Nigel M. Crawford
Michael L. Kahn
Thomas Leustek
Sharon R. Long

The elements nitrogen and sulfur are acquired by plants primarily through interaction with the soil solution. As with other mineral nutrients, the acquisition of nitrogen- and sulfur-containing ions is mediated by highly evolved morphological, physiological, and biochemical mechanisms. Unlike other mineral nutrients, however, the inorganic forms of nitrogen and sulfur are often present in soil in oxidized forms, which must be reduced for the element to be used in metabolism. These conversions take place in highly reducing environments (characterized by low E° values) and link nitrogen and sulfur assimilation with pathways that generate reducing potential. Nitrate and sulfate reduction are compartmentalized and regulated to facilitate integration with other cellular metabolism. A combination of biochemical and molecular-genetic approaches is further elucidating these pathways. Although we know more about nitrogen metabolism in plants than we do about sulfur metabolism, our understanding of sulfur is increasing impressively with the recent renewed interest in the subject and the advent of new tools with which to study it.