CsPM5.2, a phosphate transporter protein-like gene, promotes powdery mildew resistance in cucumber.

Powdery mildew (PM) is one of the most serious fungal diseases affecting cucumbers (Cucumis sativus L.). The mechanism of PM resistance in cucumber is intricate and remains fragmentary as it is controlled by several genes. In this study, we detected the major-effect Quantitative Trait Locus (QTL), PM5.2, involved in PM resistance by QTL mapping. Through fine mapping, the dominant PM resistance gene, CsPM5.2, was cloned and its function was confirmed by transgenic complementation and natural variation identification. In cultivar 9930, a dysfunctional CsPM5.2 mutant resulted from a single nucleotide polymorphism in the coding region and endowed susceptibility to PM. CsPM5.2 encodes a phosphate transporter-like protein PHO1; H3. The expression of CsPM5.2 is ubiquitous and induced by the PM pathogen. In cucumber, both CsPM5.2 and Cspm5.1 (Csmlo1) are required for PM resistance. Transcriptome analysis suggested that the salicylic acid (SA) pathway may play an important role in CsPM5.2-mediated PM resistance. Our findings help parse the mechanisms of PM resistance and provide strategies for breeding PM-resistant cucumber cultivars.

Comparative Transcriptome Analysis of Hard and Tender Fruit Spines of Cucumber to Identify Genes Involved in the Morphological Development of Fruit Spines.

The spines of cucumber fruit not only have important commercial value but are also a classical tissue to study cell division and differentiation modes of multicellular trichomes. It has been reported that CsTs (C-type Lectin receptor-like kinase) can influence the development of fruit spines. In this study, we took a pair of cucumber materials defined as hard (Ts, wild type) and tender spines (ts, mutant) and defined the developmental process of fruit spines as consisting of four stages (stage I to stage IV) by continuously observing by microscope and SEM. Comparisons of transcriptome profiles at different development stages of wild-type spines showed that 803 and 722 genes were upregulated in the stalk (stage II and stage III) and base (stage IV) development stages of fruit spines, respectively. The function analysis of DEGs showed that genes related to auxin polar transport and HD-ZIP transcription factor are significantly upregulated during the development of the stalk. bHLH transcription factors and cytoskeleton-related genes were significantly upregulated during the development of the base. In addition, stage III is the key point for the difference between wild-type and mutant spines. We detected 628 DEGs between wild type and mutant at stage III. These DEGs are mainly involved in the calcium signaling of the cytoskeleton and auxin polar transport. Coincidentally, we found that CsVTI11, a factor involved in auxin signal transmission, can interact with CsTs in vivo, but this interaction does not occur between CsVTI11 and Csts, further suggesting that CsTs may regulate the development of fruit spines by influencing cell polarity. These results provide useful tools to study the molecular networks associated with cucumber fruit spine development and elucidate the biological pathways that C-type Lectin receptor-like kinase plays in regulating the development of fruit spines.

Genome-Wide Identification and Characterization of Lectin Receptor-Like Kinase Gene Family in Cucumber and Expression Profiling Analysis under Different Treatments.

Lectin receptor-like kinases (LecRLKs) are a class of membrane proteins found in plants that are involved in diverse functions, including plant development and stress responses. Although LecRLK families have been identified in a variety of plants, a comprehensive analysis has not yet been undertaken in cucumber (Cucumis sativus L.). In this study, 46 putative LecRLK genes were identified in the cucumber genome, including 23 G-type and 22 L-type, and one C-type LecRLK gene. They were unequally distributed on all seven chromosomes, with a clustering tendency. Most of the genes in the cucumber LecRLK (CsLecRLK) gene family lacked introns. In addition, there were many regulatory elements associated with phytohormones and stress on these genes' promoters. Transcriptome data demonstrated distinct expression patterns of CsLecRLK genes in various tissues. Furthermore, we found that each member of the CsLecRLK family had its own unique expression pattern under hormone and stress treatment by the quantitative real-time PCR (qRT-PCR) analysis. This study provides a better understanding of the character and function of the LecRLK gene family in cucumber and opens up the possibility to exploring the roles that LecRLKs might play in the life cycle of cucumber.

Mapping and identification of CsUp, a gene encoding an Auxilin-like protein, as a putative candidate gene for the upward-pedicel mutation (up) in cucumber.

