Physiological, biochemical, and metabolic changes in diploid and triploid watermelon leaves during flooding.

Background: Flooding is a major stress factor impacting watermelon growth and production globally. Metabolites play a crucial role in coping with both biotic and abiotic stresses. Methods: In this study, diploid (2X) and triploid (3X) watermelons were investigated to determine their flooding tolerance mechanisms by examining physiological, biochemical, and metabolic changes at different stages. Metabolite quantification was done using UPLC-ESI-MS/MS and a total of 682 metabolites were detected. Results: The results showed that 2X watermelon leaves had lower chlorophyll content and fresh weights compared to 3X. The activities of antioxidants, such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), were higher in 3X than in 2X. 3X watermelon leaves showed lower O2 production rates, MDA, and hydrogen peroxide (H2O2) levels in response to flooding, while higher ethylene production was observed. 3X had higher levels of dehydrogenase activity (DHA) and ascorbic acid + dehydrogenase (AsA + DHA), but both 2X and 3X showed a significant decline in the AsA/DHA ratio at later stages of flooding. Among them, 4-guanidinobutyric acid (mws0567), an organic acid, may be a candidate metabolite responsible for flooding tolerance in watermelon and had higher expression levels in 3X watermelon, suggesting that triploid watermelon is more tolerant to flooding. Conclusion: This study provides insights into the response of 2X and 3X watermelon to flooding and the physiological, biochemical, and metabolic changes involved. It will serve as a foundation for future in-depth molecular and genetic studies on flooding response in watermelon.

Multi-omics integration to explore the molecular insight into the volatile organic compounds in watermelon.

A range of volatile organic compounds played an important role in the formation of watermelon fruit aroma, while due to the low content and difficulty in detection, it is often neglected in watermelon breeding programs, resulting in a decline in fruit flavor. VOCs in the flesh of 194 watermelon accessions and seven cultivars at four developmental stages were determined by SPME-GC-MS. Ten metabolites with significant differences in the natural population and positive accumulation during fruit development are considered to be the key metabolite related to watermelon fruit aroma. And the link between metabolite and, flesh color and sugar content by correlation analysis was established. The results of the genome-wide association study showed that (5E)-6,10-dimethylundeca-5,9-dien-2-one, and 1-(4-methylphenyl) ethanone were colocalized with watermelon flesh color on chromosome 4, which may be regulated by LCYB and CCD. (E)-4-(2,6,6-trimethylcyclohexen-1-yl)but-3-en-2-one is the VOC produced by the cleavage of carotenoids, which has a positive correlation with the sugar content of the fruit, and the candidate gene Cla97C05G092490 on chromosome 5 may interact with PSY to influence the accumulation of this metabolite. In addition, Cla97C02G049790 (enol reductase), Cla97C03G051490 (omega-3 fatty acid desaturase gene), LOX, and ADH may play important roles in the synthesis of fatty acids and their derived VOCs. Taken together, our findings provide molecular insights into the accumulation and natural variation of VOCs in watermelon, and give data support for breeding watermelon cultivars with better flavor.

Watermelon domestication was shaped by stepwise selection and regulation of the metabolome.

Although crop domestication has greatly aided human civilization, the sequential domestication and regulation of most quality traits remain poorly understood. Here, we report the stepwise selection and regulation of major fruit quality traits that occurred during watermelon evolution. The levels of fruit cucurbitacins and flavonoids were negatively selected during speciation, whereas sugar and carotenoid contents were positively selected during domestication. Interestingly, fruit malic acid and citric acid showed the opposite selection trends during the improvement. We identified a novel gene cluster (CGC1, cucurbitacin gene cluster on chromosome 1) containing both regulatory and structural genes involved in cucurbitacin biosynthesis, which revealed a cascade of transcriptional regulation operating mechanisms. In the CGC1, an allele caused a single nucleotide change in ClERF1 binding sites (GCC-box) in the promoter of ClBh1, which resulted in reduced expression of ClBh1 and inhibition of cucurbitacin synthesis in cultivated watermelon. Functional analysis revealed that a rare insertion of 244 amino acids, which arose in C. amarus and became fixed in sweet watermelon, in ClOSC (oxidosqualene cyclase) was critical for the negative selection of cucurbitacins during watermelon evolution. This research provides an important resource for metabolomics-assisted breeding in watermelon and for exploring metabolic pathway regulation mechanisms.

