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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
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Plasma Membrane

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
Binding to Phospholipids
Biological membranes are composed of phospholipids that contain a phosphate group (67), and aluminum can bind to this negatively charged group. Using electron paramagnetic resonance spectroscopy, Vierstra and Haug (111) demonstrated that 100mM Al at pH 4 decreased fluidity in membrane lipids of a thermophilic microorganism (Thermoplasma acidophilum Darland, Brock, Samsonoff and Conti). Using physiologically significant concentrations of aluminum, Deleers et al. (112) showed that 25 µM Al increased rigidity of membrane vesicles as indicated by the increased temperature required to maintain a specific polarization value. In addition, aluminum at30 µM could induce phase separation of phosphatidylserine (PS; a negatively charged phospholipid) vesicles, as shown by leakage of a fluorescent compound (113).

Phosphatidylcholine (PC) is the most abundant phospholipid in plasma membranes of eukaryotes, and Akeson et al. (114) showed that in vitro, Al3+ has a 560-fold greater affinity for the surface of PC than Ca2+. Further, Jones and Kochian (102) found that lipids with net negatively charged head groups such as phosphatidyl inositol (PI) had a much greater affinity for aluminum than PC with its net neutral head group. Interestingly, Delhaize et al. (115) found that expression of a wheat cDNA (TaPSS1) encoding for phosphatidylserine synthase (PSS) increased in response to excess aluminum in roots. Overexpression of this cDNA conferred aluminum resistance in one strain of yeast (Saccharomyces cerevisiae) but not in another. In addition, a disruption mutant of the endogenous yeast CHO1 gene that encodes for PSS was sensitive to aluminum (115).

Aluminum reduced membrane permeability to water as shown by a plasmometric method on root disks of red oak (116). To remove the confounding effect of aluminum binding to cell walls, Lee et al. (117) used protoplasts of red beet (Beta vulgaris L.). Within 1 min of exposure to 0.5mM Al, volumetric expansion of red beet cells was reduced under hypotonic conditions, and Lee et al. (117) hypothesized that aluminum could bridge neighboring negatively charged sites on the plasma membrane, stabilizing the membrane.

Binding of Al3+ to the exterior of phospholipids reduces the surface negative charge of membranes. Kinraide et al. (27) proposed that accumulation of aluminum at the negatively charged cell surface plays a role in rhizotoxicity and that amelioration of aluminum toxicity by cations is due to reduced negativity of the cell-surface electrical potential by charge screening or cation binding. Kinraide et al. (27) found a good correlation between the reduction in relative root length of an aluminum- sensitive wheat cultivar with aluminum activity as calculated at the membrane surface, but not in the bulk external solution. Ahn et al. (118) measured the zeta potential (an estimate of surface potential) of plasma membrane vesicles from squash (Cucurbita pepo L.) roots and showed that aluminum exposure resulted in a less negative surface potential. Measuring uptake of radioisotopes by barley roots, Nichol et al. (119) showed that influx of cations (K+, NH4+, and Ca2+) decreased whereas influx of anions (NO3-, HPO42-) increased in the presence of aluminum. They speculated that binding of Al3+ to the exterior of a plasma membrane forms a positively charged layer that retards movement of cations to the membrane surface and increases movement of anions to the surface. In contrast, Silva et al. (120) demonstrated that Mg2+ was 100-fold more effective than Ca2+ in alleviating aluminum-induced inhibition of soybean taproot elongation. They (120) suggested that such an effect could not be explained by changes in membrane surface potential and proposed that the protective effects of Mg could be due to alleviation of aluminum binding to G-protein.

Interference with Proteins Involved in Transport
In addition to phospholipids, biological membranes are composed of proteins, many of which are involved in transport functions across the membrane (5,67). Aluminum is reported to interfere with the uptake of many nutrients, perhaps through interactions with cross-membrane transporters or channels.

Transmembrane electric potential (Vm) is the difference in electric potential between the external environment and the symplasm; typically, the interior of the cell is negatively charged with respect to the outside (67). The potential depends on transient fluxes of H+ through membrane-bound H+- ATPases, as well as fluxes of K+ and other cations through membrane transporters. Measurements of net H+ flux using either a microelectrode or vibrating probe demonstrated that net inward currents of H+ occurred between 0 to 3 mm from root tips of wheat (60,121). Exposure of roots of an aluminum-sensitive wheat cultivar to 10 µM Al for 1 to 3 h inhibited H+ influx; however, there was no obligatory association between inhibition of H+ influx and inhibition of root elongation (60). Ryan et al. (60) speculated that the H+ influx near the root apex could be due to cotransport of H+ with unloaded sugars and amino acids into the cytoplasm, or a membrane more permeable to H+.

Conducting an in vitro enzyme test, Jones and Kochian (102) found little effect of aluminum on H+-ATPase activity. Similarly, Tu and Brouillette (122) found no effect of aluminum on plasma membrane-bound ATPase activity in the presence of free ATP; however, exposure of Mg2+-ATP to 18 µM Al competitively inhibited hydrolysis of ATP. Based on immunolocalization, H+-ATPases in epidermal and cortical cells (2 to 3 mm from tip) of squash roots decreased after 3 h of exposure to 50 µM Al (118). Similarly, 2 days of exposure to≥75 µM Al decreased activity of plasma membrane- bound ATPases in 1-cm root tips of five wheat cultivars (123). Since H+-ATPases generate the proton motive force that drives secondary transporters and channels (5,67), a decrease in activity of this membrane-bound enzyme could result in an overall decrease in nutrient uptake.

