Oriental Melon Research Articles

Transcriptome and metabolomics analysis revealed that CmWRKY49 regulating CmPSY1 promotes β-carotene accumulation in orange fleshed oriental melon

The flesh color of oriental melons is an important commercial trait that affects consumer preferences. To explore the mechanisms underlying the flesh color formation and regulation during fruit ripening, carotenoid-targeted metabolomic and RNA-seq analysis were conducted between white-fleshed (WF) and orange-fleshed (OF) oriental melon cultivars at different stages. The carotenoid-targeted metabolomic analysis indicated that β-carotene was the major metabolite that caused differences in flesh color between the two cultivars. Additionally, through KEGG pathway enrichment and weighted gene co-expression network (WGCNA) analysis, metabolic pathways and related transcription factors that are associated with carotenoid metabolism were selected and transcriptome data was verified using RT-qPCR. Finally, the yeast one hybrid and luciferase activity showed that the transcription factor CmWRKY49 could directly bind to the CmPSY1 promoter to activate its expression in the ‘OF’ cultivar. Transient overexpression of CmWRKY49 in ‘OF’ cultivar increased the β-carotene content, while the β-carotene content decreased when it was silenced in the same cultivar. This study provides insights into the underlying regulatory network of carotenoid metabolism in oriental melon fruit.

Identification of Key Gene Networks Controlling Soluble Sugar and Organic Acid Metabolism During Oriental Melon Fruit Development by Integrated Analysis of Metabolic and Transcriptomic Analyses.

Oriental melon (Cucumis melo var. acidulus) is one of the most economically important fruit crops worldwide. To elucidate the molecular basis related to soluble sugar and organic acid metabolism in the fruits of two oriental melon cultivars with different sweetness, we performed integrated metabolomic and transcriptomic analyses of the fruits of ‘Tianbao’ (A) with high sweetness and ‘Xiaocuigua’ (B) with low sweetness at different ripening stages. The high accumulation of sucrose, D-glucose, D-(+)-raffinose, and the relatively lower citric acid and malic acid might contribute to the sweet taste of A. By screening the differentially expressed genes (DEGs) and correlation analysis of the DEGs and differentially accumulated metabolites, we deduced that the B cultivar might promote the conversion of glucose and fructose into intermediate compounds for downstream processes such as glycolysis. The tricarboxylic acid (TCA) cycle might also be enhanced compared to A, thus resulting in the differential accumulation of soluble sugars and organic acids, ultimately causing the taste difference between the two oriental melon cultivars. Our finding provides important information for further exploring the metabolic mechanisms of soluble sugars and organic acids in oriental melon.

Metabolomic Change of Oriental Melon and Its Skin Browning by Coconut Oil and Beeswax Coating Treatment During Room Temperature Storage

Metabolomic Change of Oriental Melon and Its Skin Browning by Coconut Oil and Beeswax Coating Treatment During Room Temperature Storage

Effect of Ascorbic Acid, Salicylic Acid and Polyethylene Film Treatments on Fruit Quality and Stripes Browning of ‘Smartkkul’ Oriental Melon during Low Temperature Storage

Effect of Ascorbic Acid, Salicylic Acid and Polyethylene Film Treatments on Fruit Quality and Stripes Browning of ‘Smartkkul’ Oriental Melon during Low Temperature Storage

FKBP-type peptidyl-prolyl cis-trans isomerase interacts with the movement protein of tomato leaf curl New Delhi virus and impacts viral replication in Nicotiana benthamiana.