Genome-wide association study provides genetic insights into natural variation in watermelon rind thickness and single fruit weight.

Rind thickness and fruit weight are agronomic traits closely related to quality and yield, which have attracted much attention from consumers and breeders. However, the genetic mechanism of these two traits is still not well understood in natural populations. In this study, rind thickness and single fruit weight in 151 watermelon accessions were determined in 2019 and 2020, and genome-wide association analysis was performed by integrating phenotypic and genotype data. Abundant phenotypic variation was found in the test population, and the watermelon with thinner rind thickness tended to have smaller fruit weights. Five significant SNPs were closely associated with rind thickness on chromosome 2 by Genome-wide association study (GWAS), i.e., 32344170, 32321308, 32304738, 32328501, and 32311192. And there were 21 genes were annotated in the candidate interval, most notably, Cla97C02G044160 belonged to the MADS family, and part of the genes in this family played an important role in regulating organ size, further analysis of gene structure, gene expression level, and phylogenetic tree showed that Cla97C02G044160 was a candidate gene for regulating target traits. In conclusion, our study provides molecular insights into the natural variation of watermelon rind thickness and single fruit weight, meanwhile, providing data support for molecular marker-assisted breeding.

Genome-wide association analysis provides molecular insights into the natural variation of watermelon seed size.

Seed-consumption watermelon tend to have larger-sized seeds, while flesh-consumed watermelon often require relatively smaller seed. Therefore, the seed size of watermelon has received extensive attention from consumers and breeders. However, the study on the natural variation and genetic mechanism of watermelon seed size is not clear enough. In the present study, 100 seed weight, seed hilum length, seed length, seed width, and seed thickness in 197 watermelon accessions were examined. Furthermore, association analysis was conducted between seed size traits and high-quality SNP data. The results revealed that there was a strong correlation between the five seed traits. And seed enlargement was an important feature during watermelon seed size domestication. Meanwhile, the seed consumption biological species C. mucosospermu and C. lanatus edible seed watermelon had a significantly bigger seed size than other species's. Eleven non-repeating significant SNPs above the threshold line were obtained by GWAS analysis. Four of them on chromosome 5 were considered to be closely associated with seed size traits, i.e. S5: 32250307, S5: 32250454, S5: 32256177, S5: 32260870, which could be used as potential molecular markers for the breeding of watermelon cultivars with target seed size. In addition, combined with gene annotation information and previous reports, five genes near the four significant SNPs may regulate seed size. And qRT-PCR analysis showed that two genes Cla97C05G104360 and Cla97C05G104380, which may be involved in abscisic acid metabolism, may play an important role in regulating the seed size of watermelon. Our findings provide molecular insights into natural variation in watermelon seed size, and gives valuable information of molecular marker-assisted breeding.

An integrated transcriptome and metabolome approach reveals the accumulation of taste-related metabolites and gene regulatory networks during watermelon fruit development.

MAIN CONCLUSION: Accumulation patterns and gene regulatory networks of sugars and cucurbitacins and related primary and secondary metabolites during cultivated watermelon 'Cheng Lan' and wild watermelon 'PI 632,751' fruit development were identified. Metabolites are the end products of cellular regulatory processes and play important roles in fruit taste formation. However, comprehensive studies on the accumulation patterns of watermelon fruit metabolites and transcriptional regulatory networks are still scarce. In this study, 451 annotated metabolites were identified at four key fruit developmental stages in wild watermelon 'PI 632,751' and modern cultivated watermelon 'Cheng Lan'. Interestingly, 11 sugars and 25 major primary metabolites were mainly accumulated in 'Cheng Lan' during fruit development, which are considered to be the potential metabolites beneficial to the formation of watermelon taste. Cucurbitacins and the main flavonoids were mainly specifically accumulated in 'PI 632,751', not being considered to be responsible for the taste. Moreover, forty-seven genes involved in carbohydrate metabolism, glycolysis, and TCA cycle were highly expressed in 'Cheng Lan', which was positively correlated with the accumulation of major primary metabolites. Alternatively, seven UDP-glycosyltransferase genes are closely related to the glycosylation of cucurbitacins through co-expression analysis. Our findings established a global map of metabolite accumulation and gene regulation during fruit development in wild and cultivated watermelons and provided valuable information on taste formation in watermelon fruit.

