PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Functional characterization of the chloroplastic selenium-independent glutathione peroxidases (GPXs) in Arabidopsis thaliana. Christine Chi-Chen Chang^*,1, Barbara Karpinska¹, Lucía Jordá-Miró², Ireneusz Slesak⁴, Michael Melzer⁵, Jane Parker², Philip Mullineaux³, Stanislaw Karpinski¹, ¹ Department of Botany, Stockholm, Sweden² Department of Molecular Plant Pathology, Cologne, Germany⁴ Polish Academy of Sciences, Krakow, Poland⁵ Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany³ Dept. of Disease and Stress Biology, Norwich, U.K.

ABSTRACT- Selenium(Se)independent glutathione peroxidases (GPX, EC: 1.11.1.9) are the family of multiple isozymes that catalyse the reduction of H₂O₂, organic hydroperoxides and/or lipid hydroperoxides, with reduced glutathione (GSH). The role of plant Se-independent GPXs is not well described. In Arabidopsis thaliana, GPXs are encoded by a small gene family of eight genes (GPX8 was not reported by Rodrigues Milla et al., 2003, Plant J. 36, 602-615). Two of them encode putative chloroplastic isoforms (GPX1 and GPX7, identified by the presence of a predicted chloroplastic transit-peptide in their cDNA sequences). In order to elucidate the role and function of chloroplastic (cp)GPXs we have selected Arabidopsis transgenic RNAi lines and null mutants with reduced or inhibited expression of cpGPX. Under conditions, which promote excess excitation energy (EEE; continuous high light; 700 mol s^-1 m^-2 at 4 ^oC or excess light alone of; 2200 mol s ^-1 m ^-2), these lines showed enhanced inhibition of photosynthetic electron transport. However, control and cpGPX deficient plants recovered equally from such a stress. cpGPX deficiency was associated with increased size of intercellular spaces in foliar spongy mesophyll, reduced oxygen evolution, higher hydrogen peroxide levels and higher accumulation of starch in the chloroplasts, and was compensated for by higher ascorbate peroxidase activity, foliar anthocyanin content, and heat dissipation. Interestingly, all these cpGPX deficient lines also showed higher growth rate (indicated by fresh/dry weight ratio, but only in low light conditions). To assess whether these cpGPX deficient lines were affected in the defense responses against pathogens, they were inoculated with different bacterial strains. This analysis revealed that some lines manifested a reduced bacterial growth rate, when virulent strains were inoculated (Pseudomonas syringae pv. tomato DC3000 and P.s.t. maculicola strain ES4326). In the case of the avirulent strain P.s.t. DC3000/avrRpm1 no differences in growth rate were detected. However, more pronounced hypersensitive responses after infection were observed. Our results suggest that photo-oxidative stress tolerance, chloroplastic reactive oxygen species metabolism and defense mechanisms to virulent bacteria can be functionally linked in plants.

KEY WORDS: Glutathione peroxidase, Pathogen defense, Arabidopsis thaliana, Photo-oxidative stress