Candidate genes underlying the quantitative trait loci for root-knot nematode resistance in a Cucumis hystrix introgression line of cucumber based on population sequencing

Transcriptome Analysis Reveals the Molecular Mechanism of the Leaf Yellowing in Allotriploid Cucumber

Yellowing leaves are ideal materials for studying the metabolic pathways of photosynthetic pigment chloroplast development, and the mechanism of photosynthetic systems. Here, we obtained a triploid material HCC (2n = 3x = 26), which was derived from hybridization between the artificial tetraploid Cucumis × hytivus (2n = 4x = 38, HHCC) and the cultivated cucumber Cucumis sativus (2n = 2x = 14, CC), and this triploid HCC showed obvious leaf yellowing characteristics. Phenotypic observation results showed that chloroplast development was impaired, the chlorophyll content decreased, and photosynthesis decreased in yellowing HCC leaves. The transcriptome results indicated that HCC-GLK is significantly downregulated in HCC and participates in the regulation of leaf yellowing. GO enrichment analysis revealed that differential genes were enriched in the heme binding and tetrapyrrole binding pathways related to leaf color. KEGG enrichment analysis revealed that differential genes were predominantly enriched in photosynthesis-related pathways. The experimental results of VIGS and yeast hybridization showed that silencing the GLK gene can induce leaf yellowing in cucumber plants, and the GLK protein can affect plant chloroplast development by interacting with the CAB3C protein (light-harvesting chlorophyll a/b binding) in the plant chlorophyll synthesis pathway. The current findings have not only enhanced our understanding of the regulatory mechanism of the GLK transcription factor in cucumber but also introduced novel insights and directions for investigating the molecular mechanism underlying polyploid leaf yellowing.

Molecular Markers for Marker-Assisted Breeding for Biotic and Abiotic Stress in Melon (Cucumis melo L.): A Review

Melon (Cucumis melo L.) is a globally grown crop renowned for its juice and flavor. Despite growth in production, the melon industry faces several challenges owing to a wide range of biotic and abiotic stresses throughout the growth and development of melon. The aim of the review article is to consolidate current knowledge on the genetic mechanism of both biotic and abiotic stress in melon, facilitating the development of robust, disease-resistant melon varieties. A comprehensive literature review was performed, focusing on recent genetic and molecular advancements related to biotic and abiotic stress responses in melons. The review emphasizes the identification and analysis of quantitative trait loci (QTLs), functional genes, and molecular markers in two sections. The initial section provides a comprehensive summary of the QTLs and major and minor functional genes, and the establishment of molecular markers associated with biotic (viral, bacterial, and fungal pathogens, and nematodes) and abiotic stress (cold/chilling, drought, salt, and toxic compounds). The latter section briefly outlines the molecular markers employed to facilitate marker-assisted backcrossing (MABC) and identify cultivars resistant to biotic and abiotic stressors, emphasizing their relevance in strategic marker-assisted melon breeding. These insights could guide the incorporation of specific traits, culminating in developing novel varieties, equipped to withstand diseases and environmental stresses by targeted breeding, that meet both consumer preferences and the needs of melon breeders.

Impact of Cucurbita moschata Resistant Rootstocks on Cucumis sativus Fruit and Meloidogyne incognita Development

Plant grafting can provide resistance to nematodes. There is a distinct need to determine the role of Meloidogyne incognita-resistant rootstocks on the growth and quality of grafted cucumber plants. Cucumber (Cucumis sativus L.) cultivar Jinchun No. 4 (J) was hole grafted onto the pumpkin (Cucurbita moschata) cultivars Xiuli (X), Banzhen No. 3 (B), and its root to generate JX, JB, and JJ plants. The histopathology and M. incognita development associated with JX, JB, and JJ were analyzed under incubator and high plastic tunnel conditions. Under incubator conditions, M. incognita root galls and egg mass indices associated with the JX and JB resistant rootstocks were significantly (P < 0.05) lower than those associated with JJ susceptible rootstocks. In addition, the number of eggs were 73.3 ± 8.8% and 85.3 ± 7.7% less, respectively. The number of second-stage juveniles (J2s) in JX roots decreased by 57.1 ± 9.2% compared with that in JJ, and the giant cell and J2 development were poor in JX and JB roots. In pot experiments under a high plastic tunnel, plant height, stem diameter, leaf area, and yield of M. incognita-infected JX plants were not significantly different from those of noninoculated control. There was no significant difference in fruit weight, length, firmness, soluble solids, and color among the three grafted plants. The yield per JB plant was increased compared with that of JJ, irrespective of nematode presence. In the M. incognita-infested soil experiment in a high plastic tunnel, the yield per JX and JB plant were significantly higher than JJ (P < 0.05). Thus, the pumpkin rootstock Xiuli and Banzhen No. 3 are promising rootstocks for managing M. incognita without affecting cucumber fruit quality. Grafting provides a good basis for studying the defense mechanism of rootstocks against M. incognita.

