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The sweet melon fruit is characterized by a metabolic transition during its development that leads to extensive accumulation of the disaccharide sucrose in the mature fruit. While the biochemistry of the sugar metabolism pathway of the cucurbits has been well studied, a comprehensive analysis of the pathway at the transcriptional level allows for a global genomic view of sugar metabolism during fruit sink development. We identified 42 genes encoding the enzymatic reactions of the sugar metabolism pathway in melon. The expression pattern of the 42 genes during fruit development of the sweet melon cv Dulce was determined from a deep sequencing analysis performed by 454 pyrosequencing technology, comprising over 350,000 transcripts from four stages of developing melon fruit flesh, allowing for digital expression of the complete metabolic pathway. The results shed light on the transcriptional control of sugar metabolism in the developing sweet melon fruit, particularly the metabolic transition to sucrose accumulation, and point to a concerted metabolic transition that occurs during fruit development.

The consumption of sweeteners, natural as well as synthetic sugars, is implicated in an array of modern-day health problems. Therefore, natural nonsugar sweeteners are of increasing interest. We identify here the biosynthetic pathway of the sweet triterpenoid glycoside mogroside V, which has a sweetening strength of 250 times that of sucrose and is derived from mature fruit of luohan-guo (Siraitia grosvenorii, monk fruit). A whole-genome sequencing of Siraitia, leading to a preliminary draft of the genome, was combined with an extensive transcriptomic analysis of developing fruit. A functional expression survey of nearly 200 candidate genes identified the members of the five enzyme families responsible for the synthesis of mogroside V: squalene epoxidases, triterpenoid synthases, epoxide hydrolases, cytochrome P450s, and UDP-glucosyltransferases. Protein modeling and docking studies corroborated the experimentally proven functional enzyme activities and indicated the order of the metabolic steps in the pathway. A comparison of the genomic organization and expression patterns of these Siraitia genes with the orthologs of other Cucurbitaceae implicates a strikingly coordinated expression of the pathway in the evolution of this species-specific and valuable metabolic pathway. The genomic organization of the pathway genes, syntenously preserved among the Cucurbitaceae, indicates, on the other hand, that gene clustering cannot account for this novel secondary metabolic pathway.

Over the past century, Israel was a focal point for the improvement of local cucurbit landraces and introduction of cucurbit germplasm. Some improved open-pollinated cucurbit cultivars developed in Israel became established as market types of considerable economic importance far beyond its borders. The origin and history of these cultivars is not widely known and therefore the purpose of the present work was to collect and compare the records relevant to the development of these cultivars, and to describe them more fully. The four economically most important cultivars originated through mass-selection by amateur breeders and were named after their respective farming communities. The ‘Bet Alfa’ cucumber (Cucumis sativus) was selected from a local landrace and introduced in 1936. The ‘Malali’ watermelon (Citrullus lanatus) originated as a rogue in a local landrace and was commercialized around 1940. The ‘Ananas Yoqne‘am’ melon (Cucumis melo) was selected from a local landrace and commercialized around 1950. The ‘Ha‘Ogen’ melon was selected from a cultivar introduced from Hungary and commercialized in the 1950s. The outstanding fruit quality of these four cultivars resulted in their widespread planting in Israel and neighboring countries. Moreover, the quality of the cucumber and melon cultivars inspired successive improvements, notably introgression of disease resistance and development of hybrids, by Israeli breeder-geneticists. The ever-increasing demand for their high-quality fruits established as international market types the Bet Alfa cucumber, the Ananas Yoqne‘am melon, and the Ha‘Ogen melon and its derivative, the Galia melon, and each is intensively bred today by local and multinational seed companies.

Taste has been the subject of human selection in the evolution of agricultural crops, and acidity is one of the three major components of fleshy fruit taste, together with sugars and volatile flavour compounds. We identify a family of plant-specific genes with a major effect on fruit acidity by map-based cloning of C. melo PH gene ( CmPH ) from melon, Cucumis melo taking advantage of the novel natural genetic variation for both high and low fruit acidity in this species. Functional silencing of orthologous PH genes in two distantly related plant families, cucumber and tomato, produced low-acid, bland tasting fruit, showing that PH genes control fruit acidity across plant families. A four amino-acid duplication in CmPH distinguishes between primitive acidic varieties and modern dessert melons. This fortuitous mutation served as a preadaptive antecedent to the development of sweet melon cultigens in Central Asia over 1,000 years ago. Fruit acidity is an important factor affecting fleshy fruit taste. Here, the authors identify the PH gene that regulates fruit acidity in a number of species and report a mutation that is responsible for the diversification and evolution of the sweet melon.

The broad variability of Cucumis melo (melon, Cucurbitaceae) presents a challenge to conventional classification and organization within the species. To shed further light on the infraspecific relationships within C. melo, we compared genotypic and metabolomic similarities among 44 accessions representative of most of the cultivar-groups. Genotyping-by-sequencing (GBS) provided over 20,000 single-nucleotide polymorphisms (SNPs). Metabolomics data of the mature fruit flesh and rind provided over 80,000 metabolomic and elemental features via an orchestra of six complementary metabolomic platforms. These technologies probed polar, semi-polar, and non-polar metabolite fractions as well as a set of mineral elements and included both flavor- and taste-relevant volatile and non-volatile metabolites. Together these results enabled an estimate of “metabolomic/elemental distance” and its correlation with the genetic GBS distance of melon accessions. This study indicates that extensive and non-targeted metabolomics/elemental characterization produced classifications that strongly, but not completely, reflect the current and extensive genetic classification. Certain melon Groups, such as Inodorous, clustered in parallel with the genetic classifications while other genome to metabolome/element associations proved less clear. We suggest that the combined genomic, metabolic, and element data reflect the extensive sexual compatibility among melon accessions and the breeding history that has, for example, targeted metabolic quality traits, such as taste and flavor.

Fil: William Allwood, J.. The James Hutton Institute; Reino Unido

Melon is a highly polymorphic species that comprises an extensive number of wild and cultivated genotypes that can be sub‐clustered according to different traits. Melon exhibits extreme diversity for fruit traits. Fruits of the species vary in size, shape, sugar content, acidity, external and internal color, and aroma, traits that are basic for marketing type characterization. This variability provides an intriguing subject for investigations into the genetic and physiological basis of fruit appearance and quality, the latter of which is determined largely by taste, sugar content, and aroma. Melons accumulate a large number of volatile aroma compounds. Attempts to evaluate the differences in the aroma properties and the aroma compounds of cantaloupes, muskmelons and honeydews have been described. An understanding of the regulation of the biosynthesis of melon fruit aroma formation will permit the formulation of efficient molecular breeding tools that will lead to the development of elite long shelf‐life (LSL) varieties.