Plant Physiol, November 2002, Vol. 130, pp. 1406-1413

Interaction of Sulfate Assimilation with Carbon and Nitrogen Metabolism in Lemna minor¹

Department of Tree Physiology, University of Freiburg, D-79085 Freiburg, Germany (S.K., H.R.); Institute of Plant Sciences, CH-3013 Berne, Switzerland (M.S., P.v.B., C.B.); Institute of Biology, Free University of Berlin, Applied Genetics, D-14195 Berlin, Germany (H.H.); and Department of Chemistry and Biochemistry, University of Berne, CH-3012 Berne, Switzerland (U.K.)

Cysteine synthesis from sulfide and O-acetyl-L-serine (OAS) is a reaction interconnecting sulfate, nitrogen, and carbon assimilation. Using Lemna minor, we analyzed the effects of omission of CO₂ from the atmosphere and simultaneous application of alternative carbon sources on adenosine 5'-phosphosulfate reductase (APR) and nitrate reductase (NR), the key enzymes of sulfate and nitrate assimilation, respectively. Incubation in air without CO₂ led to severe decrease in APR and NR activities and mRNA levels, but ribulose-1,5-bisphosphate carboxylase/oxygenase was not considerably affected. Simultaneous addition of sucrose (Suc) prevented the reduction in enzyme activities, but not in mRNA levels. OAS, a known regulator of sulfate assimilation, could also attenuate the effect of missing CO₂ on APR, but did not affect NR. When the plants were subjected to normal air after a 24-h pretreatment in air without CO₂, APR and NR activities and mRNA levels recovered within the next 24 h. The addition of Suc and glucose in air without CO₂ also recovered both enzyme activities, with OAS again influenced only APR. ³⁵SO₄² feeding showed that treatment in air without CO₂ severely inhibited sulfate uptake and the flux through sulfate assimilation. After a resupply of normal air or the addition of Suc, incorporation of ³⁵S into proteins and glutathione greatly increased. OAS treatment resulted in high labeling of cysteine; the incorporation of ³⁵S in proteins and glutathione was much less increased compared with treatment with normal air or Suc. These results corroborate the tight interconnection of sulfate, nitrate, and carbon assimilation.