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

Abstract

The effect of 0.5 millimolar O-acetyl-L-serine added to the nutrient solution on sulfate assimilation of Lemna minor L., cultivated in the light or in the dark, or transferred from light to the dark, was examined. During 24 hours after transfer from light to the dark the extractable activity of adenosine 5'-phosphosulfate sulfotransferase, a key enzyme of sulfate assimilation, decreased to 10% of the light control. Nitrate reductase (EC 1.7.7.1.) activity, measured for comparison, decreased to 40%. Adenosine 5'-triphosphate (ATP) sulfurylase (EC 2.7.7.4.) and O-acetyl-L-serine sulfhydrylase (EC 4.2.99.8.) activities were not affected by the transfer. When O-acetyl-L-serine was added to the nutrient solution at the time of transfer to the dark, adenosine 5'-phosphosulfate sulfotransferase activity was still at 50% of the light control after 24 hours, ATP sulfurylase and O-acetyl-L-serine sulfhydrylase activity were again not affected, and nitrate reductase activity decreased as before. Addition of O-acetyl-L-serine at the time of the transfer caused a 100% increase in acid-soluble SH compounds after 24 hours in the dark. In continuous light the corresponding increase was 200%. During 24 hours after transfer to the dark the assimilation of 35S042-into organic compounds decreased by 80% without O-acetyl-L-serine but was comparable to light controls in its presence. The addition of O-acetyl-L-serine to Lemna minor precultivated in the dark for 24 hours induced an increase in adenosine 5'-phosphosulfate sulfotransferase activity so that a constant level of 50% of the light control was reached after an additional 9 hours. Cycloheximide as well as 6-methylpurine inhibited this effect. In the same type of experiment 0-acetyl-L-serine induced a 100-fold increase in the incorporation of label from 35S042-into cysteine after additional 24 hours in the dark. Taken together, these results show that exogenous 0-acetyl-L-serine has a regulatory effect on assimilatory sulfate reduction of L. minor in light and darkness. 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)

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

“…The decrease in APR activity and flux through sulfate assimilation was similar to that caused by prolonged cultivation of L. minor in the dark (Neuenschwander et al, 1991). The incubation in air without CO 2 resulted in decreased protein concentrations.…”

“…1B and 2B) most probably due to the presence of an amino group in this compound. OAS, as a source of reduced nitrogen, was shown to repress NR activity (Neuenschwander et al, 1991). Whether OAS is the compound signaling the sulfur status of the plant toward nitrate assimilation is a matter of debate because the regulation of nitrate uptake and reduction can also be triggered by changes in concentrations of free amino acids, which were shown to accumulate upon sulfur deficiency (Klapheck et al, 1982;Migge et al, 2000;Prosser et al, 2001).…”

“…For Cys synthesis, sulfate assimilation requires the carbon skeletons of OAS as acceptors of reduced sulfide. APR activity was induced by feeding OAS in the light and in the dark (Neuenschwander et al, 1991). Increased synthesis of OAS due to overexpression of Ser acetyltransferase affected sulfate assimilation and resulted in increased Cys and glutathione synthesis (Blaszczyk et al, 1999;Harms et al, 2000).…”

“…OAS accumulates during sulfur starvation and may thus act as a signal of the sulfur status (Kim et al, 1999). The addition of OAS increases sulfate uptake and APR activity and mRNA level also at normal sulfate levels (Neuenschwander et al, 1991;Smith et al, 1997;Koprivova et al, 2000). It is apparent that sulfate reduction is regulated by nitrogen nutrition on the transcriptional level, and OAS plays a major role in this regulation (Koprivova et al, 2000).…”

Isolation and Characterization of an Arabidopsis Mutant That Overaccumulates O-Acetyl-l-Ser

“…OAS concentration is drastically increased by sulfate deficiency or nitrate application and is positively correlated with the ratio of nitrate to sulfate in the medium (Kim et al, 1999). OASsupplemented medium enhanced both the activity of APR and the assimilation of sulfate to Cys in L. minor (Neuenschwander et al, 1991). Accumulation of mRNAs for a sulfate transporter in Hordeum vulgare and for APR in Arabidopsis was increased by OAS application (Smith et al, 1997;Koprivova et al, 2000).…”