Potassium Channels
Uptake of K+ by pea roots was depressed by aluminum (124). Similarly, exposure of mature root cells (≥10 mm from root tip) of an aluminum-sensitive wheat cultivar to 5µM Al inhibited K+ influx (121). In addition, Reid et al. (105) showed partial inhibition of Rb (analog for K+) uptake by>50µM Al in giant algal (Chara corallina) cells, and they attributed this effect to partial blocking by aluminum of K+ channels. Using the patch-clamp technique on isolated plasma membranes or whole cells from an aluminum-tolerant corn cultivar, Pineros and Kochian (125) showed that instantaneous outward K+ channels were blocked by 12µM Al, whereas inward K+ channels were inhibited by 400µM Al.

A strong dysfunction in K+ fluxes between guard cells and epidermal cells was observed in beech (Betula spp.) seedlings exposed to excess aluminum for 2 months (83). Measuring currents of inside-out membrane patches from fava bean (Vicia faba L.) guard cells, Liu and Luan (41) demonstrated that the K+ inward rectifying channel (KIRC) was inhibited by 50 µM Al when exposed on the inward-facing side of the membrane. They (41) proposed that calcium channels conduct Al3+ across the plasma membrane because, verapamil, a Ca2+ channel blocker, prevented aluminum- induced inhibition of KIRC in the whole cell configuration. In addition, Liu and Luan (41) expressed the gene, KAT1, which encodes for a KIRC, in Xenopus oocytes, injected aluminum into the cytoplasm, and observed inhibition of the KAT1 current.

Calcium Channels
Uptake by roots and translocation of 45Ca to shoots was decreased in wheat by 100 µM Al (126). Similar results occurred with 4-week-old Norway spruce seedlings, in which uptake of 45Ca was reduced by 77 to 92% by 100 to 800 µM Al (3). Net Ca2+ influx was highest between 0 and 2mm from the root apex of wheat, based on a calcium-selective vibrating microelectrode (127). Addition of 20 µM Al to roots of an aluminum-sensitive wheat cultivar resulted in a dramatic decrease in Ca2+ influx, and this effect was attributed to blockage by aluminum of a putative calcium channel (128). However, Ryan and Kochian (107) did not find an obligatory relationship between inhibition of calcium uptake and reduction of root growth in wheat. Similarly, in Chara corallina cells, aluminum inhibited calcium influx by less than 50% at 100 µM Al, and Reid et al. (105) thought it unlikely that such a small degree of inhibition would be sufficient to inhibit growth so rapidly.

Magnesium Transporters
Exposure of annual ryegrass (Lolium multiflorum Lam.) to 6.6 µM Al competitively inhibited net Mg2+ uptake (129). Interestingly, McDiarmid and Gardner (130) isolated two yeast genes, ALR1 and ALR2, that encode proteins homologous to bacterial Mg2+ and Co2 transport systems. Overexpression of these genes conferred increased tolerance to Al3+, indicating that aluminum toxicity in yeast is related to reduced Mg2+ influx (130).

Nitrate Uptake
In white clover, 3 weeks of exposure to 50 µM Al inhibited nitrate uptake as measured by nitrogen content in plants (131). In all regions of soybean roots, 15NO3- influxes were reduced within 30 min of exposure to 80 µM Al (132). In corn, 30 min of exposure to 100 µM Al decreased NO3- uptake as measured by NO3-N depletion in solution, but aluminum-induced inhibition of root elongation was not attributed to inhibition of nitrate uptake (133). Aluminum treatment for 3 days followed by measurement of 15NO3- uptake in the final hour decreased 15NO3- uptake in soybean at≥44 µM Al but increased 15NO3- uptake at aluminum levels below 10 µM, probably as a result of Al3+ amelioration of H+ toxicity (24).

Iron Uptake
Iron acquisition in Strategy II plants (gramineous plants) involves secretion of mugineic acids (MA) and uptake of MA–Fe3+ complexes (67). Chang et al. (134) demonstrated that exposure to 100mM Al for 21 h depressed biosynthesis and secretion of 2′-deoxymugineic acid in wheat.

Water Channels
Aluminum is reported to reduce permeability of the plasma membrane to water, perhaps through reduced aquaporin (water channel) activity. Milla et al. (135) found that expression of a rye (Secale cereale L.) gene encoding for aquaporin (water channel) was decreased by aluminum.

Signal Transduction

Interference with Phosphoinositide Signal Transduction
Under in vitro conditions, aluminum interacted strongly with the phosphoinositide signal transduction element, the plasma-membrane-bound phosphatidylinositol-4,5-bisphosphate (PIP2) (136). In animals, cleavage of the plasma membrane lipid, PIP2, by phospholipase C (PLC) releases inositol 1,4,5-triphosphate (IP3) into the cytoplasm. Then, IP3 could produce a signaling cascade by binding to a Ca2+ channel and releasing Ca2+ into the cytosol. In microsomal membranes of wheat roots, aluminum≥20 µM dramatically inhibited PLC activity (136). Under in vitro conditions, aluminum was shown to block the PLC-activated cleavage of PIP2 to IP3 (136).

Transduction of Aluminum Signal
Cell wall-associated kinases could serve as a connecting molecule between the cell wall and the cytoplasmic cytoskeleton. These kinases span the plasma membrane, with the extracellular portion covalently bound to pectin in the cell wall and the cytoplasmic portion containing kinase activity. Recently, expression of a cell wall associated kinase (WAK1) in arabidopsis was induced within 3 h of exposure to aluminum (89). Sivaguru et al. (89) hypothesized that WAK1 could be involved in the aluminum signal transduction pathway.
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