Growth-promoting effects of Aspergillus Elegans and the dark septate endophyte (DSE) Periconia macrospinosa on cucumber.

Cucurbits are subject to a variety of stresses that limit their sustainable production, despite their important role in ensuring food security and nutrition. Plant stress tolerance can be enhanced through fungal endophytes. In this study, two endophytes isolated from wild plant roots, were tested to determine their effect on the growth promotion of cucumber (Cucumis sativus L.) plants. The phylogenetic analysis revealed that the designated isolates were Aspergillus elegans and Periconia macrospinosa. The results of the Plant Growth Promoting Fungal (PGPF) tests showed that both Aspergillus elegans and Periconia macrospinosa have a zinc solubilizing capacity, especially A. elegans, with a solubilization index higher than 80%. Also, both have a high salt tolerance (10-15% NaCl for P. macrospinosa and A. elegans, respectively), cellulolytic activity, and inhibition indices of 40-64.53%. A. elegans and P. macrospinosa had antagonistic effects against the cucumber phytopathogenic fungi Verticillium dahliae and Fusarium oxysporum, respectively. However, A. elegans and P. macrospinosa didn't exhibit certain potential plant benefits, such as the production of hydrogen cyanide (HCN) and phosphate solubilization. The chlorophyll content and growth parameters of two-month-old cucumber plants inoculated with the fungal species were significantly better than those of the controls (non-inoculated); the shoot dry weights of inoculated plants were increased by 138% and 170% for A. elegans and P. macrospinosa, respectively; and the root colonization by fungal endophytes has also been demonstrated. In addition to the fact that P. macrospinosa has long been known as PGPF, this is the first time that the ability of A. elegans to modulate host plant growth has been demonstrated, with the potential to be used as a biofertilizer in sustainable agriculture.

Biocontrol potential of endophytic Pseudomonas strain IALR1619 against two Pythium species in cucumber and hydroponic lettuce.

The use of fungicides to manage disease has led to multiple environmental externalities, including resistance development, pollution, and non-target mortality. Growers have limited options as legacy chemistry is withdrawn from the market. Moreover, fungicides are generally labeled for traditional soil-based production, and not for liquid culture systems. Biocontrol agents for disease management are a more sustainable and environmentally friendly alternative to conventional agroprotectants. Pythium ultimum is a soil borne oomycete plant pathogen with a broad taxonomic host range exceeding 300 plants. Cucumber seedlings exposed to P. ultimum 1 day after a protective inoculation with bacterial endophyte accession IALR1619 (Pseudomonas sp.) recorded 59% survival; with the control assessed at 18%. When the pathogen was added 5 days post endophyte inoculation, 74% of the seedlings treated survived, compared to 36% of the control, indicating a longer-term effect of IALR1619. Under hydroponic conditions, IALR1619 treated leaf type lettuce cv. 'Cristabel' and Romaine cv. 'Red Rosie' showed 29% and 42% higher shoot fresh weight compared to their controls, respectively. Similar results with less growth decline were observed for a repeat experiment with IALR1619. Additionally, an experiment on hydroponic lettuce in pots with perlite was carried out with a mixture of P. ultimum and P. dissotocum after IALR1619 inoculation. The endophyte treated 'Cristabel' showed fresh weight gain, but the second cultivar 'Pensacola' yielded no increase. In summary, the endophyte IALR1619 provided short term as well as medium-term protection against Pythium blight in cucumber seedlings and may be used as an alternative to conventional fungicides in a greenhouse setting. This study also demonstrated the potential of ALR1619 as a biocontrol agent against Pythium blight in hydroponic lettuce.

Guanosine-specific single-stranded ribonuclease effectors of a phytopathogenic fungus potentiate host immune responses.

