1. Chapter 12 Assimilation of Mineral Nutrients

2. Nutrient assimilation   Incorporation of mineral nutrients into organic substances such as amino acids, nucleic acids, enzyme cofactors, lipids, and pigments

3. The Nitrogen Cycle Thiobacillus Denitrificans (Denitrification)

4. (Nitrification) Plants Microorganisms Nitrosomonas Nitrobacter Nitrococcus Thiobacillus denitrificans Lightning 75% 15% 10% N 2 + H 2 Fertilizer Food chain Bacteria or cyanobacteria

6. Flag leaf

8. NH 4 + toxicity can dissipate pH gradient ammonium ammonia

9. Root cell NO 3 - NO 3 - (Nitrate) NO 2 - (Nitrite, toxic) Nitrate reductase NADH (or NADPH) NAD + (or NADP + ) NO 2 - NH 4 + (Ammonium) NADPH nitrite reductase Plastids Glutamine Glutamate GS/GOGAT Nitrate assimilation at roots Nitrate-proton cotransporter

10. Nitrate reductase   Homodimers, two identical subunits   Polypeptide sequences are similar in eukaryotes (except the hinge regions)   Each subunit with 100kD   Each subunit contains 3 prosthetic groups: FAD, heme, and molybdenum complexed to pterin (an organic molecule).   Nitrate reductase is the main molybdenum- containing protein in vegetative tissues.

11. NAD +, H + NO 2 + The nitrate reductase dimer Three binding domains NO 2 + (+5) (+3) NAD +, H +

12. Addition of nitrate stimulates nitrate reductase mRNA accumulation and enzyme activity in roots and shoots of barley

13.  Nitrate reductase-Pi Nitrate reductase  Nitrate reductase Nitrate reductase -Pi Light [ carbohydrate ] (active form) Serine/Threonine (inactive form) protein kinase (inactive form) Phosphatase (active form) Dark [Mg +2 ] Other factors regulate nitrate reductase

14.   Regulation of nitrate reductase activity through phosphorylation and dephosphorylation provides more rapid control than can be achieved through synthesis or degradation of the enzyme (minutes versus hours).

15.   In many plants, when the roots receive small amounts of nitrate, nitrate is reduced primarily in the roots. As the supply of nitrate increases, a greater proportion of the absorbed nitrate is translocated to the shoot and assimilated there.   Generally, plants native to temperate region assimilate NO 3 - at roots.   Plants native to tropical or subtropical region assimilate NO 3 - at shoots.

16. Root cell NO 3 - NO 2 - Nitrate reductase NADH or NADPH NAD + or NADP + NO 2 - NADPH-nitrite reductase Plastids Nitrite reductase converts nitrite to ammonium in roots NH 4 +

17. Mesophyll cell NO 3 - NO 2 - Nitrate reductase NADH NAD + NO 2 - NH 4 + Ferredoxin- nitrite reductase Chloroplasts Nitrite reductase converts nitrite to ammonium in leaves

18. Nitrite reductase   Nitrite (NO 2 - ) is a highly reactive, potentially toxic ion.   Plant cells immediately transport the nitrite into chloroplasts or plastids.   The enzyme nitrite reductase reduces nitrite to ammonium.   Leaf chloroplasts and root plastids contain different forms of nitrite reductase.   Nitrite reductase is synthesized in cytoplasm with an N- terminal transit peptide.   Both nitrite reductases consist of a single 63kD polypeptide.   Each polypeptide contains two prosthetic groups, an iron- sulfur group and a specialized heme.   NO 3 -, high sucrose conc, and light induce the transcription of nitrite reductase mRNA.   Asparagine and glutamine repress the induction.

19. A. Chloroplasts Ferredoxin - a protein of 12 ~ 24 kD - containing a FeS group - e - carried by the iron Pentose phosphate pathway B. Root plastids NADPH NADP +, H + e-e- 2

20. Relative amounts of nitrate and other nitrogen compounds in the xylem exudate of various plant species R-CO-NH 2

21. Ammonium assimilation   NH 3 is almost certainly protonated to form NH 4 +.   NH 4 + is toxic to plants.   Inhibit dinitrogenase   Interfere energy metabolism by dissociating ATP formation from ETC in both mitochondria and chloroplasts.   To avoid these problems, NH 4 + generated from nitrate assimilation or photorespiration is rapidly converted into amino acids.   Conversion of ammonium to amino acids requires two enzymes, glutamine synthetase (GS) and glutamate synthase (GOGAT).

