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Chapter 16

Nitrogen and Sulfur

 

Nitrogen and sulfur are used to make diverse plant constituents. These essential macronutrients are involved in biogeochemical cycles and are often obtained from the environment in forms that must be reduced before they can be used. The reduction and assimilation reactions are integrated into general plant metabolism. They are often compartmentalized in the cell and are sometimes located in certain plant cell types or organs. Plants most commonly acquire nitrogen as NH4+ , which can be assimilated immediately into amino acids, or as NO3– , which must be reduced to NH4+ for further metabolism. Some bacteria can reduce (fix) atmospheric N2 into NH4+. The key catalyst of nitrogen fixation, nitrogenase, has two functional components. Dinitrogenase is the enzyme that actually reduces N2. It contains unusual metal clusters, including one type that generally includes an atom of Mo or V. Dinitrogenase reductase uses ATP to lower the potential of reductants obtained from metabolism to a level where they can reduce the dinitrogenase clusters.
      A few groups of plants can obtain NH4 efficiently from N2 by participating in a symbiosis with nitrogen-fixing bacteria. The best characterized nitrogen-fixing symbioses are the interactions between legumes and rhizo-bia. Bacteria infect the plant roots and trigger the formation of root nodules. Infection requires communication between the bacteria and the plant. Specific plant compounds, such as flavonoids, are released by developing roots. These are detected by the bacteria and induce the bacteria to make lipooligosaccharide Nod factors, which alter plant gene expression and cell division in roots of specific host plants. As the nodule grows, bacteria invade some plant cells and develop into nitrogen-fixing forms called bacteroids. Bacteroids are surrounded by a plant- derived membrane which controls the exchange of nutrients between the bacteriod and the plant cytoplasm. A mature nodule is organized to support the energy intensive nitrogen fixation reaction by delivering oxygen and carbon sources to the bacteroids at low concentrations of free oxygen in order to protect nitrogenase, a notoriously sensitive enzyme. Ammonia released by bacteriods is first assimilated in the plant by glutamine synthetase and other enzymes, sometimes employing nodule-specific isozymes. Assimilated nitrogen leaves the nodule as either amides or ureides, depending on the host species, which leads to distinct patterns of gene expression, compartmentation, and regulation of these final steps in different hosts.
      Nitrate uptake is mediated by high- and low-affinity proton symporters that are encoded by genes belonging to two gene families, NRT1 and NRT2. In the cytosol, nitrate is reduced to nitrite by NAD(P)H-dependent nitrate reductase, a metallo-flavoenzyme that can be inhibited by phosphorylation and 14-3-3 protein binding. In the plastids, nitrite is reduced to ammonium by nitrite reductase, which contains a 4Fe–4S center and a siroheme and uses ferredoxin as the reductant. Most of the nitrate assimilatory genes are regulated at the transcriptional level, being induced by nitrate, light, or sucrose and being repressed by ammonium or glutamine.
      Sulfur is essential for life. Its oxidation state is in constant flux as it circulates through the global sulfur cycle. As the primary producers of organic sulfur compounds, plants play a key role in the cycle by coupling photosynthesis to the reduction of sulfate, assimilating sulfur into cysteine, and metabolizing this amino acid to form methionine, glutathione, and many other compounds. The activity of the sulfur assimilation pathway responds dynamically to changes in the sulfur supply and to environmental conditions that alter the need for reduced sulfur. Through molecular-genetic analysis, many of the enzymes involved in the process have been defined, and the mechanisms for regulation are being revealed.


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