Plants activate immunity upon recognition of pathogen-associated molecular patterns. Although phytopathogens have evolved a set of effector proteins to counteract plant immunity, some effectors are perceived by hosts and induce immune responses. Here, we show that two secreted ribonuclease effectors, SRN1 and SRN2, encoded in a phytopathogenic fungus, Colletotrichum orbiculare, induce cell death in a signal peptide- and catalytic residue-dependent manner, when transiently expressed in Nicotiana benthamiana. The pervasive presence of SRN genes across Colletotrichum species suggested the conserved roles. Using a transient gene expression system in cucumber (Cucumis sativus), an original host of C. orbiculare, we show that SRN1 and SRN2 potentiate host pattern-triggered immunity responses. Consistent with this, C. orbiculare SRN1 and SRN2 deletion mutants exhibited increased virulence on the host. In vitro analysis revealed that SRN1 specifically cleaves single-stranded RNAs at guanosine, leaving a 3'-end phosphate. Importantly, the potentiation of C. sativus responses by SRN1 and SRN2, present in the apoplast, depends on ribonuclease catalytic residues. We propose that the pathogen-derived apoplastic guanosine-specific single-stranded endoribonucleases lead to immunity potentiation in plants.

Nano-selenium foliar intervention-induced resistance of cucumber to Botrytis cinerea by activating jasmonic acid biosynthesis and regulating phenolic acid and cucurbitacin.

PURPOSE AND METHODS: Botrytis cinerea is the primary disease affecting cucumber production. It can be managed by applying pesticides and cultivating disease-resistant cucumber strains. However, challenges, such as drug resistance in pathogenic bacteria and changes in physiological strains, are obstacles in the effective management of B. cinerea. Nano-selenium (Nano-Se) has potential in enhancing crop resistance to biological stress, but the exact mechanism for boosting disease resistance remains unclear. Here, we used metabolomics and transcriptomics to examine how Nano-Se, as an immune activator, induces plant resistance. RESULT: Compared with the control group, the application of 10.0 mg/L Nano-Se on the cucumber plant's leaf surface resulted in increased levels of chlorophyll, catalase (10.2%), glutathione (326.6%), glutathione peroxidase (52.2%), cucurbitacin (41.40%), and metabolites associated with the phenylpropane synthesis pathway, as well as the total antioxidant capacity (21.3%). Additionally, the expression levels of jasmonic acid (14.8 times) and related synthetic genes, namely LOX (264.1%), LOX4 (224.1%), and AOC2 (309.2%), were up-regulated. A transcription analysis revealed that the CsaV3_4G002860 gene was up-regulated in the KEGG enrichment pathway in response to B. cinerea infection following the 10.0 mg/L Nano-Se treatment. DISCUSSION: In conclusion, the activation of the phenylpropane biosynthesis and branched-chain fatty acid pathways by Nano-Se promotes the accumulation of jasmonic acid and cucurbitacin in cucumber plants. This enhancement enables the plants to exhibit resistance against B. cinerea infections. Additionally, this study identified a potential candidate gene for cucumber resistance to B. cinerea induced by Nano-Se, thereby laying a theoretical foundation for further research in this area. © 2023 Society of Chemical Industry.

ß-Aminobutyric Acid and Powdery Mildew Infection Enhanced the Activation of Defense-Related Genes and Salicylic Acid in Cucumber (Cucumis sativus L.).

Powdery mildew disease, caused by Sphaerotheca fusca, is a major disease affecting cucumbers cultivated in greenhouses. This study was conducted to find defense genes induced by ß-aminobutyric acid (BABA) and powdery mildew in cucumber. Disease severities of 25% and 5% were exhibited by the 2000 and 5000 mg/L BABA-treated cucumber, respectively. BABA did not affect the spore germination of the powdery mildew pathogen, showing that BABA is not an antifungal agent against the pathogen. In quantitative real-time PCR analysis, BABA-treated cucumber upregulated the transcriptional levels of the defense genes CsPAL, CsPR3, CsPR1, CsLOX1, CsLOX23, Cs LecRK6.1, CsWRKY20, and Cupi4 in cucumber to maximum levels at 48 h, whereas CsLecRK6.1 reached maximum expression after 24 h, and further, salicylic acid (SA) levels were significantly increased in BABA-treated cucumber plants. In addition, the cucumber infected with powdery mildew underwent a 1.6- to 47.3-fold enhancement in the defense genes PAL, PR3, PR1, Lox1, Lox 23, LecRK6.1, WRKY20, and Cupi4 compared to heathy cucumber. These results suggest that the BABA-induced defense response is associated with SA signaling pathway-dependent systemic acquired resistance (SAR) in cucumber, which is involved in plant resistance mechanisms.