Begomoviruses belonging to the family Geminiviridae are plant‐infecting DNA viruses. Begomoviral movement protein (MP) has been reported to be required for virus movement, host range determination, and symptom development. In the present study, the FK506‐binding protein (FKBP)‐type peptidyl‐prolyl cis‐trans isomerase (NbFKPPIase) of Nicotiana benthamiana was identified by a yeast two‐hybrid screening system using the MP of tomato leaf curl New Delhi virus (ToLCNDV) oriental melon (OM) isolate (MPOM) as bait. Transient silencing of the gene encoding NbFKPPIase increased replication of three test begomoviruses, and transient overexpression decreased viral replication, indicating that NbFKPPIase plays a role in defence against begomoviruses. However, infection of N. benthamiana by ToLCNDV‐OM or overexpression of the gene encoding MPOM drastically reduced the expression of the gene encoding NbFKPPIase. Fluorescence resonance energy transfer analysis revealed that MPOM interacted with NbFKPPIase in the periphery of cells. Expression of the gene encoding NbFKPPIase was induced by salicylic acid but not by methyl jasmonate or ethylene. Moreover, the expression of the gene encoding NbFKPPIase was down‐regulated in response to 6‐benzylaminopurine and up‐regulated in response to gibberellin or indole‐3‐acetic acid, suggesting a role of NbFKPPIase in plant development. Transcriptome analysis and comparison of N. benthamiana transient silencing and overexpression of the gene encoding MPOM led to the identification of several differentially expressed genes whose functions are probably associated with cell cycle regulation. Our results indicate that begomoviruses could suppress NbFKPPIase‐mediated defence and biological functions by transcriptional inhibition and physical interaction between MP and NbFKPPIase to facilitate infection.

Spatial Distribution and Migration Characteristic of Forchlorfenuron in Oriental Melon Fruit by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Spatial Distribution and Migration Characteristic of Forchlorfenuron in Oriental Melon Fruit by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Transcriptomic analysis of melon/squash graft junction reveals molecular mechanisms potentially underlying the graft union development.

Oriental melon (Cucumis melo var. makuwa Makino) has become a widely planted horticultural crop in China especially in recent years and has been subjected to the grafting technique for the improvement of cultivation and stress resistance. Although grafting has a long history in horticulture, there is little known about the molecular mechanisms of the graft healing process in oriental melon. This study aims to reveal the molecular changes involved in the graft healing process. In the present work, anatomical observations indicated that the 2, 6, and 9 DAG were three critical stages for the graft healing and therefore, were selected for the subsequent high-throughput RNA-seq analysis. A total of 1,950 and 1,313 DEGs were identified by comparing IL vs. CA and CA vs. VB libraries, respectively. More DEGs in the melon scion exhibited abundant transcriptional changes compared to the squash rootstock, providing increased metabolic activity and thus more material basis for the graft healing formation in the scion. Several DEGs were enriched in the plant hormone signal transduction pathway, phenylpropanoid biosynthesis, and carbon metabolism. In addition, the results showed that concentrations of IAA, GA3, and ZR were induced in the graft junctions. In conclusion, our study determined that genes involved in the hormone-signaling pathway and lignin biosynthesis played the essential roles during graft healing. These findings expand our current understandings of the molecular basis of the graft junction formation and facilitate the improvement and success of melon grafting in future production.

Comparison of Yield and Workload depending on Stem Training Methods in Oriental Melon Hydroponics

Comparison of Yield and Workload depending on Stem Training Methods in Oriental Melon Hydroponics

Development of Strategies to Minimize the Risk of Listeria monocytogenes Contamination in Radish, Oriental Melon, and Carrots.

Contamination by Listeria monocytogenes in packaged produce is a major concern. The purpose of this study was to find natural and affordable sanitizers to reduce L. monocytogenes contamination in agricultural products. Organic acids, ultraviolet-C (UV-C), and ethanol were analyzed either alone or in combination to assess their ability to reduce L. monocytogenes population in radish, oriental melon, and carrot samples. In radish samples, 3% malic acid combined with UV-C at a dosage of 144 mj/cm2 significantly reduced (>4 log CFU/g) the population of L. monocytogenes (1.44 ± 0.5) compared to the control sample (5.14 ± 0.09). In the case of the melon samples, exposure to UV-C at a dosage of 144 mj/cm2 combined with 3% lactic acid (2.73 ± 0.75) or 50% ethanol (2.30 ± 0.01) was effective against L. monocytogenes compared to the control sample (5.10 ± 0.19). In carrot samples, 3% lactic acid combined with 144 mj/cm2 dosage UV-C reduced L. monocytogenes population (4.48 ± 0.25) more than in the control sample (5.85 ± 0.08). These results reveal that sanitizers that are effective for one crop are less effective for another crop indicating that effective prevention methods should be customized for each crop to prevent pathogen cross contamination during postharvest washing.

