Apoplastic degradation of ascorbate: Novel enzymes and metabolites permeating the plant cell wall

Abstract

A significant minority of the vitamin C in plants is located in the apoplast. We discuss recent progress towards mapping the pathway by which apoplastic l-ascorbate is converted to oxalate plus l-threonate. At least two novel compounds have been detected as apoplastic intermediates in the pathway: namely, 4-O-oxalyl-l-threonate and cyclic oxalyl di-ester(s) of l-threonate. In addition, evidence is presented for a dehydroascorbate oxidase activity and two novel oxalyl-esterase activities involved in the pathway. The operation of the pathway may augment the proposed role of ascorbate as a pro-oxidant since several steps in the pathway potentially generate H₂O₂. We argue that, whether acting as a pro-oxidant or in its better-known capacity as an anti-oxidant, apoplastic ascorbate may loosen the cell wall and hence promote cell expansion and/or fruit softening.

Keywords:

• Ascorbate
• apoplast
• dehydroascorbate
• oxalyl esters
• reactive oxygen species (ROS)
• wall loosening

Acknowledgements

We thank Dr Bernard Dudley and Mrs Janice Miller for excellent technical assistance. M.A.G. thanks the BBSRC for a research studentship during the tenure of which this work was conducted.

Notes

A, dehydro-l-ascorbate; AH₂, l-ascorbate; AO, l-ascorbate oxidase; D, cyclic oxalyl di-ester(s) of l-threonate; F, 4-O-oxalyl-l-threonate.

Additional information

Notes on contributors

MA Green

A, dehydro-l-ascorbate; AH2, l-ascorbate; AO, l-ascorbate oxidase; D, cyclic oxalyl di-ester(s) of l-threonate; F, 4-O-oxalyl-l-threonate.