Antibacterial and antibiofilm performance of low-frequency ultrasound against Escherichia coli O157:H7 and its application in fresh produce.

Ultrasound technology has been focused on due to its unique advantages in biofilm removal compared with traditional antibacterial methods. Herein, the anti-biofilm properties of low-frequency ultrasound (LFUS) were studied against Enterohemorrhagic Escherichia coli O157: H7 (E. coli O157:H7). After ultrasonication (20 kHz, 300 W) for 5 min, the removal rate of biofilm from polystyrene sheets reached up to 99.999 %. However, the bacterial cells could not be inactivated completely even extending the duration of ultrasonic irradiation to 30 min. Fortunately, this study indicated that LFUS could efficiently weaken the metabolic capacity and biofilm-forming ability of bacterial cells separated from biofilm. It could be associated with the removal of cell surface appendages and damage to cell membrane induced by mechanical vibration and acoustic cavitation. Besides, the genetic analysis proved that the transcription level of genes involved in curli formation was significantly down-regulated during ultrasonic irradiation, thus impeding the process of irreversible adhesion and cells aggregation. Finally, the actual application effect of LFUS was also evaluated in different fresh produces model. The results of this study would provide a theoretical basis for the further application of ultrasound in the food preservation.

Protective role of iron oxide nanocomposites on disease index, and biochemical resistance indicators against Fusarium oxysporum induced-cucumber wilt disease: In vitro, and in vivo studies.

Recently nanocomposites have become a super-growth inducers as well as vital antifungal agents, which enhance plant growth and suppress plant diseases. A new strategy regarding the fabrication of humic acid (H) and boron (B) conjugated Fe2O3 nanocomposites was performed. Fe2O3 NP-B and Fe2O3 NP-H were synthesized in the presence of gamma-rays (as a direct reducing agent). Gamma-rays provoked reduction of metal ions due to the liberated reducing electrons, (e-aq), in aqueous solutions which can be considered as a direct reduction. Antifungal potential against Fusarium oxysporum, the causative agent of wilt disease in cucumber was determined. Disease index percent, metabolic resistance indicators in cucumber plant as response to promotion of systemic resistance (SR) were recorded. Results illustrated that both Fe2O3 NPs-B and Fe2O3 NPs-H nanocomposites had antifungal activity against F. oxysporumin vitro as well as in vivo. Results revealed that minimum inhibitory concentrations of Fe2O3 NPs-B and Fe2O3 NPs-H nanocomposites were 0.25 and 0.125 mM, respectively. Application of Fe2O3 NPs-B (0.25 mM) and Fe2O3 NPs-H (0.125 mM) appeared highly reduced the cucumber wilt disease symptoms incidence caused by F. oxysporum, and recorded disease severity by 83.33%. Fe2O3 NPs-B was the best treatment reducing disease indexes by 20.83% and gave highly protection against wilt disease by 75.0% and came next Fe2O3 NPs-H which reduced disease indexes by 25% and gave 69.99% protection against disease. Fe2O3 NPs-B and Fe2O3 NPs-H treatments improved morphological traits, photosynthetic pigments, osmolytes, total phenol and antioxidant enzymes activities in both infected and non-infected plants. The beneficial effects of the synthesized Fe2O3 NPs-B and Fe2O3 NPs-H nanocomposites were extended to increase not only the total phenol, and total soluble protein contents but also the activities of peroxidase (POD), and polyphenol oxidase (PPO) enzymes of the healthy and infected cucumber plants in comparison with control.

Isolation and identification of the endophytic fungus J2-3 and its disease-preventive and growth-promoting effects on cucumber.

