Profiles of potentially protective
peroxidases
One effect of water deficit is the
production of reactive oxygen species including H2O2,
which the plant is able to remove through the activity of peroxidase enzymes.
In green leaves, the major detoxifying peroxidase is ascorbate peroxidase
(APX). D’Arcy-Lameta et al., Paris, France (pp. 133–140) report their investigation of APX in drought-tolerant and
non-tolerant varieties of cowpea (Vigna unguiculata). Plants were
exposed to progressive drought by withholding water. Baseline activity of APX
was much higher in the drought-tolerant cultivar than the non-tolerant variery,
and did not increase significantly during water deficit. However, in the
non-tolerant variety, activity increased as leaf water potential fell to –1.5
MPa but declined to control levels as water deficit became more severe. Cloning
and sequencing of APX cDNAs revealed the existence of four genes, identified as
encoding isoenzymes in the cytosol, peroxisomes and chloroplasts (two genes:
one stromal and one in the thylakoids). Amounts of mRNA in leaves exposed to
progressive drought indicated an up-regulation of the genes encoding the
cytosolic and peroxisomal isoenzymes in the non-tolerant but not the tolerant
variety, in concordance with the data on total APX activity. However,
expression patterns of the chloroplast isoenzymes were more complex. In the
tolerant variety, mRNAs for stromal and thylakoid forms were present in very
low amounts in control plants; there was a dramatic increase in abundance in
the early stages of drought, peaking at approx. 1.0 MPa, after which both
declined to almost undetectable levels. In the non-tolerant variety both these
genes were expressed significantly under control conditions. The onset of
drought led to a major reduction in the mRNA encoding the thylakoid form but
expression of the stromal form increased with increasing severity of water deficit.
These data on mRNA populations thus provide clues that assays of total enzyme
activity may hide more subtle changes in individual cell compartments.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
