No abstract

SummaryThe major constraint to plant growth in acid soils is the presence of toxic aluminum (Al) cations, which inhibit root elongation. The enhanced Al tolerance exhibited by some cultivars of wheat is associated with the Al-dependent ef¯ux of malate from root apices. Malate forms a stable complex with Al that is harmless to plants and, therefore, this ef¯ux of malate forms the basis of a hypothesis to explain Al tolerance in wheat. Here, we report on the cloning of a wheat gene, ALMT1 (aluminum-activated malate transporter), that co-segregates with Al tolerance in F 2 and F 3 populations derived from crosses between near-isogenic wheat lines that differ in Al tolerance. The ALMT1 gene encodes a membrane protein, which is constitutively expressed in the root apices of the Al-tolerant line at greater levels than in the near-isogenic but Alsensitive line. Heterologous expression of ALMT1 in Xenopus oocytes, rice and cultured tobacco cells conferred an Al-activated malate ef¯ux. Additionally, ALMT1 increased the tolerance of tobacco cells to Al treatment. These ®ndings demonstrate that ALMT1 encodes an Al-activated malate transporter that is capable of conferring Al tolerance to plant cells.

The rhizosphere is the zone of soil immediately surrounding plant roots that is modified by root activity. In this critical zone, plants perceive and respond to their environment. As a consequence of normal growth and development, a large range of organic and inorganic substances are exchanged between the root and soil, which inevitably leads to changes in the biochemical and physical properties of the rhizosphere. Plants also modify their rhizosphere in response to certain environmental signals and stresses. Organic anions are commonly detected in this region, and their exudation from plant roots has now been associated with nutrient deficiencies and inorganic ion stresses. This review summarizes recent developments in the understanding of the function, mechanism, and regulation of organic anion exudation from roots. The benefits that plants derive from the presence of organic anions in the rhizosphere are described and the potential for biotechnology to increase organic anion exudation is highlighted.