Regulation of sulfate assimilation by nitrogen in Arabidopsis

Using Arabidopsis, we analyzed the effect of omission of a nitrogen source and of the addition of different nitrogen-containing compounds on the extra ctable activity and the enzyme and mRNA accumulation of adenosine 5'-phosph osulfate reductase (APR). During 72 h without a nitrogen source, the APR ac tivity decreased to 70% and 50% of controls in leaves and roots, respective ly, while cysteine (Cys) and glutathione contents were not affected. Northe rn and western analysis revealed that the decrease of APR activity was corr elated with decreased mRNA and enzyme levels. The reduced APR activity in r oots could be fully restored within 24 h by the addition of 4 mM each of NO 3-, NH4+, or glutamine (Gln), or 1 mM O-acetylserine (OAS). (SO42-)-S-35 fe eding showed that after addition of NH4+, Gln, or OAS to nitrogen-starved p lants, incorporation of S-35 into proteins significantly increased in roots ; however, glutathione and Cys labeling was higher only with Gln and OAS or with OAS alone, respectively. OAS strongly increased mRNA levels of all th ree APR isoforms in roots and also those of sulfite reductase, Cys synthase , and serine acetyltransferase. Our data demonstrate that sulfate reduction is regulated by nitrogen nutrition at the transcriptional level and that O AS plays a major role in this regulation.