Plasma membrane of younger and outer cells is the primary specific site for aluminium toxicity in roots

Experiments were carried out to identify the primary site for aluminium (Al) toxicity in roots. Al accumulated in large amounts in the younger and outer cells in roots of pea and was retarded when the ionic strength of the Al solution was high. Cell destruction was extensive in the regions with high Al accumulation. The accumulation of Al in, and potassium (K) leakage from, the root tip were in the order pea>maize>rice, the same order as their sensitivity to Al.The protoplasts from the root tip portion of pea incubated with Al showed a wrinkled and uneven surface. The protoplasts progressively shrank and eventually collapsed. Viability decreased in this process. In the control protoplasts of maize, -glucan formation was uniform on the spherical surfaces, whereas it was spotty in the Al-treated protoplasts; the cell wall material of the latter contained partly 1, 3--glucan which is known to be synthesised by 1, 3--glucan synthase embedded in the plasma membrane. These results suggest that the specific site for Al toxicity is the plasma membrane of younger and outer cells in roots and that Al tolerance depends largely on the integrity of the plasma membrane.