Protective role of chloroplastic ascorbate peroxidases in transgenic tobacco plants



Journal Title

Journal ISSN

Volume Title


Texas Tech University


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, possibly indicating higher electron flow in PSIl via 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 provided significant stress protection not only through H2O2 scavenging activity but possibly also as an electron sink. Plants that expressed high levels of sAPX maintained stomatal aperture more efficiently than control plants under gradual water deficit stress. This effect could be explained by increased H2O2 scavenging in guard cell chloroplasts. Therefore, the roles of sAPX in the chloroplasts include both H2O2 scavenging in the Mehler-APX pathway and increased electron sink under the over-burden electron transport systems in photosystems under stress.



Transgenic plants -- Effect of stress on, Physiological, Tobacco -- Effect of stress on, Oxidation