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Plant Physiol, October 1999, Vol. 121, pp. 471-478

The Role of Photosynthetic Electron Transport in the Oxidative Degradation of Chloroplastic Glutamine Synthetase1

Javier F. Palatnik, Néstor Carrillo, and Estela M. Valle*

Programa Multidisciplinario de Biología Experimental, División Biología Molecular, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, 2000 Rosario, Argentina

The stability of chloroplastic glutamine synthetase (GS; EC 6.3.1.2) was investigated under photooxidative stress using wheat (Triticum aestivum L.) leaves, chloroplasts, and chloroplast lysates. Illuminated seedlings sprayed with the superoxide radical (Obardot 2) propagator methyl viologen showed rapid GS decline dependent on MV concentration and exposure time. Degradation products of approximately 39 and 31 kD were detected when chloroplast lysates containing both stroma and thylakoids were illuminated in the presence of MV or H2O2. In all cases, GS cleavage was prevented by the addition of the electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Full protection against degradation could also be obtained by the incorporation of chelators or antioxidant enzymes. Maximal rates of degradation required the presence of transition metals and reducing compounds such as NADPH or dithiothreitol. Similar patterns of GS cleavage were obtained when seedlings were exposed to high doses of irradiation. The results indicate that chloroplastic GS is extremely prone to oxidative cleavage, and that reduced transition metals, presumably resulting from the destruction of iron-sulfur clusters by light-generated Obardot 2, play a crucial role in the degradation process. The physiological implications of GS lability to oxidative stress are discussed.

1 This work was supported by grant no. BID 802/0C-AR PICT 01-00000-01363 from the National Research Agency of Argentina. N.C. and E.M.V. are members of the National Research Council of Argentina and J.F.P. is a fellow of the same institution.

* Corresponding author; e-mail evalle{at}arnet.com.ar; fax 54-341-4390465.

© 1999 American Society of Plant Physiologists


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