Dissipation and risk assessment of forchlorfenuron and its major metabolites in oriental melon under greenhouse cultivation

Forchlorfenuron is a widely used plant growth regulator. The uptake of forchlorfenuron and its major metabolites poses a potential risk for human health. However, little is known about the dissipation of forchlorfenuron and its major metabolites in agricultural food. In this study, the metabolite 4-hydroxyphenyl-forchlorfenuron was first identified in oriental melon, which exhibited the highest level of residues of 4.42–5.12 μg/kg on the 4–7th days after application. Forchlorfenuron was found to be dissipated rapidly in melon at the recommended application rates, with half-lives ranging from 1.20 to 1.33 days. The rate of dissipation of 4-hydroxyphenyl-forchlorfenuron was greater than that of metabolism from forchlorfenuron in the oriental melon. However, the other metabolite, 3-hydroxyphenyl-forchlorfenuron, was not detected in oriental melon. The risk assessment showed that the acute and chronic dietary exposure risks of forchlorfenuron in oriental melon were 0.0011–0.0037% and 0.06–0.12%, respectively, suggesting little health risk to Chinese consumers.

First report of Cucurbit chlorotic yellows virus infecting Cucumis melo (muskmelon and oriental melon) in Korea.

In December 2018, virus-like symptoms (yellowing, vein clearing) were observed on 2% of muskmelon (Cucumis melo L.) plants in plastic houses on a farm in Gyeongsang province, Korea Total RNA from two symptomatic and two asymptomatic plants was extracted using RNeasy Plant Mini Kit (Qiagen, Germany) for high throughput sequencing (HTS). After pre-processing and Ribo-Zero rRNA removal, a cDNA library was prepared (Illumina TruSeq Stranded Total RNA kit) and sequenced (Illumina NovaSeq 6000 system: Macrogen Inc. Korea). De novo assembly of 88,222,684 HTS reads with Trinity software (r20140717) yielded 146,269 contigs of 201-28,442 bp, which were screened against the NCBI viral genome database by BLASTn. Contigs from cucumber mosaic virus (CMV), melon necrotic spot virus (MNSV), tobacco mosaic virus (TMV) and watermelon mosaic virus (WMV) were identified, all previously reported in Korea. Two contigs (8,539 and 8,040 bp) with 99.9% sequence identity to distinct cucurbit chlorotic yellows virus (CCYV) isolates (JN641883, RNA1, Taiwan; MH819191, RNA2, China) were also identified. The ten sequences most closely related to each RNA of the Korean isolate (≥99% coverage, ≥99.6% nt identity) were from Japan, China, Taiwan, or Israel. CCYV presence was confirmed by reverse transcription-PCR (RT-PCR) using newly designed specific primers, RdRp-F/RdRp-R (5'-ACCGAACACTTGGCTATCCAA-3'/5'-CTTAATGCCGCGTATGAACTCA-3') span style="font-family:'Times New Roman'; letter-spacing:-0.5pt">and HSP-F/HSP-R (5'-TGAACGACACTGAGTTCATTCCTA-3'/5'-CGCCAAGATCGTACATGAGGAA-3'), against RNA dependent RNA polymerase (RdRp; RNA1) and the heat shock protein 70 homolog (HSP70h; RNA2). Symptomatic samples yielded products of expected sizes (RdRp,450 bp; HSP70h, 510 bp) while asymptomatic samples did not. The amplicons were cloned, and two clones of each were sequenced (BIONEER, Korea; GenBank acc. nos. LC592226 and LC592227) showing 100% and 99.2% nt identity with RdRp and HSP70h genes of Chinese CCYV isolate SD (MH819190 and MH819191, respectively) and other Asian isolates. Primers specific for CMV, WMV, beet pseudo-yellows virus (BPYV) (Okuda et al., 2007), TMV (Kim et al., 2018), MNSV (F/R, 5'-ATCTCGCATTTGGCATTACTC-3'/5'-ATTTGTAGAGATGCCAACGTA-3'), cucurbit yellow stunting disorder virus (CYSDV; Zeng et al., 2011) and cucurbit aphid-borne yellows virus (CABYV; F/R, 5'-CGGTCTATTGTCTGCAGTACCA-3'/5'- GTAGAGGATCTTGAATTGGTCCTCA-3') were also used. None of these viruses were detected in the symptomatic samples, but both asymptomatic plants were positive for CMV and WMV, and one also for MNSV. In June and September 2020, muskmelon and oriental melon (Cucumis melo L. var. makuwa) plants with yellowing disease (incidence 80-90%) and whiteflies were observed in all investigated plastic houses of one muskmelon and one oriental melon farm in Gyeonggi and Jeolla provinces. Symptomatic samples (14 muskmelon; 6 oriental melon) were collected and RT-PCR tested as above; 19/20 samples were positive for CCYV, but none for the other viruses. The oriental melon sequence (LC592895, LC592230) showed 99.7% and 100% nt identity with the RdRp and HSP70h genes of Chinese isolate SD, respectively. CCYV was first reported in Japan (Okuda et al., 2010), Taiwan, and China (Huang et al., 2010; Gu et al., 2011); to our knowledge, this is the first report of CCYV infecting muskmelon and oriental melon in Korea. Whitefly-transmitted CCYV could present a serious threat of yield losses to cucurbit crops in Korea, requiring control of vector populations to prevent spread of CCYV.

