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Cucumber is one of the most widely consumed vegetables worldwide, and the fruit spine is an important fruit quality trait. Expansins play critical roles in fruit development; however, the regulation of expansins in cucumber fruit spine development has not been reported. In this study, 33 expansin genes were identified in the cucumber genome V3; additionally, expansin genes in Citrullus lanatus, Cucumis melo, Cucurbita maxima, Lagenaria siceraria, and Benincasa hispida were also identified. Phylogenetic analysis of expansin proteins in Cucurbitaceae and Arabidopsis showed that they evolved separately in each plant species. Phylogenetic analysis showed that C. maxima was derived earlier than the other five Cucurbitaceae species. The expression of CsEXPA2, CsEXPA14, and CsEXLA3 varied in cucumber lines with different fruit spine densities. A yeast two-hybrid assay showed that a putative auxin transporter encoded by numerous spine gene (ns) interacts with CsEXLA2, which may be involved in the development of the numerous spines in cucumber. These results provide novel insights into the expansins related to plant development and fruit spine development in cucumber.

Watermelon (Citrullus lanatus) is a succulent fruit and vine-like plant that is cultivated in Mexico and it generates employment and currency for the country. However, there is the need to research what local organic substrates can substitute peat moss as a culture medium to produce watermelon seedlings of good quality and at low cost. The objective of this study was to evaluate the physical and chemical properties of five local organic substrates as substitutes of the commercial substrate “Peat Moss”, for the production of seedlings of two watermelon cultivars, Sun Sweet and Jubilee. Five local organic substrates were studied: cacao husk, compost, vermicompost, bovine manure, coconut fiber and the commercial substrate “Peat Moss” as control. The response variables were percentage of germination, indicators of morphological quality and morphological quality indexes, stability of the clod, and relative efficiency of the local substrates. The best morphological indicators and morphological quality index of the seedlings were found with the substrates cacao husk and vermicompost, with a seedling quality similar to those obtained with the commercial substrate. Compost presented the lowest stability of the clod and relative efficiency. The substrates of cacao husk and vermicompost can substitute the commercial substrate “Peat Moss”, in addition to being easy to obtain and of low cost; so they are a viable alternative for rural farmers in the production of watermelon seedlings. RESUMO Melancia (Citrullus lanatus) é uma planta frutífera, ramadora, cultivada no México, gerando emprego e divisas para o país. Há necessidade de pesquisar substratos orgânicos locais ideais para substituir a turfa como meio de cultura visando produção de mudas de melancia de boa qualidade com baixo custo. Portanto, avaliou-se as propriedades físico-químicas de cinco substratos orgânicos locais (casca de cacau, composto, vermicomposto, esterco bovino, fibra de coco e o substrato comercial “Peat Moss” como controle) como substitutos do substrato comercial “Peat Moss”, para produzir mudas das cultivares Sun Sweet e Jubilee. As variáveis de resposta foram: porcentagem de germinação, indicadores de qualidade morfológica e índices de qualidade morfológica, estabilidade do torrão e eficiência relativa dos substratos locais. Os melhores indicadores morfológicos e índice de qualidade morfológica das mudas foram encontrados com os substratos casca de cacau e vermicomposto, com qualidade de mudas semelhantes às obtidas com o substrato comercial. O composto apresentou a menor estabilidade do torrão e eficiência relativa. Os substratos de casca de cacau e vermicomposto são adequados para substituir o substrato comercial “Peat Moss”, além de serem de fácil obtenção e de baixo custo, sendo uma alternativa viável para agricultores rurais na produção de mudas de melancia.

Watermelon (Citrullus lanatus) is one of the world's most important fruit crops, and China produces the most watermelons in the world. Recently, a watermelon variome consisting of 414 key resequenced accessions was reported. However, the genetic relationships and pedigree of Chinese watermelon varieties in the seed market remain unclear. In this study, 241 evenly distributed perfect single nucleotide polymorphisms (SNPs) derived from the watermelon variome were selected for variety identification. The diversity of 247 Chinese watermelon varieties was identified based on their SNP genotypes. The 247 watermelon varieties were clustered into five subpopulations: the East Asian ecotype, intermediate ecotype, small fruit with red flesh ecotype, small fruit with yellow flesh ecotype, and American ecotype. We further established the pedigree of four subpopulations, of which JingXinNo.1, ZaoChunHongYu, HuangXiaoYu and XiaoLan, and Sugarlee were the main doner of the East Asian ecotype, small fruit with red flesh ecotype, small fruit with yellow flesh ecotype, and American ecotype, respectively. Thirty-two core SNPs were selected and applied in watermelon variety identification. They were also validated by the Kompetitive allele-specific PCR (KASPar) platform. The present study furthered our understanding of the genetic relationships and pedigree of watermelon varieties in China, and will help to manage the plant variety protection in watermelon.

Citron watermelon (Citrullus lanatus var. citroides) is a multi-purpose cucurbit crop serving the food, feed and the pharmaceutical sectors. The fresh and dried young leaves, fruit and seed are sources of vital human nutrients, and unique phytochemical compounds with pharmacological and therapeutic values. Citron watermelon is indigenous to Africa but commercial products are not widely developed and the crop is largely neglected and underutilized in the region. This is attributed to lack of dedicated research and development efforts and well-defined value chains for citron watermelon genetic resources. The objective of this review was to document the nutritional and phytochemical compositions, pharmacological and therapeutic values of citron watermelon to aid future production, utilization, genetic conservation, research and product development. The review serves as a foundation information on the unique values of citron watermelon to guide future research and development. It recommends collaborative and detailed biochemical analysis efforts for product discovery and optimal use of health-promoting bioactive compounds using the diverse genetic resources available in Africa and elsewhere. This will enhance the commercial value and benefit sharing from citron watermelon in the horticulture, food and pharmaceutical industry.

Bottle gourd (Lagenaria siceraria (Molina) Standl.] and citron watermelon (Citrullus lanatus var. citroides (L.H. Bailey) Mansf. ex Greb.) are amongst the widely grown yet under-researched cucurbits genetic resources in sub-Saharan Africa (SSA). Fresh immature leaves, fruit and seed are used for food and medicinal purposes in the region. Furthermore, both species are used as rootstock and donors of novel genes for commercial production and breeding of sweet dessert watermelon cultivars. Despite their economic value and contribution to food and nutrition security in the region, bottle gourd and citron watermelon genetic resources remain under-utilized and largely un-explored for commercial product development. The objective of this review was to document the unique values of bottle gourd and citron watermelon for genetic improvement and development of value-added food and non-food products, and to highlight regional and global efforts on conservation, production, processing, commercialization and various enterprise development of these important indigenous species. The review discussed on the value of both crops as a rootstock to improve fruit yield and quality of cultivated watermelon. This is followed by important summaries on phenotypic and genetic variation of bottle gourd and citron watermelon genetic resources and implications for new cultivar design. Finally, the review highlighted value-added non-food and food-based products developed based on indigenous knowledge systems in SSA and availability and access to genetic resources of bottle gourd and citron watermelon for breeding, product design and deployment.

