Aluminium/silicon interactions in higher plants

Aluminium and silicon are usually abundant in soil mineral matter, but their availability for plant uptake is limited by low solubility and, in the case of Al, high soil pH causes precipitation of the element in insoluble forms. Al toxicity is a major problem in naturally occurring acid soils and in soils affected by acidic precipitation. Al has no known role in higher plants, and is generally known as a toxic element, whereas Si is generally regarded as a beneficial element. Recently, it has been suggested that Al toxicity can be ameliorated by Si in a variety of animal systems. In this review the evidence that amelioration of Al toxicity by Si can also occur in plants is assessed. At present such amelioration has been shown in sorghum, barley, teosinte, and soybean, but not in rice, wheat, cotton, and pea. Plant species vary considerably in the amounts of Al and Si that they transport into their tissues, and it seems that very high Si accumulation and very high Al accumulation are mutually exclusive. The mechanisms considered for amelioration are: solution effects; codeposition of Al and Si within the plant; effects in the cytoplasm and on enzyme activity; and indirect effects.