CmMYB113 regulates ethylene-dependent sucrose accumulation in postharvest climacteric melon fruit

Sucrose is mainly accumulated in mature oriental melon fruit, and it is generally regulated by ethylene. However, the molecular mechanism of sucrose accumulation induced by ethylene remains largely unknown. Here, high sucrose melon cultivar ‘HS’ and low sucrose melon cultivar ‘LW’ fruit were treated with ethylene and 1-MCP at ethylene release initial stage (27 d after anthesis, DAA), we found that the exogenous ethylene treatment promoted the sucrose accumulation, and enhanced the activity of sucrose phosphate synthase (SPS), as well as the transcript of CmSPS1 in postharvest climacteric melon fruit ‘HS’. However, both treatments had no effect on sucrose accumulation, SPS activity and its transcription in ‘LW’ fruit. In addition, exogenous ethylene increased endogenous ethylene production in ‘HS’ fruit, and had no effect in ‘LW’ fruit, except 5 d after treatment. Also, a strong positive correlation between ethylene and the CmACO1 expression level were observed in ‘HS’. The expression of transcription factor CmMYB113, screened from yeast one hybrid library, was induced by exogenous ethylene treatment in ‘HS’ rather than ‘LW’. Moreover, the yeast one hybrid and GUS activity assays indicated that CmMYB113 could activate the transcription of CmACO1 and CmSPS1 by binding directly to their promoters, respectively. From the findings, it was revealed that the targeted up-regulation of CmACO1 and CmSPS1 by CmMYB113 could be involved in ethylene-dependent sucrose accumulation, which enhanced flavor quality in climacteric melon fruit.

Abscisic acid and putrescine synergistically regulate the cold tolerance of melon seedlings

Low temperature in early spring severely endangers the growth and development of melon seedlings. Abscisic acid (ABA) and polyamines (PAs) are important signal molecules in plant response to stress. However, the issue of whether they interact to regulate melon cold tolerance remains largely uncharacterized. Here, we identified a total of 14 key genes related to ABA and PAs biosynthesis, including four CmNCEDs, and ten genes in PA pathway (one CmADC, one CmODC, four CmSAMDCs, two CmSPDSs, and two CmSPAMs). Two oriental melon cultivars (IVF571, cold-tolerant; IVF004, cold-sensitive) were selected to explore the difference of ABA and PAs biosynthesis under cold stress (15 °C/6 °C, day/night). Results showed that the expressions of CmNCED3, CmNCED3-2, CmADC, CmSAMDCs, CmSPDS2 and CmSPMS1 were significantly up-regulated. ABA and putrescine levels were significantly increased in IVF571 under cold stress. Inhibiting the biosynthesis of endogenous ABA with nordihydroguaiaretic acid (NDGA) or Put with D-Arginine (D-Arg) dramatically decreased the levels of each other and aggravated the cold injury of melon seedlings. In addition, spraying with exogenous 75 μM ABA or 1 mM Put improved the activities of superoxide dismutase, catalase and ascorbate peroxidase, and reduced the membrane lipid peroxidation damage of melon seedlings under cold stress. In all, the higher cold tolerance of IVF571 seedlings than that of IVF004 seedlings might be related to the increase in ABA and Put levels triggered by cold stress. ABA and Put could regulate the biosynthesis of each other and might act as signals to trigger the antioxidant system, thereby increasing melon cold tolerance.

