Role of O-Acetylserine in Hydrogen Sulfide Emission from Pumpkin Leaves in Response to Sulfate

Abstract

ABSTRACIÌ n the presence of excess sulfate, cysteine synthesis in pumpkin (Cucurbita pepo) leaves is not limited by sulfate reduction, but by the availability of O-acetylserine. Feeding of 0-acetylserine or its metabolic precursors S-acetyl-coenzyme-A and coenzyme A to leaf discs enhanced the incorportion of luSlsulfate into reduced sulfur compounds, mainly into cysteine, at the cost of lowered H2S emission; the uptake and reduction of sulfate is not affected by these treatments. ,-Fluoropyruvate, an inhibitor of the generation of S-acetyl-coenzyme A via pyruvate dehydrogenase, stimulated H2S emission in response to sulfate. This stimulation is overcompensated by addition of O-acetylserine, S-acetylcoenzyme A, or coenzyme A. These results indicate that, in the presence of high amounts of sulfate, excess sulfur is reduced and emitted as H2S into the atmosphere. The H2S emitted seems to be produced by liberation from a precursor of cysteine rather than by cysteine desulfhydration.path of H2S emission in response to sulfate (pathway 3). In these experiments, we observed that leaf tissues are injured by Lcysteine in a way reminiscent of injury caused by sulfite or SO2. In leaf tissue, L-cysteine is desulfhydrated and H2S is emitted as a result. However, only a fraction of the H2S produced this way is emitted from the leaf; the other fraction gives rise to the development of a pool of free sulfite, sufficiently high to cause injury to the leaf tissue (15,16, 26). If the reduction of sulfate, leading to H2S emission, proceeded via free sulfite, a pool of sulfite would develop in leaf tissue exposed to high concentrations of sulfate. Such afpool, if appreciable, would cause injury to the leaf tissue. The lack of injury symptoms associated with H2S emission in response to sulfate (26) makes a participation of free sulfite in the path of H2S biosynthesis from sulfate unlikely. Thus, desulfhydration of L-cysteine (pathway 2) or diversion from carrier-SH (pathway 1) seems to be the path of H2S biosynthesis in response to sulfate. The present experiments were performed to get information about which of these two possible pathways is responsible for the generation of H2S in leaf tissue supplied with excess sulfate.Leaf tissues of higher plants emit H2S into the atmosphere, when they are supplied with excess sulfate (6,13,14,16,24, 26), sulfite,17, 26),14,15,18). The path of H2S biosynthesis is different for each of these sulfur sources. In response to sulfite or SO2, direct light-dependent reduction, probably catalyzed by a sulfite reductase, leads to emission of H2S (6, 17), although high amounts of bisulfite are oxidized to sulfate (12,17). In response to L-cysteine, H2S emission is caused by desulfhydration of L-cysteine, probably to pyruvate, ammonium, and SH-, catalyzed by a cysteine desulfhydrase (9,14,18). The path of H2S biosynthesis from sulfate, however, is much less certain. There are at least three possible pathways of sulfate reduction which may result in the emission of H2S from this sulfur source: (a) reductio...

Regulation of Sulfate Assimilation by Light and O-Acetyl-l-Serine in Lemna minor L.

“…7C) and the protein fraction (Fig. 7D) (16). Inhibition of the synthesis of the OAS precursor acetyl-CoA by 3'fluoropyruvate enhances hydrogen sulfide emission but inhibits cysteine synthesis.…”

“…They are in agreement with the idea that this compound is a limiting factor for sulfate assimilation and seem to be in contrast to the proposed strict light control of sulfate assimilation. Light (3,21,26) and the rate of nitrate assimilation or the availability of amino acids (4,16,17,25) …”

“…OAS has been identified as a limiting factor for cysteine formation in leaf discs from pumpkin kept in the light (16), suggesting that plants use the same substrate for cysteine formation as bacteria. OAS is one product of assimilatory nitrate reduction.…”

“…Light (3,21,26) and the rate of nitrate assimilation or the availability of amino acids (4,16,17,25) have been described as regulatory factors of assimilatory sulfate reduction. The light dependence of assimilatory sulfate reduction has been demonstrated by the finding that spinach chloroplasts form [35S]cysteine from "SO4 2-in the light (26).…”

Interaction of Sulfate Assimilation with Carbon and Nitrogen Metabolism in Lemna minor

“…OAS is considered to connect these pathways as it regulates sulfate uptake and assimilation. In the presence of excess sulfate, OAS seems to be limiting for Cys synthesis (Rennenberg, 1983). Overexpression of Ser acetyltransferase, the enzyme synthesizing OAS, led to increased Cys and glutathione (GSH) concentrations in transgenic potato (Solanum tuberosum) and tobacco (Blaszczyk et al, 1999;Harms et al, 2000).…”