SummaryNa uptake in the roots of K -starved seedlings of barley, rice, and wheat was found to exhibit fast rate, low OsHKT genes were fairly constant and insensitive to changes in the K and Na concentrations of the nutrient solution. In roots, the expressions were much lower than in shoots, except for OsHKT4 and OsHKT1 in K -starved plants. We propose that OsHKT transporters are involved in Na movements in rice, and that OsHKT1 speci®cally mediates Na uptake in rice roots when the plants are K de®cient. The incidence of HKT ESTs in several plant species suggests that the rice model with many HKT genes applies to other plants.

The salt tolerance of rice (Oryza sativa) correlates with the ability to exclude Na 1 from the shoot and to maintain a low cellular Na 1 /K 1 ratio. We have identified a rice plasma membrane Na 1 /H 1 exchanger that, on the basis of genetic and biochemical criteria, is the functional homolog of the Arabidopsis (Arabidopsis thaliana) salt overly sensitive 1 (SOS1) protein. The rice transporter, denoted by OsSOS1, demonstrated a capacity for Na 1 /H 1 exchange in plasma membrane vesicles of yeast (Saccharomyces cerevisiae) cells and reduced their net cellular Na 1 content. The Arabidopsis protein kinase complex SOS2/ SOS3, which positively controls the activity of AtSOS1, phosphorylated OsSOS1 and stimulated its activity in vivo and in vitro. Moreover, OsSOS1 suppressed the salt sensitivity of a sos1-1 mutant of Arabidopsis. These results represent the first molecular and biochemical characterization of a Na 1 efflux protein from monocots. Putative rice homologs of the Arabidopsis protein kinase SOS2 and its Ca 21 -dependent activator SOS3 were identified also. OsCIPK24 and OsCBL4 acted coordinately to activate OsSOS1 in yeast cells and they could be exchanged with their Arabidopsis counterpart to form heterologous protein kinase modules that activated both OsSOS1 and AtSOS1 and suppressed the salt sensitivity of sos2 and sos3 mutants of Arabidopsis. These results demonstrate that the SOS salt tolerance pathway operates in cereals and evidences a high degree of structural conservation among the SOS proteins from dicots and monocots.Rice (Oryza sativa) is one of the most important cereal crops in tropical and temperate regions of the world. Among all common environmental stresses, salinity is a major factor decreasing the yield in rice cultivation in coastal areas and in irrigated farmlands. Problems associated with salinity are water deficit imposed by the greater osmolarity of the soil solution and the cellular damage inflicted by excessive ion accumulation in plant tissues. Comparison of rice subspecies and varieties differing in tolerance to salinity has shown that greater tolerance correlates with the ability to exclude Na 1 from the shoot and maintain a low Na 1 /K1 ratio (Golldack et al., 2003;Lee et al., 2003;Ren et al., 2005). For instance, the salt-sensitive variety IR29 accumulated Na 1 in leaves at 5-to 10-fold greater concentrations than the salt-tolerant lines BK or Pokkali (Golldack et al., 2003). In contrast, shoot K 1 concentration per se showed no relation to salinity tolerance in japonica spp. and only weak correlation in indica spp. varieties (Golldack et al., 2003;Lee et al., 2003). Because steady accumulation of Na 1 is what injures the cells of leaves at moderate salinity levels (Flowers et al., 1991;Munns, 1993), restricting the translocation of Na 1 is a mechanism for salt tolerance that plays a major role in rice (Lee et al., 2003;Ren et al., 2005). The gene SKC1/HKT8, responsible for a major quantitative trait locus imparting a high K 1 / Na 1 balance in shoots and salt tolerance, encodes...

The gene ENA1 was cloned by its ability to complement the Li+ sensitivity of a low Li+‐efflux strain. The nucleotide sequence of the cloned DNA fragment showed that there are two almost identical genes in tandem, and predicts that they encode P‐ATPases. Disruption of both genes originated a strain defective in Na+ and Li+ effluxes, and sensitive to Na+, to Li+ and to alkaline pH. By transformation with ENA1 the defective effluxes and tolerances were repaired.

Plants take up large amounts of K+ from the soil solution and distribute it to the cells of all organs, where it fulfills important physiological functions. Transport of K+from the soil solution to its final destination is mediated by channels and transporters. To better understand K+ movements in plants, we intended to characterize the function of the large KT-HAK-KUP family of transporters in rice (Oryza sativacv Nipponbare). By searching in databases and cDNA cloning, we have identified 17 genes (OsHAK1–17) encoding transporters of this family and obtained evidence of the existence of other two genes. Phylogenetic analysis of the encoded transporters reveals a great diversity among them, and three distant transporters, OsHAK1, OsHAK7, and OsHAK10, were expressed in yeast (Saccharomyces cerevisiae) and bacterial mutants to determine their functions. The three transporters mediate K+ influxes or effluxes, depending on the conditions of the experiment. A comparative kinetic analysis of HAK-mediated K+ influx in yeast and in roots of K+-starved rice seedlings demonstrated the involvement of HAK transporters in root K+ uptake. We discuss that all HAK transporters may mediate K+ transport, but probably not only in the plasma membrane. Transient expression of the OsHAK10-green fluorescent protein fusion protein in living onion epidermal cells targeted this protein to the tonoplast.