There are many problems that result from the use of a large number of chemical pesticides to control plant diseases, including pathogenic bacteria resistance, environmental contamination, and human health effects. Recently, endophytic fungi have become a significant source of bioactive fungicide products and an invaluable resource for excavating microbial pesticides. In this study, endophytic fungi with biocontrol potential were isolated and screened from Mikania micrantha leaves, stems, and roots. Fifty endophytic fungi were isolated and their antagonistic activity was studied in vitro using the confrontation culture method. The J2-3 strains from stems exhibit broad-spectrum and high activity. The strain's biological characteristics were determined by various culture conditions, and it was identified as Fusarium proliferatum by both morphological and ITS sequence analysis. Biological characteristics of the J2-3 strain were also tested. The optimum temperature for mycelium growth and sporulation was 25 °C and 30 °C, respectively. For mycelium growth, starch was the optimum carbon source, and peptone was the optimum nitrogen source for sucrose, mycelium growth, and sporulation. Mycelium growth was killed by a temperature of 60 °C, and sporulation was killed by a temperature of 55 °C. The light aided mycelium growth, and the light alternated between light and dark cycles for sporulation. Further, pot experiments were conducted to determine the antagonistic and viable effects of highly antagonistic strains on cucumber. The spore suspension's final control efficacy on cucumber wilt disease was up to 62.79% and it also promoted cucumber growth significantly. The results show that the entophytic fungus J2-3 from M. micrantha can protect cucumbers from wilt disease and promote growth.

Metabolic Study of Cucumber Seeds and Seedlings in the Light of the New, Controversial Trend of Preventive Use of Systemic Fungicides.

Cucumber is one of the most commonly produced vegetable crops. The greatest economic losses in the yields of these crops have resulted from fungal infections-powdery mildew and downy mildew. The action of fungicides not only affects the fungi, but can also lead to metabolic disorders in plants. However, some fungicides have been reported to have positive physiological effects. Our research focused on the action of two commercially available fungicides, Scorpion 325 SC and Magnicur Finito 687,5 SC, on plant metabolism. Two approaches were used to check the effect of the fungicides at the early stage of plant development when metabolic changes occur most dynamically: spraying on the leaves of cucumber seedlings and presowing seed treatment. The application of the fungicide formulation as a presowing seed treatment caused perturbations in the phytase activity, leading to disorders in the energetic status of the germinating seeds. In addition, the tested preparations changed the morphology of the germinating seeds, limiting the growth of the stem. Furthermore, the application of the tested fungicides on seedlings also showed a disruption in the energetic status and in the antioxidative system. Therefore, the use of pesticides as agents causes a "green effect" and requires a much deeper understanding of plant metabolism.

Selective deployment of virulence effectors correlates with host specificity in a fungal plant pathogen.

The hemibiotrophic fungal plant pathogen Colletotrichum orbiculare is predicted to secrete hundreds of effector proteins when the pathogen infects cucurbit crops, such as cucumber and melon, and tobacco (Nicotiana benthamiana), a distantly related Solanaceae species. Here, we report the identification of sets of C. orbiculare effector genes that are differentially required for fungal virulence to two phylogenetically distant host species. Through targeted gene knockout screening of C. orbiculare 'core' effector candidates defined based on in planta gene expression, we identified: four host-specific virulence effectors (named effector proteins for cucurbit infection, or EPCs) that are required for full virulence of C. orbiculare to cucurbit hosts, but not to the Solanaceae host N. benthamiana; and five host-nonspecific virulence effectors, which collectively contribute to fungal virulence to both hosts. During host infection, only a small subset of genes, including the host-specific EPC effector genes, showed preferential expression on one of the hosts, while gene expression profiles of the majority of other genes, including the five host-nonspecific effector genes, were common to both hosts. This work suggests that C. orbiculare adopts a host-specific effector deployment strategy, in addition to general host-blind virulence mechanisms, for adaptation to cucurbit hosts.

Deciphering the mechanism of fungal pathogen-induced disease-suppressive soil.

