The yeast (Schizosaccharomyces pombe) SOD2 (Sodium2) gene was introduced into Arabidopsis under the control of the cauliflower mosaic virus 35S promoter. Transformants were selected for their ability to grow on medium containing kanamycin. Southern-and northern-blot analyses confirmed that SOD2 was transferred into the Arabidopsis genome. There were no obvious morphological or developmental differences between the transgenic and wild-type (wt) plants. Several transgenic homozygous lines and wt plants (control) were evaluated for salt tolerance and gene expression. Overexpression of SOD2 in Arabidopsis improved seed germination and seedling salt tolerance. Analysis of Na ϩ and K ϩ contents of the symplast and apoplast in the parenchyma cells of the root cortex and mesophyll cells in the spongy tissue of the leaf showed that transgenic lines accumulated less Na ϩ and more K ϩ in the symplast than the wt plants did. The photosynthetic rate and the fresh weight of the transgenic lines were distinctly higher than that of wt plants after NaCl treatment. Results from different tests indicated that the expression of the SOD2 gene promoted a higher level of salt tolerance in vivo in transgenic Arabidopsis plants.A major factor impairing worldwide agricultural productivity is salinity, which is believed to affect nearly one-fifth of the world's irrigated land and resulted in a loss of 10 million ha of otherwise arable land each year (Boyer, 1982; Szaboles, 1987; Flowers and Yeo, 1995; Nelson et al., 1998). To solve the problems caused by salinity in agricultural areas, several approaches have been applied, such as irrigation with fresh water and improving soil drainage. However, these expensive solutions are not always practical. Therefore, the study of plant salt tolerance, with a view to identify and eventually to manipulate the genes involved in salt perception and responses, seems to be a promising approach (Zhu, 2000).Plant growth depends on mineral nutrients absorbed from the soil by roots. Although Na ϩ is a major cation present in the soil, it is not considered an essential mineral for most plants. In saline soils, high concentrations of Na ϩ disrupt the balance of other minerals such as K ϩ , thereby creating osmotic stress and causing secondary problems such as oxidative stress (Zhu, 2001). These adverse effects result in plant growth inhibition and even plant death.The mechanisms for plant cells to prevent excessive accumulation of Na ϩ in the cytosol are as follows.First, Na ϩ entry to plant cells may be restricted by selective ion uptake. However, nonselective cation channels have been proposed to mediate substantial Na ϩ entry into plant roots (Davenport and Tester, 2000; Demidchik and Tester, 2002). The cloned highaffinity potassium uptake transporter (HKT1) and low-affinity cation transporter (LCT1) affect Na ϩ permeability when expressed in yeast or oocytes, suggesting that they may also be considered Na ϩ influx transporters (Rubio et al., 1995; Schachtman et al., 1997; Liu et al., 2000).Second, inter...