Tissue-specific expression of vitamin C biosynthetic genes during fruit development in Nagpur sweet orange (Citrus sinensis L.)

Vitamin C (L-ascorbic acid, AsA) is an essential dietary antioxidant primarily obtained from fruits and vegetables, with Citrus serving as one of the most important natural sources. Despite extensive characterization of AsA content in citrus fruits, the molecular regulation of its biosynthesis, recycling and degradation across different tissues remains poorly understood. We analyzed tissue-specific expression of 16 biosynthetic genes in Nagpur sweet orange across three developmental stages. Results revealed that the L-galactose pathway, represented by GMP, GGP, GPP, GME, GDH and GLDH, was the predominant biosynthetic route, with early upregulation in all tissues, followed by tissue-specific modulation during maturation. Myo-inositol pathway genes (MyoOX and MyoIPS) displayed distinct tissue-dependent regulation, with sustained expression in pulp and late activation in albedo, suggesting a conditional role in AsA biosynthesis and cell wall metabolism. Members of the D-galacturonic acid pathway (GalUR8, GalUR10 and GalUR12) exhibited biphasic expression, peaking during fruit maturation, consistent with pectin turnover and localized AsA biosynthesis. Ascorbic acid (AsA) degradation genes (APX2, APX3 and AO) showed differential expression, with APX2 strongly induced in flavedo and pulp during early and late stages, highlighting its role in ROS management. Recycling genes (MDHAR and DHAR) were dynamically regulated, with DHAR induction at maturity in flavedo. Collectively, these findings provide the first tissue-specific molecular framework of AsA metabolism in Indian sweet orange, offering insights for metabolic engineering to enhance vitamin C content and fruit quality.