## 1 Introduction  
Ascorbic acid (AsA), which is also called vitamin C, is an essential antioxidant molecule in plant and animal metabolism. While many animals are able to synthesize ascorbate in the liver or kidneys, others, such as humans, have lost this ability due to the accumulation of mutations in the coding sequence of the last committed enzyme in the pathway (L-gallono-1,4-lactone oxidase, GULO) [1, 2]. A lack of AsA has been shown to induce various diseases in humans [3], and the human diet must incorporate fresh fruits or vegetables, which are the primary sources of AsA for humans [4]. In plants, the ascorbic acid content is regulated by the complex network of ascorbate biosynthesis, degradation, and recycling pathways. Four well-defined or presumed biosynthetic pathways for AsA have been proposed [5, 6, 7, 8], i.e., the L-galactose, galacturonate, myo-inositol, and L-glucose pathways. The specific process of the L-galactose pathway has been well clarified. Starting with D-6-phospho fructose, the synthesis of AsA requires that a total of eight enzymes, such as _GME_ (GDP-D-mannose-3/5,5-epimerase), _GGP_ (GDP-L-galactose phosphorylase), and _GPP_ (L-galactose-1-phosphate phosphatase [9, 10]) participate in catalysis. Transformation with these key genes results in very significant increases in tissue ascorbate concentrations [11, 12, 13, 14]. For example, the transient co-expression of _GGP_ and _GME_ in tobacco leaves caused a seven-fold increase in AsA content [15]. In _Arabidopsis_, _GGP_-overexpressing lines had a 2.9-fold enhancement of AsA, whereas double-gene transformation with _GGP_-_GPP_ and _GGP_-_GLDH_ led to up to a 4.1-fold AsA increase [16]. Despite the many results obtained by overexpression AsA-related genes, limited success has been reported in many horticultural plants. In light of this, attention should be shifted to the manipulation of components of the regulatory network of AsA in horticultural plants. _Actinidia_, which is commonly known as kiwifruit, is mainly planted in China, Italy, New Zealand, Iran, and Greece [17]. _Actinidia_ contains approximately 54 species [18], of which four are economically cultivated, including _Actinidia chinensis_ [PERSON] (_A. chinensis_), _Actinidia chinensis_ var. _delicosa_ A. Chevalier (_A. delicosa_), _Actinidia eriantha_ [PERSON] (_A. eriantha_), and _Actinidia arguta_ ([PERSON] & [PERSON]) [PERSON]. (_A. arguta_) [18]. Kiwifruit has been mentioned as 'the King of Vitamin C' based on its remarkable abundance of AsA, polyphenols, and other beneficial health metabolites [19]. However, the AsA content in _Actinidia_ species such as _A. latifolia_ is more than ten times that of commercial kiwifruit. Identifying the genetic basis of AsA variation between kiwifruit varieties and discovering how some'superfruits' accumulate extremely high levels of AsA should reveal new ways to more effectively manipulate the production of AsA in crops. High-throughput sequencing technologies have been used to map and study the genomes, transcriptome, proteomes, and metabolomics of living organisms, tissues, organelles, and even single cells in detail [20; 21; 22]. In recent years, RNA sequencing (RNA-seq) technology has attracted more attention for analyzing metabolism and physiology during plant growth and development, especially in fruits and vegetables. Currently, few genomic studies have been conducted on _Actinidia_ species, as only two domesticated species (_A. eriantha_ and _A. chinenses_) genomes are available in the NCBI GenBank ([[https://www.ncbi.nlm.nih.gov/assembly/?term=actinidia](https://www.ncbi.nlm.nih.gov/assembly/?term=actinidia)]([https://www.ncbi.nlm.nih.gov/assembly/?term=actinidia](https://www.ncbi.nlm.nih.gov/assembly/?term=actinidia))) (19 October 2021) [23; 24]. For some other kiwifruit species whose genomes are not publicly available, there are limitations hindering the rapid and accurate investigation of genes.