Cysteine Synthesis in the Chloroplast Is Not Required for Resistance of Arabidopsis thaliana to H2S Fumigation

Reduced sulfur is necessary for synthesis of various essential compounds in the plant cell including cysteine and glutathione. For this reason sulfide is continuously produced in plastids as intermediate of the assimilatory sulfate reduction pathway. Varying demand for sulfide during development and in response to external stress challenges a plant to rapidly re-organize the entire sulfur metabolism, since high endogenous sulfide levels are potentially toxic for the cellular metabolism. Upon prolonged exposure of Arabidopsis to 1 mu l l(-1) H2S, the endogenous sulfide level in the shoot was kept low, whereas there was a mass increase in thiol content. The cysteine and glutathione content increased approximately 30- and 3.4-fold, respectively. Notably, the dramatic changes on thiol levels were not accompanied by changes in extractable activities of the cysteine synthesizing enzymes, serine acetyltransferase and O-acetylserine(thiol) lyase (OAS-TL). It was evident that, despite the production of sulfide in plastids, its consumption by OAS-TL in the same compartment was not crucial for adaptation of Arabidopsis to high H2S levels in the environment.