Differential stimulation of ascorbate peroxidase isoforms by ozone exposure in sunflower plants

fn sunflower plants exposed to 150 nl L-1 of O-3 (4 h day(-1) for 4 days) a n enhancement in the activity of APx was found at both the apoplastic and s ymplastic levels, while stromal and thylakoid-bound chloroplastic APx activ ity did not change. PAGE analysis confirmed the presence of APx activity in the apoplast and revealed different APx isoenzyme patterns between control and O-3-treated plants in either apoplastic and cytoplasmic samples. Immun oreaction with a cytosolic APx antibody revealed, at both the apoplastic an d symplastic levels, only one band, more evident in the O-3-treated than in the control plants. These results support the hypothesis of a cytoplasmic origin for the apoplastic isoform, Thus, sunflower plants seem to adapt to O-3-induced oxidative stress through the induction of genes coding for APx proteins whose enzymatic activity enhancement at the apoplastic and cytosol ic levels may act as a biochemical defence line against ozone pollution dam age.