Soft Rot Pectobacteriaceae Research Articles

Encapsulated Predatory Bacteria Efficiently Protect Potato Tubers from Soft Rot Disease.

Soft rot Pectobacteriaceae (SRP) are a group of destructive Gram-negative phytopathogens that can infect a wide range of plant hosts, including potatoes. There are no effective control agents available against SRP, making their management challenging. We have developed a novel approach to protect potato tubers against SRP. It makes use of encapsulated predatory Bdellovibrio bacteriovorus bacteria that, upon release from a polymeric carrier, prey upon SRP. We applied a carrageenan-trehalose-based formulation containing a B. bacteriovorus HD100 predator to prevent soft rot disease development in potato tubers, under various conditions. The dried formulation exhibited very high stability over an 18-month period at room temperature (∼25°C), in contrast to unencapsulated suspensions of the predator, in which viability decreased rapidly below detection level. The rehydrated formulation was as efficient as freshly grown unencapsulated predators and provided high protection in potted potato tubers, displaying an average of 50% reduction in disease parameters (e.g., tissue decay and disease index) under controlled conditions at 7 days postinoculation and planting. The protective effect provided by this formulation was maintained in longer-term trials (28 days) conducted in larger vessels within a net house under natural climate conditions, highlighting its potential for practical application in the field.

Analysis of soft rot Pectobacteriaceae population diversity in US potato growing regions between 2015 and 2022.

Soft rot Pectobacteriaceae (SRP) bacteria are globally dispersed pathogens that cause significant economic loss in potato and other crops. Our understanding of the SRP species diversity has expanded in recent years due to advances and adoption of whole-genome sequence technologies. There are currently 34 recognized SRP species that belong to the Dickeya and Pectobacterium genera. We used whole-genome sequencing based analysis to describe the current distribution and epidemiology of SRP isolated from diseased potato samples obtained from commercial potato cropping systems in the United States. Our primary objectives in the present study were to: (1) identify the species of these SRP isolates recovered from potato samples across 14 states in the US, (2) describe the variation among SRP isolates from various US locations and track their temporal changes, and (3) evaluate the evolutionary relationships among these SRP isolates to deduce their source. We collected 118 SRP strains from diseased potato plants and tubers in 14 states between 2015 and 2022. We identified three Dickeya and eight Pectobacterium species from diseased potato samples. Dickeya dianthicola, Pectobacterium parmentieri, P. carotovorum, and P. versatile appeared to be the predominant species, constituting 83% of the isolates. Furthermore, all D. dianthicola strains studied here as well as 90% of US D. dianthicola isolates sequenced to date exhibit significant clonality. The prevalence of this specific group of D. dianthicola, temporally and geographically, aligns with the occurrence of blackleg and soft rot outbreaks in the northeastern US after 2014. The genomic diversity observed in P. parmentieri implies multiple introductions to the US from at least four distinct sources, earlier than the arrival of the predominant group of D. dianthicola. In contrast, P. carotovorum and P. versatile appear to be widespread, long-term endemic strains in the US.

Elicitation of potato plants to increase their resistance against Soft Rot Pectobacteriaceae bacteria

AbstractThe investigation aimed to assess three methods for inducing resistance in Solanum tuberosum L. (potato) plants against Soft Rot Pectobacteriaceae (SRP) bacteria. The approaches involved the incorporation of elicitors into the in vitro nutrient medium during plant cultivation, pre-planting of seeds soaked in water-containing elicitors, and irradiating in vitro cultivated potato plants with 21 kJ/m2 of ultraviolet C light (UVC). The research validated the authors’ hypothesis, that the use of specific elicitors and UV-C light could induce resistance in potato plants against SRP bacteria, identifying the most effective elicitation technique for potato plants under experimental conditions. Two elicitors, specifically 50 μM salicylic acid and 250 μg/mL proline, were determined to be the most potent under experimental conditions. Supplementation of the Murashige and Skoog medium (MS) with 50 μM salicylic acid resulted in 21% of infected plants exhibiting no symptoms of infection on the 14th-day post-inoculation, and all plants treated with 10, 30, 35, 40, 45, or 50 μM of salicylic acid displayed a diminishing rate of infection progression. Plants growing with 25 µM SA were visually characterized by faster growth rates and higher vegetative mass relative to both control and test variants. Similarly, the incorporation of 250 μg/mL proline into the MS medium caused a response with 37.5% of plants showing no disease symptoms on the 14th day post-inoculation.

