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 ...

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The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na

Rice (Oryza sativa) stands among the world's most important crop species. Rice is salt sensitive, and the undue accumulation of sodium ions (Na +) in shoots has the strongest negative correlation with rice productivity under long-term salinity. The plasma membrane Na + /H + exchanger protein Salt Overly Sensitive 1 (SOS1) is the sole Na + efflux transporter that has been genetically characterized to date. Here, the importance of SOS1-facilitated Na + flux in the salt tolerance of rice was analyzed in a reversegenetics approach. A sos1 loss-of-function mutant displayed exceptional salt sensitivity that was correlated with excessive Na + intake and impaired Na + loading into the xylem, thus indicating that SOS1 controls net root Na + uptake and long-distance Na + transport to shoots. The acute Na + sensitivity of sos1 plants at low NaCl concentrations allowed analysis of the transcriptional response to sodicity stress without effects of the osmotic stress intrinsic to high-salinity treatments. In contrast with that in the wild type, sos1 mutant roots displayed preferential down-regulation of stress-related genes in response to salt treatment, despite the greater intensity of stress experienced by the mutant. These results suggest there is impaired stress detection or an inability to mount a comprehensive response to salinity in sos1. In summary, the plasma membrane Na + /H + exchanger SOS1 plays a major role in the salt tolerance of rice by controlling Na + homeostasis and possibly contributing to the sensing of sodicity stress.

The sodium and potassium concentrations in leaf and stem have been genetically studied as physiological components of the vegetative and reproductive development in two populations of F(8) lines, derived from a salt sensitive genotype of Solanum lycopersicum cv. Cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (142 lines), and S. cheesmaniae, the C population (116 lines). Genetic parameters of ten traits under salinity and five of them under control conditions were studied by ANOVA, correlation, principal component and QTL analysis to understand the global response of the plant. Two linkage maps including some tomato flowering time and salt tolerance candidate genes encoding for SlSOS1, SlSOS2, SlSOS3, LeNHX1, LeNHX3, were used for the QTL detection. Thirteen and 20 QTLs were detected under salinity in the P and C populations, respectively, and four under control conditions. Highly significant and contributing QTLs (over 40%) for the concentrations of Na(+) and K(+) in stems and leaves have been detected on chromosome 7 in both the populations. This is the only genomic position where the concentration QTLs for both the cations locate together. The proportion of QTLs significantly affected by salinity was larger in the P population (64.3%, including all QTLs detected under control) than in the C population (21.4%), where the estimated genetic component of variance was larger for most traits. A highly significant association between the leaf area and fruit yield under salinity was found only in the C population, which is supported by the location of QTLs for these traits in a common region of chromososome C1. As far as breeding for salt tolerance is concerned, only two sodium QTLs (lnc1.1 and lnc8.1) map in genomic regions of C1 and C8 where fruit yield QTLs are also located but in both the cases the profitable allele corresponds to the salt sensitive, cultivated species. One of those QTLs, lnc1.1 might involve LeNHX3.

The endomembrane system, which sorts proteins and membranes through the secretory and endocytic pathways, is critical for cellular functions in eukary-otes (Otegui and Spitzer, 2008; Richter et al., 2009; Contento and Bassham, 2012). The plant endomembrane system comprises the endoplasmic reticulum (ER), Golgi complex, trans-Golgi network (TGN), prevacuolar compartment (PVC)/multivesicular bodies (MVB), and vacuoles (Contento and Bassham, 2012). The endomembrane system, by enabling protein modification and sorting, plays important roles in cell polarity, cytokinesis, cell wall formation, signaling, and stress tolerance (

The rootstock effect on the fruit yield of a grafted tomato variety was genetically analyzed under salinity using as rootstock two populations of F(9) lines developed from a salt sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (123 lines), and S. cheesmaniae, the C population (100 lines). There were rootstock lines from the two populations (up to 65% in the P population) that raised the fruit yield of the commercial hybrid under saline conditions. It is shown that this salt tolerance rootstock effect is a heritable trait (h (2) near 0.3), governed by at least eight QTLs. The most relevant component was the number of fruits. Thus most detected QTLs correspond to this component. In general, QTL gene effects are medium-sized, with contributions from 8.5 up to 15.9% at most, and the advantageous allele comes from the wild, salt tolerant species. Only two fruit yield QTLs on chromosomes P9 and C11 might correspond to fruit yield QTLs of the non-grafted lines indicating their root system dependence. A fruit yield QTL on chromosome 3 is acting epistatically in both populations. The epistatic interactions found were dominant and they were unveiled using the associated marker as cofactor in the composite interval mapping methodology. Therefore, an efficient and profitable utilization of wild germplasm can be carried out through the improvement of rootstocks that confer salt tolerance in terms of fruit yield to the grafted variety.

Salt tolerance has been analysed in two populations of F(7) lines developed from a salt sensitive genotype of Solanum lycopersicum var. cerasiforme, as female parent, and two salt tolerant lines, as male parents, from S. pimpinellifolium, the P population (142 lines), and S. cheesmaniae, the C population (116 lines). Salinity effects on 19 quantitative traits including fruit yield were investigated by correlation, principal component analysis, ANOVA and QTL analysis. A total of 153 and 124 markers were genotyped in the P and C populations, respectively. Some flowering time and salt tolerance candidate genes were included. Since most traits deviated from a normal distribution, results based on the Kruskal-Wallis non-parametric test were preferred. Interval mapping methodology and ANOVA were also used for QTL detection. Eight out of 15 QTLs at each population were detected for the target traits under both control and high salinity conditions, and among them, only average fruit weight (FW) and fruit number (FN) QTLs (fw1.1, fw2.1 and fn1.2) were detected in both populations. The individual contribution of QTLs were, in general, low. After leaf chloride concentration, flowering time is the trait most affected by salinity because different QTLs are detected and some of their QTLxE interactions have been found significant. Also reinforcing the interest on information provided by QTL analysis, it has been found that non-correlated traits may present QTL(s) that are associated with the same marker. A few salinity specific QTLs for fruit yield, not associated with detrimental effects, might be used to increase tomato salt tolerance. The beneficial allele at two of them, fw8.1 (in C) and tw8.1 (for total fruit weight in P) corresponds to the salt sensitive parent, suggesting that the effect of the genetic background is crucial to breed for wide adaptation using wild germplasm.