Ascorbic acid: metabolism and functions of a multi-facetted molecule

Area of Science:

• Plant biochemistry
• Molecular genetics
• Cellular biology

Background:

• Ascorbic acid (vitamin C) is a crucial plant antioxidant.
• Its biosynthetic pathway was recently proposed, involving GDP-D-mannose and L-galactose.
• The pathway's localization and interaction with other cellular processes were unclear.

Purpose of the Study:

• To provide molecular genetic evidence for the proposed plant ascorbic acid biosynthetic pathway.
• To elucidate the subcellular localization and intermediate sharing of this pathway.
• To highlight the role of ascorbate peroxidase in hydrogen peroxide (H2O2) homeostasis and ascorbate's function in plant growth.

Main Methods:

• Molecular genetic analysis in Arabidopsis thaliana.
• Transgenic potato plant studies.
• Biochemical pathway analysis focusing on GDP-sugar intermediates.

Main Results:

• Molecular genetic evidence supports the GDP-D-mannose/L-galactose pathway for ascorbic acid synthesis in plants.
• The pathway is predominantly cytosolic, except for the final step on the inner mitochondrial membrane.
• Ascorbate peroxidase identified as key in regulating H2O2 concentration, with its expression linked to redox signals.
• Ascorbate's involvement in cell division, growth, and cell wall hydroxyproline-rich glycoprotein synthesis confirmed.

Conclusions:

• The study validates the novel biosynthetic route for ascorbic acid in plants.
• Ascorbic acid synthesis is integrated with cell wall and glycoprotein metabolism.
• Ascorbate plays multifaceted roles in plant physiology, including growth regulation and oxidative stress management.