Watermelon is an important cucurbit vegetable crop grown in most of the United States. Phytophthora fruit rot of watermelon caused by Phytophthora capsici has been a major factor, limiting production for the past 15 years in the southeastern United States. The U.S. Department of Agriculture, Agricultural Research Service released five Phytophthora fruit rot-resistant germplasm lines for use in breeding programs. These lines were developed by phenotyping using a local isolate of P. capsici from South Carolina. The present study was undertaken to determine if these resistant lines had broad resistance to diverse P. capsici isolates collected from different states and crops. Five resistant germplasm lines (USVL020-PFR, USVL203-PFR, USVL782-PFR, USVL489-PFR, and USVL531-MDR) and two susceptible cultivars, Sugar Baby and Mickey Lee, used as checks were grown in a field in 2014 and 2015 to produce fruit for evaluation. Mature fruit were harvested and placed in a walk-in growth chamber and inoculated with 20 different P. capsici isolates. The chamber was maintained at 26 ± 2°C and high relative humidity (>95%) using a humidifier. All five resistant germplasm lines were significantly more resistant than the two susceptible checks to all 20 P. capsici isolates. Among the five resistant germplasm lines, USVL020-PFR, USVL782-PFR, and USVL531-MDR had broad resistance. Some P. capsici isolates induced minor lesions and rot on USVL489-PFR compared with the other resistant lines. Variation in virulence and genetic diversity among the 20 P. capsici isolates was also observed. The five watermelon germplasm lines will be useful for developing commercial watermelon cultivars with broad resistance to P. capsici.

Watermelon is traditionally grown under direct sowing system. However, due to high costs of good quality seeds of improved cultivars, seedling transplanting is a promising alternative for watermelon production. This study aimed at evaluating, during two production cycles, the performance of watermelon under direct sowing cultivation system and seedling transplanting. The field trials were carried out from May to September, 2015 and 2016, at Setor de Horticultura da Escola de Agronomia of Universidade Federal de Goiás. In 2015, the experimental design was randomized blocks (DBC) with four replicates in a 2x4 factorial scheme (two hybrids: Manchester and Talisman, and two planting methods: direct sowing and transplanting 15-, 20- and 25-day old seedlings). In 2016, the experimental design was DBC with six replicates. The treatments consisted of three direct sowing dates (0, 5 and 10 days) and three different seedling ages (15, 20 and 25 days). Number of leaves, plant height and dry mass of seedlings of different ages, vine length and virus incidence along the crop cycle were evaluated. Upon significant F-test (5%) for seedling age, regression analysis was performed, and when cultivar effect was significant, Tukey test (5%) was used. Planting methods (direct sowing X seedlings) were compared by orthogonal contrasts. In 2016, the beginning of flowering was evaluated, and in both trials, the average crop cycle for each treatment was determined. Fruit length and width, fruit mass, thickness of the white part of the rind, and soluble solids content were evaluated at harvesting. The 25-day old seedlings had higher number of leaves, as well as, higher dry mass, comparing with seedlings on the transplanting day. Yield and fruit quality were not influenced by treatments. No significant differences for virus disease incidence were detected, in both production cycles. Direct sowing reduced the crop cycle in both field trials, with no differences in production and quality of fruits. Considering the costs of establishing watermelon fields as relevant, the results suggest the importance of direct sowing as the most viable method for watermelon cultivation, due to the ease of the process, as well as, reduction in costs related to seedling production and transplanting operations. RESUMO A melancia é tradicionalmente cultivada por semeadura direta, porém, em função do alto valor das sementes de materiais melhorados, o transplantio de mudas é alternativa promissora. Este trabalho teve como objetivo avaliar, em dois ciclos de produção, o desempenho agronômico da melancia por semeadura direta e transplantio de mudas. Os experimentos foram conduzidos em campo, entre maio e setembro de 2015 e de 2016 no Setor de Horticultura da Escola de Agronomia da Universidade Federal de Goiás. O ciclo de produção de 2015 foi em delineamento de blocos ao acaso (DBC) com quatro repetições em esquema fatorial 2x4 (dois híbridos: Manchester e Talisman e dois tipos de cultivos: semeadura direta e mudas com 15, 20 e 25 dias). Na safra de 2016 foi em DBC com seis repetições. Os tratamentos foram constituídos por três datas de semeadura direta (0, 5 e 10 dias) e três idades de mudas (15, 20 e 25 dias). Avaliou-se número de folhas, altura e massa seca por muda de diferentes idades e comprimento de ramas e a incidência de viroses ao longo do ciclo da cultura. Quando o teste F (5%) foi significativo para idade da muda, realizou-se análise de regressão, e quando houve efeito da cultivar foi aplicado o teste de Tukey (5%). Métodos de plantio (semeadura direta versus mudas) foram comparados por contrastes ortogonais. Em 2016, avaliou-se o início do florescimento, e em ambos os experimentos foi determinado o ciclo médio da cultura em cada tratamento. Na fase de colheita avaliaram-se: comprimento e largura do fruto, massa do fruto, espessura da parte branca do fruto e teor de sólidos solúveis. Mudas com maior idade (25 dias) apresentaram superioridade em relação ao número de folhas e massa seca da muda no dia do transplantio. Variáveis associadas à produtividade e qualidade dos frutos não foram influenciadas pelos tratamentos. Não houve diferença significativa com relação à incidência de viroses nos dois ciclos de produção. A semeadura direta reduziu o ciclo da melancia nos dois experimentos, entretanto, sem alteração da produção e qualidade dos frutos. Considerando que o custo de implantação de lavouras de melancia é fator relevante, os resultados obtidos reafirmam a importância da semeadura direta como o método mais viável ao plantio de melancia, considerando a facilidade do método e também a redução dos custos com a produção de mudas e operações de transplantio.

Citrullus colocynthis (C. colocynthis) seed is a common and expensive cuisine in West African diet unlike the seeds from Citrullus lanatus (C. lanatus) that are agro waste. The oils from these seeds were comparatively profiled using their physicochemical properties, fatty acids and phenolic contents, in-vitro antioxidant capacity (DPPH, TEAC, FRAP, ORAC), inhibitions of lipid peroxidation and tyrosinase. Toxicities of the oils in mice were also assessed using plasma AST, ALT, cholesterol, HDL-C, liver, heart and kidney (GSH, GPx and SOD) and clastogenic effects in mouse bone marrow. Citrullus lanatus oil contains higher peroxides, saponification, saturated fatty acids and stearic acid (6-fold) levels, with a lower acid value when compared to C. colocynthis. Strong antioxidant capacity, inhibition of lipid peroxidation and tyrosinase by C. lanatus oil extract is proposed to be related to the presence of catechins that are absent in C. colocynthis. The in-vivo study indicated that C. lanatus induced significant increase in ALT while a significant increase of AST was observed in the C. colocynthis treated group. Citrullus lanatus induced a significantly (p 0.05). This work reports for the first time the presence of stearic acid and catechin in C. lanatus oil which is important for its compelling in vitro activity. High doses of both oils could evidently induce toxicological indices in mice.

Climate change is an inevitable process characterized by an abrupt increase in global temperature and a decrease in precipitations leading to drought incidents. Biostimulants could be a valuable tool for mitigating these harsh conditions. The objective of our study was to test the efficiency of two biostimulants, a silicon-based seaweed and the seaweed Ascophyllum nodosum, to mitigate the drought stress endured by watermelon transplants during the first few weeks after transplanting. In order to achieve this, three water treatments (100%, 75%, and 50% of field capacity) were applied in pots. Important growth parameters (leaf number, fresh weight, and plant area) deteriorated depending on water availability. This was also the case for the root system development displayed by root dry weight, total length, and surface area. It is the first time the OJIP transient has been evaluated after the application of A. nodosum for drought-stressed plants. Chlorophyll fluorescence parameters showed that the photosynthetic apparatus was more stressed when A. nodosum was applied, especially in the harshest conditions (i.e., 50% field capacity). Overall, the silicon-based biostimulant failed to demonstrate drought-mitigating potential compared to the non-treated counterparts. On the other hand, A. nodosum alleviated the negative effects of water deficit, especially in the harshest conditions.