Identification of Key Gene Networks Associated With Cell Wall Components Leading to Flesh Firmness in Watermelon.

Flesh firmness of watermelon is an important quality trait for commercial fruit values, including fruit storability, transportability, and shelf life. To date, knowledge of the gene networks underlying this trait is still limited. Herein, we used weighted genes co-expression network analysis (WGCNA) based on correlation and the association of phenotypic data (cell wall contents) with significantly differentially expressed genes between two materials, a near isogeneic line "HWF" (with high average flesh firmness) and inbred line "203Z" (with low average flesh firmness), to identify the gene networks responsible for changes in fruit flesh firmness. We identified three gene modules harboring 354 genes; these gene modules demonstrated significant correlation with water-soluble pectin, cellulose, hemicellulose, and protopectin. Based on intramodular significance, eight genes involved in cell wall biosynthesis and ethylene pathway are identified as hub genes within these modules. Among these genes, two genes, Cla012351 (Cellulose synthase) and Cla004251 (Pectinesterase), were significantly correlated with cellulose (r 2 = 0.83) and protopectin (r 2 = 0.81); three genes, Cla004120 (ERF1), Cla009966 (Cellulose synthase), and Cla006648 (Galactosyltransferase), had a significant correlation with water-soluble pectin (r 2 = 0.91), cellulose (r 2 = 0.9), and protopectin (r 2 = 0.92); and three genes, Cla007092 (ERF2a), Cla004119 (probable glycosyltransferase), and Cla018816 (Xyloglucan endotransglucosylase/hydrolase), were correlated with hemicellulose (r 2 = 0.85), cellulose (r 2 = 0.8), and protopectin (r 2 = 0.8). This study generated important insights of biosynthesis of a cell wall structure and ethylene signaling transduction pathway, the mechanism controlling the flesh firmness changes in watermelon, which provide a significant source to accelerate future functional analysis in watermelon to facilitate crop improvement.

Metabolome and Transcriptome Integration Reveals Insights Into Flavor Formation of 'Crimson' Watermelon Flesh During Fruit Development.

Metabolites have been reported as the main factor that influences the fruit flavor of watermelon. But the comprehensive study on the dynamics of metabolites during the development of watermelon fruit is not up-to-date. In this study, metabolome and transcriptome datasets of 'Crimson' watermelon fruit at four key developmental stages were generated. A total of 517 metabolites were detected by ultrahigh-performance liquid chromatography-electrospray ionization-tandem mass spectrometry and gas chromatography-solid-phase microextraction-mass spectrometry. Meanwhile, by K-means clustering analysis, the total differentially expressed genes were clustered in six classes. Integrating transcriptome and metabolome data revealed similar expression trends of sugars and genes involved in the glycolytic pathway, providing molecular insights into the formation of taste during fruit development. Furthermore, through coexpression analysis, we identified five differentially expressed ADH (alcohol dehydrogenase) genes (Cla97C01G013600, Cla97C05G089700, Cla97C01G001290, Cla97C05G095170, and Cla97C06G118330), which were found to be closely related to C9 alcohols/aldehydes, providing information for the formation of fruit aroma. Our findings establish a metabolic profile during watermelon fruit development and provide insights into flavor formation.

Transcriptome regulation of carotenoids in five flesh-colored watermelons (Citrullus lanatus).

