Dynamic Changes of Ascorbic Acid Occurring During Fruit Development and Ripening of Actinida Latifolia and Their Associated Molecular Mechanisms Deciphered by Weighted Gene Co-Expression Network Analysis

Background: Actinidia latifolia is an exceptional source with extremely high ascorbic acid (AsA) content. However, its transcriptome atlas is lacking and how AsA accumulates during fruit development and ripening of this special kiwifruit and its associated molecular mechanisms are still poorly understood. Results: : Herein, the dynamic changes of AsA content of six stages of A. latifolia fruit development and ripening determined by HPLC demonstrated a rapid increasing profile during the initial expansion stage with a peak around 60 days after flowering (1194.21±69.25 mg 100g -1 FW), followed by a progressive, albeit not significant decrease tendency and reached the minimum levels (1028.76±31.19 mg 100g -1 FW) at maturity. A high-quality full-length (FL) transcriptome of A. latifolia was successfully constructed by third-generation sequencing for the first time, comprising 326,926 FL non-chimeric reads, 15,505 coding sequences, 2882 transcription factors, 18,797 simple sequence repeats, 3328 long noncoding RNAs, and 231 alternative splicing events. Illumina RNA-seq in combination with weighted gene co-expression network analysis revealed a network module highly correlated (r=0.5, p=0.03) with AsA content. Gene co-expression in this network module was explained by its roles in protein processing in endoplasmic reticulum (ko04141), glycolysis/gluconeogenesis (ko00010), and carbon metabolism (ko01200). Moreover, the expression patterns of genes involved in AsA biosynthesis and metabolism validated by qRT-PCR exhibited a similar trend with AsA accumulation. Conclusions: : Overall, the dynamic changes of AsA content and associated key genes and enriched metabolic pathways were deciphered, which paves the way for genetic improvement that aims for development of kiwifruit with super-high AsA content.