One model of a disease-suppressive soil predicts that the confrontation of plant with a phytopathogen can lead to the recruitment and accumulation of beneficial microorganisms. However, more information needs to be deciphered regarding which beneficial microbes become enriched, and how the disease suppression is achieved. Here, we conditioned soil by continuously growing eight generations of cucumber inoculated with Fusarium oxysporum f.sp. cucumerinum in a split-root system. Disease incidence was found to decrease gradually upon pathogen infection accompanied with higher quantity of reactive oxygen species (ROS mainly OH• ) in roots and accumulation of Bacillus and Sphingomonas. These key microbes were proven to protect the cucumber from pathogen infection by inducing high ROS level in the roots through enrichment of pathways, including a two-component system, a bacterial secretion system, and flagellar assembly revealed by metagenomics sequencing. Untargeted metabolomics analysis combined with in vitro application assays suggested that threonic acid and lysine were pivotal to recruit Bacillus and Sphingomonas. Collectively, our study deciphered a 'cry for help' case, wherein cucumber releases particular compounds to enrich beneficial microbes that raise the ROS level of host to prevent pathogen attack. More importantly, this may be one of the fundamental mechanisms underpinning disease-suppressive soil formation.

Improving Agrobacterium tumefaciens-Mediated Genetic Transformation for Gene Function Studies and Mutagenesis in Cucumber (Cucumis sativus L.).

In the post-genomics era, Agrobacterium tumefaciens-mediated genetic transformation is becoming an increasingly indispensable tool for characterization of gene functions and crop improvement in cucumber (Cucumis sativus L.). However, cucumber transformation efficiency is still low. In this study, we evaluated the effects of several key factors affecting the shoot-regeneration rate and overall transformation efficiency in cucumber including genotypes, the age and sources of explants, Agrobacterium strains, infection/co-cultivation conditions, and selective agents. We showed that in general, North China cucumbers exhibited higher shoot-regeneration rate than US pickling or slicing cucumbers. The subapical ground meristematic regions from cotyledons or the hypocotyl had a similar shoot-regeneration efficiency that was also affected by the age of the explants. Transformation with the Agrobacterium strain AGL1 yielded a higher frequency of positive transformants than with GV3101. The antibiotic kanamycin was effective in selection against non-transformants or chimeras. Optimization of various factors was exemplified with the development of transgenic plants overexpressing the LittleLeaf (LL) gene or RNAi of the APRR2 gene in three cucumber lines. The streamlined protocol was also tested in transgenic studies in three additional genes. The overall transformation efficiency defined by the number of verified transgenic plants out of the number of seeds across multiple experiments was 0.2-1.7%. Screening among T1 OE transgenic plants identified novel, inheritable mutants for leaf or fruit color or size/shape, suggesting T-DNA insertion as a potential source of mutagenesis. The Agrobacterium-mediated transformation protocol from this study could be used as the baseline for further improvements in cucumber transformation.

Nanomaterial Size and Surface Modification Mediate Disease Resistance Activation in Cucumber (Cucumis sativus).

Crop disease represents a serious and increasing threat to global food security. Lanthanum oxide nanomaterials (La2O3 NMs) with different sizes (10 and 20 nm) and surface modifications (citrate, polyvinylpyrrolidone [PVP], and poly(ethylene glycol)) were investigated for their control of the fungal pathogen Fusarium oxysporum (Schl.) f. sp cucumerinum Owen on six-week-old cucumber (Cucumis sativus) in soil. Seed treatment and foliar application of the La2O3 NMs at 20-200 mg/kg (mg/L) significantly suppressed cucumber wilt (decreased by 12.50-52.11%), although the disease control efficacy was concentration-, size-, and surface modification-dependent. The best pathogen control was achieved by foliar application of 200 mg/L PVP-coated La2O3 NMs (10 nm); disease severity was decreased by 67.6%, and fresh shoot biomass was increased by 49.9% as compared with pathogen-infected control. Importantly, disease control efficacy was 1.97- and 3.61-fold greater than that of La2O3 bulk particles and a commercial fungicide (Hymexazol), respectively. Additionally, La2O3 NMs application enhanced cucumber yield by 350-461%, increased fruit total amino acids by 295-344%, and improved fruit vitamin content by 65-169% as compared with infected controls. Transcriptomic and metabolomic analyses revealed that La2O3 NMs: (1) interacted with calmodulin, subsequently activating salicylic acid-dependent systemic acquired resistance; (2) increased the activity and expression of antioxidant and related genes, thereby alleviating pathogen-induced oxidative stress; and (3) directly inhibited in vivo pathogen growth. The findings highlight the significant potential of La2O3 NMs for suppressing plant disease in sustainable agriculture.