Looking for Resistance to Soft Rot Disease of Potatoes Facing Environmental Hypoxia.

Plants are exposed to various stressors, including pathogens, requiring specific environmental conditions to provoke/induce plant disease. This phenomenon is called the "disease triangle" and is directly connected with a particular plant-pathogen interaction. Only a virulent pathogen interacting with a susceptible plant cultivar will lead to disease under specific environmental conditions. This may seem difficult to accomplish, but soft rot Pectobacteriaceae (SRPs) is a group virulent of pathogenic bacteria with a broad host range. Additionally, waterlogging (and, resulting from it, hypoxia), which is becoming a frequent problem in farming, is a favoring condition for this group of pathogens. Waterlogging by itself is an important source of abiotic stress for plants due to lowered gas exchange. Therefore, plants have evolved an ethylene-based system for hypoxia sensing. Plant response is coordinated by hormonal changes which induce metabolic and physiological adjustment to the environmental conditions. Wetland species such as rice (Oryza sativa L.), and bittersweet nightshade (Solanum dulcamara L.) have developed adaptations enabling them to withstand prolonged periods of decreased oxygen availability. On the other hand, potato (Solanum tuberosum L.), although able to sense and response to hypoxia, is sensitive to this environmental stress. This situation is exploited by SRPs which in response to hypoxia induce the production of virulence factors with the use of cyclic diguanylate (c-di-GMP). Potato tubers in turn reduce their defenses to preserve energy to prevent the negative effects of reactive oxygen species and acidification, making them prone to soft rot disease. To reduce the losses caused by the soft rot disease we need sensitive and reliable methods for the detection of the pathogens, to isolate infected plant material. However, due to the high prevalence of SRPs in the environment, we also need to create new potato varieties more resistant to the disease. To reach that goal, we can look to wild potatoes and other Solanum species for mechanisms of resistance to waterlogging. Potato resistance can also be aided by beneficial microorganisms which can induce the plant's natural defenses to bacterial infections but also waterlogging. However, most of the known plant-beneficial microorganisms suffer from hypoxia and can be outcompeted by plant pathogens. Therefore, it is important to look for microorganisms that can withstand hypoxia or alleviate its effects on the plant, e.g., by improving soil structure. Therefore, this review aims to present crucial elements of potato response to hypoxia and SRP infection and future outlooks for the prevention of soft rot disease considering the influence of environmental conditions.

Genetic Loci of Plant Pathogenic Dickeya solani IPO 2222 Expressed in Contact with Weed-Host Bittersweet Nightshade (Solanum dulcamara L.) Plants.

Dickeya solani, belonging to the Soft Rot Pectobacteriaceae, are aggressive necrotrophs, exhibiting both a wide geographic distribution and a wide host range that includes many angiosperm orders, both dicot and monocot plants, cultivated under all climatic conditions. Little is known about the infection strategies D. solani employs to infect hosts other than potato (Solanum tuberosum L.). Our earlier study identified D. solani Tn5 mutants induced exclusively by the presence of the weed host S. dulcamara. The current study assessed the identity and virulence contribution of the selected genes mutated by the Tn5 insertions and induced by the presence of S. dulcamara. These genes encode proteins with functions linked to polyketide antibiotics and polysaccharide synthesis, membrane transport, stress response, and sugar and amino acid metabolism. Eight of these genes, encoding UvrY (GacA), tRNA guanosine transglycosylase Tgt, LPS-related WbeA, capsular biosynthesis protein VpsM, DltB alanine export protein, glycosyltransferase, putative transcription regulator YheO/PAS domain-containing protein, and a hypothetical protein, were required for virulence on S. dulcamara plants. The implications of D. solani interaction with a weed host, S. dulcamara, are discussed.

