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Background Fruit skin color play important role in commercial value of cucumber, which is mainly determined by the content and composition of chlorophyll and anthocyanins. Therefore, understanding the related genes and metabolomics involved in composition of fruit skin color is essential for cucumber quality and commodity value. Results The results showed that chlorophyll a, chlorophyll b and carotenoid content in fruit skin were higher in Lv (dark green skin) than Bai (light green skin) on fruit skin. Cytological observation showed more chloroplast existed in fruit skin cells of Lv. A total of 162 significantly different metabolites were found between the fruit skin of the two genotypes by metabolome analysis, including 40 flavones, 9 flavanones, 8 flavonols, 6 anthocyanins, and other compounds. Crucial anthocyanins and flavonols for fruit skin color, were detected significantly decreased in fruit skin of Bai compared with Lv. By RNA-seq assay, 4516 differentially expressed genes (DEGs) were identified between two cultivars. Further analyses suggested that low expression level of chlorophyll biosynthetic genes, such as chlM, por and NOL caused less chlorophylls or chloroplast in fruit skin of Bai. Meanwhile, a predicted regulatory network of anthocyanin biosynthesis was established to illustrate involving many DEGs, especially 4CL, CHS and UFGT. Conclusions This study uncovered significant differences between two cucumber genotypes with different fruit color using metabolome and RNA-seq analysis. We lay a foundation to understand molecular regulation mechanism on formation of cucumber skin color, by exploring valuable genes, which is helpful for cucumber breeding and improvement on fruit skin color.

Drought, one of the crucial environmental constraints, seriously threats the quality and yield in chieh-qua. Therefore, cultivating drought-tolerant variety is greatly necessary for its normal growth under water deficiency. However, at present, molecular knowledge on drought resistance is mostly unclear in chieh-qua. In the study, characteristics of two diverse genetic chieh-qua variety, A39 (drought-resistance) and H5 (drought-sensitivity), were analyzed. Under drought stress, H5 exerted high water loss rate, increased malonaldehyde (MDA) content, and decreased enzyme activity of glutathione peroxidase (GSH-PX) and superoxide dismutase (SOD) compared with A39. In addition, based on the transcriptome results, we obtained a total of 1821 (511 up-regulated and 1310 down-regulated) and 2114 (1282 up-regulated and 832 down- regulated) differentially expressed genes (DEGs) in the A39 versus H5 under normal and water-deficiency stress, respectively. Several DEGs involved in the cuticle synthesis (cytochrome P450 genes: CYP94A2, CYP86B1, CYP86A7), carbohydrate metabolism, and plant hormone signal transduction (small auxin-up RNA genes: SAUR32, SAUR72; JA-induced genes: TIFY 10A, TIFY 10C; ABA related genes: PYL2, PYL4) were explored and related to drought resistance. These expression patterns observed in the RNA-seq data were further confirmed with quantitative real-time PCR (qRT-PCR). In all, these results not only provided a new insight into analyzing genes of drought response, but also laid a foundation for isolating crucial genes involved in drought stress in chieh-qua.

Heat stress (HS) negatively influences plant development and growth, especially production and quality. Cucumber is a widely cultivated plant in the gourd family Cucurbitaceae that is often exposed to high temperatures during summer and protected cultivation. In this study, we performed whole-genome re-sequencing of two pools, one heat-tolerant and one heat-sensitive, of the F2 population derived from L-9 (heat-resistant) and A-16 (heat-sensitive). The genetic analysis showed that the heat resistance of L-9 cucumber seedlings was controlled by a single recessive gene. By combining bulked segregant analysis (BSA) technology, the crucial gene related to HS was preliminarily mapped to a 1.08-Mb region on chromosome 1. To fine-map the locus, Indel markers were designed according to the genomic sequence. Finally, the gene was narrowed to a 550-kb region flanked by two Indel markers, namely Indel-H90 and Indel-H224, that contained 56 candidate genes. Re-sequencing results indicated that 10 candidate genes among the 56 in the candidate region showed single base pair differences in the exons. Quantitative reverse-transcription polymerase chain reaction showed that 6 genes among the 10 candidate genes were significantly decreased when exposed to high temperatures. These results not only were useful for the isolation and characterization of the key genes involved in HS but also provided a basis for understanding the mechanism of heat tolerance regulation.

