Does aluminium affect root growth of maize through interaction with the cell wall - plasma membrane - cytoskeleton continuum?

The mechanism of aluminium-induced inhibition of root elongation is still n ot well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises exp erimental evidence which offers new avenues in the understanding of Al toxi city and resistance in maize. Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm. The most Al-sensitive apical root zone appeared to be th e 1-2 mm segment. Aluminium-induced prominent alterations in both the micro tubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1-2 mm zone using monoclonal antibodies. Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell wall s through pre-treatment of intact maize plants with 150 mM NaCl for 5 days. NaCl-adapted plants with higher pectin content accumulated more Al in thei r root apices and they were more Al-sensitive as indicated by more severe i nhibition of root elongation and enhanced callose induction by Al. This spe cial role of the pectic matrix of the cell walls in the modulation of Al to xicity is also indicated by a close positive correlation between pectin, Al , and Al-induced callose contents of 1 mm root segments along the 5 mm root apex. On the basis of the presented data we suggest that the rapid disorga nisation of the cytoskeleton leading to root growth inhibition may be media ted by interaction of Al with the apoplastic side of the cell wall - plasma membrane - cytoskeleton continuum.