BACKGROUND: Fruit flesh color in watermelon (Citrullus lanatus) is a great index for evaluating the appearance quality and a key contributor influencing consumers' preferences. But the molecular mechanism of this intricate trait remains largely unknown. Here, the carotenoids and transcriptome dynamics during the fruit development of cultivated watermelon with five different flesh colors were analyzed. RESULTS: A total of 13 carotenoids and 16,781 differentially expressed genes (DEGs), including 1295 transcription factors (TFs), were detected in five watermelon genotypes during the fruit development. The comprehensive accumulation patterns of carotenoids were closely related to flesh color. A number of potential structural genes and transcription factors were found to be associated with the carotenoid biosynthesis pathway using comparative transcriptome analysis. The differentially expressed genes were divided into six subclusters and distributed in different GO terms and metabolic pathways. Furthermore, we performed weighted gene co-expression network analysis and predicted the hub genes in six main modules determining carotenoid contents. Cla018406 (a chaperone protein dnaJ-like protein) may be a candidate gene for ß-carotene accumulation and highly expressed in orange flesh-colored fruit. Cla007686 (a zinc finger CCCH domain-containing protein) was highly expressed in the red flesh-colored watermelon, maybe a key regulator of lycopene accumulation. Cla003760 (membrane protein) and Cla021635 (photosystem I reaction center subunit II) were predicted to be the hub genes and may play an essential role in yellow flesh formation. CONCLUSIONS: The composition and contents of carotenoids in five watermelon genotypes vary greatly. A series of candidate genes were revealed through combined analysis of metabolites and transcriptome. These results provide an important data resource for dissecting candidate genes and molecular basis governing flesh color formation in watermelon fruit.

Identification of key gene networks controlling organic acid and sugar metabolism during watermelon fruit development by integrating metabolic phenotypes and gene expression profiles.

The organoleptic qualities of watermelon fruit are defined by the sugar and organic acid contents, which undergo considerable variations during development and maturation. The molecular mechanisms underlying these variations remain unclear. In this study, we used transcriptome profiles to investigate the coexpression patterns of gene networks associated with sugar and organic acid metabolism. We identified 3 gene networks/modules containing 2443 genes highly correlated with sugars and organic acids. Within these modules, based on intramodular significance and Reverse Transcription Quantitative polymerase chain reaction (RT-qPCR), we identified 7 genes involved in the metabolism of sugars and organic acids. Among these genes, Cla97C01G000640, Cla97C05G087120 and Cla97C01G018840 (r2 = 0.83 with glucose content) were identified as sugar transporters (SWEET, EDR6 and STP) and Cla97C03G064990 (r2 = 0.92 with sucrose content) was identified as a sucrose synthase from information available for other crops. Similarly, Cla97C07G128420, Cla97C03G068240 and Cla97C01G008870, having strong correlations with malic (r2 = 0.75) and citric acid (r2 = 0.85), were annotated as malate and citrate transporters (ALMT7, CS, and ICDH). The expression profiles of these 7 genes in diverse watermelon genotypes revealed consistent patterns of expression variation in various types of watermelon. These findings add significantly to our existing knowledge of sugar and organic acid metabolism in watermelon.

Genetic Mapping and Discovery of the Candidate Gene for Black Seed Coat Color in Watermelon (Citrullus lanatus).

Seed coat color is an important trait highly affecting the seed quality and flesh appearance of watermelon (Citrullus lanatus). However, the molecular regulation mechanism of seed coat color in watermelon is still unclear. In the present study, genetic analysis was performed by evaluating F1, F2 and BC1 populations derived from two parental lines (9904 with light yellow seeds and Handel with black seeds), suggesting that a single dominant gene controls the black seed coat. The initial mapping result revealed a region of interest spanning 370 kb on chromosome 3. Genetic mapping with CAPS and SNP markers narrowed down the candidate region to 70.2 kb. Sequence alignment of the three putative genes in the candidate region suggested that there was a single-nucleotide insertion in the coding region of Cla019481 in 9904, resulting in a frameshift mutation and premature stop codon. The results indicated that Cla019481 named ClCS1 was the candidate gene for black seed coat color in watermelon. In addition, gene annotation revealed that Cla019481 encoded a polyphenol oxidase (PPO), which involved in the oxidation step of the melanin biosynthesis. This research finding will facilitate maker-assisted selection in watermelon and provide evidence for the study of black seed coat coloration in plants.