Abstract
Ascorbte peroxidase (APX) catalyzes the scavenging of hydrogen peroxide and plays an important role for reactive oxygen scavenging in chloroplasts. However, chloroplastic APX is highly sensitive to inactivation by hydrogen peroxide and is inactivated under severe stress conditions. Therefore the enzyme is thought to be one of the primary targets of photooxidative damages. In this study, we investigated the role of chloroplastic APX in stress tolerance by overexpression of stromal APX (sAPX). We produced transplastomic tobacco plants which overexpress sAPX from tobacco under the control of psbA promoter. The transplastomic tobacco showed a 27-32 fold increase in total APX activity compared with WT. However, the chlorophyll content and Fv/Fm of the transplastomic lines (APX7 and APX11-1) were lower than WT under non-stressed condition. In addition, the treatment of APX7 with low temperature stress (15 degree C for 4 weeks) caused a significant bleaching and decreases in Fv/Fm, ΦII, total APX activity and sAPX protein level. These results indicate that the sAPX overexpressing lines unexpectedly suffer from photooxidative damages caused by low temperature stress.
