Mutagenized Arabidopsis seedlings (ecotype Columbia) were screened for the ability to grow photoautotrophically on solid medium containing 200 mM NaCl. A novel mutant line, designated pst1 (for photoautotrophic salt tolerance1 ), was obtained. There were no significant differences between pst1 and wild-type plants with regard to their ability to induce proline as an osmoregulatory solute. In addition, the content of monovalent cations in pst1 plants grown with or without salt stress was equal to that in the wild type. We observed that light, even at moderate intensities, increased the effects of salt stress on wild-type plants. The pst1 seedlings were nearly 10 times more tolerant to methyl viologen than were wild-type seedlings. We also found that the activities of the active oxygen scavengers superoxide dismutase and ascorbate peroxidase were enhanced significantly in pst1 plants. The pst1 plants also were tolerant to other stresses, such as high light intensity and toxic monovalent cations. The recessive nature of the pst1 mutation indicates that the potential for salt-stress tolerance is blocked in wild-type Arabidopsis.
INTRODUCTIONArid areas are expanding gradually but steadily on earth. It is known that salt accumulation caused by unsuitable irrigation practices decreases crop productivity (Epstein et al., 1980). Under these conditions, high concentrations of NaCl arrest plant development and lead to plant death. Several interacting events are triggered in plants by salt stress, including the inhibition of enzyme activities in metabolic pathways, decreased uptake of nutrients in roots, decreased carbonuse efficiency, and the denaturation of protein and membrane structures. However, the complete response of plants to salt stress has not been explained systematically (Benzel and Reuveni, 1994).Several mechanisms of defense and adaptation to water deficiency resulting from salt stress have been well studied. Adaptation to water deficiency can involve the accumulation of osmolytes, such as proline (Igarashi et al., 1997), glycinebetaine, and sugar alcohols (Bohnert et al., 1995), or lateembryo-abundant (Lea) proteins (Dure, 1993). With this knowledge, researchers have attempted to improve salt tolerance in plants by increasing levels of osmolytes. For example, osmolyte concentrations have been manipulated by constitutively expressing genes for a protective protein (Xu et al., 1996) or genes for key enzymes involved in the production of osmosolytes occasionally associated with protective activities (Tarczynski et al., 1993;Thomas et al., 1995;Hayashi et al., 1997;Karakas et al., 1997).Photosynthesis, one of the most important metabolic pathways in plants, is a target of salt stress. Abscisic acid produced in response to salt stress (Leung et al., 1994) decreases turgor in guard cells and thus limits the CO 2 available for photosynthesis. During water stress, reduction of chloroplast stromal volume (Gupta and Berkowitz, 1988) and generation of active oxygen species (Price and Hendry, 1991) also are thought ...