Svx Peptidases of Phytopathogenic Pectolytic Bacteria: Structural, Catalytic and Phytoimmune Properties.

Svx proteins are virulence factors secreted by phytopathogenic bacteria of the Pectobacterium genus into the host plant cell wall. Svx-encoding genes are present in almost all species of the soft rot Pectobacteriaceae (Pectobacterium and Dickeya genera). The Svx of P. atrosepticum (Pba) has been shown to be a gluzincin metallopeptidase that presumably targets plant extensins, proteins that contribute to plant cell wall rigidity and participate in cell signaling. However, the particular "output" of the Pba Svx action in terms of plant-pathogen interactions and plant immune responses remained unknown. The Svx proteins are largely unexplored in Dickeya species, even though some of them have genes encoding two Svx homologs. Therefore, our study aims to compare the structural and catalytic properties of the Svx proteins of Pba and D. solani (Dso) and to test the phytoimmune properties of these proteins. Two assayed Dso Svx proteins, similar to Pba Svx, were gluzincin metallopeptidases with conservative tertiary structures. The two domains of the Svx proteins form electronegative clefts where the active centers of the peptidase domains are located. All three assayed Svx proteins possessed phytoimmunosuppressory properties and induced ethylene-mediated plant susceptible responses that play a decisive role in Pba-caused disease.

Soft rot pathogen Dickeya dadantii 3937 produces tailocins resembling the tails of Peduovirus P2

Tailocins are nanomolecular machines with bactericidal activity. They are produced by bacteria to contribute to fitness in mixed communities, and hence, they play a critical role in their ecology in a variety of habitats. Here, we characterized the new tailocin produced by Dickeya dadantii strain 3937, a well-characterized member of plant pathogenic Soft Rot Pectobacteriaceae (SRP). Tailocins induced in D. dadantii were ca. 166 nm long tubes surrounded by contractive sheaths with baseplates having tail fibers at one end. A 22-kb genomic cluster involved in their synthesis and having high homology to the cluster coding for the tail of the Peduovirus P2 was identified. The D. dadantii tailocins, termed dickeyocins P2D1 (phage P2-like dickeyocin 1), were resistant to inactivation by pH (3.5–12), temperature (4–50°C), and elevated osmolarity (NaCl concentration: 0.01–1 M). P2D1 could kill a variety of different Dickeya spp. but not any strain of Pectobacterium spp. tested and were not toxic to Caenorhabditis elegans.

Multilocus sequence and phenotypic analysis of Pectobacterium and Dickeya type strains for identification of soft rot Pectobacteriaceae from symptomatic potato stems and tubers in Pennsylvania

Outbreaks of potato blackleg and soft rot caused by Pectobacterium species and more recently Dickeya species across the U.S. mid-Atlantic region have caused yield loss due to poor emergence as well as losses from stem and tuber rot. To develop management strategies for soft rot diseases, we must first identify which members of the soft rot Pectobacteriaceae are present in regional potato plantings. However, the rapidly expanding number of soft rot Pectobacteriaceae species and the lack of readily available comparative data for type strains of Pectobacterium and Dickeya hinder quick identification. This manuscript provides a comparative analysis of soft rot Pectobacteriaceae and a comprehensive comparison of type strains from this group using rep-PCR, MLSA and 16S sequence analysis, as well as phenotypic and physiological analyses using Biolog GEN III plates. These data were used to identify isolates cultured from symptomatic potato stems collected between 2016 and 2018. The isolates were characterized for phenotypic traits and by sequence analysis to identify the bacteria from potatoes with blackleg and soft rot symptoms in Pennsylvania potato fields. In this survey, P. actinidiae, P. brasiliense, P. polonicum, P. polaris, P. punjabense, P. parmentieri, and P. versatile were identified from Pennsylvania for the first time. Importantly, the presence of P. actinidiae in Pennsylvania represents the first report of this organism in the U.S. As expected, P. carotorvorum and D. dianthicola were also isolated. In addition to a resource for future work studying the Dickeya and Pectobacterium associated with potato blackleg and soft rot, we provide recommendations for future surveys to monitor for quarantine or emerging soft rot Pectobacteriace regionally.

