Ascorbate peroxidase activity, not the mRNA level, is enhanced in salt‐stressed Raphanus sativus plants

The effect of salt on ascorbate peroxidase (EC 1.11.1.11) activity and mRNA level in radish ( Raphanus sativus L. cv. Fakir) was investigated. cDNA encoding a radish ascorbate peroxidase was obtained and sequenced. Sequence data analysis showed that the coding region was well conserved among higher plants. The radish cDNA was used as a probe to measure the steady‐state level of ascorbate peroxidase mRNA in plants subjected to salt stress (100 m M NaCl). The levels of transcripts encoding ascorbate peroxidase were almost identical in control or salt‐grown radish plants in both leaves and roots. The activity of soluble ascorbate peroxidase was enhanced by the salt treatment in both leaves and roots. The salt‐induced increase in ascorbate peroxidase activity requires days to become significant and can be considered as a late response. The increase in activity, but not in mRNA level, suggests that the salt‐induced ascorbate peroxidase expression is probably the consequence of post‐transcriptional events.