477:	Cleavage of chloroplastic glutamine synthetase in wheat under oxidative stress.

Authors:	Palatnik , Javier , F.(A)Carrillo, Nestor(A)Valle , Estela , M.(A)
Affiliations:	(A): PROMUBIE, Fac. Cs. Bioquimicas y Farmaceuticas, UNR
Presenter:	Palatnik , Javier F., palatnik@arnet.com.ar

We investigated the stability of chloroplastic glutamine synthetase (GS), which catalyses the rate-limiting step of photorespiration, under photooxidative stress, using wheat leaves, chloroplasts, and chloroplast lysates. Illuminated seedlings sprayed with the superoxide radical propagator methyl viologen (MV) showed rapid GS decline, dependent on MV concentration and exposure time. Degradation products of ~39 kD and ~31 kD were detected when chloroplast lysates containing both stroma and thylakoids were illuminated in the presence of MV or hydrogen peroxide. In all cases, GS cleavage was prevented by the addition of the electron transport inhibitor DCMU. Full protection against degradation could also be obtained by the incorporation of chelating agents, whereas hydroxyl radical scavengers were ineffective. Maximal rates of degradation required the presence of transition metals and reducing compounds such as NADPH or DTT. The addition of catalase or superoxide dismutase also inhibited GS damage. 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 superoxide radicals, play a crucial role in the degradation process. Photorespiration may contribute to the survival of C3 plants under conditions of photooxidative stress, by dissipating the excess of photochemical energy and recycling NADP+. The extreme sensitivity of GS to oxidative conditions suggests that photorespiration could work as a protective mechanism only when the level of reactive oxygen species are kept low enough to ensure GS stability.