BACKGROUND: Pedicel orientation can affect the female flower orientation and seed yield in cucumber. A spontaneous mutant possessing upward growth of pedicels was identified in the wild type inbred strain 9930 and named upward-pedicel (up). The morphological and genetic analyses of up were performed in this study. In order to clone the up gene, 933 F2 individuals and 524 BC1 individuals derived from C-8-6 (WT) and up were used for map-based cloning. RESULTS: up was mapped to a 35.2 kb physical interval on chromosome 1, which contains three predicted genes. Sequencing analysis revealed that a 5-bp deletion was found in the second exon of Csa1G535800, and it led to a frameshift mutation resulting in a premature stop codon. The candidate gene of CsUp (Csa1G535800) was further confirmed via genomic and cDNA sequencing in biparental and natural cucumber populations. Sequencing data showed that a 4-bp deletion was found in the sixth exon of Csa1G535800 in CGN19839, another inbred line, and there was also a mutation of an amino acid in Csa1G535800 that could contribute to the upward growth of pedicels in CGN19839. Moreover, it was found that Csa1G535800 exhibited strong expression in the pedicel of WT, suggesting its important role in development of pedicel orientation. Thus, Csa1G535800 was considered to be the candidate gene of CsUp. CONCLUSIONS: CsUp encodes an Auxilin-like protein and controls pedicel orientation in cucumber. The identification of CsUp may help us to understand the mechanism of pedicel orientation development and allow for investigation of novel functions of Auxilin-like proteins in cucumber.

A High-Density Genetic Linkage Map for Cucumber (Cucumis sativus L.): Based on Specific Length Amplified Fragment (SLAF) Sequencing and QTL Analysis of Fruit Traits in Cucumber.

High-density genetic linkage map plays an important role in genome assembly and quantitative trait loci (QTL) fine mapping. Since the coming of next-generation sequencing, makes the structure of high-density linkage maps much more convenient and practical, which simplifies SNP discovery and high-throughput genotyping. In this research, a high-density linkage map of cucumber was structured using specific length amplified fragment sequencing, using 153 F2 populations of S1000 × S1002. The high-density genetic map composed 3,057 SLAFs, including 4,475 SNP markers on seven chromosomes, and spanned 1061.19 cM. The average genetic distance is 0.35 cM. Based on this high-density genome map, QTL analysis was performed on two cucumber fruit traits, fruit length and fruit diameter. There are 15 QTLs for the two fruit traits were detected.

Identification and mapping of Tril, a homeodomain-leucine zipper gene involved in multicellular trichome initiation in Cucumis sativus.

KEY MESSAGE: Using map-based cloning of Tril gene, we identified a homeodomain-leucine zipper gene involved in the initiation of multicellular trichomes (including the spines of fruit) in cucumber. ABSTRACT: Fruit spines are a special type of trichome that impacts the quality and appearance of cucumber (Cucumis sativus L.) fruit. Scanning electron microscopy revealed that the trichome-less (tril) mutant originating from European greenhouse cucumber has a completely glabrous phenotype on cotyledons, hypocotyls, young leaves, fruits, and fruit stalks. Genetic analysis revealed that tril was inherited as a recessive allele at a single locus. Using 1058 F2 individuals derived from a cross between cucumber tril mutant CGN19839 and the micro-trichome (mict) mutant 06-2, tril was mapped to chromosome 6, and narrowed down to a 37.4 kb genomic region which carries seven predicted genes. Genetic and molecular analyses revealed that gene Cucsa.045360 is a possible candidate gene for the differentiation of epidermal cells to trichomes. It is a member of the class IV homeodomain-leucine zipper (HD-Zip IV) family and encodes homeodomain and START domain, sharing 66.7% predicted amino acid sequence identity to PROTODERMAL FACTOR2 (PDF2) and 35.0% to GLABRA2 (GL2) of Arabidopsis. The homeobox domain had changed amino acid sequence because of an insertion in tril mutant. The results of genetic analysis and transcriptome profiling indicated that the Tril gene had an epistatic effect on the Mict gene in trichome development. Phenotypes of the tril mutant such as glabrous fruits and female flowers at every node could be used in developing new cultivars.

Transcriptome profiling of trichome-less reveals genes associated with multicellular trichome development in Cucumis sativus.

Trichomes on plants, similar to fine hairs on animal and human bodies, play important roles in plant survival and development. They also represent a useful model for the study of cell differentiation. Although the regulatory gene network of unicellular trichome development in Arabidopsis thaliana has been well studied, the genes that regulate multicellular trichome development remain unclear. We confirmed that Cucumis sativus (cucumber) trichomes are multicellular and unbranched, but identified a spontaneous mutant, trichome-less (tril), which presented a completely glabrous phenotype. We compared the transcriptome profilings of the tril mutant and wild type using the Illumina HiSeq 2000 sequencing technology. A total of 991 genes exhibited differential expression: 518 were up-regulated and 473 were down-regulated. We further identified 62 differentially expressed genes that encoded crucial transcription factors and were subdivided into seven categories: homeodomain, MADS, MYB, and WRKY domains, ethylene-responsive, zinc finger, and other transcription factor genes. We further analyzed the tissue-expression profiles of two candidate genes, GLABRA2-like and ATHB51-like, using qRT-PCR and found that these two genes were specifically expressed in the epidermis and trichomes, respectively. These results and the tril mutant provide useful tools to study the molecular networks associated with multicellular trichome development.

