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  • Micronutrients and beneficial elements in horticultural crops between critical aspects and new opportunities for the production

    and quality

    Processes at the soil/root interface involved in plant nutrient acquisition

    Prof. Stefano Cesco

    stefano.cesco@unibz.it

  • Essential mineral element (or mineral nutrient) as proposed by Arnon and Stout (1939). 1. A given plant must be unable to complete its lifecycle in the

    absence of the element 2. The function of the element must not be replaceable by another

    element. 3. The element must be directly involved in plant metabolism for

    example, as a component of an essential plant constituent such as an enzyme or it must be required for a distinct metabolic step such as an enzyme reaction.

    According to this strict definition, an element which alleviates the toxic effects of another element (e.g., Si for Mn toxicity), or one which simply replaces another element (e.g., Na for K) may not be described as essential for plant growth

  • Micronutrients are defined as substances in foods that are essential for human health and are required in small amounts Micronutrient malnutrition affects about 1/3 of the global population

  • 1. Iron (Fe): a) availability in soil and metabolic functions, b) mechanisms for acquisition and use efficiency, c) interactions with other nutrients such as N and S, d) possible strategies for biofortification

    2. Silicon (Si): beneficial effect in relation to productivity and to mechanisms for acquisition of other nutrients

    3. Nickel (Ni): with respect to the acquisition of N and the quality of the edible product

    4. Selenium (Se): with respect to the acquisition of other nutrients (S and N) and the quality of the edible product

  • Iron

  • IRON

    General

    Fe is the second most abundant metal in the earths crust after Al

    Solubility of Fe is, however, extremely low, especially in aerated alkaline soils (concentrations of ionic Fe3+ and Fe2+ are below 10-15 M)

    Chelates of Fe(III) and occasionally of Fe(II) are therefore the dominant forms of soluble Fe in soil and nutrient solutions As a rule, Fe(II) is taken up preferentially compared with Fe(III), but this also depends on the plant species

    Fe is normally found in most soils being the fourth most abundant

    element in the lithosphere

  • Soil availability as a function of pH

    Solubility of Fe(III) is very low at neutral pH, and even lower at pH 8, typical of alkaline soils (30 % of soils worldwide).

    http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/iron-chlorosis/

    Solubility of inorganic iron species in equilibrium with iron oxides in well-aerated soils in comparison to the requirement of soluble iron at the root surface of various plant species (Rmheld and Marschner, 1986)

  • Soil availability as a function of pH

    Solubility of Fe(III) is very low at neutral pH, and even lower at pH 8, typical of alkaline soils (30 % of soils worldwide).

    http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/iron-chlorosis/

    Solubility of inorganic iron species in equilibrium with iron oxides in well-aerated soils in comparison to the requirement of soluble iron at the root surface of various plant species (Rmheld and Marschner, 1986)

  • Acquisition mechanism in dicots

    Strategy I

    Apoplasm Symplasm

    H+ ATP

    ADP H+

    Fe(III)-chelate NADH

    Fe(II) NAD+

    Fe(II) Fe(II)

    AHA2

    IRT1

    FRO2

    Fe

  • + Fe - Fe

    Fe(III)-chelate reductase activity [nmol Fe(II) gpf

    -1h-1]

    64 2388

    16-day-old Cucumber plants

    Fe Acquisition mechanism in dicots

  • IRON

    General

    In long-distance transport in the xylem, there is a predominance of Fe(III) complexes

    As a transition element, Fe is characterized by the relative ease by which it may change its oxidation state: Fe3+ Fe2+

    and by its ability to form octahedral complexes with various ligands

    Depending on the ligand, the redox potential of Fe(II/III) varies widely

  • General

    Due to the high affinity of Fe for various ligands (e.g., organic acids or inorganic phosphate) ionic Fe3+ or Fe2+ do not play a role in short- or long-distance transport in plants

    In aerobic systems many low-molecular-weight iron chelates, and free iron in particular (either Fe3+ or Fe2+), produce reactive oxygen species (ROS) such as superoxide radical and hydroxyl radical and related compounds,

    These radicals are highly toxic and responsible for peroxidation of polyunsaturated fatty acids of membrane lipids and proteins To prevent oxidative cell damage, Fe has to be either tightly bound or incorporated into structures (e.g., heme and non-heme proteins) which allow controlled reversible oxidationreduction reactions

