Evidence for an intracellular sulfur cycle in cucumber leaves

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Abstract
H2S emission from cucumber (Cucumis sativus L.) leaf discs supplied with L-cysteine in the dark is inhibited 80–90% by aminooxyacetic acid (AOA), an inhibitor of pyridoxal-phosphate dependent enzymes. Exposure to L-cysteine in the light enhanced the emission of H2S in response to this sulfur source. Turning off the light reduced the emission of H2S to the rate observed in continuous dark; turning on the light enhanced the emission of H2S to the rate observed in continuous light. Therefore, in the light H2S emission in response to L-cysteine becomes a partially light-dependent process. Treatment with cyanazine, an inhibitor of photosynthetic electron transport, reduced H2S emission in the light to the rate observed in continuous dark, but did not affect H2S emission in the dark. In leaf discs pre-exposed to L-cysteine in the light, treatment with cyanazine+ AOA inhibited the emission of H2S in response to L-cysteine completely. Therefore, only part of the H2S emitted in response to this sulfur source is derived from a light-independent, but pyridoxal-phosphate-dependent process; the balance of the H2S emitted is derived from a light-dependent process that can be inhibited by cyanazine. When cucumber leaf discs were supplied with a pulse of L-[35S]cysteine, radioactively labeled H2S was emitted in two waves, one during the first hour of exposure to L-cysteine, and a second after 3–4 h; unlabeled H2S, however, was emitted continuously. The second wave of emission of labeled H2S was not observed in pulse-chase experiments in which sulfate or cyanazine were added to the treatment solution after 3 h of exposure to L-cysteine, or when the lights was turned off. The labeling pattern of sulfur compounds inside cucumber cells supplied with a pulse of L-[35S]cysteine showed that the labeled H2S released from L-cysteine partially enters first the sulfite, then the sulfate pool of the cells. The radioactively labeled sulfate, however, is not incorporated into L-cysteine, but enters the H2S pool of the cells again. These observations are consistent with the idea of an intracellular sulfur cycle in plant cells. The L-cysteine taken up by the leaf discs seems to be desulfhydrated in a light-independent, but pyridoxal-phosphate-dependent process. The H2S synthesized this way may be partially released into the atmosphere; the other part of the H2S produced in response to L-cysteine may be oxidized to sulfite, then to sulfate, which is subsequently reduced via the light-depent sulfate assimilation pathway. In the presence of excess L-cysteine, synthesis of additional cysteine may be inhibited, and the sulfide moiety may be split off carrier bound sulfide to enter the H2S pool of the cells again. It is suggested that the function of this sulfur cycle may be regulation of the free cysteine pool.