In many countries of Europe and Eastern Asia, watermelon production is mainly based on the use of grafted seedlings. Upon grafting, seedlings undergo a period of healing where artificial lighting is provided by light-emitting diodes in controlled chambers in order to accelerate and improve the healing process. The objective of our study was to test the effect of light quality on the final product (i.e., seedlings ready for transplanting) in the nursery, as well as to evaluate the possible implications on fruit quality after field cultivation. Narrow-band blue (B) and red (R) wavelengths, 64–36% R-B (36B), 76–24% R-B (24B), 88–12% R-B (12B), and 83–12% R-B plus 5% far-red (12B+FR) wavelengths were tested. 12B+FR enhanced the root dry weight, root architecture, and maximum photosynthetic rate, while RB combinations generally showed better root system development with increased blue portion. R light induced inferior root dry weight and quality indices (root/shoot and shoot–dry–weight/length ratios), lower gas exchange parameters, and chlorophyll content, but high shoot length and leaf area. B light led to inferior root architecture, lower stem diameter, leaf area, and maximum photosynthetic rate. Both R and B wavelengths showed decreased concentration of macronutrients and trace elements. After field cultivation, fruit quality (i.e., morphology and color), and valuable nutritive characteristics (i.e., phenolics, carotenoids, lycopene, antioxidants) maintained high quality irrespective of light treatments. Overall, 12B+FR performed well in almost all qualitative parameters including the morphology, the root development, and photosynthesis, while also maintaining high fruit quality.

Over the years, the use of organic materials in farming has reduced due to the increase in the use of chemical fertilizers which are rich in readily available plant nutrients. Intensive use of chemical fertilizers may have depressing effect on yield of watermelon. The field experiment were conducted at the Experimental Clinic of the Laboratory “Selection of vegetable and melon crops” and in the laboratory “Biosafety and Biocontrol of vegetable and melon crops” of Regional Branch “Kainar” of the LLP “Kazakh Research Institute of Fruit and Vegetable Growing” which is located in the foothill zone of the southeast of Kazakhstan, to study the effects of different types of organic fertilizers (cow manure, poultry manure and biohumus) and recommended chemical fertilizer (NPK) on the characteristics of watermelon (growth parameters, yield and quality) of southeast of Kazakhstan. There were significant differences among the treatments in relation to fruit yield of watermelon, growth paremeters and quality properties (dry matter, total sugar, Vitamin C and NO3-N). Recommended fertilizer dose (N90P60K60) had the highest fruit yield of watermelon and growth paremeters followed by manure applied at 40 t ha-1. All the fertilizer treatments had higher yield of watermelon than control. From this study, the use of manure as an organic fertilizer in the cultivation of watermelon could be used as alternative to chemical fertilizer. It is recommended that manure at 40 t ha-1 be adopted for watermelon cultivation in in the Southeast of Kazakhstan.

The investigation aimed to analyze the qualitative attributes of ‘Sugar Baby’ variety of watermelon. Ripe fruit consisted of 68.30% flesh, 31.07% rind, and 1.58% seeds. Freshly extracted juice had a pH, total soluble solids, lycopene, ascorbic acid, and total phenolic content of 5.64, 8.90°Bx, 5.23 mg/100 mL juice, 8.22 mg/100 mL juice, and 15.48 mg GAE/100 mL juice, respectively, with a color intensity value of 45.37. Juice had a total aerobic bacteria and yeast & mold counts of 5.69 and 5.68 log cfu/mL, respectively. The predominant sugar and organic acid were fructose (31.10 mg/mL juice) and malic acid (3.38 mg/mL juice). Six key flavor compounds were identified such as 1-Dodecyne; 6-Nonenal (Z); propanoic acid 2-oxo-methyl ester; 2-propanol, 1-methoxy; 5-heptene, 2-one, 6-methyl and hexanal. The rind contains pectin (20.08%) and citrulline (13.36 mg/g and 9.78 mg/g in the skin and peel, respectively). Solvent extraction from dried seeds resulted in 38.66% oil.

The economically profitable production of crops is related, among other factors, to seed quality, the production system, and the water used in irrigation or preparation of nutrient solutions. Therefore, the objective was to evaluate the phenology, production, and vigor of seeds of mini watermelons grown in saline nutrient solution and different substrates. In the fruit and seed production phase, the experiment occurred in a greenhouse with five electrical conductivities of water for nutrient solution preparation, ECw (0.5, 2.4, 4.0, 5.5, and 6.9 dS m−1), and two growing substrates (coconut fiber and sand). We evaluated the physiological quality of seeds previously produced under the five electrical conductivities of water and two substrates. High salinities for the hydroponic cultivation of the mini watermelon cultivar ‘Sugar Baby’ accelerated fruit maturation and crop cycle, decreasing fruit size. However, in both substrates, the seed production of mini watermelons, seed viability, and seed vigor occurred adequately with a reject brine of 6.9 dS m−1 in the hydroponic nutrient solution. The seed production of ‘Sugar Baby’ mini watermelons using reject brine in a hydroponic system with coconut fiber and sand substrates is viable in regions with water limitations.

This study aimed to analyze the evolution of Benincasa hispida (DGA) in the Cucurbitaceae family, as well as the phylogenetic relationships of the phenylalanine ammonia-lyase (PAL) gene family in six Cucurbitaceae species. Whole genome sequencing of DGA was performed and compared to genomes of several Cucurbitaceae species that were downloaded from public databases in order to analyze the evolution of DGA in the Cucurbitaceae family. Moreover, we studied the expansion and contraction of gene families during evolution and the phylogenetic relationships of the PAL gene family in six Cucurbitaceae species. Phylogenetic analysis showed that among the Cucurbitaceae species, Cucumis melo and Cucumis sativus shared the same ancestor; Lagenaria siceraria and Citrullus lanatus shared the same ancestor; and Cucurbita maxima (CMA), Cucurbita moschata (CMO), and Cucurbita pepo (CPE) shared the same ancestor. Notably, DGA was located on a separate branch, indicating that it was the only species of its genus. CPE and CMO had closer kinship, suggesting a significant effect of regional factors on Cucurbitaceae species evolution. Moreover, there was a large amount of gene loss in the genome during evolution from the common ancestor to DGA. The PAL gene family was highly conserved during the evolution of the Cucurbitaceae family.

The production of transgenic watermelon through Agrobacterium tumefaciens-mediated transformation with in vitro regeneration system is a time-consuming, labor-intensive and genotype-dependent process. To acquire large number of transgenic watermelons in a shorter period of time, the half-seed explants were infected with Agrobacterium strain EHA 105 harboring pCAMBIA1301 with bar and gus genes. The factors affecting in planta transformation efficiency, such as co-cultivation duration, acetosyringone concentration, sonication duration and vacuum infiltration were assessed in the present study. The half-seed explants were sonicated for 3 min and 2 min vacuum infiltrated in Agrobacterium suspension and co-cultivated for 3 days with 100 µM acetosyringone showed maximum transformation efficiency. The transformed watermelon plants were selected against BASTA® and GUS, PCR, Southern hybridization analysis confirmed the transgene integration. The amenability of this established protocol was analyzed on four genotypes, in which the response of all genotypes was positive, whereas Arka manik showed the higher transformation efficiency of 17%. The transformation protocol developed in the present study is efficient, economical and expeditious without the taking part of any tissue culture phases and produce a large number of transgenic lines within a short period of 56 days.

