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.

Potassium (K ϩ ) is essential for plant growth and is the most abundant cation in plants, making up 3% to 5% of the plant's total dry weight (Marschner, 1995). K ϩ is involved in enzyme function, the maintenance of turgor pressure, leaf, and stomatal movement, and cell elongation (Kochian and Lucas, 1988;Schroeder et al., 1994; Maathuis and Sanders, 1996; Maathuis et al., 1997;Very and Sentenac, 2003). Plants have multiple mechanisms for K ϩ uptake from soil and translocation to various plant tissues to help them respond to changing environmental conditions and the varying K ϩ requirements in different tissues. Epstein and colleagues provided the first evidence of the operation of at least two (high-and lowaffinity) K ϩ uptake systems in plants (Epstein et al., 1963; Kochian and Lucas, 1988). The two transport systems were proposed to play roles in uptake that correspond with external K ϩ concentrations. More recently, it has been shown that there is functional overlap between high-and low-affinity uptake mechanisms (Hirsch et al., 1998;Santa-Maria et al., 2000). In addition to the uptake mechanisms' differences in affinity, the high-affinity uptake mechanisms have been shown to be inducible, whereas the low-affinity systems may be constitutive (Glass, 1976(Glass, , 1983 Fernando et al., 1990). Root epidermal cells play an important role in high-and low-affinity K ϩ acquisition (Kochian and Lucas, 1983;Gassmann and Schroeder, 1994; Jungk, 2001). At least two K ϩ channels, a nonselective cation channel and a K ϩ transporter, have been shown to be active in root epidermal cells; therefore, it is likely that more than two different proteins are involved in high-and low-affinity K ϩ uptake (Very and Sentenac, 2003).It is also likely that the translocation of K ϩ involves multiple transport proteins. It has been shown that a specific K ϩ channel is involved in xylem loading, but other proteins are implicated in this process because the deletion of this channel only partially reduced xylem K ϩ concentrations (Gaymard et al., 1998). Many steps are involved in the radial movement of K ϩ from the surface of the root to the xylem; the molecular details of this process are not understood (Tester and Leigh, 2001). After K ϩ is released into the xylem, it moves to the shoots. Then, it must be unloaded in the leaves. K ϩ is also retranslocated from leaves to other parts of the plant, such as roots or other sink tissues. Retranslocation occurs via the phloem, where K ϩ channels have been identified as being involved in phloem loading (Deeken et al., 2002; Philippar et al., 2003).There are five major families of K ϩ transporters that have been identified in Arabidopsis (Maser et al., 2001). The contribution of many of these transporters to cellular or whole-plant K ϩ homeostasis is not yet clear. The largest gene family of K ϩ transporters in Arabidopsis is the AtKT/KUP family; 13 genes are encoded by this family (Very and Sentenac, 2003). These transporters were originally identified in Escherichia coli as KUPs (K ϩ uptak...

SummaryAlthough glycine-rich RNA-binding protein 2 (GRP2) has been implicated in plant responses to environmental stresses, the function and importance of GRP2 in stress responses are largely unknown. Here, we examined the functional roles of GRP2 in Arabidopsis thaliana under high-salinity, cold or osmotic stress. GRP2 affects seed germination of Arabidopsis plants under salt stress, but does not influence seed germination and seedling growth of Arabidopsis plants under osmotic stress. GRP2 accelerates seed germination and seedling growth in Arabidopsis plants under cold stress, and contributes to enhancement of cold and freezing tolerance in Arabidopsis plants. No differences in germination between the wild-type and transgenic plants were observed following addition of abscisic acid (ABA) or glucose, implying that GRP2 affects germination through an ABAindependent pathway. GRP2 complements the cold sensitivity of an Escherichia coli BX04 mutant and exhibits transcription anti-termination activity, suggesting that it has an RNA chaperone activity during the cold adaptation process. Mitochondrial respiration and catalase and peroxidase activities were affected by expression of mitochondrial-localized GRP2 in Arabidopsis plants under cold stress. Proteome analysis revealed that expression of several mitochondrial-encoded genes was modulated by GRP2 under cold stress. These results provide new evidence indicating that GRP2 plays important roles in seed germination, seedling growth and freezing tolerance of Arabidopsis under stress conditions, and that GRP2 exerts its function by modulating the expression and activity of various classes of genes.

The aluminum (Al)-induced secretion of citrate has been regarded as an important mechanism for Al resistance in soybean (Glycine max). However, the mechanism of how Al induces citrate secretion remains unclear. In this study, we investigated the regulatory role of plasma membrane H 1 -ATPase activity and expression were higher in an Al-resistant cultivar than in an Al-sensitive cultivar. Al activated the threonine-oriented phosphorylation of plasma membrane H 1 -ATPase in a dose-and time-dependent manner. Taken together, our results demonstrated that up-regulation of plasma membrane H 1 -ATPase activity was associated with the secretion of citrate from soybean roots.