Background Photosynthesis is a fundamental process for plant growth and development dependent on a precise network, including formation of chloroplast and chlorophyll synthesis. Chloroplast development deficiency could lead to albinism in higher plant. Results Here, we report a cucumber albino recessive mutant that processed white cotyledons under light condition and is unable to produce first true leaf. Meanwhile, albino mutant could grow out creamy green cotyledons under dark condition but died after exposing to light. Using fluorescence microscopy and transmission electron microscope (TEM), impaired chloroplasts were observed. We identified 7 and 3 differentially expressed genes (DEG) involved in Chlorophyll metabolism and Methylerythritol 4-phosphate (MEP) pathway through transcriptome analysis, respectively. We also examined the reported homologous genes for albino mutants from other plants. Two of 12 genes, TOC159 and DXS1, were up-regulated in cucumber albino mutants as well. The reliability of RNA sequencing results were further confirmed by real-time quantitative PCR (qPCR). Conclusions Taken together, we elaborate the differences between albino mutant and normal seedlings from a single cucumber progeny. This mutant is a new material to study protoplast development.

Temperature rising caused by global warming has imposed significant negative effects on crop qualities and yields. To get the well-known molecular mechanism upon the higher temperature, we carefully analyzed the RNA sequencing-based transcriptomic responses of two contrasting chieh-qua genotypes: A39 (heat-tolerant) and H5 (heat-sensitive). In this study, twelve cDNA libraries generated from A39 and H5 were performed with a transcriptome assay under normal and heat stress conditions, respectively. A total of 8705 differentially expressed genes (DEGs) were detected under normal conditions (3676 up-regulated and 5029 down-regulated) and 1505 genes under heat stress (914 up-regulated and 591 down-regulated), respectively. A significant positive correlation between RNA-Seq data and qRT-PCR results was identified. DEGs related to heat shock proteins (HSPs), ubiquitin-protein ligase, transcriptional factors, and pentatricopeptide repeat-containing proteins were significantly changed after heat stress. Several genes, which encoded HSPs (CL2311.Contig3 and CL6612.Contig2), cytochrome P450 (CL4517.Contig4 and CL683.Contig7), and bHLH TFs (CL914.Contig2 and CL8321.Contig1) were specifically induced after four days of heat stress. DEGs detected in our study between these two contrasting cultivars would provide a novel basis for isolating useful candidate genes of heat stress responses in chieh-qua.

Fusarium wilt results in undesirable effects on the quality and production of chieh-qua (Benincasa hispida Cogn. var. Chieh-qua How). Fusaric acid (FA), a secondary metabolite of biotin produced by pathogens of genus Fusarium, induced resistant responses in chieh-qua, however, the physiological and molecular mechanism(s) of FA resistance remains largely unknown. In our study, ‘A39’ (FA-resistant cultivar) exhibited decreased malondialdehyde (MDA) content and increased superoxide dismutase (SOD) enzyme activity when exposed to FA compared with ‘H5’ (FA-susceptible cultivar). More apoptosis cells existed in ‘H5’ than ‘A39’ after 2 days of FA treatment. RNA-seq results revealed that a total of 2968 and 3931 differentially expressed genes (DEGs) were detected under normal conditions (1562 up-regulated and 1406 down-regulated) and FA treatment (2243 up-regulated and 1688 down-regulated), respectively. Interestingly, DEGs associated with pathogen-related protein and ethylene (ET) biosynthesis and signal pathways were most significantly changed during FA stress. Notably, several crucial genes encoding pathogenesis-related protein (CL4451.Contig2, CL2175.Contig4), peroxidase (Unigene49615 and CL11695.Contig2), and ET-responsive transcription factors (TFs) (CL9320.Contig1, CL9849.Contig3, CL6826.Contig2, CL919. Contig6, and CL518.Contig7) were specifically induced after FA treatment. Collectively, the study provides molecular data for isolating candidate genes involved in FA resistance, especially ET related genes in chieh-qua.

As an important fruit vegetable, fruit-related traits have become one of the breeding hotspots of wax gourd (Benincasa hispida). The fruit traits have an important role on the quality and yield of wax gourd, and QTL mapping of these traits will provide solid basis for fruit character improvement as well as high-yield breeding. In this study, the inheritance of four fruit-related traits, including fruit weight (FW), fruit length (FL), fruit diameter (FD) and flesh thickness (FT), was analyzed. All the frequency distribution of the four traits in an F2 population showed a single peak skewed distribution, which proved them to be quantitative. Subsequently, based on the high-density genetic map using 140 F2 individuals, nine QTLs associated with the four traits were detected on chromosome 3, 4, 5, 6, 9, 10 and 11. Four of the nine QTLs had major effect, which were responsible for more than 10.0% of phenotypic variance. Furthermore, the major QTLs of FW, FD and FT shared similar location intervals, which implied that this location might contain some pleiotropic genes. This is the first report on QTL mapping of quantitative trait in wax gourd, which plays an important role for fine mapping of these important fruit traits.