22. Ammonium can be assimilated by one of several processes GS GOGAT

23. Glutamine synthetase (GS)   Glutamine synthetase Glutamate + NH4 + -----------------> Glutamine GS   Two classes of glutamine synthetase (GS)   in cytosol   The cytosolic form GS express in germinating seeds or in the vascular bundles of roots and shoots, and produce glutamine for intracellular nitrogen transport   in root plastids and in shoot chloroplasts   GS in root plastids : generating amide nitrogen (glutamine & asparagine) for local consumption.   The GS in shoot chloroplast reassimilates photorespiratory NH 4 +.

24. Germinating seeds/vascular bundles of roots and shoots NO 2 - NH 4 + Plastids Glutamate Glutamine GS Ammonium assimilation via GS/GOGAT Nitrite reductase NH 4 + GOGAT 2 Glutamate 2-oxoglutarate

25. Root cell NO 3 - NO 2 - Nitrate reductase NADH or NADPH NAD + or NADP + NO 2 - NADPH nitrite reductase Plastids Ammonium assimilation in roots NH 4 + (Ammonium) GS/NADH- GOGAT Glutamate

26. Mesophyll cell NO 3 - NO 2 - Nitrate reductase NADH NAD + NO 2 - NH 4 + Ferredoxin-nitrite reductase GS/Fd-GOGAT Chloroplasts Ammonium assimilation in leaves Glutamate

27.   Light and carbohydrate levels alter the expression of the plastid forms of the enzyme, but they have little effect on the cytosolic forms.

28. Glutamate synthase (glutamine:2-oxo- glutarate aminotransferase, GOGAT)   Plants contain two types of GOGAT.   NADH-GOGAT   Ferredoxin-GOGAT (Fd-GOGAT)   NADH-GOGAT NADH-GOGAT   NADH-GOGAT is located in plastids of roots or vascular bundles of leaves.   In roots, NADH-GOGAT is involved in the assimilation of NH 4 + absorbed from the rhizosphere.   Fd-GOGAT Fd-GOGAT   Fd-GOGAT is found in chloroplasts and serves in photorespiratory nitrogen metabolism.

29. Ammonium can be assimilated via an alternative pathway

30. Alternative pathway for assimilating ammonium   Two types of Glutamate dehydrogenase (GDH) :   NADH-GDH is found in mitochondria.   NADPH-GDH is found in chloroplasts.   Although both forms are relatively abundant, they cannot substitute for the GS-GOGAT pathway for assimilation of ammonium, and their primary function is in deaminating glutamate during the reallocation of nitrogen.

31. Transamination reactions transfer nitrogen   Once assimilated into glutamine and glutamate, nitrogen is incorporated into other amino acids via transamination reactions.   Aminotransferase catalyze the transamination reaction.   Aminotransferase are found in the cytoplasm, chloroplasts, mitochondria, glyoxysomes, and peroxisomes.

32. An example of aminotransferase

33. Root plastids/shoot chloroplast NO 2 - NH 4 + GS Ammonium Assimilation via GS/GOGAT Glutamine Nitrite reductase Glutamate 2 Glutamate GOGAT 2-oxoglutarate Oxaloacetate 2-oxoglutarate + Aspartate Glutamine Asparagine Glutamate Asparagine synthetase (AS) Aspartate aminotransferase Glutamate Transamination reactions

34. Asparagine and glutamine link carbon and nitrogen metabolism   Asparagine serves not only as a protein precursor, but as a key compound for nitrogen transport and storage because of its stability and high nitrogen-to- carbon ratio (1:2).   The major pathway for asparagine synthesis involves the transfer of the amide nitrogen from glutamine to asparagine.   Asparagine synthetase (AS) catalyzes the asparagine synthesis reaction.   AS is found in the cytosol of leaves and roots, and in nitrogen-fixing nodules.

35. amide

36. Biosynthetic pathways for the carbon skeletons of the 20 amino acids in plants

37. Amino acid biosynthesis   The nitrogen-containing amino groups derives from transamination reactions with glutamine or glutamate.   The carbon skeleton for amino acids derive from 3-phosphoglycerate, phosphoenolpyruvate, or pyruvate generated during glycolysis, or from  - ketoglutarate or oxaloacetate generated in the citric acid cycle.

38.   Human and most animals cannot synthesize certain amino acids.   Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine and arginine => essential amino acids   Obtain these essential amino acids from their diet.   In contrast, plants synthesize all the 20 amino acids.

