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 plasma membrane sodium/proton exchanger Salt-OverlySensitive 1 (SOS1) is a critical salt tolerance determinant in plants. The SOS2-SOS3 calcium-dependent protein kinase complex upregulates SOS1 activity, but the mechanistic details of this crucial event remain unresolved. Here we show that SOS1 is maintained in a resting state by a C-terminal auto-inhibitory domain that is the target of SOS2-SOS3. The auto-inhibitory domain interacts intramolecularly with an adjacent domain of SOS1 that is essential for activity. SOS1 is relieved from auto-inhibition upon phosphorylation of the auto-inhibitory domain by SOS2-SOS3. Mutation of the SOS2 phosphorylation and recognition site impeded the activation of SOS1 in vivo and in vitro. Additional amino acid residues critically important for SOS1 activity and regulation were identified in a genetic screen for hypermorphic alleles.ion transport | salinity | sodium tolerance S alinity is a major problem in agriculture because the total area of salt-affected soils, including saline and sodic soils, exceeds 900 million ha (1). Salt-affected soils reduce both the ability of crops to take up water and the availability of mineral nutrients. Often, the high sodium (Na) content relative to other cations is the main factor affecting plant growth by causing a set of metabolic derangements (2). Because most crop species have only very limited capacities to cope with excess Na, the elucidation of Na tolerance mechanisms in plants is of paramount importance (2). Plant ion transporters mediating Na fluxes have recently been cloned and characterized, and the knowledge of the regulatory mechanisms of transporter abundance and activity in response to environmental, hormonal, and developmental signals is critical for understanding salinity tolerance (3). The plasma membrane Na/H antiporter SOS1 is essential for the salt tolerance of various model plants, including Arabidopsis thaliana (4) and its halophytic relative Thellungiella salsuginea (5), tomato (6), and the moss Physcomitrella patens (7). SOS1 is thought to mediate Na efflux at the root epidermis and longdistance transport from roots to shoots (4, 6) while protecting individual cells from Na toxicity (7-9). SOS1 is also indirectly required for the uptake of potassium (K) in the presence of Na, although the mechanistic basis is not fully understood (7,8,10). Both the protein kinase SOS2 and its associated calcium-sensor subunit SOS3 are required for the posttranslational activation of SOS1 Na/H exchange activity in Arabidopsis (11,12), and a similar regulatory module operates also in cereals (13).To understand further the mechanism(s) of SOS1 regulation, we identified the SOS2-dependent phosphorylation site and began to dissect the structure-function relationship in the SOS1 protein.Our results indicate that the SOS1 C-terminal domain comprises an auto-inhibitory domain the activity of which is counteracted by SOS2-dependent phosphorylation upon salinity stress. Results SOS1 ResiduesPhosphorylated by the SOS2 Protein Kinase. We have ...

EudraCT (2011-003243-21).