Comparative abundance and diversity of populations of the Pseudomonas syringae and Soft Rot Pectobacteriaceae species complexes throughout the Durance River catchment from its French Alps sources to its delta

Comparative abundance and diversity of populations of the Pseudomonas syringae and Soft Rot Pectobacteriaceae species complexes throughout the Durance River catchment from its French Alps sources to its delta

Potato tuber origin and microbial composition determines resistance against soft rot Pectobacteriaceae

Soft rot Pectobacteriaceae (SRP) are the causative agents of soft rot and blackleg in potato. In this study, we investigated if potato seed lots of the same cultivar, but originating from different fields, inoculated with the same density of SRP and planted in the same field, showed differences in blackleg disease incidence. We tested if these differences were correlated with the microbial community composition in tuber, and the soil where the mother tubers were grown, as the microbiome is known to play a large role in plant disease resistance. We found that tubers from seed lots with a high disease incidence had a different microbial community composition than tubers from seed lots with a low disease incidence. Several taxa could be identified that were on average more abundant in seed lots with a low disease incidence. However, the taxa that differed in abundance were different between the two growing seasons. Most of the taxa that differed in abundance between seed lots with high and low disease incidence were also present in the soil of the fields from which the tubers originated. However, the taxa did not differ in abundance between the different fields. This raises the question as to how these taxa are recruited by the tuber.

Pseudomonas PA14H7: Identification and Quantification of the 7-Hydroxytropolone Iron Complex as an Active Metabolite against Dickeya, the Causal Agent of Blackleg on the Potato Plant.

Soft rot Pectobacteriaceae (SRP), such as Pectobacterium and Dickeya, are phytopathogenic agents responsible for blackleg disease on several crops, such as potatoes, affecting the yield and depressing the seed production quality. However, neither conventional nor biocontrol products are available on the market to control this disease. In this study Pseudomonas PA14H7, a bacteria isolated from potato rhizosphere, was selected as a potential antagonist agent against Dickeya solani. In order to understand the mechanism involved in this antagonism, we managed to identify the main active molecule(s) produced by PA14H7. Cell-free supernatant (CFS) of PA14H7 cultures were extracted and analyzed using LC-MS, GC-MS, and NMR. We further correlated the biological activity against Dickeya solani of extracted CFS-PA14H7 to the presence of 7-hydroxytropolone (7-HT) complexed with iron. In a second time, we have synthesized this molecule and determined accurately using LC-UV, LC-MS, and GC-MS that, after 48 h incubation, PA14H7 released, in its CFS, around 9 mg/L of 7-HT. The biological activities of CFS-PA14H7 vs. synthetic 7-HT, at this concentration, were evaluated to have a similar bacteriostatic effect on the growth of Dickeya solani. Even if 7-HT is produced by other Pseudomonas species and is mostly known for its antibacterial and antifungal activities, this is the first description of its involvement as an effective molecule against pectinolytic bacteria. Our work opens the way for the comprehension of the mode of action of PA14H7 as a biocontrol agent against potato blackleg.

Genotypic and phenotypic uniformity among the population of Pectobacterium atrosepticum strains isolated during three growing seasons from potato fields in Poland

