Methylation Profiling of Biosynthetic Genes Reveals The Role of D-Galacturonic Acid Reductase In Ascorbic Acid Accumulation In Tomato Fruit

Ascorbic acid (AsA) is an important nutrient component contributing to major flavor value of tomato fruit and human health. Although transcription regulation of AsA biosynthetic genes have been well demonstrated, epigenetic modification underlying AsA accumulation remains unclear. In this study, we exposed immature tomato fruits to a methyltransferase inhibitor (5-azacytidine) and detected the impacts on AsA accumulation. Inhibition of DNA methylation enhanced AsA accumulation in tomato leaves and fruits. We further isolated a AsA biosynthetic gene, SlGalUR5, which encodes a D-galacturonic acid reductase. SlGalUR5 showed reduced DNA methylation levels and higher transcription levels in Slmet1 mutant while have converse pattern in Sldml2 mutant. 5-azacytidine treatment significantly decreased DNA methylation levels of SlGalUR5 in fruits. Conversely, transcription profiles of SlGalUR5 and enzyme activity of GalUR were enhanced in 5-azacytidine–treated fruits. Our finding revealed a new insight into epigenome modification of SlGalUR5 involved in ascorbic acid accumulation and provide a potential means of increasing AsA levels for tomato breeding.