Changes in composition during ripening and storage of melons

Time-Series Transcriptome of Cucumis melo Reveals Extensive Transcriptomic Differences with Different Maturity

As the most important melon cultivar grown in the north-western provinces of China, Hami melon (Cucumis melo) produces large edible fruits that serve as an important dietary component in the world. In general, as a climacteric plant, melon harvested at 60% maturity results in a product with bad quality, while the highest-quality product can be guaranteed when harvesting at 90% maturity. In order to clarify the genetic basis of their distinct profiles of metabolite accumulation, we performed systematic transcriptome analyses between 60% and 90% maturity melons. A total of 36 samples were sequenced and over 1.7 billion reads were generated. Differentially expressed genes in 60% and 90% maturity melons were detected. Hundreds of these genes were functionally enriched in the sucrose and citric acid accumulation process of C. melo. We also detected a number of distinct splicing events between 60% and 90% maturity melons. Many genes associated with sucrose and citric acid accumulation displayed as differentially expressed or differentially spliced between different degrees of maturity of Hami melons, including CmCIN2, CmSPS2, CmBGAL3, and CmSPS2. These results demonstrate that the phenotype pattern differences between 60% and 90% maturity melons may be largely resulted from the significant transcriptome regulation.

Model-assisted comparison of sugar accumulation patterns in ten fleshy fruits highlights differences between herbaceous and woody species

BACKGROUND AND AIMS

Sugar concentration is a key determinant of fruit quality. Soluble sugars and starch concentrations in fruits vary greatly from one species to another. The aim of this study was to investigate similarities and differences in sugar accumulation strategies across ten contrasting fruit species using a modelling approach.

METHODS

We developed a coarse-grained model of primary metabolism based on the description of the main metabolic and hydraulic processes (synthesis of compounds other than sugar and starch, synthesis and hydrolysis of starch, and water dilution) involved in the accumulation of soluble sugars during fruit development.

KEY RESULTS

Statistical analyses based on metabolic rates separated the species into six groups according to the rate of synthesis of compounds other than sugar and starch. Herbaceous species (cucumber, tomato, eggplant, pepper and strawberry) were characterized by a higher synthesis rate than woody species (apple, nectarine, clementine, grape and kiwifruit). Inspection of the dynamics of the processes involved in sugar accumulation revealed that net sugar importation, metabolism and dilution processes were remarkably synchronous in most herbaceous plants, whereas in kiwifruit, apple and nectarine, processes related to starch metabolism were temporally separated from other processes. Strawberry, clementine and grape showed a distinct dynamic compared with all other species.

CONCLUSIONS

Overall, these results provide fresh insights into species-specific regulatory strategies and into the role of starch metabolism in the accumulation of soluble sugars in fleshy fruits. In particular, inter-specific differences in development period shape the co-ordination of metabolic processes and affect priorities for carbon allocation across species. The six metabolic groups identified by our analysis do not show a clear separation into climacteric and non-climacteric species, possibly suggesting that the metabolic processes related to sugar concentration are not greatly affected by ethylene-associated events.

Transcriptome profiling of Cucumis melo fruit development and ripening

Hami melon (Cucumis melo) is the most important melon crop grown in the north-western provinces of China. In order to elucidate the genetic and molecular basis of developmental changes related to size, flesh, sugar and sour content, we performed a transcriptome profiling of its fruit development. Over 155 000 000 clean reads were mapped to MELONOMICS genome, yielding a total of 23 299 expressed genes. Of these, 554 genes were specifically expressed in flowers, and 3260 genes in fruit flesh tissues. The 7892 differentially expressed genes (DEGs) were related to fruit development and mediated diverse metabolic processes and pathways; 83 DEGs and 13 DEGs were possibly associated with sucrose and citric acid accumulation, respectively. The quantitative real-time PCR results showed that six out of eight selected candidate genes displayed expression trends similar to our DEGs. This study profiled the gene expression related to different growing stages of flower and fruit at the whole transcriptome level to provide an insight into the regulatory mechanism underlying Hami melon fruit development.

Medieval emergence of sweet melons, Cucumis melo (Cucurbitaceae)

BACKGROUND

Sweet melons, Cucumis melo, are a widely grown and highly prized crop. While melons were familiar in antiquity, they were grown mostly for use of the young fruits, which are similar in appearance and taste to cucumbers, C. sativus. The time and place of emergence of sweet melons is obscure, but they are generally thought to have reached Europe from the east near the end of the 15th century. The objective of the present work was to determine where and when truly sweet melons were first developed.

METHODS

Given their large size and sweetness, melons are often confounded with watermelons, Citrullus lanatus, so a list was prepared of the characteristics distinguishing between them. An extensive search of literature from the Roman and medieval periods was conducted and the findings were considered in their context against this list and particularly in regard to the use of the word 'melon' and of adjectives for sweetness and colour.

