The distal part of the transition zone is the most aluminum-sensitive apical root zone of maize

For a better understanding of Al inhibition of root elongation, knowledge of the morphological and functional organization of the root apex is a prerequisite. We developed a polyvinyl chloride-block technique to supply Al (90 micromolar monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured during a period of 5 h. After Al treatment, callose (5 h) and Al (1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root apex. Only applying Al to the three apical 1-mm root zones inhibited root elongation after 1 h. The order of sensitivity was 1 to 2 0 to 1 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was found, independently of whether Al was applied to individual apical root zones or uniformly to the whole-root apex. We conclude from these results that the distal part of the transition zone of the root apex, where the cells are undergoing a preparatory phase for rapid elongation (F. Baluska, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112: 3-4), is the primary target of Al in this Al-sensitive maize cultivar