REVIEW: PART OF A HIGHLIGHT SECTION ON PLANT -SOIL INTERACTIONS AT LOW PH The role of the root apoplast in aluminium-induced inhibition of root elongation and in aluminium resistance of plants: a review

†Background Aluminium (Al) toxicity is the most important soil constraint for plant growth and development inacid soils. The mechanism of Al-induced inhibition of root elongation is still not well understood, and it is amatter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic.†Scope The present review focuses on the role of the apoplast in Al toxicity and resistance, summarizing evi-dence from our own experimental work and other evidence published since 1995.†Conclusions The binding of Al in the cell wall particularly to the pectic matrix and to the apoplastic face of theplasma membrane in the most Al-sensitive root zone of the root apex thus impairing apoplastic and symplasticcell functions is a major factor leading to Al-induced inhibition of root elongation. Although symplastic lesionsof Al toxicity cannot be excluded, the protection of the root apoplast appears to be a prerequisite for Al resistancein both Al-tolerant and Al-accumulating plant species. In many plant species the release of organic acid anionscomplexing Al, thus protecting the root apoplast from Al binding, is a most important Al resistance mechanism.However, there is increasing physiological, biochemical and, most recently also, molecular evidence showing thatthe modification of the binding properties of the root apoplast contributes to Al resistance. A further in-depthcharacterization of the Al-induced apoplastic reaction in the most Al-sensitive zone of the root apex is urgentlyrequired, particularly to understand the Al resistance of the most Al-resistant plant species.Key words: Aluminium, aluminum, resistance, apoplast, cell wall, pectin, root elongation.