Effects of Green Manures on Rhizosphere Fungal Community Composition of Cucumber Seedlings.

Green manures are usually used to improve soil health and increase crop productivity. The activity and composition of the soil microbial community could be altered by green manures. High-throughput amplicon sequencing was used to assess the effects of green manures of wheat (Triticum aestivum) and Indian mustard (Brassica juncea) on the rhizosphere fungal community composition of cucumber (Cucumis sativus L). Results showed that green manures of wheat and Indian mustard altered the composition but not the diversity of rhizosphere fungal communities of cucumber Contents of inorganic N, Olsen P, and available K in bulk soil decreased by green manure treatments. Ascomycota, Zygomycota, and Basidiomycota were the predominant phyla in soil, and their relative abundance changed after treatment with wheat and Indian mustard. The relative abundance of Basidiomycota was increased in the green manure of wheat, while that of Zygomycota was decreased in the green manure of Indian mustard. The relative abundance of Ascomycota increased in both wheat and Indian mustard. Green manures of wheat and Indian mustard also increased the relative abundances of unclassified Sordariomycetes and Fusarium spp., whereas they decreased the relative abundances of Pseudallescheria, Mortierella, Kernia, and unclassified Chaetomiaceae spp. Compared with other treatments, green manures of wheat increased the relative abundance of Waitea sp. and decreased the relative abundance of Cephaliophora sp. Indian mustard increased the relative abundance of Humicola sp.

Whole-Genome Sequencing of Fusarium oxysporum f. sp. cucumerinum Strain Race-4 Infecting Cucumber in China.

Fusarium oxysporum f. sp. cucumerinum, which causes root and vascular wilting, is one of the most devastating diseases infecting cucumber. Here, we report the first genome resource with high-quality assembly for F. oxysporum f. sp. cucumerinum strain Race-4, which is primarily endemic to China. The genome was 59.11 Mb in size and consisted of 48 scaffolds with an N50 of 3.87 Mb using PacBio long reads (301.77×) sequencing, and encodes 14,898 proteins from analyzing RNA-seq data. Gene annotations identified pathogen-host interaction genes, fungal virulence factors, secreted proteins, transcription factors, and secondary metabolite biosynthesis gene. Moreover, functional genes reported in previous studies were also identified in the genome of Race-4. These genes and genome resource may play important roles in understanding F. oxysporum f. sp. cucumerinum-cucumber interactions and will be useful for further research.

Development of Powdery Mildew Resistance in Cucumber Using CRISPR/Cas9-Mediated Mutagenesis of CsaMLO8.

Powdery mildew (PM) diseases may severely limit the production of various crops, including members of the family Cucurbitaceae. Successful PM infection relies on the Mildew Resistance Locus O (MLO) plant gene family, which encodes susceptibility factors essential for fungus penetration into the host cell. In cucumber (Cucumis sativus), natural mutations in CsaMLO8 confer resistance to the PM pathogen Podosphaera xanthii. Here, we used CRISPR/Cas9-mediated mutagenesis to generate PM resistance in the susceptible cucumber cultivar Ilan. Two transgene-free Csamlo8 CRISPR mutant lines (Csamlo-cr-1 and Csamlo-cr-2) were isolated, the first with a 5-bp deletion in exon 1, and the second harboring a 1,280-bp deletion and 10-bp insertion between exons 1 and 5. Both lines showed high resistance to PM under semicommercial growth conditions in the summer growing seasons of 2019 and 2021. These results provide the basis for generating transgene-free powdery mildew resistance in cucumber in any genetic background. This method can directly be employed on commercial cultivars and hybrid parental lines, and thereby substantially shorten and simplify the breeding process for PM resistance in cucumber.

Taxonomic response of bacterial and fungal populations to biofertilizers applied to soil or substrate in greenhouse-grown cucumber.