Drought stress is one of the most serious threats to cucumber quality and yield. To gain a good understanding of the molecular mechanism upon water deficiency, we compared and analyzed the RNA sequencing-based transcriptomic responses of two contrasting cucumber genotypes, L-9 (drought-tolerant) and A-16 (drought-sensitive). In our present study, combining the analysis of phenotype, twelve samples of cucumber were carried out a transcriptomic profile by RNA-Seq under normal and water-deficiency conditions, respectively. A total of 1008 transcripts were differentially expressed under normal conditions (466 up-regulated and 542 down-regulated) and 2265 transcripts under drought stress (979 up-regulated and 1286 down-regulated). The significant positive correlation between RNA sequencing data and a qRT-PCR analysis supported the results found. Differentially expressed genes (DEGs) involved in metabolic pathway and biosynthesis of secondary metabolism were significantly changed after drought stress. Several genes, which were related to sucrose biosynthesis (Csa3G784370 and Csa3G149890) and abscisic acid (ABA) signal transduction (Csa4M361820 and Csa6M382950), were specifically induced after 4 days of drought stress. DEGs between the two contrasting cultivars identified in our study provide a novel insight into isolating helpful candidate genes for drought tolerance in cucumber.

This study aims to investigate key genes involved in molecular regulatory networks of cucumber sex determination. Genome-wide high-throughput RNA sequencing was performed for young apical buds of gynoecious and weak female cucumber at three growth stages (one-leaf one-bud, three-leaf one-bud, and five-leaf one-bud). Seven comparisons from the same cultivar at three different stages and at the same stage between the two cultivars were analyzed, and the results revealed that compared with differentially expressed genes (DEGs) in weak female cucumber, more genes were upregulated at the one-leaf one-bud stage and downregulated at the three-leaf one-bud stage in gynoecious cucumber. In addition, there were four kinds of gene expression trends (0, 1, 6, and 7), which were significantly enriched in gynoecious cucumber, while only two kinds of gene expression trends (5 and 6) were significantly enriched in weak female cucumber. Together with the data of the Gene Ontology (GO), pathway, gene expression trends and qRT-PCR, nine genes were identified and considered as candidate genes that may be involved in sex differentiation regulation in cucumber. These genes included Cs-MCM6, Cs-ACT3, Cs-XRCC4, Cs-MCM2, Cs-CDC45, Cs-Dpri, Cs-H2B, Cs-CDC20 and Cs-CNGC1. Among these genes, five genes (Cs-MCM6, Cs-MCM2, Cs-CDC45, Cs-Dpri, and Cs-CDC20) were involved in the cell cycle pathway, suggesting that the cell cycle pathway may play an important role in sex determination in cucumber.

The WRKY transcription factors play an important role in plant resistance for biotic and abiotic stresses. In the present study, we cloned 10 WRKY gene homologs (CqWRKY) in Chieh-qua (Benincasa hispida Cogn. var. Chieh-qua) using the rapid-amplification of cDNA ends (RACE) or homology-based cloning methods. We characterized the structure of these CqWRKY genes. Phylogenetic analysis of these sequences with cucumber homologs suggested possible structural conservation of these genes among cucurbit crops. We examined the expression levels of these genes in response to fusaric acid (FA) treatment between resistant and susceptible Chieh-qua lines with quantitative real-time PCR. All genes could be upregulated upon FA treatment, but four CqWRKY genes exhibited differential expression between resistant and susceptible lines before and after FA application. CqWRKY31 seemed to be a positive regulator while CqWRKY1, CqWRKY23 and CqWRKY53 were negative regulators of fusaric resistance. This is the first report of characterization of WRKY family genes in Chieh-qua. The results may also be useful in breeding Chieh-qua for Fusarium wilt resistance.

Pumpkin (Cucurbita moschata Duch.) is an important vegetable crop cultivated worldwide. In this study, the pumpkin transcriptome was sequenced by RNA-seq using the Illumina Hiseq 2000. A total of 52,849,316 clean sequencing reads, 66,621 contigs and 62,480 unigenes were postulated. Based on similarity searches with known proteins, 47,899 genes (76.66 % of the unigenes) were annotated: 47,596, 34,368 and 16,700 mapped in Nr, Swissprot and COG classifica- tions, respectively; 21,164 were annotated with 44 gene ontology functional categories; and 13,728 were anno- tated to 269 pathways by searching the Kyoto Encyclo- pedia of Genes and Genomes pathway database. A total of 7,814 simple sequence repeats (SSRs) were identified in these unigenes and 4,794 pairs of primers were designed for application of SSRs. To date, 35 SSRs have been validated in 12 pumpkin varieties and can separate the pumpkin varieties into Cucurbita maxima and Cucurbita moschata. In addition, the expression of eight photoperiod-related unigenes were studied in different pumpkin plants and it was deduced that they may contribute to late flowering and light insensitive- ness. This research will provide an important platform to facilitate gene discovery for functional genome studies of pumpkin and to conduct SSR discovery for breeders for use in pumpkin breeding.