Aluminum as an Essential Nutrient


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

Aluminum as an Essential Nutrient

Speculation that aluminum is an essential nutrient has persisted for at least 70 years (330); yet to date, there is no conclusive evidence for its essentiality in the diets of animals or humans (6,7). One of the earliest speculations about the essentiality was by E. E. Smith, president of the New York Academy of Sciences in the early 1900s. In his 1928 book on aluminum, he described the effects of adding different elements to milk on the growth and fertility of rats consuming only a milk diet (330,331). Aluminum was one of the added elements that appeared to be necessary for normal fertility and survival of offspring. On this basis, and the fact that aluminum was present in tissues of the rat, Smith concluded that aluminum ‘exercises a true and essential biological function.’ This early research with milk diets must be considered equivocal, however, and has never been repeated.

Since this early work, few studies have directly addressed the question of aluminum’s essentiality. In 1980, the National Academy of Sciences reviewed the existing research and stated that ‘aluminum has not been proven to be essential to animals, but indirect evidence suggests it may be’ (332). The indirect evidence included accumulation of aluminum in regenerating bone, stimulation of certain enzyme systems, effective use as an adjuvant, and a report that aluminum stimulated growth in poultry.

Despite this optimism, recent reviews conclude that the evidence for the essentiality of aluminum remains quite limited (6,7). The reports of aluminum accumulation in regenerating bone, stimulation of certain enzymes, and the often-cited ability of aluminum to combine with fluoride and activate the guanine nucleotide (GTP) binding regulatory element of adenylate cyclase (333) are actions of aluminum that have never been proven to be required for normal biological function in any organism. This leaves, then, two isolated studies indicating that a deficiency of aluminum in the diet may modestly inhibit the growth of goats and chickens as the only support for essentiality (6,7). These studies, however, have yet to be validated by others. If aluminum is ever shown to be essential, it appears that the levels required in the diet are so low (less than 200 µg kg-1 diet in the goat study) that dietary deficiency would be very rare.