Significance of the root apoplast for aluminium toxicity and resistance of maize

W. J. Horst, M. Kollmeier, N. Schmohl, M. Sivaguru, Y. Wang, H. H. Felle, R. Hedrich, W. Schröder, A. Staß

Research output: Chapter in Book/Report/Conference proceedingChapter


The mechanism of aluminium-induced inhibition of root elongation is still not well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises evidence from own experimental work and the literature which could contribute to the understanding of Al toxicity and resistance in maize focussing on the role of the apoplast. Short-term application of Al to specific zones of the root apex revealed that the 1-2 mm distal transition zone is the most Al-sensitive apical root zone. The inhibition of basipetal transport of auxin by Al indicates that auxin is part of the Al-signal transduction from this to the 3-6 mm central elongation zone. Al induced callose formation, membrane depolarization, and prominent alterations in the cytoskeleton especially in this zone. Accumulation of Al in the root apex is modulated by the pectin content of the cell walls and the degree of methylation of the pectin. After short-term Al supply, Al accumulates particularly in the cell walls of the outer cortical cells modifying the apoplastic transport of higher molecular weight solutes. Apoplastic Al also affects plasma-membrane properties as expressed by changes in the trans-membrane potential, ion fluxes and enhanced callose synthesis. Higher Al resistance of root apices is sensitively reflected by a lower Al-induced callose formation. Si-enhanced Al resistance was not related to Al exclusion but to modification of the Al-binding properties of the cell walls. Genotypic Al resistance is related to less accumulation of Al due to lower negativity of the cell walls and the plasma membrane, and particularly to an enhanced release of organic acid anions via an Al-induced citrate and malate-permeable large conductance anion channel in the most Al-sensitive root zone. In an Al-resistant cultivar the channel open probability was greater than in the Al-sensitive cultivar and a higher level of citrate in the root could be maintained. It is concluded that the apoplast of the most Al-sensitive root zone plays an important role in Al toxicity and resistance in maize. Although symplastic lesions of Al toxicity cannot be excluded, the protection of the apoplast appears to be a prerequisite for Al resistance. An in-depth molecular characterisation of Al-induced apoplastic reaction in the most sensitive root zone is urgently required.

Original languageEnglish (US)
Title of host publicationThe Apoplast of Higher Plants
Subtitle of host publicationCompartment of Storage, Transport and Reactions: The Significance of the Apoplast for the Mineral Nutrition of Higher Plants
Number of pages18
ISBN (Electronic)9781402058431
ISBN (Print)9781402058424
StatePublished - Jan 1 2007
Externally publishedYes


  • Aluminum
  • Callose
  • Cell wall
  • Organic acids
  • Pectin
  • Silicon

ASJC Scopus subject areas

  • Agricultural and Biological Sciences(all)


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