Natural resistance-associated macrophage proteins (NRAMPs) are able to transport various metal ions across cell membranes, which play an important role in plant growth and development. Here, a survey of cucumber (Cucumis sativus L.) and watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] genomes found a total of five CsNRAMPs and four ClNRAMPs, respectively. Based on the phylogenetic relationships, CsNRAMPs and ClNRAMPs were clustered into three groups (I, II, and III). Five orthologous pairs were identified between cucumber and watermelon genome, and they were clustered on the same branch of the phylogenetic tree. The number of introns in CsNRAMPs and ClNRAMPs ranged from 3 to 13 and the genes from group I were more fragmented than those in group II. Subsequently, analysis of promoter sequences found that five putative transcription factors could act on NRAMPs. Moreover, CsNRAMPs and ClNRAMPs were differentially regulated by deficiencies of iron (Fe), manganese (Mn), copper (Cu), or zinc (Zn), along with toxicities of Fe, Mn, Cu, Zn, or cadmium (Cd). Functional analysis by heterologous expression in yeast indicated that CsNRAMP4 and ClNRAMP3 participate in Cd transport. Overall, the comprehensive analysis of CsNRAMPs and ClNRAMPs reported herein may pave the way for further investigations examining the regulation and functions of this gene family in cucumber and watermelon.

Background Kinesin (KIN) as a motor protein is a versatile nano-machine and involved in diverse essential processes in plant growth and development. However, the kinesin gene family has not been identified in watermelon, a valued and nutritious fruit, and yet their functions have not been characterized. Especially, their involvement in early fruit development, which directly determines the size, shape, yield and quality of the watermelon fruit, remains unclear. Results In this study, we performed a whole-genome investigation and comprehensive analysis of kinesin genes in C. lanatus. In total, 48 kinesins were identified and categorized into 10 kinesin subfamilies groups based on phylogenetic analysis. Their uneven distribution on 11 chromosomes was revealed by distribution analysis. Conserved motif analysis showed that the ATP-binding motif of kinesins was conserved within all subfamilies, but not the microtubule-binding motif. 10 segmental duplication pairs genes were detected by the syntenic and phylogenetic approaches, which showed the expansion of the kinesin gene family in C. lanatus genome during evolution. Moreover, 5 ClKINs genes are specifically and abundantly expressed in early fruit developmental stages according to comprehensive expression profile analysis, implying their critical regulatory roles during early fruit development. Our data also demonstrated that the majority of kinesin genes were responsive to plant hormones, revealing their potential involvement in the signaling pathways of plant hormones. Conclusions Kinesin gene family in watermelon was comprehensively analyzed in this study, which establishes a foundation for further functional investigation of C. lanatus kinesin genes and provides novel insights into their biological functions. In addition, these results also provide useful information for understanding the relationship between plant hormone and kinesin genes in C. lanatus.

Citron watermelon [Citrullus lanatus var. citroides (L.H. Bailey) Mansf. ex Greb.] is an important genetic resource serving for food, feed, and breeding dessert watermelon (C. lanatus var. lanatus) and other cross-compatible species. Wide phenotypic variation exists among South African citron watermelon landraces which are yet to be systematicaly assessed for diverse uses and cultivar design. The objective of this study was to assess phenotypic diversity of citron watermelon landrace accessions of South Africa and to select desirable genotypes with suitable agronomic and horticultual traits for direct production, breeding and conservation. Thirty six citron watermelon landrace accessions were evaluated under field conditions in two seasons using a 6 × 6 alpha lattice design with three replicates. Data on key qualitative and quantitative traits were collected and subjected to non-parametric and parametric statistical analyses. The accessions showed wide phenotypic variation and unique traits for genetic improvement. Positive and significant correlations (p

e studied the effects of exogenously applied palmitic acid (PA) on reactive oxygen species (ROS) production, defense-related enzyme activity and malondialdehyde (MDA) content of roots and analyzed the response of defense-related genes in Fusarium oxysporum f.sp.niveum (FON) inoculated watermelon (Citrullus lanatus (Thunb.) Matsum. et Nakai). Our results showed that as compared to control, PA reduced the morbidity of Fusarium wilt by 33.3 % and disease severity by 53.2 %, increased the level of ROS of watermelon roots at early infection stage and the activity of defense related enzymes, and decreased the MDA content of watermelon roots. Moreover, PA up-regulated the expression of disease defense-related genes. These results suggested that PA promoted the accumulation of ROS and up-regulated the transcript level of defense-related genes and eventually induced the systemic resistance in watermelon against FON.

The loss of methyl bromide led vegetable growers to rely more heavily on herbicides to control weeds. Although herbicides can be effective, limited options in vegetable production challenge growers. Identifying new, effective tools to be applied over plastic mulch before planting, for improved weed control with minimal crop injury, would be beneficial. The objective of these experiments was to evaluate the persistence of preplant applications of glyphosate (1,125 or 2,250 g ae ha-1) plus 2,4-D (1,065 or 2,130 g ae ha-1) or dicamba (560 g ae ha-1) over plastic mulch, using analytical techniques and subsequent yellow squash and watermelon response. Glyphosate and 2,4-D were not analytically detected at damaging concentrations on plastic mulch when at least 3.5 cm of rainfall was received after application and before planting. In addition, bioassay results showing less than 10% visual injury for either squash and watermelon, with no growth or yield suppression observed, supported analytical results. In contrast, dicamba concentrations on plastic mulch, regardless of rainfall amount or time between application and planting, remained at damaging levels. Squash yields were reduced by dicamba applied 1 to 30 d before planting, whereas watermelon was more resilient. 2,4-D plus glyphosate applied preplant over plastic mulch can provide an additional herbicide option for vegetable growers. More research is needed to understand the impact of residual activity of 2,4-D when transplants land directly in holes in plastic mulch at the time of application. The relationship of dicamba with plastic mulch is complex, because the herbicide cannot be easily removed by rainfall. Thus, dicamba should not be included in a weed management system in plasticulture vegetable production. Nomenclature: Glyphosate; 2; 4-D; dicamba; squash; Cucurbita pepo L.; watermelon; Citrullus lanatus (Thunb.) Matsum. & Nakai

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai was a climacteric variety with a high respiration rate and ethylene accumulation. Therefore the fruit matures and softeness quickly during post-harvest period. Calcium chloride was popularly utilized as stabilizing agent. 1-methylcyclopropene (1-MCP) has been known to be highly effective inhibitor of ethylene reaction. This research evaluated the synergistic effect of CaCl2 and 1-methylcyclopropene (1-MCP) treatment to weight loss, firmness, total soluble solid, carotenoid, ascorbic acid and decay rate of watermelon during storage. Results showed that a combination of 2.5% CaCl2 and 0.6 ppm 1-MCP in 20 min of immersion could extend watermelon shelf life for 15 days. After 15 days of ambient storage, the weight loss (1.43±0.02 %), firmness (4.38±0.00 N), total soluble solid (13.60±0.01 oBrix), carotenoid (16.31±0.02 µg/100g), ascorbic acid (13.36±0.03 mg/100g), decay rate (0.47±0.02 %) were clearly presented. Meanwhile, the treatment of 2.5% CaCl2 alone showed the weight loss (2.11±0.02 %), firmness (3.03±0.02 N), total soluble solid (10.83±0.02 oBrix), carotenoid (12.97±0.03 µg/100g), ascorbic acid (9.57±0.02 mg/100g), decay rate (2.14±0.01 %). The incorporation of CaCl2 and 1-MCP created a synergistic effect on the improved quality of watermelon fruit during ambient storage.