Neural Network Based Prediction of Soluble Solids Concentrationin Oriental Melon Using VIS/NIR Spectroscopy

Highlights Non-destructive soluble solids content prediction model for oriental melon was developed based on NIR spectrum data. Not only the classical ML or Neural-Network methods, but also the mixture of both techniques have also been tried. Comparing the various pre-processing methods, the MSC-PLS-ANN model showed the best results. MSC-PLS-ANN model demonstrated 6% of improvement in RMSE score over the PLSR model, which is commonly used in commercial products Abstract. Models for predicting the soluble solids concentration (SSC) of oriental melons were developed and evaluated by applying near infrared spectroscopy and an artificial neural network technique. For the evaluation, a total of 300 oriental melons, both ripe and unripe, were mixed together and sampled. To develop an SSC prediction model, the actual SSC values of specimens having the same spectra as those of the visible/near infrared wavelength bands were measured. The measured spectra were preprocessed using eight methods [Multiplicative Scatter Correction (MSC), Standard Normal Variate (SNV), Robust Normal Variate, Savitzky-Golay 1st and 2nd; Min-Max Normalization; Robust Normalization; Standardization], and the SSC prediction model was developed by applying three techniques (Partial Least Squared Regression [PLSR], Artificial Neural Network [ANN], and Convolutional Neural Network [CNN]). Among them, the PLSR technique also applied a Variable Importance in Projection (VIP) method for wavelength selection. Among the PLSR-based SSC prediction models, the SNV-preprocessed PLSR model showed the best SSC prediction performance (RMSEtest, 0.67; R2test, 0.81). Among the ANN-based models, the MSC-preprocessed PLS-ANN model showed the best SSC prediction performance (RMSEtest: 0.63, R2test: 0.83). Among the CNN-based models, the DeepSpectra model was applied, but showed the lowest prediction performance (RMSEtest: 0.79, R2test: 0.74). In conclusion, among the three SSC prediction algorithms tested in this study, the PLS-ANN-based prediction model showed the best SSC prediction performance, which was found to be higher than that of the PLSR-based SSC prediction model applied to the sugar sorters currently used in agricultural products at processing centers. Keywords: Artificial Neural Network, Convolution Neural Network, Korean melon, VIP-PLSR, VIS/NIR spectroscopy.

CmLOX10 positively regulates drought tolerance through jasmonic acid -mediated stomatal closure in oriental melon (Cucumis melo var. makuwa Makino)

Drought stress severely impairs plant growth and production. Lipoxygenase (LOX), a master regulator for lipid peroxidation, is critical for direct or indirect response to abiotic stresses. Here, we found that drought stress induced the transcription of CmLOX10 in leaves of oriental melon seedlings. Reverse genetic approaches and physiological analyses revealed that silencing CmLOX10 increased drought susceptibility and stomatal aperture in oriental melon seedlings, and that ectopic overexpression of CmLOX10 in Arabidopsis enhanced drought tolerance and decreased the stomatal aperture. Moreover, the transcription of jasmonic acid (JA)-related genes and JA accumulation were significantly induced in CmLOX10-overexpressed Arabidopsis, which were reversely suppressed in CmLOX10-silenced seedlings during the stage of drought stress. Foliar application of JA further verified that JA enhanced drought tolerance and induced stomatal closure in leaves of melon seedlings. In addition, the feedback regulation of CmLOX10 was induced by JA signaling, and the expression level of CmMYC2 was increased by JA and drought treatment. Yeast one-hybrid analysis showed that CmMYC2 directly bound to the promoter of CmLOX10. In summary, we identified the important roles of CmLOX10 in the regulation of drought tolerance in oriental melon seedlings through JA- mediated stomatal closure and JA signaling-mediated feedback through CmMYC2.

Modified Atmosphere and Humidity Film Reduces Browning Susceptibility of Oriental Melon Suture Tissue during Cold Storage.

