Interaction with Phytohormones
The spatial separation between the most aluminum-sensitive site, the DTZ, and the root region that exhibits reduced cell elongation, the EZ, indicates that a signaling pathway is involved. Perhaps, the phytohormones, auxin (IAA) or cytokinin, are involved in the transduction of an aluminum-stress signal.

Auxin Corn roots were observed to curve away from unilaterally applied aluminum (149). Similar results were found for snapbean roots that curved away from an agar surface containing aluminum (52). Hasenstein and Evans (150) showed that aluminum inhibited basipetal transport of indoleacetic acid (IAA), perhaps resulting in the tropic root response. Kollmeier et al. (151) confirmed this result, showing that exogenous 3H-IAA application to the meristematic zone of corn roots with aluminum application to the DTZ resulted in decreased basipetal transport of auxin to the EZ. They also showed that exogenous IAA application to the EZ partially ameliorated the aluminum-induced (Al applied to DTZ) inhibition of root elongation. Kollmeier et al. (151) hypothesized that aluminum inhibition of auxin transport mediated the aluminum signal between the DTZ and EZ. Sivaguru et al. (74) speculated that aluminum-induced callose in plasmodesmata could be a primary factor in aluminum inhibition of root growth through disturbance of auxin transport.

Cytokinin
Bean root elongation was inhibited after 360 min of exposure to 6.5 µM Al (152). Ethylene evolution as well as the level of zeatin (a cytokinin) from root tips increased after 5 min of aluminum exposure. Massot et al. (152) suggested a role for cytokinin and ethylene in transduction of aluminum- induced stress signal.


Oxidative Stress
Aluminum is redox inactive and is not able to initiate oxidation of lipids or proteins on its own. Yet, lipid peroxidation has been observed in barley roots after 3 h incubation with aluminum (100 µM AICI3, pH, 4.3) (153). Similarly, in pea roots, increase of lipid peroxidation and inhibition of root elongation occurred after 4 h of exposure to 10 µM aluminium (154). Sakihama and Yamasaki (153) proposal that aluminum stabilizes the oxidized form of phenolics (normally unstable), resulting in phenoxyl radicals that initiate lipid peroxidation. Alternatively, aluminum could increase formation of reactive oxygen species (ROS). Cell defense against ROS includes the enzymes, superoxide dismutase (SOD) and glutathione peroxidase (PX), which reduce ROS (153). If levels of these enzymes are not sufficient, then ROS could lead to oxidation of lipids, proteins, and DNA, and even cell death. In corn, 24 h of exposure to aluminum increased activities of SOD and PX, and increased protein oxidation in the aluminum-sensitive genotype (155).



Another possibility proposed by Ikegawa et al. (156) is aluminum-enhanced, Fe(II)-medicated peroxidation of lipids as a cause of cell death. Exposure of tobacco suspension cultures to aluminum alone for 24 h resulted in aluminum accumulation but no significant cell death (156). Addition of Fe(II) (a redox active metal) to cells with accumulated aluminum after 12 h resulted in enhanced lipid peroxidation and cell death. Lipid peroxidation does not appear to be the mechanism involved in reduction of root elongation (154). In pea roots, treatment with an antioxidant prevented aluminum-enhanced lipid peroxidation, reduced callose formation, but did not prevent aluminuminduced inhibition of root elongation (154).


Interestingly, three of four cDNA up-regulated by aluminum stress in Arabidopsis thaliana encoded genes were induced also by oxidative stress (157). Similarly, the vast majority of isolated cDNAs, whose expression increased in response to aluminum toxicity in sugarcane (Saccharum officinarum L.), showed greater expression in response to oxidative stress (158). These results indicate that oxidative stress is an important component of the plant’s response to aluminum toxicity. Overexpression of a tobacco gene encoding for glutathione S-transferase (parB) in Arabidopsis thaliana conferred a degree of aluminum resistance as well as resistance to oxidative stress induced by diamide, providing genetic evidence of a linkage between aluminum stress and oxidative stress in plants (159).


Binding to Internal Membranes in Chloroplasts
As discussed earlier, one long-term effect of aluminum toxicity is the suppression of photosynthetic activity (79,90). Photosynthetic ¹⁴CO₂ fixation of isolated spinach (Spinacia oleracea L.) chloroplasts was inhibited by 10µM Al at pH 7 (160). Hampp and Schnabel (160) attributed this effect to damage of the membrane system. Aluminum exposure of wheat for 14 days decreased the maximum photochemical yield F_v/F_m of photosystem II, (ratio of variable fluorescence over maximum fluorescence, as measured by a fluorometer) (161). Moustakas and Ouzounidou (161) attributed this effect to loss of Ca²⁺, Mg²⁺, and K⁺ from chloroplasts. Seventy days of aluminum exposure decreased F_v/F₀, or the ratio of variable fluorescence over initial fluorescence (162). Pereira et al. (162) speculated that this decrease was an indicator of aluminum-induced structural damage in the thylakoids. In the cyanobacterium, Anabaena cylindrica Lemm., aluminum was found to degrade thylakoid membranes (163).




Binding to Nuclei
Aluminum entered soybean root cells and was associated with nuclei only after 30 min of exposure to 1.45 µM Al (164). In corn root tips, high chromatin fragmentation and loss of plasma membrane integrity occurred after 48 h exposure to 36 µM Al (155). However, Al³⁺ binding to DNA is very weak and cannot compete with phosphate, ATP, or other organic ligands such as citrate (47,48). Martin (47) stated that the observed association of aluminum with nuclear chromatin must be due to its complexation to other ligands and not to DNA.