 4,182). Several SMs (X1, X2, X5, X9, X12, and X16) completely inhibited the growth of BS-X isolates (n = 68 X. gardneri, 55 X. perforans, 4 X. vesicatoria, and 32 X. euvesicatoria) at ≥12.5 µM by disrupting Xanthomonas cell integrity through weakening of the cell membrane and formation of pores. These SMs were also effective against biofilm-embedded, copper- and streptomycin-resistant Xanthomonas strains while having minimal impact on other plant pathogenic (n = 20) and beneficial bacteria (n = 12). Furthermore, these SMs displayed equivalent antimicrobial activity against BS-X in seeds and X. gardneri in seedlings compared with conventional control methods (copper sulfate and streptomycin) at similar concentrations while having no detectable toxicity to tomato tissues. SMs X2, X5, and X12 reduced X. gardneri, X. perforans, X. vesicatoria, and X. euvesicatoria populations in artificially infested seeds ≤3.4-log CFU/seed 1 day postinfection (dpi) compared with the infested untreated control (P ≤ 0.05). SMs X1, X2, X5, and X12 reduced disease severity ≤72% and engineered bioluminescent X. gardneri populations ≤3.0-log CFU/plant in infected seedlings at 7 dpi compared with the infected untreated control (P ≤ 0.05). Additional studies are needed to increase the applicability of these SMs for BS management in tomato production.

A horizontally acquired expansin gene increases virulence of the emerging plant pathogen Erwinia tracheiphila

Erwinia tracheiphila is a bacterial plant pathogen that causes a fatal wilt infection in some cucurbit crop plants. Wilt symptoms are thought to be caused by systemic bacterial colonization through xylem that impedes sap flow. However, the genetic determinants of within-plant movement are unknown for this pathogen species. Here, we find that E. tracheiphila has horizontally acquired an operon with a microbial expansin (exlx) gene adjacent to a glycoside hydrolase family 5 (gh5) gene. Plant inoculation experiments with deletion mutants in the individual genes (Δexlx and Δgh5) and the full operon (Δexlx-gh5) resulted in decreased severity of wilt symptoms, decreased mortality rate, and impaired systemic colonization compared to the Wt strain. Co-inoculation experiments with Wt and Δexlx-gh5 rescued the movement defect of the mutant strain, suggesting that expansin and GH5 function extracellularly. Together, these results show that expansin-GH5 contributes to systemic movement through xylem, leading to rapid wilt symptom development and higher rates of plant death. The presence of expansin genes in diverse species of bacterial and fungal wilt-inducing pathogens suggests that microbial expansin proteins may be an under-appreciated virulence factor for many pathogen species.

Discovery and Characterization of Low-Molecular Weight Inhibitors of Erwinia tracheiphila

Plant pathogenic bacteria in the genus Erwinia cause economically important diseases, including bacterial wilt of cucurbits caused by Erwinia tracheiphila. Conventional bactericides are insufficient to control this disease. Using high-throughput screening, 464 small molecules (SMs) with either cidal or static activity at 100 µM against a cucumber strain of E. tracheiphila were identified. Among them, 20 SMs (SM1 to SM20), composed of nine distinct chemical moiety structures, were cidal to multiple E. tracheiphila strains at 100 µM. These lead SMs had low toxicity to human cells and honey bees at 100 µM. No phytotoxicity was observed on melon plants at 100 µM, except when SM12 was either mixed with Silwet L-77 and foliar sprayed or when delivered through the roots. Lead SMs did not inhibit the growth of beneficial Pseudomonas and Enterobacter species but inhibited the growth of Bacillus species. Nineteen SMs were cidal to Xanthomonas cucurbitae and showed >50% growth inhibition against Pseudomonas syringae pv. lachrymans. In addition, 19 SMs were cidal or static against Erwinia amylovora in vitro. Five SMs demonstrated potential to suppress E. tracheiphila when foliar sprayed on melon plants at 2× the minimum bactericidal concentration. Thirteen SMs reduced Et load in melon plants when delivered via roots. Temperature and light did not affect the activity of SMs. In vitro cidal activity was observed after 3 to 10 h of exposure to these five SMs. Here, we report 19 SMs that provide chemical scaffolds for future development of bactericides against plant pathogenic bacterial species.