Oriental melons have a relatively short shelf life as they are harvested during the summer season and susceptible to cold-induced injuries. Typical chilling injury when stored at 4 °C is expressed as browning of the fruit suture. To prolong the shelf life and reduce browning of the fruit, the effects of modified atmosphere packaging (MAP), X-tend modified atmosphere (MA)/modified humidity (MH) bulk packaging (XF), and polyethylene (PE) packaging, on oriental melons were investigated during storage at 4 °C and 10 °C for 14 days and under retail display conditions at 20 °C. The O2 concentrations in PE packages stored at 4 °C and 10 °C ranged from 17.4 to 18.5%, whereas those in XF packages were reduced to 16.3–16.6%. The CO2 content of XF package (4.2–4.6%) was higher than that of PE package (1.4–1.9%) stored at 4 °C or 10 °C. Relative humidity (RH) saturated in the PE packages but not in the XF packages after seven days of storage. Furthermore, PE packages performed better at maintaining melon weight and firmness than XF packages during storage at 10 °C for 14 days and under retail display conditions at 20 °C. PE and XF packages effectively reduced the browning index of the peel and white linear sutures of oriental melons compared with the unpackaged control during cold storage at 4 °C, and this observation was maintained at the retail display condition at 20 °C. The enhanced CO2 levels, reduced O2 levels, and optimal RH values that were provided by the MAP, prevented the browning symptoms, and improved the marketability and shelf life of oriental melons.

SNP marker assay and candidate gene identification for sex expression via genotyping-by-sequencing-based genome-wide associations (GWAS) analyses in Oriental melon (Cucumis melo L.var.makuwa)

The Oriental melon is an important delicious fruit crop across East Asia. Single-nucleotide polymorphisms (SNPs) are considered to be a useful genotyping tool both to detect genetic diversity and to protect breeder rights. This study used genotyping-by-sequencing (GBS) to detect genome-wide SNPs by analyzing 48 commercial Oriental melon varieties to generate 5640 filtered SNPs. Based on the high polymorphism information content (PIC), genetic and physical distances, a subset of 192 SNPs was selected as putative SNPs for validation via the Fluidigm JunoTM system. Of these, 164 SNPs were successfully validated in 87 Oriental melon varieties with a validation ratio of 85.41 %. Furthermore, these 87 Oriental melon varieties were classified into ten distinct groups, based on 164 SNP markers. Therefore, these large set of SNP markers detected here has several application, such as genetic diversity studies, varietal identification and marker-assisted selection (MAS) in the Oriental melon. Genome-wide associations (GWAS) analyses detected significant 18 SNPs associated with various morphological traits, including two novel SNPs for sex expression on chromosomes 1 and 8 that were not colocalized with previous studies. The four potential candidate genes such as MELO3C015898, MELO3C015904, MELO3C024563, and MELO3C024565 were predicted within the GWAS-SNPs regions for sex expression in the Oriental melon. Hence, identification of these candidate genes has provided the foundation to study the molecular genetic mechanism of sex expression trait in the Oriental melon.

ITRAQ and RNA-Seq analyses revealed the effects of grafting on fruit development and ripening of oriental melon (Cucumis melo L. var. makuwa).

Rootstocks are among the primary factors that influence fruit yield and quality as well as melon development. To understand the differences in the molecular mechanisms and gene expression networks of fruit development between grafted and nongrafted plants in oriental melon, we performed a comprehensive analysis of the transcriptome and proteome dynamic gene/protein expression profiles during fruit development in oriental melon (Cucumis melo L. var. makuwa). Using pairwise comparisons between grafted and nongrafted samples by transcriptome analysis, we identified a large number of candidate genes involved in hormonal signaling pathways, transcription factors, resistance-related biosynthetic pathways and photosynthesis-related metabolic pathways. Many transcription factor-encoded genes were significantly more strongly expressed in the grafted samples, for example, AP2/ERF, C2H2, MYB, bHLH, and AUX/IAA, which are well-known participants in the regulation of developmental processes and hormonal signaling metabolism. Some differentially expressed genes (DEGs) were enriched in flavonoid biosynthesis and phenylpropanoid biosynthesis and determined plant resistance. In addition, some differentially expressed proteins (DEPs) were enriched in photosynthesis-related pathways, which could improve fruit quality and yield. Moreover, through weighted gene coexpression network analyses, we identified modules of coexpressed genes and hub genes specifically related to grafting for different fruit developmental stages. The results suggested that graft-related modules and hub genes were primarily associated with photosynthate metabolism and hormonal signaling pathways. The results obtained in this study provide a valuable resource for dissecting the role of candidate genes governing graft-related metabolism in oriental melon fruit, suggesting an interesting correlation with the effects of rootstock on fruit development.

