Oxidative stress induced in tobacco leaves by chloroplast over-expression of maize plastidial transglutaminase

Abstract

As part of a project aiming to characterize the role of maize plastidial transglutaminase (chlTGZ) in the plant chloroplast, this paper presents results on stress induced by continuous chlTGZ over-expression in transplastomic tobacco leaves. Thylakoid remodelling induced by chlTGZ over-expression in young leaves of tobacco chloroplasts has already been reported (Ioannidis et al. in Biochem Biophys Acta 1787:1215–1222, 2009). In the present work, we determined the induced alterations in the photosynthetic apparatus, in the chloroplast ultrastructure, and, particularly, the activation of oxidative and antioxidative metabolism pathways, regarding ageing and functionality of the tobacco transformed plants. The results revealed that photochemistry impairment and oxidative stress increased with transplastomic leaf age. The decrease in pigment levels in the transformed leaves was accompanied by an increase in H2O2 and lipid peroxidation. The rise in H2O2 correlated with a decrease in catalase activity, whereas there was an increase in peroxidase activity. In addition, chlTGZ over-expression lead to a drop in reduced glutathione, while Fe-superoxide dismutase activity was higher in transformed than in wild-type leaves. Together with the induced oxidative stress, the over-expressed chlTGZ protein accumulated progressively in chloroplast inclusion bodies. These traits were accompanied by thylakoid scattering, membrane degradation and reduction of thylakoid interconnections. Consequently, the electron transport between photosystems decrease in the old leaves. In spite of these alterations, transplastomic plants can be maintained and reproduced in vitro. These results are discussed in line with chlTGZ involvement in chloroplast functionality.