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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
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Genotypic Differences in Aluminum Response of Plants

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
Comparative studies of aluminum effects in 22 species in seven plant families have established that some species or genotypes within species can resist aluminum toxicity (82). Foy (165) proposed 'tailoring the plant to fit the soil; in other words, he suggested that it was more economical to develop mineral-stress-resistant plants than to correct the soil for nutrient deficiencies or toxicities. This statement is particularly true for acid subsoils, where it is not economically feasible to lime at such depths, or for developing countries, where farmers cannot afford the high-input costs of lime.

Screening Tests
Screening for genotypic differences in response to aluminum toxicity can be conducted in pots or in fields with aluminum-toxic soil. A more rapid screening test for differences in aluminum tolerance among species or genotypes within species utilizes the aluminum-induced inhibition of root elongation as a measure of aluminum sensitivity (166). These tests are conducted with varying levels of aluminum in solution at an acid pH (≤4.5) to maintain a high activity of Al3+, the phytotoxic ion. Some researchers have found a poor correlation between plant responses in soil with those in nutrient solution (167). Others have found a good correlation (168-171).

Hematoxylin stains extracellular aluminum phosphate compounds that result from aluminum damage to root cells (172). Another quick screening test is to stain roots grown in an aluminumcontaining solution with hematoxylin and to assess the intensity of staining (173). With wheat, Scott et al. (174) found a good agreement between root elongation results and those using hematoxylin. However, Bennet (175) warned that many aspects of hematoxylin staining are not well understood and that aluminum-treated roots do not always respond to hematoxylin even when symptoms of aluminum toxicity occurred. Further, sometimes roots will stain in the absence of aluminum (175).

Moore et al. (176) proposed that recovery of root elongation after 48 h of exposure to aluminum is a better measure of irreversible damage to the root apical meristem. Hecht-Buchholz (177) reported that aluminum toxicity in barley caused stunted roots, destruction of root cap cells, swelling, and destruction of both root epidermal and cortical cells. She found large differences between cultivars and proposed that aluminum resistance could be attributed to greater resistance of the root meristem of the aluminum-tolerant genotype to irreversible destruction. Lazof and Holland (28) suggested that root recovery experiments in soybean, pea, and snapbean allowed separation of H+ toxicity effects from Al3+ toxicity effects. Zhang et al. (178) showed that root regrowth after aluminum stress could be used to improve aluminum tolerance in triticale (Triticosecale spp.).

Aluminum tolerance is a heritable trait in sorghum (179), barley (180), wheat (181,182), rice (Oryza sativa L.) (183), soybean (184), and Arabidopsis thaliana (185). With sorghum, Magalhaes (cited in 179) has found a pattern of inheritance of aluminum tolerance that is consistent with a single locus. With barley, Tang et al. (180) confirmed that aluminum tolerance segregation in F2 genotypes was due to a single gene, Alp, and they proposed the use of molecular markers in selection of aluminum tolerance in barley genotypes without the need for field trials, soil bioassays, or solution culture tests. In wheat, controversy exists over the number and location of genes that are involved in aluminum tolerance (181,182). In rice, nine different genomic regions on eight chromosomes have been associated with genetic control of plant response to aluminum, indicating that aluminum tolerance is a multigenic trait (183). Similarly, with soybean, aluminum tolerance is likely to be governed by 3 to 5 genes (184). In Arabidopsis, two quantitative trait loci occurring on two chromosomes could account for 43% of total variability in aluminum tolerance among a recombinant inbred population (185). A recent review of genetic analysis of aluminum tolerance in plants is found in Kochian et al. (179).

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