The plasma membrane Na+/H+ antiporter SOS1 is essential for salt tolerance in tomato and affects the partitioning of Na+ between plant organs

The woody plant poplar has a functionally conserved salt overly sensitive pathway in response to salinity stress

In Arabidopsis thaliana, the salt overly sensitive (SOS) pathway plays an essential role in maintaining ion homeostasis and conferring salt tolerance. Here we identified three SOS components in the woody plant Populus trichocarpa, designated as PtSOS1, PtSOS2 and PtSOS3. These putative SOS genes exhibited an overlapping but distinct expression pattern in poplar plants and the transcript levels of SOS1 and SOS2 were responsive to salinity stress. In poplar mesophyll protoplasts, PtSOS1 was specifically localized in the plasma membrane, whereas PtSOS2 was distributed throughout the cell, and PtSOS3 was predominantly targeted to the plasma membrane. Heterologous expression of PtSOS1, PtSOS2 and PtSOS3 could rescue salt-sensitive phenotypes of the corresponding Arabidopsis sos mutants, demonstrating that the Populus SOS proteins are functional homologues of their Arabidopsis counterpart. In addition, PtSOS3 interacted with, and recruited PtSOS2 to the plasma membrane in yeast and in planta. Reconstitution of poplar SOS pathway in yeast cells revealed that PtSOS2 and PtSOS3 acted coordinately to activate PtSOS1. Moreover, expression of the constitutively activated form of PtSOS2 partially complemented the sos3 mutant but not sos1, suggesting that PtSOS2 functions genetically downstream of SOS3 and upstream of SOS1. These results indicate a strong functional conservation of SOS pathway responsible for salt stress signaling from herbaceous to woody plants.

The SOS1 transporter of Physcomitrella patens mediates sodium efflux in planta

• SOS1 is an Na(+)/H(+) antiporter that plays a central role in Na(+) tolerance in land plants. SOS1 mediation of Na(+) efflux has been studied in plasma-membrane vesicles and deduced from the SOS1 suppression of the Na(+) sensitivity of yeast mutants defective in Na(+) -efflux. However, SOS1-mediated Na(+) efflux has not been characterized in either plant or yeast cells. Here, we use Physcomitrella patens to investigate the function of SOS1 in planta. • In P. patens, a nonvascular plant in which the study of ion cellular fluxes is technically simple, the existence of two SOS1 genes suggests that the Na(+) efflux remaining after the deletion of the ENA1 ATPase is mediated by a SOS1 system. Therefore, we cloned the P. patens SOS1 and SOS1B genes (PpSOS1 and PpSOS1B, respectively) and complementary DNAs, and constructed the PpΔsos1 and PpΔena1/PpΔsos1 deletion lines by gene targeting. • Comparison of wild-type, and PpΔsos1 and PpΔena1/PpΔsos1 mutant lines revealed that PpSOS1 is crucial for Na(+) efflux and that the PpΔsos1 line, and especially the PpΔena1/PpΔsos1 lines, showed excessive Na(+) accumulation and Na(+)-triggered cell death. The PpΔsos1 and PpΔena1/PpΔsos1 lines showed impaired high-affinity K(+) uptake. • Our data support the hypothesis that PpSOS1 mediates cellular Na(+) efflux and that PpSOS1 enhances K(+) uptake by an indirect effect.

A comparative study of salt tolerance parameters in 11 wild relatives of Arabidopsis thaliana

Salinity is an abiotic stress that limits both yield and the expansion of agricultural crops to new areas. In the last 20 years our basic understanding of the mechanisms underlying plant tolerance and adaptation to saline environments has greatly improved owing to active development of advanced tools in molecular, genomics, and bioinformatics analyses. However, the full potential of investigative power has not been fully exploited, because the use of halophytes as model systems in plant salt tolerance research is largely neglected. The recent introduction of halophytic Arabidopsis-Relative Model Species (ARMS) has begun to compare and relate several unique genetic resources to the well-developed Arabidopsis model. In a search for candidates to begin to understand, through genetic analyses, the biological bases of salt tolerance, 11 wild relatives of Arabidopsis thaliana were compared: Barbarea verna, Capsella bursa-pastoris, Hirschfeldia incana, Lepidium densiflorum, Malcolmia triloba, Lepidium virginicum, Descurainia pinnata, Sisymbrium officinale, Thellungiella parvula, Thellungiella salsuginea (previously T. halophila), and Thlaspi arvense. Among these species, highly salt-tolerant (L. densiflorum and L. virginicum) and moderately salt-tolerant (M. triloba and H. incana) species were identified. Only T. parvula revealed a true halophytic habitus, comparable to the better studied Thellungiella salsuginea. Major differences in growth, water transport properties, and ion accumulation are observed and discussed to describe the distinctive traits and physiological responses that can now be studied genetically in salt stress research.