Pectobacterium atrosepticum (Pba)are phytopathogenic, pectinolytic, non-sporulating, gram-negative rods causing blackleg and soft rot diseases in potato. They are ubiquitously present under temperate climate conditions and contribute to significant economic losses. Currently, there are no efficient control methods against soft rot Pectobacteriaceae (SRP). As limitation of the spread of SRP is solely prevention-based, we undertook broad characterization on both genotypic and phenotypic levels of the P. atrosepticum isolates acquired during monitoring of potato fields in Poland in 2013, 2014 and 2016. Of REP, ERIC and BOX-based genomic fingerprinting, the BOX-based approach turned out to be the most informative and allowed for classification of 118 Pba strains into six groups. The present study indicated that IV and I BOX profiles dominated among the investigated population. Finally, 23 Pba strains representing all BOX groups and originating from various growing seasons were selected for genotypic, phylogenetic and phenotypic studies. rpoS-based phylogeny revealed intraspecies variation (16 SNP sites) among the studied Pba strains, in contrast to the analyses relying on gyrA (1 SNP) and recA (0 SNP) sequences. Pba strains showed higher potency to macerate potato tissue at 20 °C than 28 °C. These isolates exhibited rather uniform production of plant cell wall degrading enzymes (pectinases, cellulases and proteases), lipases, siderophores and biofilm, however in the majority of cases inferior in comparison to Dickeya solani and Pectobacterium carotovorum strains. In conclusion, this study revealed genotypic and phenotypic uniformity in addition to high virulence potential of Pba strains isolated from potato fields during several growing seasons in Poland.

Phytochemical Screening and Antibacterial Activity of Taxus baccata L. against Pectobacterium spp. and Dickeya chrysanthemi

The yew tree (Taxus baccata L.) is considered in folklore a symbol of immortality due to its qualities of longevity and regeneration. Despite its poisonous reputation, the yew tree has a long history of medicinal use, particularly in the form of extracts from its leaves and bark. In the work presented herein, gas chromatography–mass spectrometry (GC–MS) chemical profiling was applied to the aqueous ammonia/hydromethanolic extracts of several plant organs of T. baccata, leading to the identification of different bioactive compounds than those previously characterized by high-performance liquid chromatography with tandem mass spectrometry (HPLC–MS/MS) in other extraction media. The leaf aqueous ammonia extract was rich in 2-hexylthiophene and 3-O-methyl-d-fructose; 9-octadecenoic and hexadecanoic acid were the main constituents of the bark aqueous ammonia extract; and the fruit hydromethanolic extract contained methyl 2-O-methyl-α-d-xylofuranoside, 1,3-dioxolane derivatives, and erysimoside. The antimicrobial activity of the extracts was assayed against four bacterial pathogens responsible for the soft rot and blackleg diseases of potatoes, viz. Pectobacterium carotovorum subsp. carotovorum, Pectobacterium atrosepticum, Pectobacterium parmentieri, and Dickeya chrysanthemi, resulting in minimum inhibitory concentration (MIC) values as low as 187 μg·mL−1. Bioassays on potato slices confirmed the efficacy of the leaf extract at this dose when applied as a preventive treatment before artificial inoculation with P. carotovorum subsp. carotovorum. In view of this high activity, these extracts may find application in the integrated pest management of soft rot Pectobacteriaceae (SRP) diseases.

Natural Infections of Potato Plants Grown from Minitubers with Blackleg-Causing Soft Rot Pectobacteriaceae.

Information on the infection incidence of blackleg-causing soft rot Pectobacteriaceae (BL-SRP) in potato crops grown from minitubers (PB1-crop) and the distribution of BL-SRP in individual plants was collected during a two-year survey conducted at five potato growers located in the Netherlands. In the last weeks before haulm destruction, leaves, stems, and tubers of 100 or 200 plants were analyzed separately for the presence of Pectobacterium parmentieri, P. brasiliense, P. atrosepticum, and Dickeya spp. Extracted plant parts enriched for BL-SRP were analyzed with TaqMan assays specific for the detection of blackleg-causing BL-SRP. In 2019, low incidences of P. parmentieri (1-6%) in leaves were found at four growing sites. At one farm, reactions were detected in TaqMan assays for D. zeae and D. chrysanthemi in leaves. In 2020, the crops of two growers were largely free from BL-SRP. At one farm, a high infection incidence (21%) was found for D. fangzhongdai in tubers. The isolated pathogen was able to cause potato blackleg. At two other farms, high infection incidences in tubers were found with P. brasiliense (35-39%) and P. parmentieri (12-19%), whereas the incidence of P. brasiliense in leaves was also high (8%). In conclusion, high infection incidences with BL-SRP in potatoes can be found in a PB1 crop at the end of the growing season. Infections in individual plants were found either in tubers or in leaves. The potential sources of initial infection are discussed.