39.   High levels of light and carbohydrates stimulate GS and GOGAT, inhibit AS, and thus favor nitrogen assimilation into glutamine and glutamate, compounds that are rich in carbon and participate in the synthesis of new plant materials.   In contrast, energy-limiting conditions inhibit GS and GOGAT, stimulate AS, and thus favor nitrogen assimilation into asparagine, a compound that is rich in nitrogen and sufficient stable for long-distance transport or long-term storage.

40. N:C ratio Allantoin ( 尿囊素 ) = 1:1 Allantoic acid = 1:1 Asparagine = 1:2 Citrulline= 1:2 Glutamine= 1:2.5 Glutamate= 1:5 The more nitrogen ratio the molecule contains, the higher efficiency the molecule transports nitrogen.

41. (Synthesized in peroxisome) (ER) (amide : glutamine & asparagine )

42. p.310 Fig.12-21

43. Biological nitrogen fixation

44.   Biological nitrogen fixation accounts for most of the fixation of atmospheric N 2 into ammonium.   N 2 + 8H + + 16ATP ---------------> 2NH 3 + H 2 + 16ADP + 16Pi nitrogenase   Two major types of nitrogen fixation :   Prokaryotes that are free-living in the soil, lake, and ocean.   Symbiotic bacteria

45. (Nonsymbiotic N 2 fixation)

46. N 2 fixation   Symbiotic N 2 fixation   Rhizobia – Legume   Unicellular   G(-)   Rhizobial symbiosis   Actinomycetes – Nonleguminous plants   Frankia spp, Alnus (alder), Myrica (bayberry), Casuarina (Australian pine)   Filamentous bacteria – G(+)   Live freely in soil, but fix nitrogen in symbiotic association with host.   Actinorhizal symbiosis   Nonsymbiotic N 2 fixation

47. N 2 fixation  Symbiotic N 2 fixation  Nonsymbiotic N 2 fixation  Cyanobacteria (Blue-green algae) : Nostoc, Anabaena, Calothrix, etc.  Other bacteria  Aerobic : Azotobacter, Beijerinckia, Derxia, etc.  Facultative : Bacillus, Klebsiella, etc.  Anaerobic : Nonphotosynthetic - Clostridium, Methanococcus Photosynthetic – Rhodospirillum, Chromatium

48. Cyanobacterium Anabaena

50. Heterocysts   Thick-walled cells   Differentiate when filamentous cyanobacteria are deprived of NH 4 +.   Heterocysts lack photosystem II, so they do not generate oxygen.   Exist among aerobic cyanobacteria that fix nitrogen.

51. Root nodules on soybean

52. 紫花苜蓿 蠶豆 豌豆 苜蓿 黃豆, 大豆 扁豆 菜豆

54. Symbiosis   The symbiosis between legumes and rhizobia is not obligatory.   Legume seedlings can be unassociated with rhizobia throughout their life cycle.   Rhizobia also occur as free-living organisms in the soil.   Under nitrogen-limited conditions, the symbionts seek out one another through an elaborate exchange of signals.

55. Nodule formation   Recognition and Colonization   Chemotaxis (response to flavonoids and betaine) pea roots – Rhizobium leguminosarum   Flavonoids (host) -> activate expression of nod genes -> Nod factor (lipo-chito-oligosaccharide)   Lectin – Nod factor -------------------------> initiate nodule formation e.g. pea lectin gene -> white clover -> nodulated by R. leguminosarum bv. viciae (pea)   Rhicadhesin : a calcium-binding protein (of all rhizobia)   Invasion of root hairs   Nodulation

56. Lectin Flavonoids Nod factor nod D nod boxnodA nodB nodC Root hair Rhizobia Nod factor Root Constitutive expression The common nod genes – nodA, nodB, and nodC – are found in all rhizobial strains and are required for synthesizing the basic structure.

59. Lectin Flavonoids Nod factor nod D nod boxnodA nodB nodC Root hair Rhizobia Nod factor Root Constitutive expression The common nod genes – nodA, nodB, and nodC – are found in all rhizobial strains and are required for synthesizing the basic structure.

