Chapter 12 Assimilation of Mineral Nutrients BIOL 5130/6130 Bob Locy Auburn University Chapter 12.01. Introduction - 01 Chapter 12.02. Nitrogen in the Environment - 02 02.01. Nitrogen passes through several forms in a biogeochemical cycle Chapter 12.02. Nitrogen in the Environment - 03 02.02. Unassimilated ammonium or nitrate may be dangerous – Although nitrate and ammonium are preferred nitrogen sources to support plant growth and development, both substances can be toxic at highlevels. – Ammonium ion dissipates the proton gradient – Nitrate can geneate toxic biproducts. Chapter 12.03. Nitrate Assimilation - 04 - + - + NO3 + NAD(P)H + H NO2 + NAD(P) + H20 03.01a. Many factors regulate nitrate reductase – Tight control of NR activity is required to keep toxic levels of nitrite from accumulating in cells – The induction of nitrate reductase activity is regulated transcriptionally by the control of NR mRNA levels. Chapter 12.03. Nitrate Assimilation - 05 03.01b. Many factors regulate nitrate reductase – Nitrate reductase activity is also regulated posttranslationally by a number of physiological parameters. – This control involves activating 14-3-3 protein, and phosphorylation of a serine residue in the hinge region of the protein between the MoCo and the heme. – Phosphorylation activates and dephosphorylation inhibits NR activity. Chapter 12.03. Nitrate Assimilation - 06 03.02. Nitrite reductase converts nitrite to ammonium - + + NO2 + 6 Fdred + 8 H NH4 + 6 Fdox + 2H20 – Nitrite derived from nitrate reductase is immediately taken up by choroplasts (in leaves) or plastids (in roots), where it is reduced to amonia by nitrite reductase (Nit) Chapter 12.03. Nitrate Assimilation - 07 03.03. Both roots and shoots assimilate nitrate – Varies from one species to another – GENERALLY – temperate species are root assimilators, tropical species are leaf assimilators Chapter 12.04. Ammonia Assimilation - 08 04.01. Converting ammonium to amino acids requires two enzymes – Glutamine synthetase – 2 isoforms, one cytosol, the other chloroplast/plastid – Two different types of glutamate synthase (GOGAT), NADH requiring (non-green plastids) and Fd requiring (chloroplast). Chapter 12.04. Ammonia Assimilation - 09 04.02. Ammonium can be assimilated via an alternative pathway Chapter 12.04. Ammonia Assimilation - 10 04.03. Transamination reactions transfer nitrogen Chapter 12.04. Ammonia Assimilation - 11 04.04. Asparagine and glutamine link carbon and nitrogen metabolism Chapter 12.06. Biological Nitrogen Fixation - 02 • The Conversion of nitrogen gas (N2) into ammonia (NH3) iside a biological system is referred to as biological nitrogen fixation. 06.01a. Free-living and symbiotic bacteria fix nitrogen Chapter 12.06. Biological Nitrogen Fixation - 02 06.01b. Free-living and symbiotic bacteria fix nitrogen Chapter 12.06. Biological Nitrogen Fixation - 02 06.02. Nitrogen fixation requires anaerobic conditions – Nitrogenase, the enzyme responsible for the conversion of N2 into ammonium requires an anaerobic environment and is labile in the prsence of oxygen. – In legume of alder nodules a specialized protein called leghemoglobin is responsible for creating a microaerophilic envirnment that protects nitrogenase. – In free living nitrogen fixers, they either only fix nitrogen when they are anaerobic, or a capable of creating anaeorobic conditions. – The heterocyst of Anabena is an example of this. 06.03. Symbiotic nitrogen fixation occurs in specialized structures – Examples are root nodules and heterocysts discussed above. Chapter 12.06. Biological Nitrogen Fixation - 02 06.04. Establishing symbiosis requires an exchange of signals – Nodulin genes are plant genes involved in the production of notdules – Nodulation genes are bacterial genes required for nodulation. – Nodulation is a complex process involving signalling and cross talk between the two organisms involved. 06.05. Nod factors produced by bacteria act as signals for symbiosis – Bacteria produce host specific Nod factors. – Specific species of plants respond to different Nod factors. Chapter 12.06. Biological Nitrogen Fixation - 02 06.06a. Nodule formation involves several phytohormones A. Rhizobia bind to an emerging root hair in response to a chemical attractant produced by the plant. B. In response to signals produced by the bacterial, the plant root hair curls and the bacteria grow within the curl. Chapter 12.06. Biological Nitrogen Fixation - 02 06.06b. Nodule formation involves several phytohormones C. Degradation of the root hair wall allows infection to occur, and the formation of an infection thread. D. When the infection thread reaches the end of the root hair wall, the infection thread membrane fuses with the plasma membrane. Chapter 12.06. Biological Nitrogen Fixation - 02 06.06c. Nodule formation involves several phytohormones E. The bacterial cells are released into the middle lamella, and ultimately work their way across a network of cells via the formation of subsequent branched infection threads. F. Once in the target cells, bacterial vessiculate into plant membrane surrounded vessicles in the cytosol of the cell, and form bacteroids. Chapter 12.06. Biological Nitrogen Fixation - 02 06.07. The nitrogenase enzyme complex fixes N2 – The nitrogenase is a complex enzyme with at least 2 types of subunits, the Fe protiens, and the MoFe proteins. – Neither of these have nitrogenase activity alone. – The electron path through the enzyme is complex but known. Chapter 12.06. Biological Nitrogen Fixation - 02 06.08. Amides and ureides are the transported forms of nitrogen – Ureides are translocated from nodules to other parts of the plant in the xylem in many tropical legumes such as soybean, peanut, and southern peas. – In temperate legumes, glutamine and asparagines are the primary forms of nitrogen translocated from the nodule in the xylem. Peas, clover, and alfalfa are examples of amide translocators. Chapter 12.07. Sulfur Assimilation - 02 07.01. Sulfate is the absorbed form of sulfur in plants 07.02. Sulfate assimilation requires the reduction of sulfate to cysteine 07.03. Sulfate assimilation occurs mostly in leaves Chapter 12.07. Sulfur Assimilation - 02 07.04. Methionine is synthesized from cysteine – Other sulfur containing compounds are made from APS via the PAPS pathway. END Chapter 12 Assimilation of Mineral Nutrients Supplemental topics and study questions Can be found at: http://4e.plantphys.net/chapter.php?ch=12 .