Pumpkin rootstock affects watermelon scion growth, fruit yield, and quality, but the mechanisms of related key enzymes and photosynthesis remain unclear. In this study, net photosynthetic rate (PN), chlorophyll (Chl) fluorescence parameters, fruit mass and sugar accumulation, and related key enzyme activities were determined during the fruit development stage in diploid and triploid watermelon lines and corresponding pumpkin rootstock-grafted lines. The results showed that pumpkin rootstock increased PN and Chl fluorescence parameters of diploid and triploid watermelon, indicating that pumpkin rootstock could increase photosynthesis, the utilization efficiency of light energy of diploid and triploid watermelon lines. Pumpkin-grafted lines had higher alkaline α-galactosidase activity and lower activities of insoluble acid invertase, sucrose phosphate synthase, and sucrose synthase than corresponding own-root lines in diploid and triploid watermelon. It indicates that pumpkin rootstock could increase the unloading of photoassimilates and reduce the conversion of photoassimilates into sucrose in diploid and triploid watermelon fruits.

The growing realization of nutritional and health benefits, and the potential of watermelon (Citrullus lanatus L.) as a cash-producing crop has renewed interest in reducing loss in its post-harvest...

Two new phenolic glycosides, 2-caffeoyl-3-hydroxy-3-methylbutyric 4′-β- d -glucopyranosyloxy-3′-hydroxybenzyl ester and 2-caffeoyl-3-hydroxy-3-methylbutyric 4′-β- d -glucopyranosyloxybenzyl ester (citrulluside H and citrulluside T) were isolated from young fruits of Citrullus lanatus. Their structures were elucidated by spectroscopic analyses. These constituents exhibited weak antioxidant activity.

To determine the allelopathic potential of phenolic acids on watermelon (Citrullus lanatus Thunb), we studied the inhibitory activities of 21 phenolic acids on watermelon seeds germination. The quantitative structure-activity relationship (QSAR) method using the comparative molecular field analysis and comparative molecular similarity index analysis was used to elucidate the relationships between the inhibitory activities and structural characteristics. The results showed that half-maximal inhibitory concentrations (IC50) were in ~µg/mL. The QSAR calculations suggested that the allelopathic inhibitory activities were increased when substituents had large moieties at 1/2/3/4-positions, negative charge at 1/3-position, a H-bond donor at 4-position and a H-bond acceptor at 2-position.

Pythium species cause root and stem rot in watermelon (Citrullus lanatus), but cucurbit rootstocks used to graft watermelon have not been evaluated for resistance. P. aphanidermatum and P. myriotylum were inoculated onto 15 nongrafted watermelon, citron (Citrullus amarus), bottle gourd (Lagenaria siceraria), and interspecific hybrid squash (Cucurbita maxima × C. moschata) cultivars in a growth chamber. Watermelon was more susceptible than bottle gourd and interspecific hybrid squash at 20 and 30°C. Twenty-one cultivars were inoculated in a field with an equal blend of both Pythium species. Interspecific hybrid squash was less susceptible than bottle gourd and watermelon in 2018 and 2019. Seedless watermelon cultivar Tri-X 313 was grafted to one citron, one bottle gourd, and three interspecific hybrid squash rootstocks. Plants were inoculated in the field as described. Grafting to interspecific hybrid squash rootstocks reduced disease incidence compared with nongrafted controls in 2018 and 2019. Mefenoxam and propamocarb applied at transplanting did not affect disease compared with non-fungicide-treated plots. Grafting to interspecific hybrid squash Camelforce significantly increased total and marketable fruit numbers and total weight in 2019 compared with the nongrafted control. In summary, interspecific hybrid squash was consistently resistant to Pythium, demonstrating resistance and utility in watermelon grafting.

This study aimed to evaluate the gas exchanges and growth of ‘Sugar Baby’ watermelon under different strategies of irrigation with saline water and potassium fertilization, in an experiment conducted in a protected environment. The experiment was conducted in randomized blocks, in 8 x 3 factorial arrangement, corresponding to eight water salinity management strategies applied at the phenological stages of the crop (NS = no stress along the cycle; VE = salt stress during vegetative stage; VE/FL = salt stress during vegetative and flowering stages; FL = salt stress during flowering stage; FL/FR = salt stress during flowering and fruiting stages; FR = salt stress during fruiting stage; FR/MAT = salt stress during fruiting and maturation stages; MAT = salt stress during fruit maturation stage) and three potassium doses – KD (corresponding to 50, 100 and 150% of the recommendation), with three replicates. Two levels of water salinity were used, high and low values of electrical conductivity (ECw = 0.8 and 4.0 dS m-1). Watermelon growth and gas exchanges, especially transpiration, stomatal conductance, CO2 assimilation rate, instantaneous carboxylation efficiency, and absolute and relative growth rates of stem diameter were compromised by water salinity at the stages of flowering, fruiting and maturation. The potassium dose with 50% of K2O recommendation can be used in the cultivation of watermelon, without negatively affecting gas exchange and growth.

Most of the modern-day improved watermelon varieties succumb to various biotic and abiotic stresses mainly because of their narrow genetic base. Insights into the genetic diversity and population structure are crucial for broadening the genetic base and improving the adaptive value. The present experiment was conducted to study the genetic diversity and population structure of a germplasm panel comprising 336Citrullussp. accessions. Another objective was to formulate a core collection of IndianCitrullussp. accessions. Data from 23 highly polymorphic microsatellite markers were used for genetic diversity and population structure analysis while both molecular and phenotypic data from 17 traits were used to formulate the core set. The markers yielded a total of 69 alleles with an average of three alleles per locus. Initially, the accessions clustered into two populations and an admixture group. Intra-population analysis revealed three and two statistically distinct subpopulations in Pop I and Pop II, respectively. The exotic collections were predominant in Pop I-A, Pop II-A and Pop II-B while the Indian accessions were preponderant in Pop I-B and Pop I-C. Pop I-B recorded the maximum magnitude of gene diversity and the highest number of alleles. The well-adapted Indian landraces could be deployed in future watermelon improvement programmes. The formulated core collection (n= 46; 23.71% of the entire collection studied) would ease in maintenance of the diversity present among indigenousCitrullussp. accessions; would ease trait search while exploring Indian diversity and can be pooled with other collection(s) to form a global core of watermelon.