Survey of Soft Rot Pectobacteriaceae Infecting Potatoes in South Africa

Survey of Soft Rot Pectobacteriaceae Infecting Potatoes in South Africa

Comparative Pathogenomic Analysis of Two Banana Pathogenic Dickeya Strains Isolated from China and the Philippines.

Dickeya is a major and typical member of soft rot Pectobacteriaceae (SRP) with a wide range of plant hosts worldwide. Previous studies have identified D. zeae as the causal agent of banana soft rot disease in China. In 2017, we obtained banana soft rot pathogen strain FZ06 from the Philippines. Genome sequencing and analysis indicated that FZ06 can be classified as D. dadantii and represents a novel subspecies of D. dadantii, which we propose to name as subsp. paradisiaca. Compared with Chinese banana soft rot pathogenic strain D. zeae MS2, strain FZ06 has a similar host range but different virulence; FZ06 is significantly less virulent to banana and potato but more virulent to Chinese cabbage and onion. Characterization of virulence factors revealed obviously less production of pectate lyases (Pels), polygalacturonases (Pehs), proteases (Prts), and extrapolysaccharides (EPSs), as well as lower swimming and swarming motility and biofilm formation in strain FZ06. Genomic comparison of the two strains revealed five extra gene clusters in FZ06, including one Stt-type T2SS, three T4SSs, and one T4P. Expression of cell wall degrading enzyme (CWDE)-encoding genes is significantly lower in FZ06 than in MS2.

The Diversity and Abundance of Soft Rot Pectobacteriaceae Along the Durance River Stream in the Southeast of France Revealed by Multiple Seasonal Surveys.

Although irrigation water is frequently assessed for the presence of plant pathogens, large spatial and temporal surveys that provide clues on the diversity and circulation of pathogens are missing. We evaluate the diversity of soft rot Pectobacteriaceae (SRP) of the genera Dickeya and Pectobacterium over 2 years in a temperate, mixed-use watershed. The abundance of isolated strains correlates with the agricultural gradient along the watershed with a positive correlation found with temperature, nitrate, and dissolved organic carbon water concentration. We characterized 582 strains by amplification and sequencing of the gapA gene. Multilocus sequence analysis, performed with three housekeeping genes for 99 strains, and core genome analysis of 38 sequenced strains, confirmed for all the strains but one, the taxonomic assignation obtained with the sole gapA sequence. Pectobacterium spp. (549 isolates) were far more abundant than Dickeya spp. (33 isolates). Dickeya spp. were only observed in the lower part of the river when water temperature was >19°C, and we experimentally confirmed a decreased fitness of several Dickeya spp. at 8°C in river water. D. oryzae dominates the Dickeya spp. and P. versatile and P. aquaticum dominate the Pectobacterium spp., but their repartition along the watershed was different, with P. versatile being the only species regularly recovered all along the watershed. Excepting P. versatile, the Dickeya and Pectobacterium spp. responsible for disease outbreak on crops were less abundant or rarely detected. This work sheds light on the various ecological behaviors of different SRP types in stream water and indicates that SRP occupation is geographically structured.

Isolation and Genome Analysis of Pectobacterium colocasium sp. nov. and Pectobacterium aroidearum, Two New Pathogens of Taro.

Bacterial soft rot is one of the most destructive diseases of taro (Colocasia esculenta) worldwide. In recent years, frequent outbreaks of soft rot disease have seriously affected taro production and became a major constraint to the development of taro planting in China. However, little is known about the causal agents of this disease, and the only reported pathogens are two Dickeya species and P. carotovorum. In this study, we report taro soft rot caused by two novel Pectobacterium strains, LJ1 and LJ2, isolated from taro corms in Ruyuan County, Shaoguan City, Guangdong Province, China. We showed that LJ1 and LJ2 fulfill Koch’s postulates for taro soft rot. The two pathogens can infect taro both individually and simultaneously, and neither synergistic nor antagonistic interaction was observed between the two pathogens. Genome sequencing of the two strains indicated that LJ1 represents a novel species of the genus Pectobacterium, for which the name “Pectobacterium colocasium sp. nov.” is proposed, while LJ2 belongs to Pectobacterium aroidearum. Pan-genome analysis revealed multiple pathogenicity-related differences between LJ1, LJ2, and other Pectobacterium species, including unique virulence factors, variation in the copy number and organization of Type III, IV, and VI secretion systems, and differential production of plant cell wall degrading enzymes. This study identifies two new soft rot Pectobacteriaceae (SRP) pathogens causing taro soft rot in China, reports a new case of co-infection of plant pathogens, and provides valuable resources for further investigation of the pathogenic mechanisms of SRP.

