Sodium transporters in plants. Diverse genes and physiological functions.

Soil salinity represents an increasing threat to agricultural production. High sodium (Na) concentrations in soils are toxic to most higher plants. More than 40% of irrigated lands worldwide show increased salt levels. Several studies have shown that under saline conditions, Na influx into root cells occurs via Na permeable transporters (Amtmann et al., 1997; Roberts and Tester, 1997; Tyerman et al., 1997), which in turn elevates the cytoplasmic sodium concentration and causes toxicity (Kingsbury and Epstein, 1986). Channels and transporters for nutrient cations such as Ca and K allow sodium influx. Furthermore, other Na transporters that are more specific for sodium transport play diverse roles in protecting plant cells and whole tissues from Na toxicity. Important mechanisms that contribute to Na tolerance are Na efflux transporters (DuPont, 1992; Shi et al., 2002) and Na transporters that mediate Na sequestration into vacuoles (Blumwald and Poole, 1985, 1987). Moreover, recent research also has shown that plasma membrane Na influx can mediate Na tolerance by reducing vascular long-distance Na transfer to leaves, thus protecting photosynthetically active tissues from salinity stress (Mäser et al., 2002a; Berthomieu et al., 2003). Here we review findings of plant Na transporter genes that have been shown to contribute to important and specific mechanisms during plant salinity stress.