FINDINGS

Medieval lexicographies and an illustrated Arabic translation of Dioscorides' herbal suggest that sweet melons were present in Central Asia in the mid-9th century. A travelogue description indicates the presence of sweet melons in Khorasan and Persia by the mid-10th century. Agricultural literature from Andalusia documents the growing of sweet melons, evidently casabas (Inodorous Group), there by the second half of the 11th century, which probably arrived from Central Asia as a consequence of Islamic conquest, trade and agricultural development. Climate and geopolitical boundaries were the likely causes of the delay in the spread of sweet melons into the rest of Europe.

Metabolism of soluble sugars in developing melon fruit: a global transcriptional view of the metabolic transition to sucrose accumulation

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.

A genetic map of melon highly enriched with fruit quality QTLs and EST markers, including sugar and carotenoid metabolism genes

A genetic map of melon enriched for fruit traits was constructed, using a recombinant inbred (RI) population developed from a cross between representatives of the two subspecies of Cucumis melo L.: PI 414723 (subspecies agrestis) and 'Dulce' (subspecies melo). Phenotyping of 99 RI lines was conducted over three seasons in two locations in Israel and the US. The map includes 668 DNA markers (386 SSRs, 76 SNPs, six INDELs and 200 AFLPs), of which 160 were newly developed from fruit ESTs. These ESTs include candidate genes encoding for enzymes of sugar and carotenoid metabolic pathways that were cloned from melon cDNA or identified through mining of the International Cucurbit Genomics Initiative database (http://www.icugi.org/). The map covers 1,222 cM with an average of 2.672 cM between markers. In addition, a skeleton physical map was initiated and 29 melon BACs harboring fruit ESTs were localized to the 12 linkage groups of the map. Altogether, 44 fruit QTLs were identified: 25 confirming QTLs described using other populations and 19 newly described QTLs. The map includes QTLs for fruit sugar content, particularly sucrose, the major sugar affecting sweetness in melon fruit. Six QTLs interacting in an additive manner account for nearly all the difference in sugar content between the two genotypes. Three QTLs for fruit flesh color and carotenoid content were identified. Interestingly, no clear colocalization of QTLs for either sugar or carotenoid content was observed with over 40 genes encoding for enzymes involved in their metabolism. The RI population described here provides a useful resource for further genomics and metabolomics studies in melon, as well as useful markers for breeding for fruit quality.

Cloning and expression analysis of a UDP-galactose/glucose pyrophosphorylase from melon fruit provides evidence for the major metabolic pathway of galactose metabolism in raffinose oligosaccharide metabolizing plants

The Cucurbitaceae translocate a significant portion of their photosynthate as raffinose and stachyose, which are galactosyl derivatives of sucrose. These are initially hydrolyzed by alpha-galactosidase to yield free galactose (Gal) and, accordingly, Gal metabolism is an important pathway in Cucurbitaceae sink tissue. We report here on a novel plant-specific enzyme responsible for the nucleotide activation of phosphorylated Gal and the subsequent entry of Gal into sink metabolism. The enzyme was antibody purified, sequenced, and the gene cloned and functionally expressed in Escherichia coli. The heterologous protein showed the characteristics of a dual substrate UDP-hexose pyrophosphorylase (PPase) with activity toward both Gal-1-P and glucose (Glc)-1-P in the uridinylation direction and their respective UDP-sugars in the reverse direction. The two other enzymes involved in Glc-P and Gal-P uridinylation are UDP-Glc PPase and uridyltransferase, and these were also cloned, heterologously expressed, and characterized. The gene expression and enzyme activities of all three enzymes in melon (Cucumis melo) fruit were measured. The UDP-Glc PPase was expressed in melon fruit to a similar extent as the novel enzyme, but the expressed protein was specific for Glc-1-P in the UDP-Glc synthesis direction and did not catalyze the nucleotide activation of Gal-1-P. The uridyltransferase gene was only weakly expressed in melon fruit, and activity was not observed in crude extracts. The results indicate that this novel enzyme carries out both the synthesis of UDP-Gal from Gal-1-P as well as the subsequent synthesis of Glc-1-P from the epimerase product, UDP-Glc, and thus plays a key role in melon fruit sink metabolism.

Sex determination in plants

The majority of flowering plants produce flowers that are "perfect." These flowers are both staminate (with stamens) and pistillate (with one or more carpels). In a small number of species, there is spatial separation of the sexual organs either as monoecy, where the male and female organs are carried on separate flowers on the same plant, or dioecy, where male and female flowers are carried on separate male (staminate) or female (pistillate) individuals. Sex determination systems in plants, leading to unisexuality as monoecy or dioecy, have evolved independently many times. In dioecious plant species, the point of divergence from the hermaphrodite pattern shows wide variation between species, implying that the genetic bases are very different. This review considers monoecious and dioecious flowering plants and focuses on the underlying genetic and molecular mechanisms. We propose that dioecy arises either from monoecy as an environmentally unstable system controlled by plant growth substances or from hermaphroditism where the underlying mechanisms are highly stable and control does not involve plant growth substances.