Lecture12 - MCDB321 Plant Physiology MCDB321 Lecture 12...

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Unformatted text preview: MCDB321 Plant Physiology MCDB321 Lecture 12 Mineral Assimilation Feb. 17, 2011 0. Electron transport/ATP synthesis in mitochondria A. 4 protein complexes A. B. NADH/FADH2 oxidation B. C. alternative oxidase C. 1. Nitrogen assimilation A. nitrate uptake& nitrate-NH3 conversion A. B. NH3 assimilation (GS-GOGAT/GDH) B. C. nodule formation and nitrogen fixation C. 2. Sulfur assimilation 3. Metal ion assimilation A. two general mechanisms: coordination bond/salt bridge A. B. Iron assimilation: soil acidification, Fe(III)-Fe(II) reduction, and B. the production of phytosiderophores the 190 x 106 tons 8% HNO3 85 x106 tons Nitrogen is present in many forms in the biosphere Nitrogen Most plants have many kinds of nitrate transporters Most Low-affinity NO3- transporters (operates at high [NO3-]ext) and high-affinity NO3- transporters (operates at low [NO3-]ext, are often induced by NO3-). and ext, are The Arabidopsis genome encodes 53 nitrate-transporter genes CHL1 is shown to be a dual-affinity transporter and can function as a nitrate sensor Nitrate Assimilation The first step is the conversion of nitrate to nitrite in the cytosol The in 1. NO3- + NAD(P)H + H+ + 2 e- ------> NO2- + NAD(P)+ + H2O NO NO catalyzed by nitrate reductase nitrate [the main molybdenum(Mo)-containing protein in vegetative tissue] [the 2. Nitrate, light, & carbohydrates stimulates nitrate reductase at both 2. transcriptional & translation levels. transcriptional 3. When roots receive small amounts of NO3-, nitrate is reduced nitrate mainly in the roots.. mainly Nitrite is a highly toxic ion Nitrite Plant cells immediately transport NO2- from the cytosol into chloroplasts in chloroplasts lleaves or plastids in roots eaves plastids NO2- + 6 Fdred + 8H+ + 6 e- ------> NH4+ + 6Fdox + 2H2O NH Nitrate and light induce the transcription of nitrite reductase mRNA. ammonium is highly toxic to both plants and animals! ammonium Ammonium Assimilation Ammonium Plant cells avoid ammonium toxicity by rapidly converting NH4+ into amino acids The GS-GOGAT cycle GS-GOGAT 1. Cytosolic form: producing Gln for intracellular nitrogen transport 2. Plastid form: A. root plastid: for local consumption A. B. shoot chloroplasts: reassimilation of photorespiratory NH 4+ B. Two types of glutamate synthase NADH: in plastids of nonphotosynthetic tissue Assimilation of NH4+ absorbed from rhizosphere or Assimilation assimilation of glutamine translocated from roots or senescing leaves assimilation Fd: involved in photorespiratory nitrogen metabolism. N-starvation activates GS (high NH3 affinity) and GOGAT but represses GDH (low NH3 affinity) Ammonium can also be assimilated via an alternative pathway Ammonium NADH-dependent form: mitochondria NADPH-form: chloroplast Main function: deaminate glutamate The activity is induced by NH3 (accompanied by increased respiration rate?) The Transamination reactions transfer nitrogen into Aspartate and Asparagine Aspartate A key compound for N transport and storage due to its high N/C ratio (2N/4C) Biological Nitrogen Fixation N2 + 16 ATP + 8e- + 2H+ --> NH3 + 16 ADP + 16 Pi + H2 Some plants can join in a symbiotic relationship with bacteria that are able to fix N 2 principally the legumes with Rhizobium bacteria (live in the soil), but also Alder (Alnus) with Frankia (an actinomycete) Each Rhizobium species/strain requires a specific host plant Plants secret flavenoids to attract bacteria bacterium produces Nod factors, which are recognized by the root cells Nodule Formation II Nodule 1. in response to Nod factors, the root hair exhibits abnormal curling growth 2. bacteria penetrate the root through an infection thread (invagination of the root cell plasma membrane) 1. bacteria are "packaged" within membrane derived from the root cell [plasma membrane=peribacteroid membrane] 2. they stop dividing and differentiate in bacteroids 3. Meanwhile, the root cortical cells dedifferentiate, then divide to form a mass of cells=nodule primordium Nitrogen fixation in bacteroids is catalyzed by a bacterial enzyme nitrogenase 1. 2 components, Fe protein (Fe-S cofactors) and the MoFe protein (Mo-Fe-S), both act together to reduce N2 2. the reduction of N2 to 2 NH3, a 6 e- transfer, appears to be coupled with the reduction of 2 protons to evolve H2 (requires 2 additional e-) 3. e- are provided by reduced ferredoxinRed 4. 16 ATPs are required for the reaction 5. VERY sensitive to O2 Oxygen paradox nitrogenase is very sensitive to O2 but the cell needs a lot of ATPs and reducing power from respiration [a plant consumes 12 g of organic carbon per 1 g of N2 fixed] Nodule maintains low [O2] the nodule has a layer of cells (nodule cortex) with low permeability to O2 the plant produces leghemoglobin-carrying O2 for bacterial respiration The bacteria has a high-affinity cbb3-type cytochrome oxidase Sulfur assimilation Sulfur 1. 2. 3. Most of the sulfur in plants derives from SO42- absorbed via the H+/SO42- symporter in roots The first H+/SO42- transporter AST68 was discovered by complementing a yeast mutant SO42- is transported via xylem to the shoot where it is unloaded and transported into the chloroplast, transfer to various compounds in the cytosol via sulfotransferase. the Schema of plant sulfur assimilation and subcellular localization of its major steps. Green color represents plastids, brown mitochondria and blue vacuoles. cytosolic sulfation 6 e-/6 FdRed 2 e-/2 GSH reduced glutathione reduced =2ATP activation of SO42- Cations taken up by plant cells form complexes with organic compounds Cations though coordination bonds though coordination (O or N donates unshared electrons to form a bond with the cation nutrient) (O Electrostatic bonds form due to the attraction of the cations Electrostatic with a negatively charged group on an organic compound Iron assimilation Iron Plants obtain Fe from soil where it is present mainly as Fe3+ in hydroxides [Fe(OH)2+, Fe(OH)3, and Fe(OH)4-], which are very insoluble at neutral pH 3 mechanisms for helping Fe absorption by plants • Soil acidification (increase Fe solubility) • Reduction of Fe3+ to Fe2+ catalyzed by Fe-chelating reductase • Releasing chelating compounds that form stable, soluble Fe complexes Ferrochelatase Ferrochelatase Transgenic barley with enhanced ability to synthesize phytosiderophore Transgenic grew better than the control barley in alkaline soils grew ...
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