Alternative pathways leading to ascorbate biosynthesis in plants: lessons from the last 25 years.

This article critically reviews the evidence supporting three alternative biosynthetic pathways for l-ascorbic acid (AsA, vitamin C) production in plants: the d-galacturonate, l-gulose, and myo-inositol pathways. It summarizes feeding experiments, gene expression analyses, and transgenic studies across multiple plant species that demonstrate these pathways contribute to AsA synthesis, often enhancing AsA content and improving tolerance to abiotic stresses such as salt, heat, and drought. While the classical d-mannose/l-galactose pathway remains the primary route, these alternative pathways appear vital for maintaining AsA levels under stress or when the main pathway's flux is reduced. The review highlights the need for further characterization of the enzymes and regulatory mechanisms involved in these alternative routes to better understand their roles in plant physiology and crop improvement.