This report describes the characterisation of a gene (APX20) from tomato pl
ants that encodes a cytosolic isoform of ascorbate peroxidase which is invo
lved in the detoxification of intracellular H2O2 Expression analysis of pro
moter-GUS fusions in transgenic plants reveals that the gene is under stric
t developmental control, becoming transcriptionally active in cells which a
re apparently undergoing mechanical stimulation generated during the differ
ent phases of growth. We show that the APX20 gene contains a large 5' leade
r intron which is required to confer constitutive gene expression in leaves
, but not in other organs of the plant. Based on these observations, we pro
pose that the observed trancriptional regulation of this gene may constitut
e a basic mechanism for deployment of antioxidative defences in plants, whi
ch act to limit the deleterious effects of H2O2 generated during normal pla
nt development.