Micro-trichome as a class I homeodomain-leucine zipper gene regulates multicellular trichome development in Cucumis sativus.

Plant trichomes serve as a highly suitable model for investigating cell differentiation at the single-cell level. The regulatory genes involved in unicellular trichome development in Arabidopsis thaliana have been intensively studied, but genes regulating multicellular trichome development in plants remain unclear. Here, we characterized Cucumis sativus (cucumber) trichomes as representative multicellular and unbranched structures, and identified Micro-trichome (Mict), using map-based cloning in an F2 segregating population of 7,936 individuals generated from a spontaneous mict mutant. In mict plants, trichomes in both leaves and fruits, are small, poorly developed, and denser than in the wild type. Sequence analysis revealed that a 2,649-bp genomic deletion, spanning the first and second exons, occurred in a plant-specific class I homeodomain-leucine zipper gene. Tissue-specific expression analysis indicated that Mict is strongly expressed in the trichome cells. Transcriptome profiling identified potential targets of Mict including putative homologs of genes known in other systems to regulate trichome development, meristem determinacy, and hormone responsiveness. Phylogenic analysis charted the relationships among putative homologs in angiosperms. Our paper represents initial steps toward understanding the development of multicellular trichomes.

Transcriptome analysis in Cucumis sativus identifies genes involved in multicellular trichome development.

The regulatory gene network of unicellular trichome development in Arabidopsis thaliana has been studied intensively, but that of multicellular remains unclear. In the present study, we characterized cucumber trichomes as representative multicellular and unbranched structures, but in a spontaneous mutant, mict (micro-trichome), all trichomes showed a micro-size and stunted morphologies. We revealed the transcriptome profile using Illumina HiSeq 2000 sequencing technology, and determined that a total of 1391 genes exhibited differential expression. We further validated the accuracy of the transcriptome data by RT-qPCR and found that 43 genes encoding critical transcription factors were likely involved in multicellular trichome development. These 43 candidate genes were subdivided into seven groups: homeodomain, MYB-domain, WRKY-domain, bHLH-domain, ethylene-responsive, zinc finger and other transcription factor genes. Our findings also serve as a powerful tool to further study the relevant molecular networks, and provide a new perspective for investigating this complex and species-specific developmental process.

Construction of a high density integrated genetic map for cucumber (Cucumis sativus L.).

The high-density consensus map was constructed based on the GY14 × PI 183967 map from an inter-subspecific cross and the extended S94 × S06 map from an intra-subspecific cross. The consensus map was composed of 1,369 loci, including 1,152 SSR loci, 192 SRAP loci, 21 SCAR loci and one STS locus as well as three gene loci of fruit external quality traits in seven chromosomes, and spanned 700.5 cM, of which 682.7 cM (97.5%) were covered by SSR markers. The average genetic distance and physical interval between loci were 0.51 cM and ~268 kbp, respectively. Additionally, the physical position of the sequence-associated markers aligned along the assembled cucumber genome sequence established a relationship between genetic maps and cucumber genome sequence and to a great extent validated the order of markers in individual maps and consensus map. This consensus map with a high marker density and well-ordered markers is a saturated and reliable linkage map for genetic analysis of cucumber or the Cucurbitaceae family of plants.

[Expression analysis of the unisex-determine gene M in cucumber].

Sex determination in plants has various mechanism; however, a single gene locus controlling unisexual expression is unique in the Cucurbitaceae plants, particularly cucumber (Cucumis sativus L.) and melon (Cucumis melo L.). In this study, with quantitative RT-PCR method, two sets of near-isogenic lines (NILs) were used to analyze the expression pattern of gene CsACS2 (GenBank accession number FJ529216). Additionally, chemical applications (AgNO3 and AVG) were used to investigate the effect of the plant endogenous ethylene on CsACS2 expression. Expression analysis reveals that endogenous ethylene, which might be derived from F or M itself, could activate the expression of M gene.

[Variations in chromosomal number of regenerated melon plants from somatic embryos].

Chromosomal number of different of somatic embryos regenerated plants were investigated in melon variety "xiaomaigua" and "Qingpilurou". Certain variations of chromosomal number were found among the regenerated plants compared with normal sample,and range of variation covered from 2n = 23-24 to 2n = 13-48 with the increase of generation,the rate from 3.3% to 30%. The results indicated that degree of variation in chromosomal number of somatic embryos regenerated melon plants increased with the time of culture, and those cultured in one to two months had the least variation. It was also found that degree of chromosomal number variations varied with melon varieties.