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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
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Plant Mechanisms of Aluminum Tolerance

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
Mechanisms of internal tolerance of aluminum involve: (a) complexation with organic acids, (b) complexation with phenolics, (c) complexation with silicon, (d) sequestration in the vacuole or other storage organs, and (e) trapping of aluminum in cells.

Complexation with Organic Acids
In the leaves of aluminum-accumulating hydrangea, Ma et al. (257) used molecular sieve chromatography to determine that citrate eluted at the same time as aluminum and that the molar ratio of aluminum to citric acid was approximately 1:1. In the aluminum accumulator, buckwheat, aluminum was complexed with citrate in the xylem (258), but with oxalic acid in vacuoles of leaf cells (259,260). In the aluminum accumulator, Melastoma malabathricum L., aluminum citrate occurred in the xylem sap and was then transformed into aluminum oxalate for storage in leaves (261,262).

Complexation with Phenolics
In aluminum-accumulating tea, Nagata et al. (263) used 27Al-NMR to demonstrate that aluminum was bound to catechin in young leaves and buds; in mature leaves, aluminum-phenolic acid and aluminum-organic acid complexes were found. Interestingly, Ofei-Manu et al. (227) showed that at pH 7 (cytoplasmic pH), aluminum binding capacity is in the order: quercetin>catechin, chlorogenic acid, morin>organic acids. Among ten woody plant species and two marker crop species, a positive linear correlation was found between root phenolic compounds and aluminum tolerance, based on aluminum-inhibited root elongation (227).

Complexation with Silicon
Cocker et al. (229) proposed that amelioration of aluminum toxicity by silicon is due to formation of an aluminosilicate compound in the root apoplast. Hodson and Sangster (264) proposed that codeposition of aluminum and silicon in needles of conifers is responsible for aluminum detoxification by silicon. Hodson and Evans (228) reviewed the evidence in support of various mechanisms of silicon amelioration of aluminum toxicity, and they divided plants into four groups: (a) aluminum accumulators in arborescent dicots, (b) silicon accumulators in grasses, (c) gymnosperms and arborescent dicots with moderate amounts of aluminum and silicon, and (d) herbaceous dicots that exclude aluminum and silicon. Obviously, aluminum can codeposit with silicon only in plants that accumulate both elements. Aluminum was deposited in phytoliths (hydrated silica deposits) of conifers, graminaceous plants, and dicots in the Ericaceae family (265,266). Using x-ray microanalysis, Hodson and Sangster (267) found codeposition of aluminum and silicon in the outer tangential wall of the endodermis of sorghum. In Faramea marginata Cham., a woody member of the Rubiaceae family that is known to accumulate aluminum and silicon in leaves, colocalization of aluminum and silicon in a molar ratio of 1:2 occurred in the cortex of stem sections and throughout leaves (268). A good review of aluminum and silicon interactions can be found in Hodson and Evans (228), Cocker et al. (229), and Hodson and Sangster (264).

Sequestration in the Vacuole or in Other Organelles
Aluminum ions could be sequestered in vacuoles or other storage organelles where they would not affect metabolism in the cytoplasm adversely. The presence of 50 µM Al increased pyrophosphatedependent and ATP-dependent H+ pump activity in tonoplast membrane vesicles isolated from barley roots, and Kasai et al. (269) hypothesized that Al3+ was sequestered in the vacuole perhaps by an Al/nH+ exchange reaction. Interestingly, expression of two 51 kDa proteins is strongly induced in an aluminum-tolerant wheat cultivar, and only weakly expressed in an aluminum-sensitive wheat cultivar (270). Sequence analysis of the purified peptides showed that one is homologous to the B subunit of the vacuolar H+-ATPase (V-ATPase) (270).

In an aluminum-tolerant unicellular red alga (Cyanidium caldarium Geitler), aluminum accumulated in spherical electron-dense bodies in the cytoplasm near the nucleus (271). These bodies contained high levels of iron and phosphorus, and the researchers speculated that they might be iron-storage sites under normal culture conditions. Interestingly, transferrin, an iron carrier, is the main protein that binds Al3+ in the blood plasma of animals (47).


Trapping of Aluminum in Cells
Fiskesjo (272) proposed that aluminum could be trapped in root border cells, which were then detached and sloughed away from roots. Consistent with this hypothesis, detached root border cells of snap bean were killed by aluminum within 2 h of aluminum exposure (70). A punctated pattern of cell death was observed in aluminum-tolerant wheat roots after 8 h of exposure to aluminum, with an increase in oxalate oxidase activity and H2O2 production after 24 h (273). Delisle et al. (273) speculated that cell death could be a means for root tip cells to trap or exclude aluminum from live tissues. Interestingly, a hypersensitive cell death response is a common means for plants to trap pathogens, not allowing them to spread to other cells. Many genes up-regulated by aluminum in wheat are similar to pathogenesis-related genes (274).

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