Melons Have “Jumping Sequences” that Induces Fruit Ripening: Genetic Study
Wed, April 21, 2021

Melons Have “Jumping Sequences” that Induces Fruit Ripening: Genetic Study


Melon has a diverse nutrient profile and is one of the most economically important fruit crops in the world. It is believed to have originally diversified in Asia and is known to exhibit a wide natural variation, particularly in fruit phenotypes. One notable feature of melon is the coexistence of both climacteric (ripen after being picked) and non-climacteric fruit types. As opposed to climacteric fruits, non-climatic fruits do not ripen further once harvested. Such substantial natural variation when it comes to fruit ripening physiology and the economic losses due to the short shelf life of other produce makes it an interesting focus of the study.

Retrotransposons and how it affects the gene expression in melons

A team of researchers from the University of Tsukuba in Japan even recently found that melon has hidden depths called retrotransposons or sometimes called the “jumping sequences.” It is a type of genetic component that copy and paste themselves into different genomic locations and may change how genes are expressed. The retrotransposons may affect gene expression of melon that induces fruit ripening, the authors explained via Science Daily.

Professor Hiroshi Ezura, the lead author of the study, said the Harukei-3 melon genome produces ethylene during ripening. Ethylene is a plant hormone that plays an important role in the regulation of the climacteric fruit-ripening traits, like shelf-life. If grown in the right seasons, the Harukei-3 melons produce a considerably sweeter fruit than other melon accessions. This is also why it has been used for a long time in Japan as a standard type for breeding high-grade muskmelon since it has an attractive appearance and a sweet taste. The word “harukei” in Japan is a line suitable for growing in the spring, their study reads.



Comparing the Harukei-3 with other melon genomes

The team examined the ethylene-related gene expression of the fruit by assembling its whole genome sequence. They used a third-generation nanopore sequencing that is paired with next-generation sequencing and optical mapping.  Then, they compared the genome of Harukei-3 with other melon genomes. They found that there are genome-wide presence/absence polymorphisms (PAPs) of retrotransposon-related sequences between melon accessions. A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population.

Comparison between Harukei-3 and DHL92, which is the first melon genome, also enabled the team to identify 24,758 one-to-one orthologue gene pairs. Further comparison based on 10 melon genome assemblies identified 415 retrotransposons. Of these, 160 (38.6%) showed fruit ripening-inducible expression. About 59.4% of the neighboring genes also showed similar expression patterns. In short, it co-expressed with neighboring genes.

Prof. Ezura and colleagues said that retrotransposons contributed to the alteration of gene expression during the diversification of melon genomes and could potentially affect the fruit ripening-inducible gene expression.

Senior author Dr. Ryoichi Yano also shared that some retrotransposon-related sequences in the fruit were transcribed when the fruits were subjected to heat stress. They added that the Harukei-3 genome assembly is now available in the Melonet-DB database and it will contribute to the functional genomic study of melons, particularly reverse genetics.

Plant genetics

When advancements in plant genetics and genomics are used in breeding, it can help support higher cultivation and production of crops that are resistant to pests, drought, and pathogens. So far, scientific advances in plant genomics and genetics include producing a more affordable price for vegetables and fruits, having an abundant staple corps for producing dairy products and meat, development of sustainable sources of energy, and reductions in agriculture-related pollution. This is according to the US Department of Agriculture, which is not involved in the study.



World’s top melon producing countries

Atlas Big shares that China is the world’s leading melon producer with 16,009,584 tones yearly production and 11.486kg production per person. It is followed by Turkey (1,854,356t), Iran (1,615,642t), Egypt (1,060,619t), India (1,028,650t), Kazakhstan (898,004t), United States of America (783,950 t), Spain (661,897t), Italy (632,322t), Guatemala (623,726t), and Brazil (596,430t).

In Japan, the yield amount of melon is decreasing by 29.4% from 2006 to 2018, according to Japan Crops.  In 2006, the total yield amount of melon amounted to 216,600t and it slightly increased in 2007 to 221,300t. However, in 2018 the yield amount stood only at 12,900t. The harvesting season for melon in the country usually starts in May and ends in September.

There are different ways to use melons. Since it is full of water, the fruit is hydrating and refreshing, making it an ideal summer ingredient. One way to use the fruit is by pairing it with salty ingredients, like feta or olives, to make a salad. It can also be prepared as a salsa dish by cutting it into small cubes and add lime juice, herbs, chiles, and scallions. You may also use it as an agua fresca by blending it with strawberries, sugar, and water.

Others use melon for soup while some as a frozen dessert. The preparation as a dessert it simple. Just freeze the melon puree and then scrape it into fluffy ice or just freeze the fruit as ice cube trays for mini ice pops, as recommended by Food & Wine magazine.

Registered dietitian Ansley Hill, LD, who is also not involved in the study, said that a honeydew melon  (melon species muskmelon) is rich in nutrients and may help reduce the risk of high blood pressure and heart disease. Since it is low in sodium and rich in potassium, the honeydew melon can help one maintain a healthy blood pressure level. The fruit moreover contains nutrients that are vital to maintaining and repairing strong bones. These nutrients include magnesium, vitamin K, and folate. Honeydew melons also contain carbs that can raise blood sugar temporarily and has fiber that improves blood sugar control over time.

The University of Tsukuba study is important as there has been a growing health-related concern in the usage of artificial fruit ripening agents. The proper practice has to be followed in inspecting and harvesting. Other countries, such as in Bangladesh, have even prohibited the use of certain chemicals to ripen fruits and they have a law that penalizes any person who is mixing, selling, and/or using illegally ripened fruits.