Dickeya Manipulates Multiple Quorum Sensing Systems to Control Virulence and Collective Behaviors.

Soft rot Pectobacteriaceae (SRP), typical of Pectobacterium and Dickeya, are a class of Gram-negative bacterial pathogens that cause devastating diseases on a wide range of crops and ornamental plants worldwide. Quorum sensing (QS) is a cell-cell communication mechanism regulating the expression of specific genes by releasing QS signal molecules associated with cell density, in most cases, involving in the vital process of virulence and infection. In recent years, several types of QS systems have been uncovered in Dickeya pathogens to control diverse biological behaviors, especially bacterial pathogenicity and transkingdom interactions. This review depicts an integral QS regulation network of Dickeya, elaborates in detail the regulation of specific QS system on different biological functions of the pathogens and hosts, aiming at providing a systematic overview of Dickeya pathogenicity and interactions with hosts, and, finally, expects the future prospective of effectively controlling the bacterial soft rot disease caused by Dickeya by quenching the key QS signal.

Genomic and Functional Dissections of Dickeya zeae Shed Light on the Role of Type III Secretion System and Cell Wall-Degrading Enzymes to Host Range and Virulence.

ABSTRACTDickeya zeae is a worldwide destructive pathogen that causes soft rot diseases on various hosts such as rice, maize, banana, and potato. The strain JZL7 we recently isolated from clivia represents the first monocot-specific D. zeae and also has reduced pathogenicity compared to that of other D. zeae strains (e.g., EC1 and MS2). To elucidate the molecular mechanisms underlying its more restricted host range and weakened pathogenicity, we sequenced the complete genome of JZL7 and performed comparative genomic and functional analyses of JZL7 and other D. zeae strains. We found that, while having the largest genome among D. zeae strains, JZL7 lost almost the entire type III secretion system (T3SS), which is a key component of the virulence suite of many bacterial pathogens. Importantly, the deletion of T3SS in MS2 substantially diminished the expression of most type III secreted effectors (T3SEs) and MS2’s pathogenicity on both dicots and monocots. Moreover, although JZL7 and MS2 share almost the same repertoire of cell wall-degrading enzymes (CWDEs), we found broad reduction in the production of CWDEs and expression levels of CWDE genes in JZL7. The lower expression of CWDEs, pectin lyases in particular, would probably make it difficult for JZL7 to break down the cell wall of dicots, which is rich in pectin. Together, our results suggest that the loss of T3SS and reduced CWDE activity together might have contributed to the host specificity and virulence of JZL7. Our findings also shed light on the pathogenic mechanism of Dickeya and other soft rot Pectobacteriaceae species in general.IMPORTANCE Dickeya zeae is an important, aggressive bacterial phytopathogen that can cause severe diseases in many crops and ornamental plants, thus leading to substantial economic losses. Strains from different sources showed significant diversity in their natural hosts, suggesting complicated evolution history and pathogenic mechanisms. However, molecular mechanisms that cause the differences in the host range of D. zeae strains remain poorly understood. This study carried out genomic and functional dissections of JZL7, a D. zeae strain with restricted host range, and revealed type III secretion system (T3SS) and cell wall-degrading enzymes (CWDEs) as two major factors contributing to the host range and virulence of D. zeae, which will provide a valuable reference for the exploration of pathogenic mechanisms in other bacteria and present new insights for the control of bacterial soft rot diseases on crops.