Impact of elevated H 2S on metabolite levels, activity of enzymes and expression of genes involved in cysteine metabolism

Abstract

The effects of elevated atmospheric hydrogen sulfide (H 2S) levels (0.25, 0.5, and 0.75 μl l –1) have been investigated in a short-term exposure experiment (3–48 h) on the model plant Arabidopsis thaliana (L.) Heynh. in comparison to untreated control plants. The most pronounced effects of H 2S fumigation could be observed on the metabolite level: The contents of the thiols cysteine and glutathione were increased up to 20- and fourfold, respectively. A direct positive correlation of the thiol contents with the H 2S concentrations applied was observed. To elucidate the molecular basis for the increased thiol levels, enzyme activities, messenger RNA and protein steady-state levels of cysteine-synthesizing and degrading pathways have been determined. The enzyme activities of O-acetyl- l-serine(thiol)lyase (OAS-TL) (EC 4.2.99.8) and l-cysteine desulfhydrase (EC 4.4.1.–) proteins were not significantly higher at elevated H 2S levels in comparison to untreated control plants. 3-Mercaptopyruvate sulfurtransferase (EC 2.8.1.2) activity was slightly higher after the longest H 2S exposure times. Elevated H 2S levels of 0.25 and 0.5 μl l –1 had promoting effects on both mRNA and protein levels of cysteine-synthesizing and degrading enzymes whereas the highest H 2S concentrations caused lower levels of expression combined with mild symptoms of oxidative stress, as the consequence of its phytotoxicity. The differences in the expression of the three different OAS-TL isoforms (cytoplasmic, plastidic and mitochondrial) by H 2S were very small. Increasing concentrations of H 2S and longer exposure times to H 2S let to a reduction in the pool of O-acetyl- l-serine, the second precursor of cysteine, and N-acetyl- l-serine in the leaves and shoots, indicating a substrate depletion in agreement with the increased thiol levels.