60.   NodA is an N-acyltransferase that catalyzes the addition of a fatty acyl chain. NodA   NodB is a chitin-oligosaccharide deacetylase that removes the acetyl group from the terminal nonreducing sugar.   NodC is a chitin-oligosaccharide synthase that links N-acetyl-D-glucosamine monomers. Nod Factors

61. = 1-4 General structure for Nod factors 18C 16C 20C (varied unsaturation (n=2~3)  -1,4-linked N-acetyl- D -glucosamine backbone NodA NodC NodB NodE NodF NodL NodP NodQ NodH etc

62. Flavonoids nod D nod boxnodF nodE nodL Rhizobiz Root nod boxnodP nodQ nodH The host-specific nod genes – nodP, nodQ, and nodH, or nodF, nodE, and nodL - differ among rhizobial species and determine the host range. Lectin Nod factor

63.   NodE and NodF determine the length and degree of saturation of the fatty acyl chain.   NodL adds specific substitutions at the reducing or nonreducing sugar moieties of the chitin backbone.   The host-specific nod genes – nodP, nodQ, and nodH, or nodF, nodE, and nodL - differ among rhizobial species and determine the host range.

64. = 1-4 General structure for Nod factors 18C 16C 20C (varied unsaturation (n=2~3)  -1,4-linked N-acetyl- D -glucosamine backbone NodA NodC NodB NodE NodF NodL NodP NodQ NodH etc

66.  The binding of flavonoids to the NodD gene product is one of the major determinants of rhizobial host specificity because each plant species produces its own specific set of flavonoid molecules and each rhizobial species recognizes only a limited number of flavonoid structures.  Some strains, such as R. leguminosarum bv. trifolii, have a very narrow host range, responding to only a few kinds of flavonoids, while others, such as Rhizobium sp. strain NGR234, have broad host range and respond to a much larger number of different flavonoids. Determination of host specificity

67. Lectin Flavonoids Nod factor nod D nod boxnodA nodB nodC Root hair Rhizobia Nod factor Root Constitutive expression Recognition

68.   A particular legume host responds to a specific Nod factor.   The legume receptors for Nod factor appear to involve special lectins (sugar binding proteins) produced in the root hairs.   Nod factors activate these lectins, increasing their hydrolysis of phosphoanhydride bonds of nucleoside di- and triphosphates.   This lectin activation directs particular rhizobia to appropriate hosts and facilitates attachment of the rhizobia to the cell walls of a root hair.

69.   A major goal of agricultural biotechnology research is the development, by genetic manipulation, of Rhizobium strains that can increase plant productivity more than naturally occurring strains can.   However, to date, no simple genetic means has been devised to use nod genes to enable inoculated strains of Rhizobium to outcompete indigenous strains.   The indigenous strains might be more efficient at nodulation and, as a consequence, might prevent an inoculated strain from becoming established.   Nevertheless, host specificity can be altered by the transfer of a nodD gene from a broad-specificity rhizobial strain to one with a narrow specificity.

72. Bacteroid

74. Stages in the initiation and development of a soybean root nodule. (A) Events involved in the initiation of the nodule: (1) the root excretes substances; (2) these substances attract rhizobia and stimulate them to produce cell- division factors; (3) cells in the root cortex divide to form the primary nodule meristem. (B) Stages of infection and nodule formation: (4) bacteria attach to the root hair; (5) cells in the pericycle near the xylem poles are stimulated to divide; (6) the infection thread forms and extends inward as the primary nodule meristem and the pericylce continue to divide; (7) the two masses of dividing cells fuse into a single clump while the infection thread continues to grow; (8) the nodule elongates and differentiates, including the vascular connection to the root stele. Bacteroids are released into the cells in the center.

75. Nitrogen fixation by the nitrogenase enzyme complex

76. N 2 + 8H + + 16ATP ---------------> 2NH 3 + H 2 + 16ADP + 16Pi Nitrogenase Nitrogen fixation by the nitrogenase enzyme complex

77. Nitrogenase (Dinitrogenase)  Only prokaryotes have this enzyme.  ~20% of the total protein in the cell  contains two components – Fe protein and MoFe protein  Fe protein  contains 2 identical subunits (dimer) – nif H  Each subunit has MW of 30 ~ 72 kD.  Each subunit contains an Fe-S cluster (4Fe and 4S 2- ).  MoFe protein  2 pairs of identical subunits (tetramer, nif D & nif K)  A total of MW 180 ~ 235kD  Each subunit has 2 Mo-Fe-S clusters.  Both Fe proteins and MoFe proteins are sensitive to O 2.

