Impact of elevated H2S on metabolite levels, activity of enzymes and expression of genes involved in cysteine metabolism

The effects of elevated atmospheric hydrogen sulfide (H2S) 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 H2S 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 H2S 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 H2S levels in comparison to untreated control plants. 3-Mercaptopyruvate sulfurtransferase (EC 2.8.1.2) activity was slightly higher after the longest H2S exposure times. Elevated H2S 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 H2S 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 H2S were very small. Increasing concentrations of H2S and longer exposure times to H2S 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.