Specific Environmental Temperature and Relative Humidity Conditions and Grafting Affect the Persistence and Dissemination of Salmonella enterica subsp. enterica Serotype Typhimurium in Tomato Plant Tissues

Little is known about the abiotic factors contributing to the preharvest persistence of Salmonella in tomato tissues. Therefore, we investigated the effects of specific environmental conditions and contamination methods on the persistence and dissemination of Salmonella enterica subsp. enterica serotype Typhimurium (JSG626) in tomato plants. When plants were sprayed on the leaves with a JSG626-contaminated solution, JSG626 persistence in the phyllosphere (bacteria located on the surface of the inoculated foliage and stem tissues) was lower at higher temperatures (30°C day/25°C night) than at lower temperatures (20°C day/15°C night). However, wounding cotyledons with contaminated tools improved JSG626 persistence and the internalization rate (2.27%) in planta compared to spray inoculation (0.004%). The systemic dissemination of JSG626 to other tissues increased when contaminated plants were grown under low relative humidity (<40%); however, JSG626 was only detected in the root systems at later sampling times (between 21 and 98 days postinoculation [dpi]). Further, after tomato scions were grafted onto rootstocks using contaminated cutting tools, dissemination of JSG626 was preferentially basipetal and occasionally acropetal in the plants, with higher persistence rates and loads of JSG626 in root systems compared to foliar tissues. JSG626 was detected in the grafting point and root systems up to 242 dpi; however, none of the fruits harvested from contaminated plants between 90 and 137 dpi were positive for JSG626. This study demonstrates that environmental temperature and relative humidity could be good indicators for estimating the persistence of Salmonella enterica in tomato plants. Further, root systems may represent a risk for long-term persistence of Salmonella enterica in tomato plants.IMPORTANCE Tomatoes are one of the most widely produced vegetables around the world; however, fresh tomatoes have been connected to multiple wide-scale salmonellosis outbreaks over the past decades. Salmonella is commonly found in the environment and can persist in hostile conditions for several weeks before being internalized into plant tissues, where it is protected from conventional sanitation methods. In addition to biotic factors (host, inoculum size, and phytobiome), abiotic factors (environmental conditions) may affect the persistence of Salmonella in crop production. This study demonstrates that specific environmental conditions, the inoculation method, and the inoculum density affect the persistence and dissemination of JSG626 in tomato plant tissues. Our findings enhance the understanding of interactions between Salmonella enterica and fresh produce and may lead to the development of novel management practices on farms.

An Introduced Crop Plant Is Driving Diversification of the Virulent Bacterial Pathogen Erwinia tracheiphila

Erwinia tracheiphila is the causal agent of bacterial wilt of cucurbits, an economically important phytopathogen affecting an economically important phytopathogen affecting few cultivated Cucurbitaceae few cultivated Cucurbitaceae host plant species in temperate eastern North America. However, essentially nothing is known about E. tracheiphila population structure or genetic diversity. To address this shortcoming, a representative collection of 88 E. tracheiphila isolates was gathered from throughout its geographic range, and their genomes were sequenced. Phylogenomic analysis revealed three genetic clusters with distinct hrpT3SS virulence gene repertoires, host plant association patterns, and geographic distributions. Low genetic heterogeneity within each cluster suggests a recent population bottleneck followed by population expansion. We showed that in the field and greenhouse, cucumber (Cucumis sativus), which was introduced to North America by early Spanish conquistadors, is the most susceptible host plant species and the only species susceptible to isolates from all three lineages. The establishment of large agricultural populations of highly susceptible C. sativus in temperate eastern North America may have facilitated the original emergence of E. tracheiphila into cucurbit agroecosystems, and this introduced plant species may now be acting as a highly susceptible reservoir host. Our findings have broad implications for agricultural sustainability by drawing attention to how worldwide crop plant movement, agricultural intensification, and locally unique environments may affect the emergence, evolution, and epidemic persistence of virulent microbial pathogens.IMPORTANCEErwinia tracheiphila is a virulent phytopathogen that infects two genera of cucurbit crop plants, Cucurbita spp. (pumpkin and squash) and Cucumis spp. (muskmelon and cucumber). One of the unusual ecological traits of this pathogen is that it is limited to temperate eastern North America. Here, we complete the first large-scale sequencing of an E. tracheiphila isolate collection. From phylogenomic, comparative genomic, and empirical analyses, we find that introduced Cucumis spp. crop plants are driving the diversification of E. tracheiphila into multiple lineages. Together, the results from this study show that locally unique biotic (plant population) and abiotic (climate) conditions can drive the evolutionary trajectories of locally endemic pathogens in unexpected ways.

