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Two genes in Arabidopsis thaliana encoding GDP‐l‐galactose phosphorylase are required for ascorbate biosynthesis and seedling viability

Two genes in Arabidopsis thaliana encoding GDP‐l‐galactose phosphorylase are required for... Plants synthesize ascorbate from guanosine diphosphate (GDP)‐mannose via l‐galactose/l‐gulose, although uronic acids have also been proposed as precursors. Genes encoding all the enzymes of the GDP‐mannose pathway have previously been identified, with the exception of the step that converts GDP‐l‐galactose to l‐galactose 1‐P. We show that a GDP‐l‐galactose phosphorylase, encoded by the Arabidopsis thaliana VTC2 gene, catalyses this step in the ascorbate biosynthetic pathway. Furthermore, a homologue of VTC2, At5g55120, encodes a second GDP‐l‐galactose phosphorylase with similar properties to VTC2. Two At5g55120 T‐DNA insertion mutants (vtc5‐1 and vtc5‐2) have 80% of the wild‐type ascorbate level. Double mutants were produced by crossing the loss‐of‐function vtc2‐1 mutant with each of the two vtc5 alleles. These show growth arrest immediately upon germination and the cotyledons subsequently bleach. Normal growth was restored by supplementation with ascorbate or l‐galactose, indicating that both enzymes are necessary for ascorbate generation. vtc2‐1 leaves contain more mannose 6‐P than wild‐type. We conclude that the GDP‐mannose pathway is the only significant source of ascorbate in A. thaliana seedlings, and that ascorbate is essential for seedling growth. A. thaliana leaves accumulate more ascorbate after acclimatization to high light intensity. VTC2 expression and GDP‐l‐galactose phosphorylase activity rapidly increase on transfer to high light, but the activity of other enzymes in the GDP‐mannose pathway is little affected. VTC2 and At5g55120 (VTC5) expression also peak in at the beginning of the light cycle and are controlled by the circadian clock. The GDP‐l‐galactose phosphorylase step may therefore play an important role in controlling ascorbate biosynthesis. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Plant Journal Wiley

Two genes in Arabidopsis thaliana encoding GDP‐l‐galactose phosphorylase are required for ascorbate biosynthesis and seedling viability

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References (109)

Publisher
Wiley
Copyright
Copyright © 2007 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0960-7412
eISSN
1365-313X
DOI
10.1111/j.1365-313X.2007.03266.x
pmid
17877701
Publisher site
See Article on Publisher Site

Abstract

Plants synthesize ascorbate from guanosine diphosphate (GDP)‐mannose via l‐galactose/l‐gulose, although uronic acids have also been proposed as precursors. Genes encoding all the enzymes of the GDP‐mannose pathway have previously been identified, with the exception of the step that converts GDP‐l‐galactose to l‐galactose 1‐P. We show that a GDP‐l‐galactose phosphorylase, encoded by the Arabidopsis thaliana VTC2 gene, catalyses this step in the ascorbate biosynthetic pathway. Furthermore, a homologue of VTC2, At5g55120, encodes a second GDP‐l‐galactose phosphorylase with similar properties to VTC2. Two At5g55120 T‐DNA insertion mutants (vtc5‐1 and vtc5‐2) have 80% of the wild‐type ascorbate level. Double mutants were produced by crossing the loss‐of‐function vtc2‐1 mutant with each of the two vtc5 alleles. These show growth arrest immediately upon germination and the cotyledons subsequently bleach. Normal growth was restored by supplementation with ascorbate or l‐galactose, indicating that both enzymes are necessary for ascorbate generation. vtc2‐1 leaves contain more mannose 6‐P than wild‐type. We conclude that the GDP‐mannose pathway is the only significant source of ascorbate in A. thaliana seedlings, and that ascorbate is essential for seedling growth. A. thaliana leaves accumulate more ascorbate after acclimatization to high light intensity. VTC2 expression and GDP‐l‐galactose phosphorylase activity rapidly increase on transfer to high light, but the activity of other enzymes in the GDP‐mannose pathway is little affected. VTC2 and At5g55120 (VTC5) expression also peak in at the beginning of the light cycle and are controlled by the circadian clock. The GDP‐l‐galactose phosphorylase step may therefore play an important role in controlling ascorbate biosynthesis.

Journal

The Plant Journal – Wiley

Published: Nov 1, 2007

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