78. The nitrogenase reaction in bacteroids Ferredoxin - 12 ~ 24 kD - a FeS group - e - carried by the iron - nif F Fe +3 Fe +2 Fe +3 Fe +2 NADH NAD +, H + TCA cycle nif H nif D, nif K

79. Fe +2 Fe +3 Fe +2 Fe +3

80. Proposed structure of the iron-molybdenum cofactor bound to a molecule of dinitrogen (N 2 ) MoFe cofactor of MoFe protein

81. MoFe proteins can reduce numerous substrates Under natural conditions it reacts only with N 2 and H +.

82. Dinitrogenase is sensitive to O 2   Live anaerobically   Bacteroids   Nitrogen-fixation vesicles (multilayered envelope consisting of hopanoids-steroid lipids)   Leghemoglobin   O 2 -binding proteins   located in the cytoplasm of the bacteroid-infected cells (nodule cells) at high concentration (~ 30% of total proteins or 700  M).   Control the release of O 2 in the region of bacteroids. How to reconcile the conflicting demands of O 2 for respiratory (producing ATP for nitrogen fixation) and the sensitivity of dinitrogenase to O 2 ?

83. Leghemoglobin   An oxygen-binding heme protein that help transport only enough oxygen to the respiring symbiotic bacterial cells.   Leghemoglobin has a high affinity for oxygen, about ten times higher than the  chain of human hemoglobin.   Present in the cytoplasm of infected nodule cells at high concentration (~ 30% of total proteins or 700  M in soybean nodules)   Leghemoglobin contains two parts : globin portion and heme portion.   The host plant produces the globin portion in response to infection by bacteria; the bacterial symbiont produces the heme portion.

84. O 2 leghemoglobin-O 2 H2OH2O In soybean, 12 g of carbon are required to fix 1 g of N 2 => Nitrogen fixation is energy demanding. Peribacteroid membrane

85. Genetic engineering of the nitrogenase gene cluster  The first nif genes identified by complementation were isolated from klebsiella pneumoniae.  The entire set of the nif genes are arranged in a single cluster that occupies approximately 24 Kb.  The cluster contains of 7 operons that encode 20 proteins.  Alfalfa plants that were inoculated by the Rhizobium meliloti contain an extra copy of the nifA gene (a positive activator) grew larger and produced more biomass than plants treated with the nontransformed strain.  Genetic modification of plants with the entire nif genes cluster would not be effective because the normal level of oxygen in the host cell would inactivate nitrogenase.

86. Isolation of nif genes by genetic complementation pLAFR1

87. The nif gene cluster

89. Ferredoxin (Fe protein) (MoFe protein)

90. N 2 + 8H + + 8e - +16ATP ---------------> 2NH 3 + H 2 + 16ADP + 16Pi Nitrogenase   Accompany of H 2 envolving during N 2 fixation is energy consuming.   N 2 fixer contains O 2 -dependent “uptake hydrogenase” which recovers some of the energy lost to H 2 production by coupling H 2 oxidation to ATP production.

91. Recycling of the hydrogen gas produced by nitrogen fixation H2H2 H+H+ e-e-

94. Some strains of B. japonicum could use hydrogen as an energy source for growth under low-oxygen concentration condition.

95. O 2 leghemoglobin-O 2 H2OH2O

96.   Amides and ureides are the major transported forms of nitrogen.   Amide ( 醯胺, R-CO-NH 2 ) : glutamine, asparagine   Ureides ( 醯脲 ): allantoic acid ( 尿囊酸 ), allantoin ( 尿囊 素 ), and citrulline ( 瓜氨酸 ),

97. (Synthesized in peroxisome) (ER)

98. The major ureide compounds used to transport nitrogen

99. NH 4 + is reassimilated via GS/GOGAT systems allantoinase Urate oxidase Release NH 4 + Other tissue peroxisome

101. Legume   Tropical legume   Soybean, cowpeas, Southern peas, kidney beans, peanut, and mung beans   Use ureides for nitrogen transport => ureide exporters   Temperate legume   Peas, lupins ( 羽扇豆 ), alfalfa, clovers, broad bean, and lentil ( 扁豆 )   Use glutamine or asparagine (amide) for nitrogen translocation => amide exporters

102. N:C ratio Allantoin ( 尿囊素 ) = 1:1 Allantoic acid = 1:1 Asparagine = 1:2 Citrulline= 1:2 Glutamine= 1:2.5 Glutamate= 1:5 The more nitrogen ratio the molecule contains, the higher efficiency the molecule transports nitrogen.

103. Importance of sulfur in cells   Confer disulfide bridge in proteins   Participate in Fe-S clusters for electron transport   Several enzymes need sulfur.   Some secondary metabolites contain sulfur.

