Influence of Nitrogen Availability on Sulfate Assimilation Pathway in Arabidopsis thaliana

Sulfate and nitrate, which contain sulfur and nitrogen in their most highly oxidized form, are the dominant species available to many plants for covering their needs for these elements (1–3). In amino acids, including sulfur containing amino acids cysteine and methionine, both sulfur and nitrogen are present in reduced form, therefore, sulfate and nitrate have to be reduced for their synthesis. The dominant portion of amino acids is used for protein synthesis hence the SIN ratio in plants remains quite stable. Plants appear to possess a mechanism to coordinate assimilatory sulfate and nitrate reduction so that the appropriate proportions of both sulfur containing and other amino acids are available for protein synthesis (4–7). These reports also indicate that the second enzyme of assimilatory sulfate reduction APS reductase is highly regulated by different nutritional conditions. In this study we were particularly interested in the regulation of APS reductase activity, mRNA expression, and protein accumulation under different conditions of nitrogen availability in Arabidopsis thaliana plants