Tansley review no. 22 - The apoplast and its significance for plant mineral nutrition

It has only recently become apparent that the apoplast plays a major role i n a diverse range of processes, including intercellular signalling, plant-m icrobe interactions and both water and nutrient transport. Broadly defined, the apoplast constitutes all compartments beyond the plasmalemma - the int erfibrillar and intermicellar space of the cell walls, and the xylem, inclu ding its gas- and water-filled intercellular space - extending to the rhizo plane and cuticle of the outer plant surface. The physicochemical propertie s of cell walls influence plant mineral nutrition, as nutrients do not simp ly pass through the apoplast to the plasmalemma but can also be adsorbed or fixed to cell-wall components. Here, current progress in understanding the significance of the apoplast in plant mineral nutrition is reviewed. The c ontribution of the root apoplast to short-distance transport and nutrient u ptakes is examined particularly in relation to Na+ toxicity and Al3+ tolera nce. The review extends to long-distance transport and the role of the apop last as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage and sol ute exchange with the atmosphere.