Reductions in the quality and yield of crops continuously produced in the same location for many years due to annual increases in soil-borne pathogens. Environmentally-friendly methods are needed to produce vegetables sustainably and cost effectively under protective cover. We investigated the impact of biofertilizers on cucumber growth and yield, and changes to populations of soil microorganisms in response to biofertilizer treatments applied to substrate or soil. We observed that some biofertilizers significantly increased cucumber growth and decreased soil-borne pathogens in soil and substrate. Rhizosphere microbial communities in soil and substrate responded differently to different biofertilizers, which also led to significant differences in microbial diversity and taxonomic structure at different times in the growing season. Biofertilizers increase the prospects of re-using substrate for continuously producing high-quality crops cost-effectively from the same soil each year while at the same time controlling soil-borne disease.

Changes in rhizospheric microbiome structure and soil metabolic function in response to continuous cucumber cultivation.

With the increasing reliance on intensive arable agriculture, analysis of the problems associated with continuous cropping has become a global research focus. Here, high-throughput sequencing and nontargeted metabolomics were used to evaluate the responses of soil microbial community structure and soil metabolic function to continuous cucumber cultivation (from 1 to 18 years of continuous cultivation) in greenhouses. Continuous cucumber cropping resulted in increased soil nutrient concentrations, but decreased concentrations of available nutrients. The abundance of several bacterial genera associated with nutrient cycling, such as Bacillus and Sphingomonas, was reduced by continuous cucumber cultivation. The abundance of several beneficial fungal genera, including pathogen antagonists (e.g. Chaetomium, Mortierella, Aspergillus, and Penicillium), were found to gradually decrease in response to the increased duration of continuous cropping. 3-amino-2-naphthoic acid and L-valine increased initially and then decreased as the cropping continued, which were related to fatty acid metabolism and amino acid biosynthesis. We also confirmed a close association between microbial community structure and soil metabolites. This study linked the changes in microbial community structure and metabolites in the rhizosphere soil and provided new insights into soil-microbial interactions in continuous cucumber culture systems.

A chitinase CsChi23 promoter polymorphism underlies cucumber resistance against Fusarium oxysporum f. sp. cucumerinum.

Fusarium wilt disease, caused by Fusarium oxysporum f. sp. cucumerinum (Foc), leads to widespread yield loss and quality decline in cucumber. However, the molecular mechanisms underlying Foc resistance remain poorly understood. We report the mapping and functional characterisation of CsChi23, encoding a cucumber class I chitinase with antifungal properties. We assessed sequence variations at CsChi23 and the associated defence response against Foc. We functionally characterised CsChi23 using transgenic assay and expression analysis. The mechanism regulating CsChi23 expression was assessed by genetic and molecular approaches. CsChi23 was induced by Foc infection, which led to rapid upregulation in resistant cucumber lines. Overexpressing CsChi23 enhanced fusarium wilt resistance and reduced fungal biomass accumulation, whereas silencing CsChi23 causes loss of resistance. CsHB15, a homeodomain leucine zipper (HD-Zip) III transcription factor, was found to bind to the CsChi23 promoter region and activate its expression. Furthermore, silencing of CsHB15 reduces CsChi23 expression. A single-nucleotide polymorphism variation -400 bp upstream of CsChi23 abolished the HD-Zip III binding site in a susceptible cucumber line. Collectively, our study indicates that CsChi23 is sufficient to enhance fusarium wilt resistance and reveals a novel function of an HD-Zip III transcription factor in regulating chitinase expression in cucumber defence against fusarium wilt.

Role of Salmonella Newport cell surface structures on bacterial attachment and transfer during cucumber peeling.

Fresh cucumbers have been recognized as a vehicle in foodborne disease outbreaks since several multistate outbreaks of salmonellosis linked to fresh cucumbers occurred in the United States. Little is known about how microbial cell surface characteristics that are known to affect adhesion can influence bacterial cross-contamination and transfer. This study investigated the role of S. Newport cell surface components on bacterial attachment and transfer in cucumbers. Wild type Salmonella Newport and its transposon mutants were used to inoculate cucumbers. Attachment strength of S. Newport wild type to cucumber was not significantly different than that of mutants. Log10 percent transfer of mutant strains to edible flesh was not different from the wild type. Significantly less wild type Salmonella remained on the peel and transferred to the peeler than one mutant did, but not the other. Our results suggest that while curli and cellulose enhance Salmonella attachment to surface of cucumbers, there appear to be other mechanisms and factors that govern Salmonella transfer in cucumbers.