Watermelon (Citrullus lanatus) is one of the most popular fruits in Cameroon and the world at large. However, the extreme sensitivity of watermelon to parasites and climatic vagaries makes its cultivation demanding of chemical inputs that can have negative impacts on human health and the environment. In Cameroon, there is a slow improvement of fruit yield in watermelon breeding due to the lack of natural heritable genetic variation, which is a prerequisite for genetic improvement of crops. Such variation can be created through either random or targeted processes on genotypes with appropriate doses of radiation. Genetic improvement by induced mutagenesis appears today alongside hybridization as an alternative method of creating new plant varieties. However, the success of this approach is determined by the application of an appropriate and ideal dose of mutagen. The objective of this study was to evaluate the radiosensitivity of the two most cultivated watermelon varieties in Cameroon to gamma radiation from 60Co in order to determine an optimal dose or lethal dose 50 (LD50) for the induction of the genetic variability necessary for genetic improvement. Seeds of the Kaolack and Crimson sweet watermelon varieties were irradiated with five doses of gamma radiation (100, 200, 300, 400 and 600 Gy) in the laboratory of the International Atomic Energy Agency in Seibersdorf, Austria. These seeds were cultivated in a greenhouse following an utterly randomized device with three repetitions, and parameters such as the germination rate, the survival rate and the shoot length of plants were evaluated. High rates of 90% and 75% germination were obtained, respectively, for the control treatments of Kaolack and Crimson sweet, while the lowest rates were 35% at 600 Gy for Kaolack and 30% at 400 Gy for Crimson sweet. The highest survival rate of plants (96.66%) was obtained with the control seeds of the Kaolack. This variety had the lowest survival rate (45.6%) at 600 Gy dose. Statistical analysis of data obtained helped to estimate the ideal LD50 doses based on growth reduction of seedlings’ heights after gamma-ray treatment. Using a linear regression model based on parameters like plant size, the LD50 doses for Kaolack and Crimson sweet were calculated at 225.40 Gy and 221.56 Gy, respectively, and predicted between 200 and 250 Gy. These results show that the two varieties evaluated were radiosensitive as clearly expressed in the parameters evaluated, where the values decreased with the increase in the irradiation dose. The LD50 doses from this study could be safely applied as reference doses for large-scale gamma irradiation of watermelon genotypes to create desirable agronomic traits in the mutation breeding efforts.

Fusarium wilt of watermelon (Citrullus lanatus), caused by the soilborne fungus Fusarium oxysporum f. sp. niveum, is the most serious disease of watermelon in South Carolina and other southeastern U.S. states. Isolates of F. oxysporum collected from field-grown plants, greenhouse-grown seedlings, and field soil between 1999 and 2018 were inoculated onto three differential watermelon cultivars to identify races. Of 197 isolates obtained from plants, 12% were nonpathogenic, 2% were race 0, 23% were race 1, and 63% were race 2. One collection of isolates from greenhouse seedlings was exclusively race 1 and the other was exclusively race 2. Seventeen of 81 soil isolates were pathogenic: five were race 1 and 12 were race 2. Reactions of C. amarus PI 296341-FR, Carolina Strongback, and SP-6, cultigens with resistance to race 2, did not differ significantly among five highly virulent race 2 isolates and a standard race 2 isolate, indicating a lack of a race 3 phenotype. Forma specialis-specific primers matched phenotypic race identification for 74% of the isolates. Race-specific primers based on a secreted-in-xylem elicitor present in race 0 and 1 isolates matched phenotypic race identification for 66% of the isolates. Because a majority of the F. oxysporum f. sp. niveum isolates from South Carolina were race 2, integrated management practices should be used until commercial cultivars with resistance to race 2 are available.

Melon necrotic spot virus (MNSV) was detected in field-grown Cucumis melo (rockmelon) and Citrullus lanatus (watermelon) plants in the Sunraysia district of New South Wales and Victoria, Australia, in 2012, 2013, and 2016, and in two watermelon seed lots tested at the Australian border in 2016. High-throughput sequencing was used to generate near full-length genomes of six isolates detected during the incursions and seed testing. Phylogenetic analysis of the genomes suggests that there have been at least two incursions of MNSV into Australia and none of the field isolates were the same as the isolates detected in seeds. The analysis indicated that one watermelon field sample (L10), the Victorian rockmelon field sample, and two seed interception samples may have European origins. The results showed that two isolates (L8 and L9) from watermelon were divergent from the type MNSV strain (MNSV-GA, D12536.2) and had 99% nucleotide identity to two MNSV isolates from human stool collected in the United States (KY124135.1, KY124136.1). These isolates also had high nucleotide pairwise identity (96%) to a partial sequence from a Spanish MNSV isolate (KT962848.1). The analysis supported the identification of three previously described MNSV genotype groups: EU-LA, Japan melon, and Japan watermelon. To account for the greater diversity of hosts and geographic regions of the MNSV isolates used in this study, it is suggested that the genotype groups EU-LA, Japan melon, and Japan watermelon be renamed to groups I, II, and III, respectively. The divergent isolates L8 and L9 from this study and the stool isolates from the United States formed a fourth genotype group, group IV. Soil collected from the site of the Victorian rockmelon MNSV outbreak was found to contain viable MNSV and the virus vector, a chytrid fungus, Olpidium bornovanus (Sahtiyanci) Karling, 18 months after the initial MNSV detection. This is a first report of O. bornovanus from soil sampled from an MNSV-contaminated site in Australia.

Watermelon (Citrullus lanatus) is an important fruit crop in the arid region of western India. The modern watermelon cultivars share a narrow genetic base and are susceptible to various stresses. Thus, there is a need to expand its genetic base through distant hybridization by interspecific crossing with C. colocynthis. An attempt was made to develop interspecific hybrids between lanatus and colocynthis followed by hybridity testing of F1 through SSR markers and biochemical analysis of F2. Out of nine primers, eight primers could successfully show amplification in parents and hybrids out of which two markers could correctly discriminate the true hybrid. The morphological analysis demonstrated that F1 progeny resembles much with C. colocynthis for most of the characters. Biochemical analysis of pulp of selected F2 plants and parents indicated that in many F2 plants biochemicals were higher than parents which suggest that the blending of genes from both species made a favourable effect. Diterpene was only present in watermelon and not detected in C. colocynthis or any of the F2 plants. GC–MS based fatty acid profiling of nine identified fatty acids namely palmitic acid, stearic acid, oleic acid, linoleic acid and linolenic acid was done. It can be concluded that the interspecific hybrids between water melon and C. colocynthis as also confirmed by molecular markers and superior F2 plants can be used in watermelon breeding program for enhancing vital phytochemicals.

The virome of babaco (Vasconcellea x heilbornii) —a non-traditional fruit crop native to Ecuador— was investigated by high-throughput sequencing (HTS) on plants obtained from a commercial nursery. Six virus-like sequences were detected, including the full length of papaya ringspot virus (PRSV) and an RNA-dependent-RNA-polymerase (RdRp) sequence with homology to papaya virus Q. Three RNA sequences were found with homology, respectively, to apple latent spherical virus (genus Cheravirus, 71% nt identity), cherry leaf roll virus (genus Nepovirus, 54% nt identity) and Citrullus lanatus cryptic virus (genus Deltapartitivirus, 66% nt identity); whereas a DNA pararetrovirus-like sequence with homology to citrus endogenous pararetrovirus (58% nt identity) was also detected. RT-PCR-based virus surveys on a total of 284 samples collected from three provinces revealed that the partitivirus- and pararetrovirus-like sequences were present in 100% of tested plants; whereas the other virus sequences were detected in up to 68% of plants and were associated with different symptoms. This work provides information on the occurrence and prevalence of PRSV and five additional virus-like sequences in babaco, a vegetatively propagated crop, supporting the need for a virus-free certification program.