The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato

NHX-type antiporters in the tonoplast have been reported to increase the salt tolerance of various plants species, and are thought to mediate the compartmentation of Na(+) in vacuoles. However, all isoforms characterized so far catalyze both Na(+)/H(+) and K(+)/H(+) exchange. Here, we show that AtNHX1 has a critical involvement in the subcellular partitioning of K(+), which in turn affects plant K(+) nutrition and Na(+) tolerance. Transgenic tomato plants overexpressing AtNHX1 had larger K(+) vacuolar pools in all growth conditions tested, but no consistent enhancement of Na(+) accumulation was observed under salt stress. Plants overexpressing AtNHX1 have a greater capacity to retain intracellular K(+) and to withstand salt-shock. Under K(+)-limiting conditions, greater K(+) compartmentation in the vacuole occurred at the expense of the cytosolic K(+) pool, which was lower in transgenic plants. This caused the early activation of the high-affinity K(+) uptake system, enhanced K(+) uptake by roots, and increased the K(+) content in plant tissues and the xylem sap of transformed plants. Our results strongly suggest that NHX proteins are likely candidates for the H(+)-linked K(+) transport that is thought to facilitate active K(+) uptake at the tonoplast, and the partitioning of K(+) between vacuole and cytosol.

Salt-dependent regulation of a CNG channel subfamily in Arabidopsis

BACKGROUND

In Arabidopsis thaliana, the family of cyclic nucleotide-gated channels (CNGCs) is composed of 20 members. Previous studies indicate that plant CNGCs are involved in the control of growth processes and responses to abiotic and biotic stresses. According to their proposed function as cation entry pathways these channels contribute to cellular cation homeostasis, including calcium and sodium, as well as to stress-related signal transduction. Here, we studied the expression patterns and regulation of CNGC19 and CNGC20, which constitute one of the five CNGC subfamilies.

RESULTS

GUS, GFP and luciferase reporter assays were used to study the expression of CNGC19 and CNGC20 genes from Arabidopsis thaliana in response to developmental cues and salt stress. CNGC19 and CNGC20 were differentially expressed in roots and shoots. The CNGC19 gene was predominantly active in roots already at early growth stages. Major expression was observed in the phloem. CNGC20 showed highest promoter activity in mesophyll cells surrounding the veins. Its expression increased during development and was maximal in mature and senescent leaves. Both genes were upregulated in the shoot in response to elevated NaCl but not mannitol concentrations. While in the root, CNGC19 did not respond to changes in the salt concentration, in the shoot it was strongly upregulated in the observed time frame (6-72 hours). Salt-induction of CNGC20 was also observed in the shoot, starting already one hour after stress treatment. It occurred with similar kinetics, irrespective of whether NaCl was applied to roots of intact plants or to the petiole of detached leaves. No differences in K and Na contents of the shoots were measured in homozygous T-DNA insertion lines for CNGC19 and CNGC20, respectively, which developed a growth phenotype in the presence of up to 75 mM NaCl similar to that of the wild type.

CONCLUSION

Together, the results strongly suggest that both channels are involved in the salinity response of different cell types in the shoot. Upon salinity both genes are upregulated within hours. CNGC19 and CNGC20 could assist the plant to cope with toxic effects caused by salt stress, probably by contributing to a re-allocation of sodium within the plant.

The Na(+)/H(+) exchanger SOS1 controls extrusion and distribution of Na(+) in tomato plants under salinity conditions

Maintaining a high K(+)/Na(+) ratio in the cell cytosol, along with the transport processes implicated in the xylem and phloem loading/unloading of Na(+) in plants (long-distance transport) are key aspects in plant salt tolerance. The Ca(2+)-dependent SOS pathway regulating Na(+) and K(+) homeostasis and long-distance Na(+) transport has been reported in Arabidopsis. However, Arabidopsis might not be the best model to analyze the involvement of the SOS pathway in long-distance Na(+) transport due to the very short stem of these plants which do not allow a precise dissection of the relative content of Na(+) in stem versus leaf. This separation would be critical to assess the role of SOS1 in xylem loading/unloading, Na(+) export by roots, retention in stems and the differential distribution/accumulation in old leaves. Towards this goal, tomato might represent a superior model due to its anatomical structure and agricultural significance. We recently demonstrated the key role played by the plasma membrane Na(+)/H(+) antiporter SlSOS1 in salt tolerance in tomato by maintaining ion homeostasis under salinity stress and in the partitioning of Na(+) in plant organs.