When it comes to the must-have food for the Spring Festival, sweet and sour, delicious and nutritious longan must be on the list. As a band fruit, it is native to South Asia and is cultivated in places like southwestern India, Sri Lanka, and southern China.
Generally speaking, the part we eat is the flesh of fresh longan, which also has another name called aril. And the sugar accumulation it exhibits, mainly in the form of sucrose.
Unlike many fruits, longan does not differ much in color from the peel in its ripe and unripe state, which makes harvesting difficult. At the same time, once the pulp is fully ripe, the sugar content and sweetness will decrease, commonly known as "sugar reduction".
Therefore, in order to improve the quality and harvesting efficiency of longan pulp, it is important to explore the reasons for the decline in sugar content of pulp.
According to reports, in previous reports of this kind, researchers in this field usually analyzed the reasons for the decline in pulp sugar from the perspective of physiology and biochemistry.
Recently, a research team from the Institute of Pomology of the Guangdong Academy of Agricultural Sciences has studied the molecular mechanism and found that the main role is sucrose metabolism and transport.
The researchers selected three 10-year-old varieties of Shi Kip longan grown under similar planting conditions and collected 17 samples at different stages of development (days after flowering). At the same time, four biological replicates were prepared at each stage, each of which included six fruits.
Subsequently, transcriptome and metabolome analysis were performed on these sample tissues, and 873 metabolites and 26,584 annotated single genes were identified.
Then, they came to the following conclusions.
First, the changes in sugar content in aril tissues are closely related to gene expression patterns related to starch and sucrose metabolism.
Specifically, the upregulation of glycolysis, tricarboxylic acids, fermentation, and energy metabolism played a promoting role in the process of sugar decline in Longan shikip. In addition, soluble acid invertase is also an important factor leading to the decline of sugar content in longan.
Compared with Lidongben longan, the soluble acid invertase activity and sucrose phosphate synthase activity of longan were higher and the activity of sucrose phosphate synthase was lower, which may be the direct reason for the faster degradation rate of sucrose in this longan. (Editor's note: Shi Kip, Lidong Ben and Ci H are different varieties of longan.) ）
Second, gene expression in the glycolytic pathway, as well as cellulose and lignin content, increased during the sugar decline phase compared to the sugar accumulation phase.
In addition, in addition to the inevitable respiratory consumption during the development of longan fruit, the interruption of sucrose transport is also the key to the sudden decline of longan sucrose content.
In short, glycolysis, cellulose and lignin synthesis, and disruption of sucrose transport are the main reasons for the decline in sugar content in longan arils.
Based on this result, the research group constructed a regulatory model for the decline of sugar content in longan pulp.
During the sugar accumulation stage, the sucrose synthesized by the leaves will be transported to the aril through the sucrose transporter, and respiratory depletion will occur.
Then, when the sucrose in the aril reaches the peak level, it will begin to inhibit the expression of sucrose transporter, and then disrupt the normal transport process of sucrose.
Of the sucrose metabolites produced, some of them are consumed by respiration, and the other part is used to synthesize cellulose and lignin, which will reduce the sugar content of longan pulp.
Fig丨Regulatory model of sugar decline in longan (**lwt - food science and technology).
Recently, the related ** article was published in LWT - Food Science and Fruit with the title of "Disrupted Sugar Transport and Continued Sugar Consumption Lead to Sugar Decline in ripe 'Shixia' Longan Fruit". technology[1].
Fig丨Related** (LWT - Food Science and Technology).
Dr. Huang Shilian of the Institute of Pomology of the Guangdong Academy of Agricultural Sciences is the first author of the **, and Guo Dongliang, associate researcher of the Institute of Pomology of the Guangdong Academy of Agricultural Sciences, is the corresponding author of the **.
It is understood that in order to delay the decline of sugar content in longan pulp and improve the quality of the fruit, researchers in this field usually use means to suppress respiratory consumption.
For example, during the ripening of longan pulp, respiratory inhibitors such as rotenone, potassium azide, and sodium azide are sprayed. However, this method can bring certain harm to humans and the environment.
Now, based on this research, the team offers a new way to delay the decline in sugar content in longan pulp by regulating sugar transport rather than inhibiting respiratory exertion.
In ripe fruits, transient transformation by Agrobacterium can be used to increase the expression of sucrose transporters or transcription factors.
Alternatively, virus-induced gene silencing technology can be used to silence the gene expression of transcriptional repressors, and finally achieve sustained expression of sucrose transporters. Allow the sugar to continue to be transported to the longan aril to delay the decline of sugar content.
In fact, since there are not many studies related to longan pulp sucrose transporters, on the basis of this research, the research group plans to continue to study its biological function and the role it plays in the process of sugar reduction.
References: 1s., huang, x., lv.et al. disrupted sugar transport and continued sugar consumption lead to sugar decline in ripe ‘shixia’ longan fruit. lwt-food science and technology 191, 115620(2023).
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