The genetics and nature of gene action involved in resistance to Fusarium oxysporum f. sp. niveum race 2 causing wilt in watermelon was studied. Five resistant accessions were crossed with a common susceptible parent to develop six generations for each combination. The populations were screened in two consecutive assays during June to September 2019. Based on the segregation of F2, the resistance in IC0523048, EC794455 and EC759804 was found to be governed by two major complementary dominant genes. However, the deviation from the expected ratio in BC1 progeny suggested the involvement of modifying genetic factors. The F2 and backcross progenies of BIL-53 × IIHR-9 and BIL-53 × EC794421 did not follow any of the ratios tested. Allelism test revealed the absence of any allelic relationship between IIHR9 and IC0523048 for resistance. Generation mean analysis revealed presence of epistasis in BIL-53 × EC794455 and BIL-53 × EC759804 while in the other three crosses, simple additive-dominance interaction was observed. These resistant accessions and the inheritance information generated in this study would help in incorporation of resistance to Fusarium wilt race 2 into elite commercial backgrounds.

Dopamine acts as a strong water-soluble antioxidant that mediates many physiological processes in plants. In the present study, we utilized two cultivars to investigate the effect of dopamine in mitigating chilling stress in watermelon (Citrullus lanatus) seedlings. Under chilling stress, exogenous application of dopamine (100 μM) increased seedling health index by 23.32% (cv. Xi Nong NO. 8) and 35.19% (cv. Jing Xin NO. 1) compared to chilling-stressed seedlings alone. As the stress was prolonged, chilling treatment caused a significant reduction in net rates of photosynthesis and chlorophyll contents. Dopamine supplementation significantly mitigated these adverse effects caused by chilling stress. After 8 days of chilling stress, supplementation of dopamine to chilling treatment seedlings further increases in levels of proline in cv. Xi Nong NO. 8 and cv. Jing Xin NO. 1 by 45.24% and 58.28%, respectively. Our results indicated that application of dopamine further enhanced antioxidant enzymes, which contributed to alleviate oxidative stress-induced chilling treatment. In addition, application of dopamine significantly improved polyamine levels by stimulating enzymes involved in the polyamine metabolism. From these outcomes, it is clear that supplementation of dopamine mitigates the harmful effects of chilling stress in watermelon seedlings through modulation of osmolytes, antioxidant enzymes, and polyamine metabolism.

Vine decline or sudden wilt caused by a complex of pathogens with unknown etiology is emerging as most destructive disease of watermelon in the southern part of India. In this direction ICAR-IIHR, has identified a prebred line derived from Citrullus amarus viz., BIL-53 to be field tolerant to this disease. This study was planned to understand the genetics of vine decline resistance in this accession. The BIL-53 used as resistant parent was crossed with icebox accession, IIHR‐140–152, a susceptible genotype to develop F1, F2, BC1 and BC2 generations and observe the segregation for disease reaction. The results in F2 and backcross progenies suggested that resistance is governed by a single recessive gene. BIL-53 is also known to possess resistance to WBNV and hence, this line can be used in resistance breeding programs to develop combined resistance to vine decline and watermelon bud necrosis orthotospovirus employing back cross breeding. Further, this line can be tested as a rootstock in overcoming vine decline as an immediate measure.

In vitro gynogenesis is an important tool used in haploid or homozygous double-haploid plant breeding. However, because of low repeatability, embryoid induction rate and quality, the molecular mechanisms remain poorly understood. Heat shock treatment can promote the transformation of the gametophytic pathway into the sporophyte pathway, which induces the occurrence of haploid. In this study, unfertilized ovaries were heat shocked for 0 h (A0) before flowering and for 0 h (A1), 4 h (A3), 8 h (A5), 12 h (A7), and 24 h (A8), respectively, at 37°C at the first day of the flowering stage. The ovule enlargement rate was increased from 0% at 25°C to 96.8% at 37°C (24 h treatment). Thus, we aimed to investigate the gene expression patterns in unfertilized ovules of watermelon after different periods of heat shock by using RNA-Seq technology. The results showed that compared with A3, A5, A7, and A8, the biosynthesis of amino acid, glycine, serine and threonine metabolic pathways in A1 has changed significantly. This indicated that heat shock treatment affected the synthesis and transformation of amino acids during ovule expansion. The transcriptome data suggested gene expressions of ovule growth were significantly changed by heat-specific influences. The results provide new information on the complex relationship between in vitro gynogenesis and temperature. This provides a basis for further study of the mechanism of heat shock affecting the expansion of watermelon ovule.

Seedlessness is a valuable agronomic trait in watermelon (Citrullus lanatus) breeding. Conventional less seed watermelons are mainly triploid, which has many disadvantages due to unbalanced genome content. Less seed watermelon can be achieved at the diploid level when certain reproductive genes are mutated or by chromosome translocation, which leads to defects during meiosis. However, the formation mechanism of diploid less seed watermelons remains largely unknown. Here, we identified a spontaneous mutant line, watermelon line “148”, which can set seeds normally when self-pollinated. A total of 148 × JM F1hybrid plants exhibited seed number reductions to 50.3% and 47.3% of those of the two parental lines, respectively, which are considered to be less seed. Examination of pollen viability and hybridization experiments revealed that F1hybrids produce semisterile pollen and ovules. Further cytological observations indicated that semisterility was a result of a reciprocal translocation of chromosomes, which exhibited one quadrivalent ring of four chromosomes at prometaphase I during meiosis. RT-qPCR analysis indirectly confirmed that the semisterile phenotype is caused by chromosome translocation rather than disruption of specific meiotic gene expression. F2population genetic analysis indicated that the “148” watermelon line is a homozygous translocation and that the less seed phenotype of the F1hybrid is prompted by one chromosome fragment translocation. The translocated fragment was further fine mapped to a 2.09 Mb region on chromosome 6 by whole-genome resequencing and genetic map cloning procedures. Our work revealed that a 2.09 Mb chromosome fragment translocation on chromosome 6, causing meiotic defects at metaphase I during meiosis, leads to diploid less seed watermelon. Our findings provide a new promising method for less seed watermelon breeding at the diploid level, as well as a fragment size reference for breeding less seed watermelon through artificially induced chromosome translocation.

Efficient genetic transformation has the potential to advance research and breeding in watermelon (Citrullus lanatus), but regeneration from tissue culture remains challenging. Previous work showed that expressing a fusion of two interacting transcription factors, GROWTH-REGULATING FACTOR4 (GRF4) and GRF-INTERACTING FACTOR1 (GIF1), improved regeneration in wheat (Triticum aestivum). By overexpressing a chimeric fusion of ClGRF4 and ClGIF1, we achieved highly efficient transformation in watermelon. Mutating the mi396 microRNA target site in ClGRF further boosted the transformation efficiency up to 67.27% in a genotype-independent manner. ClGRF4-GIF1 can also be combined with clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) genome editing tools to achieve highly efficient gene editing in watermelon, which we used to successfully create diploid seedless watermelon. This research thus puts forward a powerful transformation tool for future watermelon research and breeding.

