Conservation of the Salt Overly Sensitive Pathway in Rice

Abstract

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...

RAS1, a quantitative trait locus for salt tolerance and ABA sensitivity in Arabidopsis

“…The salt overly sensitive (SOS) pathway of salt tolerance is crucial for maintaining ion homeostasis under salt stress. This pathway is conserved across diverse plant species, including rice (3,4). The SOS2 ser/thr protein kinase regulates both SOS1-mediated Na + extrusion from the cytosol and vacuolar Na + /H + antiportermediated Na + sequestration into the vacuole (5,6).…”

Activation of the plasma membrane Na/H antiporter Salt-Overly-Sensitive 1 (SOS1) by phosphorylation of an auto-inhibitory C-terminal domain

“…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).…”

Plant Abiotic Stress: Insights from the Genomics Era

“…Chenopodium quinoa, a halophyte native to the Andes Mountains, was found to contain 2 AtSOS1 homologs (Maughan et al, 2009), with future work to include complementation of a mutant sos1 Arabidopsis line with the homologues from C. quinoa. Homologues of AtSOS1 have also been identified for multiple glycophyte plant species such as rice (Oryza sativa), the seagrass Cymodocea nodosa, and Populus trichocarpa, the woody perennial poplar tree (Martínez-Atienza et al, 2007, Garciadeblás et al, 2007, Tang et al, 2007. Not surprisingly, also conserved are genes controlling sodium entry, such as the previously mentioned K+ channel HKT1 and also genes controlling vacuole compartmentalization, of which the following discussion will focus primarily on AtNHX1, a gene encoding a vacuolar Na+/H+ exchanger.…”