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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
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Cell Wall

Aluminum-Accumulating Plants
Beneficial Effects of Aluminum in Plants
  Growth Stimulation
  Inhibition of Plant Pathogens
Aluminum Absorption and Transport within Plants
  Phytotoxic Species
  Aluminum Speciation in Symplasm
  Radial Transport
Aluminum Toxicity Symptoms in Plants
  Short-Term Effects
    - Inhibition of Root Elongation
    - Disruption of Root Cap Processes
    - Callose Formation
    - Lignin Deposition
    - Decline in Cell Division
  Long-Term Effects
    - Suppressed Root and Shoot Biomass
    - Abnormal Root Morphology
    - Suppressed Nutrient Uptake and Translocation
    - Restricted Water Uptake and Transport
    - Suppressed Photosynthesis
    - Inhibition of Symbiosis with Rhizobia
Mechanisms of Aluminum Toxicity in Plants
  Cell Wall
    - Modification of Synthesis or Deposition of Polysaccharides
  Plasma Membrane
    - Binding to Phospholipids
    - Interference with Proteins Involved in Transport
      - H+ -ATPases
      - Potassium Channels
      - Calcium Channel
      - Magnesium Transporters
      - Nitrate Uptake
      - Iron Uptake
      - Water Channels
    - Signal Transduction
      - Interference with Phosphoinositide Signal Transduction
      - Transduction of Aluminum Signal
    - Disruption of the Cytoskeleton
    - Disturbance of Calcium Homeostasis
    - Interaction with Phytohormones
      - Auxin
      - Cytokinin
    - Oxidative Stress
    - Binding to Internal Membranes in Chloroplasts
    - Binding to Nuclei
Genotypic Differences in Aluminum Response of Plants
  Screening Tests
Plant Mechanisms of Aluminum Avoidance or Tolerance
  Plant Mechanisms of Aluminum Avoidance
    - Avoidance Response of Roots
    - Organic Acid Release
    - Exudation of Phosphate
    - Exudation of Polypeptides
    - Exudation of Phenolics
    - Alkalinization of Rhizosphere
    - Binding to Mucilage
    - Binding to Cell Walls
    - Binding to External Face of Plasma Membrane
    - Interactions with Mycorrhizal Fungi
  Plant Mechanisms of Aluminum Tolerance
    - Complexation with Organic Acids
    - Complexation with Phenolics
    - Complexation with Silicon
    - Sequestration in Vacuole or in Other Organelles
    - Trapping of Aluminum in Cells
Aluminum in Soils
  Locations of Aluminum-Rich Soils
  Forms of Aluminum in Soils
  Detection or Diagnosis of Excess Aluminum in Soils
    - Extractable and Exchangeable Aluminum
    - Soil-Solution Aluminum
  Indicator Plants
Aluminum in Human and Animal Nutrition
  Aluminum as an Essential Nutrient
  Beneficial Effects of Aluminum
    - Beneficial Effects of Aluminum in Animal Agriculture
    - Beneficial Uses of Aluminum in Environmental Management and Water Treatment
  Toxicity of Aluminum to Animals and Humans
    - Toxicity to Wildlife
    - Toxicity to Agricultural Animals
      - Toxicity to Ruminants (Cattle and Sheep)
      - Toxicity to Poultry
    - Toxicity to Humans
      - Overview of Aluminum Metabolism
      - Overview of the Biochemical Mechanisms of Aluminum Toxicity
Aluminum Concentrations
  In Plant Tissues
    - Aluminum in Roots
    - Aluminum in Shoots
  Soil Analysis
Pectins are a mixture of heterogenous polysaccharides rich in D-galacturonic acid; one major function is to provide charged structures for ion exchange in cell walls (67). Under acidic conditions, aluminum binds strongly to negatively charged sites in the root apoplast, sites consisting mostly of free carboxyl groups on pectins. Klimashevskii and Dedov (103) isolated cell walls from pea roots, exposed them to aluminum, and found that aluminum decreased plasticity and elasticity of cell walls. Blamey et al. (104) demonstrated in vitro a rapid sorption of aluminum by calcium pectate and proposed that aluminum phytotoxicity is due to strong binding between aluminum and calcium pectate in cell walls. Reid et al. (105) proposed that aluminum could disrupt normal cell wall growth either by reducing Ca2+ concentration below that required for cross-linking of pectic residues or through formation of aluminum cross-linkages that alter normal cell wall structure. Using x-ray microanalysis, Godbold and Jentschke (106) showed that aluminum displaced calcium and magnesium from root cortical cell walls of Norway spruce. Using a vibrating calcium-selective microelectrode, Ryan and Kochian (107) observed that addition of aluminum commonly resulted in an initial efflux of calcium from wheat roots, probably due to displacement of calcium from cell walls.

Pectin is secreted in a highly esterified form from the symplasm to the apoplast, where demethylation takes place by pectin methylesterase (PME), resulting in free carboxylic groups available to bind aluminum (108). Transgenic potato (Solanum tuberosum L.) overexpressing PME is more sensitive to aluminum based on inhibition of root elongation relative to unmodified control plants, indicating that increased binding sites for aluminum in the apoplast are associated with increased aluminum sensitivity (108).

Modification of Synthesis or Deposition of Polysaccharides
In addition to external binding to cell wall components, aluminum also could interfere with the internal synthesis or deposition of cell wall polysaccharides. Exposure of wheat seedlings to 10 µM Al for 6 h decreased mechanical extensibility of subsequently isolated cell walls (109). Tabuchi and Matsumoto (109) showed that aluminum treatment modified cell wall components, increasing the molecular mass of hemicellulosic polysaccharides, thus decreasing the viscosity of cell walls, and perhaps restricting cell wall extensibility.

Uridine diphosphate glucose (UDGP) is the substrate for cellulose synthesis. Using 31P-NMR, Pfeffer et al. (87) demonstrated that a 20-h exposure of excised corn roots to 0.1mM Al decreased UDGP by 65%, and they speculated that such suppression could limit production of cell wall polysaccharides. In barley, one of the most aluminum-sensitive cereals, callose was excreted from the junction between the root cap and the root epidermis after 38 min of exposure to 37 µM Al, and Kaneko et al. (110) proposed that aluminum-induced inhibition of root elongation could be due to reduced cell wall synthesis caused by a shortage of substrate to form polysaccharides.

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