Expression of spinach ascorbate peroxidase isoenzymes in response to oxidative stresses

We studied the response of each ascorbate peroxidase (APX) isoenzyme in spi nach leaves under stress conditions imposed by high light intensity, drough t, salinity, and applications of methyl viologen and abscisic acid. The ste ady-state transcript level of cytosolic APX remarkably increased in respons e to high-light stress and methyl viologen treatment, but not in response t o the other stress treatments. The transcript levels of the chloroplastic ( stromal and thylakoid-bound) and microbody-bound APX isoenzymes were not ch anged in response to any of the stress treatments. To explore the responses of the APX isoenzymes to photooxidative stress, the levels of transcript a nd protein and activities of each isoenzyme were studied during high-light stress and following its recovery. The cytosolic APX activity increased in parallel with transcript abundance during high-light stress, while the prot ein level was not altered. The other isoenzymes showed no significant chang es in transcript and protein levels and activities, except for the gradual decrease in chloroplastic isoenzyme activities.