Characterization of the mitochondrial genome of Cucumis hystrix and comparison with other cucurbit crops

The mitochondria ofCucumis genus contain several intriguing features such as paternal inheritance and three-ring genome structure. However, the evolutionary relationships of mitochondria inCucumisremain elusive. Here, we assembled the mitochondrial genome ofC. hystrixand performed a comparative genomic analysis with other crops inthe Cucurbitaceae. The mitochondrial genome ofC. hystrixhas three circular-mapping chromosomes of lengths 1,113,461 bp, 110,683 bp, and 92,288 bp, which contain 73 genes including 38 protein-coding genes, 31tRNAgenes, and 4rRNAgenes. Repeat sequences, RNA editing, and horizontal gene transfer events were identified. The results of phylogenetic analyses, collinearity and gene clusters revealed thatC. hystrixis closer toC. sativus than to C. melo. Meanwhile, wedemonstrated mitochondrial paternal inheritance inC. hystrixbymolecular markers. In comparison with other cucurbitcrops, wefound amarker foridentification of germplasm resources ofCucumis. Collectively, our findings provide a tool to help clarify the paternal lineage within that genus in the evolution of Cucumis.

Negative regulation of root-knot nematode parasitic behavior by root-derived volatiles of wild relatives of Cucumis metuliferus CM3

Root-knot nematodes (RKN; Meloidogyne spp.) cause a significant decrease in the yield of cucumber crops every year. Cucumis metuliferus is an important wild germplasm that has resistance to RKN in which plant root volatiles are thought to play a role. However, the underlying molecular mechanism is unclear. To investigate it, we used the resistant C. metuliferus line CM3 and the susceptible cucumber line Xintaimici (XTMC). CM3 roots repelled Meloidogyne incognita second-stage larvae (J2s), while the roots of XTMC plants attracted the larvae. CM3 and XTMC were found to contain similar amounts of root volatiles, but many volatiles, including nine hydrocarbons, three alcohols, two aldehydes, two ketones, one ester, and one phenol, were only detected in CM3 roots. It was found that one of these, (methoxymethyl)-benzene, could repel M. incognita, while creosol and (Z)-2-penten-1-ol could attract M. incognita. Interestingly, creosol and (Z)-2-penten-1-ol effectively killed M. incognita at high concentrations. Furthermore, we found that a mixture of CM3 root volatiles increased cucumber resistance to M. incognita. The results provide insights into the interaction between the host and plant-parasitic nematodes in the soil, with some compounds possibly acting as nematode biofumigation, which can be used to manage nematodes.

Transcriptomic and Histological Analysis of the Response of Susceptible and Resistant Cucumber to Meloidogyne incognita Infection Revealing Complex Resistance via Multiple Signaling Pathways