High concentrations of CPPU promotes cucurbitacin B accumulation in melon (Cucumis melo var. makuwa Makino) fruit by inducing transcription factor CmBt

N-(2-Chloro-4-pyridyl)-N′-phenylurea (CPPU) is a cytokinin-like plant growth regulator, which application in melon fruit set often produced bitter fruit caused by cucurbitacin B (Cu B) accumulation. However, more evidence is required to uncover the role of CPPU in regulating Cu B synthesis. In this study, two oriental melon cultivars ‘YMR’ (easy to present bitter fruit in maturation) and ‘HDB’ (hardly produce bitter fruit at maturity) were used. Four concentrations of CPPU (2.5, 5, 10 and 20 mg L−1) were set and hand pollination was used as control. Cu B accumulated in roots and fruit of 7 days after flower (7 DAF), which in ‘YMR’ was higher than those in ‘HDB’, and consistent with Cu B biosynthetic genes expression patterns. Furthermore, Cu B content in fruit significantly increased with CPPU concentrations and reached the highest level at 7 DAF, then decreased after 14 DAF, and which treated by 20 mg L−1 CPPU was always higher than that of controls and other low CPPU concentrations. Meanwhile, fruit bitterness evaluation suggested 20 mg L−1 CPPU increased the occurrence of bitterness during melon maturation. Transcription analysis suggested that the expression of Cu B biosynthetic genes (CmBi, Cm710, CmACT) and CmBt, the fruit-specific transcription factor, were significantly induced by 20 mg L−1 CPPU. Transient over-expression of CmBt in young fruit of ‘YMR’ increased Cu B biosynthetic genes (CmBi, CmACT, Cm710 and Cm890) expression and promoted Cu B accumulation. Taken together, this study demonstrates that 20 mg L−1 CPPU promotes Cu B accumulation in melon fruit by inducing CmBt and its biosynthetic genes.

Lignin synthesized by CmCAD2 and CmCAD3 in oriental melon (Cucumis melo L.) seedlings contributes to drought tolerance.

CmCAD2 and CmCAD3 function more positively than CmCAD1 in oriental melon for lignin synthesis which is important to ensure internal water status and thus for drought tolerance. Well-lignification may be the guarantee of efficient axial water transport and barrier of lateral water flow in oriental melon tolerating drought stress, however remains to be verified. As an important enzyme in monolignol synthesis pathway, five cinnamyl alcohol dehydrogenase (CAD) genes were generally induced in melon seedlings by drought. Here we further revealed the roles of CmCAD1, 2, and 3 in lignin synthesis and for drought tolerance. Results found that overexpressing CmCAD2 or 3 strongly recovered CAD activities, lignin synthesis and composition in Arabidopsis cadc cadd, whose lignin synthesis is disrupted, while CmCAD1 functioned modestly. In melon seedlings, silenced CmCAD2 and 3 individually or collectively decreased CAD activities and lignin depositions drastically, resulting in dwarfed phenotypes. Reduced lignin, mainly composed by guaiacyl units catalyzed by CmCAD3, is mainly due to the limited lignification in tracheary elements and development of Casparion strip. While CmCAD1 and 2 exhibited catalysis to p-coumaraldehyde and sinapaldehyde, respectively. Compared with CmCAD1, drought treatments revealed higher sensitivity of CmCAD2 and/or 3 silenced melon seedlings, accompanying with lower relative water contents, water potentials and relatively higher total soluble sugar contents. Slightly up-regulated expressions of aquaporin genes together with limited lignification might imply higher lateral water loss in stems of silenced lines. In Arabidopsis, CmCAD2 and 3 transgenic lines enhanced cadc cadd drought tolerance through recovering lignin synthesis and root development, accompanying with decreased electrolyte leakage ratios and increased RWCs, thus improved survival rates. Briefly, lignin synthesized by CmCAD2 and 3 functions importantly for drought tolerance in melon.