104. Sulfur assimilation   Most of the sulfur in higher plants derives from sulfate (SO 4 -2 ) absorbed from the soil solution via an H + -SO 4 -2 symporter.   The first step in the synthesis of sulfur-containing organic compounds is the reduction of sulfate (+6) to cysteine (-4).   The enzymes involved in cysteine synthesis have been found in the cytosol, plastids and mitochondria.   Sulfate assimilation occurs mainly in leaves.   Sulfur assimilated in leaves is exported as glutathione via the phloem to sites of protein synthesis. Glutathione : r-glutamylcysteinylglycine

105. Reduced Oxidized

107. Phosphate assimilation   Plant roots absorb phosphate (H 2 PO 4 - ) via H + - H 2 PO 4 - symporter from the soil.   The main entry point of phosphate into assimilatory pathway is ATP.   The phosphate is subsequently incorporated into a variety of organic compounds, including sugar phosphates, phospholipids, and nucleotides.

109. Cation assimilation   Plants assimilate macronutrient cations (P, Mg, and Ca) and micronutrient cations (Cu, Fe, Mn, Co, Na, and Zn).   Cations taken up by plant cells form complexes with organic compounds.   Cations form coordination bonds and electrostatic bonds with carbon compounds.

110. Examples of coordination complexes A major constituent of pectins

111. Examples of electrostatic complexes

112. Uptake of iron from soil   Most of the iron in the plant is found in the heme molecule of cytochromes within the chloroplasts and mitochondria.   In addition, iron also exists in iron-sulfur proteins (ferredoxin).

113. The ferrochelatase reaction A precursor of heme group in cytochrome

114.  Supplied FeSO4 or Fe(NO3)2, iron is present primarily as ferric iron (Fe +3 ) in oxides such as Fe(OH) 3 and can precipitate out of solution. At neutral pH, ferric iron is highly insoluble, and the Ksp is 2X10 -39.  If phosphate salts are present, insoluble iron phosphate Fe2(PO4)3 will form.  How to increase iron solubility in soil and its availability for plants?  Soil acidification increases the solubility of ferric iron.  Reduction of ferric iron (Fe +3 ) to the more soluble ferrous form (Fe +2 ) at the root surface and release the iron from the chelator. (continued) Uptake of iron from soil

115. Problem in maintaining availability of iron  Root cells may reduce the Fe +3 to Fe +2.  After uptake into the root, iron is kept soluble by chelation with organic chelator citric acid.  Chelator agents such as citric acid, caffeic acid, tartaric acid, EDTA, and DTPA are used to solve the problems.

116. DTPA The chelator DTPA and chelated to iron p.78 Fig. 5.2 DTPA-Fe

117. Two strategies for plants to uptake Fe   Strategy I   Nongrain dicot (leafy vegetable)   Pea, tomato, soybean   Strategy II   Monocot grain   Barley, maize, oat

118. Cytoplasm rhizosphere Proton pump H+H+ H+H+ ATP ADP + Pi Fe +3 Fe +2 NADH NAD + Fe +2 Iron transporter Plasma membrane Ferritin Strategy I : nongrain dicot (leafy vegetable) Ferric reductase Fe +3 Soil colloid Chelator

119. In soil or nutrient solution  Fe +2 [SO 4 -2 or (NO3 - ) 2 ] -> Fe(OH) 3 or Fe 2 (PO 4 ) 3 (precipitate)  Fe +3 + chelator (EDTA, citric acid, caffeic acid, and tartaric acid) -> chelator-Fe +3 -> root surface -> chelator-Fe +3 -> chelator + Fe +2  Fe +2 is absorbed by roots (Fe +2 + citric acids) -> xylem  chelator diffuse back to soil or solution

120. Chelators

121. DTPA The chelator DTPA and chelated to iron p.78 Fig. 5.2 DTPA-Fe

123. 增加米中鐵的含量  從黃豆 (Soybean) 選殖 ferritin gene 並轉殖入米中 Seed Leaf Stem Root Control14119170956 (  g-Fe/g-DW) Fk1 38104162962 FK113598175966

124. Fe +3

125. Strategy I nongrain dicot (leafy vegetable) Phytosidero- phores (PS) Strategy II Monocot grain (Graminae)

126. Phytosiderophores (PS)  PS are nitrogen containing compounds.  PS are found in members of family, Graminae.  PS are synthesized and released under iron stress.  PS have high affinity for Fe +3 and can scavenge iron from rhizosphere.  Iron-PS complexes (ferrisiderophore) is then reabsorbed into the roots.

128. End