Watermelon (Citrullus lanatus) is one of the most popular crops in Korea, with over 100 million units produced annually. As watermelon cultivation increases, the damage caused by plant viruses in watermelon farms is also increasing. In July 2020, some watermelons cultivated on farms in Uiryeong showed typical viral symptoms, such as yellowing and necrosis. In previous studies, two plant viruses, cucurbit aphid-borne yellows virus (CABYV) and cucurbit chlorotic yellows virus (CCYV), have been reported as causal agents of yellowing disease in the cucurbitaceae plant in Korea. To identify the virus(es) associated with the symptomatic watermelon plants, 11 samples were collected. Total RNA was extracted from each sample using the Plant RNA Prep kit (Biocube System, Gwacheon, Korea). RT-PCR was performed using primer sets specific to CABYV and CCYV to detect each virus (Kwak et al. 2018, Wintermantel et al. 2019). CABYV was detected in one sample, and CCYV was detected in 8 samples. Every sample presented similar yellowing symptoms; however, neither virus was detected in the remaining two samples. To investigate unknown viruses, a transcriptome library was constructed using total RNA of the watermelons and sequenced using a NovaSeq 6000 sequencer (Illumina, San Diego, CA). The reads were de novo assembled and annotated using the KEGG virus genome database with the NCBI BLAST utility. All procedures of next generation sequencing were performed by Macrogen (Seoul, Korea). Three large viral contigs were identified, and additional BLAST analyses for nucleotides (nt) and proteins indicated that they were CABYV, CCYV, and melon aphid-borne yellows virus (MAYBV). A total of 247,198 reads were mapped to reference MABYV sequence (GenBank Accession Number NC_010809), and the sequencing depth was 6,575X. The contig (MW505927) had a size of 5,677 nt and showed 100% coverage and 96% identity with known complete MABYV sequences (JQ700307 and EU000534). To confirm the presence of MABYV, RT-PCR was performed using specific primer sets targeting MABYV (MABYV-262-F, 5'-GAACCGTCGACGCACTTCAAAGAGTA-3' and Polero-uni-R, 5'-GATYTTATAYTCATGGTAGGCCTTGAG-3'; Knierim et al. 2010). The expected size of 262 bp was obtained from 5 out of 11 samples, including the two samples mentioned above. MABYV belongs to the genus Polerovirus and has been reported in cucurbit crops in China, Taiwan, and Thailand (Xiang et al. 2008, Knierim et al. 2010, Cheewachaiwit et al. 2017). According to the farmer, outbreak of aphids had previously occurred and were controlled with pesticides. Since aphids are known to be vectors of poleroviruses, we surmise that the watermelons were infected with MABYV by the aphids at that time. To monitor the outbreak of MABYV, watermelon farms in Uiryeong will be continuously investigated. To our knowledge, this is the first report of MABYV in Korea.

DNA methylation plays an important role against adverse environment by reshaping transcriptional profile in plants. To better understand the molecular mechanisms of watermelon response to osmotic stress, the suspension cultured watermelon cells were treated with 100mM mannitol, and then a methylated cytosines map was generated by whole genome bisulfite sequencing (WGBS). Combined with transcriptome sequencing, the effects of osmotic stress on differentially methylated expressed genes (DMEGs) were assessed. It was found that genes related to plant hormone synthesis, signal transduction, osmoregulatory substance-related and reactive oxygen species scavenging-related enzyme could rapidly respond to osmotic stress. The overall methylation level of watermelon decreased after osmotic stress treatment, and demethylation occurred in CG, CHG, and CHH contexts. Moreover, differentially methylated expressed genes (DMEGs) were significantly enriched in RNA transport, starch and sucrose metabolism, plant hormone signal transduction and biosynthesis of secondary metabolites, especially in biosynthesis of osmolytes synthase genes. Interestingly, demethylation of a key enzyme gene Cla014489 in biosynthesis of inositol upregulated its expression and promoted accumulation of inositol, which could alleviate the inhibition of cell growth caused by osmotic stress. Meanwhile, a recombinant plasmid pET28a-Cla014489 was constructed and transferred into Escherichia coli BL21 for prokaryotic expression and the expression of ClMIPS protein could improve the tolerance of E. coli to osmotic stress. The effect of methylation level on the expression properties of inositol and its related genes was further confirmed by application of DNA methylation inhibitor 5-azacytidine. These results provide a preliminary insight into the altered methylation levels of watermelon cells in response to osmotic stress and suggest a new mechanism that how watermelon cells adapt to osmotic stress.

Salinization of cultivated soils is a global phenomenon mainly caused by agricultural practices and deteriorates plant production. Biostimulants are products which can be applied exogenously to enhance the plants’ defense mechanism and improve their developmental characteristics, also under abiotic stresses. We studied the potential of two biostimulants, Ascophyllum nodosum (Asc) seaweed and a silicon-based (Si), to alleviate the saline conditions endured by watermelon transplants. Three salinity (0 mM, 50 mM, and 100 mM NaCl) treatments were applied in watermelon seedlings transplanted in pots, while the two biostimulants were sprayed in the foliar in the beginning of the experiment. Relative water content was improved by Asc in the high salinity level. The plant area, leaf number, and shoot dry weight deteriorated in relation to the salinity level. However, the root system (total root length and surface area) was enhanced by 50 mM salt, as well as Asc in some cases. The OJIP transient of the photosynthetic apparatus was also evaluated. Some OJIP parameters diminished in the high salinity level after Asc application. It is concluded that after salt stress Asc provoked a positive phenotypic response, while Si did not alleviate the salinity stress of transplanted watermelon.

Watermelon (Citrullus lanatus) is a globally important Cucurbitaceae crop in which grafting is commonly used to improve stress tolerance and enhance nutrient utilization. However, the mechanism underlying grafting-enhanced nutrient assimilation remains unclear. Here, we demonstrate the possible involvement of a novel Cucurbitaceae miRNA, ClmiR86, in grafting-enhanced phosphate-starvation tolerance via CALCINEURIN B-LIKE INTERACTING PROTEIN KINASE 5 (ClCIPK5) suppression in watermelon. Transcript analyses revealed that the induction of ClmiR86 expression was correlated with the downregulation of ClCIPK5 in squash-grafted watermelon under phosphate starvation. In addition, the differential expression of ClmiR86 in various watermelon genotypes was consistent with their phosphate utilization efficiency. Furthermore, ClmiR86 overexpression in Arabidopsis enhanced root growth and phosphate uptake under phosphate starvation and promoted inflorescence elongation under normal conditions. These results suggest that the ClmiR86–ClCIPK5 axis is involved in phosphate starvation response as well as grafting-enhanced growth vigor and phosphate assimilation. The present study provides valuable insights for investigating long-distance signaling and nutrient utilization in plants.

Melatonin is a pleiotropic signaling molecule that regulates plant growth and responses to various abiotic stresses. The last step of melatonin synthesis in plants can be catalyzed by caffeic acid O-methyltransferase (COMT), a multifunctional enzyme reported to have N-acetylserotonin O-methyltransferase (ASMT) activity; however, the ASMT activity of COMT has not yet been characterized in nonmodel plants such as watermelon (Citrullus lanatus). Here, a total of 16 putative O-methyltransferase (ClOMT) genes were identified in watermelon. Among them, ClOMT03 (Cla97C07G144540) was considered a potential COMT gene (renamed ClCOMT1) based on its high identities (60.00–74.93%) to known COMT genes involved in melatonin biosynthesis, expression in almost all tissues, and upregulation under abiotic stresses. The ClCOMT1 protein was localized in the cytoplasm. Overexpression of ClCOMT1 significantly increased melatonin contents, while ClCOMT1 knockout using the CRISPR/Cas-9 system decreased melatonin contents in watermelon calli. These results suggest that ClCOMT1 plays an essential role in melatonin biosynthesis in watermelon. In addition, ClCOMT1 expression in watermelon was upregulated by cold, drought, and salt stress, accompanied by increases in melatonin contents. Overexpression of ClCOMT1 enhanced transgenic Arabidopsis tolerance against such abiotic stresses, indicating that ClCOMT1 is a positive regulator of plant tolerance to abiotic stresses.