Masao Sasaki

Al-induced inhibition of root elongation in corn, Zea mays L. is overcome by Si addition

The effect of silicic acid on Al-induced inhibition of root elongation was investigated in corn roots (Zea mays L. cv. golden cross bantam) in 100 μt M CaCl2 solution at pH 4.3. Twenty μt M Al inhibited root elongation (20 h) about 70%, however, inhibition was alleviated by addition of silicic acid. The alleviative effect increased with higher silicic acid concentrations. The concentration of Al3+, the toxic species, in solution was decreased to about 15, 10, and 5 μt M, respectively, from the initial concentration of 20 μt M by addition of silicic acid at 500, 1000, and 2000 μt M Si. Under the same concentration of Al3+, Al-induced inhibition of root elongation showed the same extent regardless of the addition of silicic acid or not by comparing 5 μt M Al treatment with 20 μt M Al + 2000 μt M Si treatment, and 10 μt M Al treatment with 20 μt M Al +1000 μt M Si treatment. Viability of cells on the root tip surface was decreased by Al addition. Cell viability was not improved by addition of silicic acid under the same concentration of Al3+. All these facts suggest that the alleviative effect of silicic acid on Al toxicity resulted from decreasing toxic Al3+ concentration by forming Al-Si complexes rather than from other physiological effects of silicic acid in corn roots.

Early events induced by aluminum stress in elongating cells of wheat root

The relation between the toxic effects of alminum (Al) on growth and cell viability in roots of wheat (Triticum aestivum L. cv Atlas 66) was investigted. Root elongation of wheat was reduced after 3 h of treatment with 50 μM AlCl3 at pH 4.7. Only the apical region of the root treated with Al was stained with hematoxylin. Al treatment decreased the length and increased the diameter of the cells at the elongation zone of the root. Staining with propidium iodide which can permeate through only a damaged plasma membrane showed that treatment with Al for longer than 3 h decreased cell viability. These findings suggest that inhibition of cell elongation by Al is partially accompanied by cell death. Lignin was deposited in the cell wall of the second and third layers of the cortex in the elongation zone of the root. Also the disruption of microtubules ocurred in elongating cells treated with 50 μM Al for 3 h. The effects of Al stress on root elongation and cell viability in the elongation zone were inhibited by 1 m M Ca.

Aluminum inhibits growth and stability of cortical microtubules in wheat (Triticum aestivum) roots

Abstract Aluminum (Al) occurs abundantly in soil and solubilized aluminum ions in acid soil inhibit plant growth, in particular, root growth. Although several toxic effects of Al on plant growth have been reported, the mechanism of Al toxicity remains to be clarified.

Comparison of the early response to aluminum stress between tolerant and sensitive wheat cultivars: Root growth, aluminum content and efflux of K+

Abstract In order to characterize the mechanism of Al tolerance (Atlas 66) and Al sensitivity (Scout 66) in two cultivars of wheat (Triticum aestivum L.), the early responses to Al stress under acidic conditions were investigated. Marked inhibition of root elongation of Scout was observed upon treatment with 10 μM AlCl3 for less than 3 h. The inhibition of root elongation of Scout was reversed within 3 days when the treated samples were transferred to a solution without Al. However, treatment for 6 h with AlCl3 repressed root elongation almost completely and irreversibly. Root elongation of Atlas was only partially inhibited by the treatment with 10 μM AlCl3 for more than 6 h. Levels of Al in two portions of roots, namely, portions 0–5 mm and 5–10 mm from the tip, were lower in Atlas than those in Scout. In Atlas the levels of Al on a fresh weight basis in both portions were very similar, while the level of Al in the portion 0–5 mm from the tip was almost double than that in the 5–10 mm portion in Scout. ...

Involvement of plasma membrane potential in the tolerance mechanism of plant roots to aluminium toxicity

H+-ATPase activity of a plasma membrane-enriched fraction decreased after the treatment of barley (Hordeum vulgare) seedlings with Al for 5 days. A remarkably high level of Al was found in the membrane fraction of Al-treated roots. A long-term effect of Al was identified as the repression of the H+-ATPase of plasma membranes isolated from the roots of barley and wheat (Triticum aestivum) cultivars, Atlas 66 (Al-tolerant) and Scout 66 (Al-sensitive). To monitor short-term effects of Al, the electrical membrane potentials across plasma membranes of both wheat cultivars were compared indirectly by measuring the efflux of K+ for 40 min under various conditions. The rate of efflux of K+ in Scout was twice that in Atlas at low pH values such as 4.2. Vanadate, an inhibitor of the H+-ATPase of the plasma membrane, increased the efflux of K+. Al repressed this efflux at low pH, probably through an effect on K+ channels, and repression was more pronounced in Scout. Al strongly repressed the efflux of K+ irrespective of the presence of vanadate. Ca2+ also had a repressive effect on the efflux of K+ at low pH. The effect of Ca2+, greater in Scout, might be related to the regulation of the net influx of H+, since the effect was negated by vanadate. The results suggest that extracellular low pH may cause an increase in the influx of H+, which in turn is counteracted by the efflux of K+ and H+. These results suggest that the ability to maintain the integrity of the plasma membrane and the ability to recover the electrical balance at the plasma membrane through a net influx of H+ and the efflux of K+ seem to participate in the mechanism of tolerance to Al stress under acidic conditions.

Putative Ca2+ channels of plasma membrane vesicles are not involved in the tolerance mechanism of aluminum in aluminum tolerant wheat (Triticum aestivum L.) cultivar

Abstract We compared the effect of aluminum on the calcium (45Ca2+) influx through putative calcium channels of plasma membrane vesicles isolated from two wheat (Triticum aestivum L.) cultivars differing in Al sensitivity. Bepridil, a calcium channel blocker, and aluminum suppressed the influx in both cultivars. However, no differences were observed in the effects of these treatments between the two cultivars.

Increase of ATP-dependent H+ pump activity of tonoplast of barley roots by aluminium stress: Possible involvement of abscisic acid for the regulation

Aluminium (Al) stress increases ATP-dependent H+ transport activity of tonoplast-enriched membrane vesicles prepared from barley roots. Also, Al stress significantly increased ABA content in roots. Treatment of roots with abscisic acid (ABA), as in the case of Al stress, increased ATP-dependent H+ transport activity of tonoplastenriched membrane vesicle preparations. Vanadate treatment, which is expected to reduce the plasma membrane H+-ATPase activity as does Al stress, increased both ABA content in roots and the H+ transport activity of tonoplast membrane vesicles. Any treatment with Al, ABA and vanadate increased nitrate-inhibitible ATPase activity of prepared membrane vesicles. These results suggest that the increase of ABA in Al-stressed roots may play a key role in the activation of H+-pump of the tonoplast.