Kinetics and nature of aluminium rhizotoxic effects: a review

Acid soils with elevated levels of soluble aluminium (Al) comprise ~40% of the world's arable land, but there remains much uncertainty regarding the mechanisms by which Al is rhizotoxic. This review examines the kinetics of the toxic effects of Al on the root elongation rate (RER), its effects on root tissues, and its location at a subcellular level. Depending upon the concentration and plant species, soluble Al decreases the RER in a median time of 73min, but in as little as 5min in soybean. This is initially due to a decreased rate at which cells expand anisotropically in the elongation zone. Thereafter, rhizodermal and outer cortical cells rupture through decreased cell wall relaxation. It is in this region where most Al accumulates in the apoplast. Subsequently, Al impacts root growth at a subcellular level through adverse effects on the plasma membrane (PM), cytoplasm, and nucleus. At the PM, Al alters permeability, fluidity, and integrity in as little as 0.5h, whilst it also depolarizes the PM and reduces H(+)-ATPase activity. The Al potentially crosses the PM within 0.5h where it is able to bind to the nucleus and inhibit cell division; sequestration within the vacuole is required to reduce the toxic effects of Al within the cytoplasm. This review demonstrates the increasing evidence of the importance of the initial Al-induced inhibition of wall loosening, but there is evidence also of the deleterious effects of Al on other cellular processes which are important for long-term root growth and function.