  • Heme Proteins

    Iron-containing Constituents of Redox Systems

    The most well known heme proteins are the cytochromes, which contain a heme Feporphyrin complex as a prosthetic group

    Role of Fe in the biosynthesis of heme coenzymes and chlorophyll

  • Heme Proteins

    Iron-containing Constituents of Redox Systems

    susceptible to low supply of Fe under Fe deficiency, the activity of both enzymes rapidly decreases in

    plant tissues, particularly catalase in genotypes susceptible to Fe deficiency, for example tomato

    1. An example of the first type of reaction is the detoxification of H2O2 in chloroplasts catalysed by ascorbate peroxidase

    2. In the second type of reaction, cell wall-bound peroxidases catalyse the polymerization of phenols to lignin.

    catalase and peroxidases

    Catalase facilitates detoxification of H2O2 to water and O2 according to the reaction:

    Peroxidases catalyse the following reactions

  • Heme Proteins

    Iron-containing Constituents of Redox Systems

    The alterations in cell wall formation of rhizodermal cells under Fe deficiency may be related to impaired peroxidase activity

    biosynthesis of lignin and suberin require phenolic compounds and H2O2 as substrates

    The formation of H2O2 is catalysed by the oxidation of NADH at the plasma membrane/cell wall interface

    Taiz-Zeiger - Plant Physiology

  • Heme Proteins

    Iron-containing Constituents of Redox Systems

    In Fe-deficient roots, peroxidase activity is strongly depressed Consequently, H2O2 production is increased and phenolics are accumulate and then released at higher rates from the roots

    Certain phenolics, such as caffeic acid, are very effective in chelation and reduction of inorganic Fe(III), and a component of Strategy I in Fe acquisition

    In response to Fe deficiency, red clover releases high amounts of phenolics which contribute to utilization and remobilization of root apolastic Fe

    https://dl.sciencesocieties.org/publications/books/abstracts/sssabookseries/micronutrientsi2/145?access=0&view=article

  • Fe-S Proteins

    Iron-containing Constituents of Redox Systems

    Fe is coordinated to the thiol group of cysteine or to inorganic S as clusters, or to both

    The most well-known Fe-S protein is ferredoxin, which acts as an electron transmitter in a number of metabolic processes according to the principle

    Due to the involvement of Fe at various steps in nitrate reduction, positive correlations between Fe supply, ferredoxin concentration and nitrate reduction are to be expected

    https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/redox.htm

  • Nitrogen Assimilation Ammonium Assimilation

    Two isoforms of GOGAT. One form accepts electrons from reduced ferredoxin (from photosystem I) the other from NADPH from respiration

    the ferredoxin-linked GOGAT isoform dominates in leaves, particularly in the chloroplasts of phloem companion cells in leaf veins , whereas the NADPH isoform is prevalent in roots

    Glutamate synthase (GOGAT)

    Buchanan-Gruissem-Jones Biochemistry &MBP

    Fe-S Proteins

    Iron-containing Constituents of Redox Systems

  • Fe-S Proteins

    Iron-containing Constituents of Redox Systems

    The isoenzymes of superoxide dismutase (SOD) contain Fe as a metal component of the prosthetic group (FeSOD) 1. may contain Cu, Zn, Mn or Fe as metal components

    2. detoxify superoxide anion free radicals (O2

    -) by formation of H2O2

    http://textbookofbacteriology.net/nutgro_4.html

  • Fe-S Proteins

    Iron-containing Constituents of Redox Systems

    Aconitase is an Fe-S protein which catalyses the isomeration of citrate to isocitrate in the tricarboxylic acid cycle

    1. Fe, as metal component of the prosthetic group, is required for stability and activity of the enzyme

    2. The Fe cluster of the enzyme is responsible for the spatial orientation of the substrates (citrate and isocitrate)

    In Fe-deficient plants, aconitase activity is lower and reactions in the tricarboxylic acid cycle are disturbed leading to organic acids accumulation, particularly citric and malic acid

    Similar increases in concentration of organic acids were also found in xylem exudates and leaf apoplasmic fluids of Fe-deficient plants indicating the Fe transport as stable, water soluble Fe-citrate complexes

    Taiz-Zeiger - Plant Physiology

  • Other Fe-requiring Enzymes

    Iron-containing Constituents of Redox Systems

    Along the ethylene biosynthetic pathway, in the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene, a two-step one-electron ox

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