Predominant role of the L-galactose pathway in L-ascorbic acid biosynthesis in fruits and leaves of the Capsicum annuum L. chili pepper

L-ascorbic acid (vitamin C) is critical for animals and plants since it is a powerful antioxidant and cofactor involved in a wide number of biochemical reactions. Because humans cannot synthesize vitamin C, they must acquire it through the daily diet. Chili pepper fruits are an excellent source of ascorbic acid (AsA). However, how AsA is synthesized in the chili pepper remains poorly understood. Four pathways for AsA synthesis in plants have been reported namely the L-galactose, L-gulose, D-galacturonate, and myo-inositol pathways. In the present study, we investigated the pathways involved in vitamin C synthesis in the Serrano Tampiqueno 74 chili pepper. Our results showed a higher incorporation of labeled L-galactose into AsA compared to the incorporation of labeled myo-inositol and D-galacturonic acid. Based on these results, the L-galactose pathway is the predominant mechanism for AsA synthesis in the fruits and leaves of this chili pepper. In addition, qPCR analysis of the expression of genes involved in the L-galactose pathway revealed that, in general, GMP, GME, and GGP/VTC2 were most highly expressed. A positive correlation between gene expression and AsA levels in pericarp tissues during chili pepper fruit development and ripening was not observed. Expression analysis of GalUR and MIOX, which are involved in the D-galacturonic and myo-inositol pathways of AsA biosynthesis, respectively, also did not exhibit a positive correlation with ascorbic acid content in chili pepper fruits.