Regulation of ascorbic acid synthesis in plants

Abstract

Ascorbic acid (AsA) is a major antioxidant and plays an important role in plant growth and development. There are two aspects to improve AsA content, including de novo synthesis and recycling from its oxidized form. However, the knowledge of regulatory mechanisms of AsA synthesis pathways and metabolism still remains limited. We have determined that AsA synthesis process was modulated on both transcriptional and post-transcriptional levels in Arabidopsis. GDP-mannose pyrophosphorylase (VTC1) is the initial AsA biosynthetic enzyme in L-galactose pathway, we previously showed that Arabidopsis ERF98 transcriptionally activates gene expression of VTC1 to improve AsA content and respond salt stress; recent discovery of the interaction between photomorphogenic factor COP9 signalosome (CSN) subunit CSN5B and VTC1 indicates that CSN5B promotes VTC1 degradation in the dark, which keeps the change of AsA content from day to night. This mini-review integrates previous reports and recent evidence to better understand the regulatory mechanisms involved in AsA synthesis.

Keywords: CSN5B; ERF98; L-galactose pathway; VTC1; ascorbic acid; regulation.

Figure 1. Overview of the reported regulators on L-galactose pathway of AsA synthesis. The broken line box shows L-galactose pathway. Enzymes: (1) GDP-Man pyrophosphorylase (VTC1);(2) GDP-Man-3′,5′-epimerase (GME);(3) GDP-L-Gal phosphorylase (VTC2/VTC5); (4) L-Gal 1-phosphate phosphatase (GPP);(5) L-Gal dehydrogenase (GalDH); (6) L-GalL dehydrogenase (GalLDH). Salt stress triggers ERF98 expression and subsequently VTC1 higher expression on transcriptional level, finally improving AsA content to respond stress, but the modulation of ERF98 on other enzymes needs further research. Light inhibits AMR1 expression, which further deactivates the expression of the synthesis enzyme genes mediated by an unknown protein. When the light goes out, CSN5B carry out function to inhibit AsA synthesis on post-transcriptional level through interaction with VTC1 directly to promote its degradation.