The root-knot nematode (RKN), Meloidogyne incognita, is a devastating pathogen for cucumber (Cucumis sativus L.) specially in production under protected environments or continuous cropping. High level RKN resistance has been identified in African horned melon Cucumis metuliferus (CM). However, the resistance mechanism remains unclear. In this study, the comparative analysis on phenotypic and transcriptomic responses in the susceptible cucumber inbred line Q24 and the resistant CM, after M. incognita infection, was performed. The results showed that, in comparison with Q24, the CM was able to significantly reduce penetration numbers of second stage juveniles (J2), slow its development in the roots resulting in fewer galls and smaller giant cells suggesting the presence of host resistance in CM. Comparative transcriptomes analysis of Q24 and CM before and after M. incognita infection was conducted and differentially expressed genes (DEGs) associated with host resistance were identified in CM. Enrichment analyses revealed most enriched DEGs in Ca²⁺ signaling, salicylic acid (SA)/jamonate signaling (JA), as well as auxin (IAA) signaling pathways. In particular, in CM, DEGs in the Ca²⁺ signaling pathway such as those for the calmodulin and calcium-binding proteins were upregulated at the early stage of M. incognita infection; genes for SA/JA synthesis/signal transduction were markedly activated, whereas the IAA signaling pathway genes were inhibited upon infection suggesting the importance of SA/JA signaling pathways in mediating M. incognita resistance in CM. A model was established to explain the different molecular mechanisms on M. incognita susceptibility in cucumber and resistance to M. incognita infection in CM.

Global Profiling of lncRNAs Expression Responsive to Allopolyploidization in Cucumis

Long non-coding RNAs (lncRNAs) play critical regulatory roles in various biological processes. However, the presence of lncRNAs and how they function in plant polyploidy are still largely unknown. Hence, we examined the profile of lncRNAs in a nascent allotetraploid Cucumis hytivus (S₁₄), its diploid parents, and the F₁ hybrid, to reveal the function of lncRNAs in plant-interspecific hybridization and whole genome duplication. Results showed that 2206 lncRNAs evenly transcribed from all 19 chromosomes were identified in C. hytivus, 44.6% of which were from intergenic regions. Based on the expression trend in allopolyploidization, we found that a high proportion of lncRNAs (94.6%) showed up-regulated expression to varying degrees following hybridization. However, few lncRNAs (33, 2.1%) were non-additively expressed after genome duplication, suggesting the significant effect of hybridization on lncRNAs, rather than genome duplication. Furthermore, 253 cis-regulated target genes were predicted for these differentially expressed lncRNAs in S₁₄, which mainly participated in chloroplast biological regulation (e.g., chlorophyll synthesis and light harvesting system). Overall, this study provides new insight into the function of lncRNAs during the processes of hybridization and polyploidization in plant evolution.

Tomato and Melon Meloidogyne Resistant Rootstocks Improve Crop Yield but Melon Fruit Quality Is Influenced by the Cropping Season

Four rotation sequences consisting of ungrafted tomato cv. Durinta - melon cv. Paloma or tomato grafted onto the resistant rootstock 'Aligator' - melon grafted onto the resistant Cucumis metuliferus accession BGV11135, and in reverse order, were conducted from 2015 to 2017 in a plastic greenhouse infested or not with Meloidogyne incognita to determine the plant tolerance (T), the minimum relative crop yield (m) and fruit quality. The relationship between M. incognita densities in soil at transplanting (Pi) of each crop and the crop yield was assessed and T and m were estimated by the Seinhorst's damage model. In addition, the volume and the number of nuclei of single giant cells and the number of giant cells, its volume and the number of nuclei per feeding site in susceptible tomato and melon were compared to those in the resistant tomato and C. metuliferus 15 days after nematode inoculation in pot test. The relationship between the Pi and the relative crop yield fitted the Seinhorst's damage model in both ungrafted and grafted tomato and melon, but not for all years and cropping seasons. The estimated T for ungrafted and grafted tomato did not differ but m was lower in the former (34%) than the latter (67%). Sodium concentration in fruits from ungrafted but not from grafted tomato increased with nematode densities in spring 2015 and 2016. The estimated ungrafted melon T did not differ from the grafted melon cultivated in spring, but it did when it was cultivated in summer. The relative crop yield of ungrafted melon was lower (2%) than the grafted cultivated in spring (62%) and summer (20%). Sodium concentration in melon fruits from ungrafted plants increased with nematode densities. No variations in fruit quality from grafted melon cultivated in spring were found, although less dry matter and soluble solid content at highest nematode densities were registered when it was cultivated in summer. Lower number of giant cells per feeding site was observed in both susceptible tomato germplasms compared to the resistant ones but they were more voluminous and held higher number of nuclei per giant cell and per feeding site.