Novel Imidazole and Methoxybenzylamine Growth Inhibitors Affecting Salmonella Cell Envelope Integrity and its Persistence in Chickens

The control of Salmonella from farm to fork is challenging due to the emergence of antimicrobial-resistant isolates and the limited effects of current control methods. Advanced chemical technologies have made accessible a wide range of uncharacterized small molecules (SMs) with encouraging chemical properties for antimicrobial treatment. Of the 4,182 SMs screened in vitro, four cidal SMs were effective at 10 µM and higher against several serotypes, antibiotic-resistant, and biofilm embedded Salmonella enterica subsp. enterica serotype Typhimurium by altering cell membrane integrity. The four SMs displayed synergistic effects with ciprofloxacin, meropenem and cefeprime against Salmonella. Further, the SMs were not pernicious to most eukaryotic cells at 200 μM and cleared internalized Salmonella in infected Caco-2, HD11, and THP-1 cells at 6.25 µM and higher. The SMs also increased the longevity of Salmonella-infected Galleria mellonella larvae and reduced the population of internalized Salmonella Typhimurium. Two of the SMs (SM4 and SM5) also reduced S. Typhimurium load in infected chicken ceca as well as its systemic translocation into other tissues, with minimal impact on the cecal microbiota. This study demonstrated that SMs are a viable source of potential antimicrobials applicable in food animal production against Salmonella.

Evaluation of Ralstonia solanacearum Infection Dynamics in Resistant and Susceptible Pepper Lines Using Bioluminescence Imaging

Bacterial wilt, incited by Ralstonia solanacearum, is a major disease affecting pepper (Capsicum annuum) production worldwide. The most effective management tactic is the deployment of wilt-resistant varieties. However, the lack of a nondestructive method to measure invasiveness and spatio-temporal distribution of R. solanacearum, a vascular pathogen, in planta limits better understanding of pepper resistance and plant-pathogen interactions. We evaluated the resistance of 100 pepper lines using R. solanacearum strain Rs-SY1 (phylotype I, isolated from a sweet pepper in South China). Based on the disease severity index (DSI) values, the elite inbred line BVRC 1 and the small-fruited accessions PI 640435 and PI 640444 were identified as resistant (DSI: 1.2, 1.8, and 1.9 out of 4.0, respectively). In order to evaluate bacterial infection dynamics in planta in real time, we generated seven bioluminescent R. solanacearum strains (BL-Rs1 to BL-Rs7) using vector pXX3 carrying luxCDABE genes, and selected BL-Rs7 for inoculation due to its similarity with parent strain Rs-SY1 in morphology, pathogenicity, and highest light emission in vitro. Luminescence intensity was strongly correlated to bacterial population in planta (R² = 0.88). The utility of the bioluminescence assay was validated by comparing R. solanacearum infection dynamics in real-time in vivo between resistant line BVRC 1 and susceptible line BVRC 25. The distribution and multiplication of BL-Rs7 strain in resistant line BVRC 1 was conspicuously limited in plants inoculated in either roots or stem compared with susceptible line BVRC 25. These results suggest that pepper line BVRC 1 may resist colonization by interfering with R. solanacearum multiplication in the roots and stem.