Protective role of chloroplastic ascorbate peroxidases in transgenic tobacco plants

Ascorbate peroxidase (APX) is a heme-containing antioxidant enzyme that detoxifies H2O2 using ascorbate as an electron donor. Arabidopsis chloroplastic APXs---stromal APX (sAPX) and thylakoid-membrane bound APX (tAPX)---were cloned and transformed into wildtype tobacco plants (Nicotiana tabacum L. Xanthi NN) to investigate their protective roles during exposure to various oxidative stresses. The APX activity from chloroplast extracts of transgenic plants that express Arabidopsis sAPX were 30 times higher than wildtype plants. These plants also had a 3-fold increase in dehydroascorbate reductase activity. Although transgenic plants that harbor Arabidopsis tAPX showed high mRNA expression, they had no significant increase in APX activity compared to wildtype plants. During exposure to oxidative stress, plants that express sAPX showed increased protection against photoinhibition caused by chilling temperature and high light conditions. However, no significant protection was found during oxidative stress caused by methyl viologen treatments. Under water deficit stress, plants that express Arabidopsis sAPX had significantly higher quantum yield than the wildtype plants, indicating higher electron flow in PSII by the Mehler-APX reaction as an electron sink. The increased photosynthetic rates of Arabidopsis sAPX-expressing plants under gradual water deficit and severe water deficit stress suggest that increased sAPX activity had significant protection mechanisms not only as H2O 2 scavenging activity but possibly also as an electron sink. High levels of sAPX expression in the chloroplasts of guard cells had effect on maintaining stomatal aperture by increased H2O2 scavenging activity only under gradual water deficit stress not severe water deficit stress. Therefore, the roles of sAPX in the chloroplasts are both a H2O2 scavenger of the Mehler-APX pathway and an electron sink under the over-burden electron transport systems in photosystems under the stresses.