Arabidopsis–rice–wheat gene orthologues for Na+ transport and transcript analysis in wheat–L. elongatum aneuploids under salt stress

Structure, Function, and Regulation of the Plasma Membrane Na+/H+ Antiporter Salt Overly Sensitive 1 in Plants

Physiological studies have confirmed that export of Na⁺ to improve salt tolerance in plants is regulated by the combined activities of a complex transport system. In the Na⁺ transport system, the Na⁺/H⁺ antiporter salt overly sensitive 1 (SOS1) is the main protein that functions to excrete Na⁺ out of plant cells. In this paper, we review the structure and function of the Na⁺/H⁺ antiporter and the physiological process of Na⁺ transport in SOS signaling pathway, and discuss the regulation of SOS1 during phosphorylation activation by protein kinase and the balance mechanism of inhibiting SOS1 antiporter at molecular and protein levels. In addition, we carried out phylogenetic tree analysis of SOS1 proteins reported so far in plants, which implied the specificity of salt tolerance mechanism from model plants to higher crops under salt stress. Finally, the high complexity of the regulatory network of adaptation to salt tolerance, and the feasibility of coping strategies in the process of genetic improvement of salt tolerance quality of higher crops were reviewed.

Plasticity in Triticeae centromere DNA sequences: a wheat × tall wheatgrass (decaploid) model

Centromeres mediate chromosome attachment to microtubules and maintain the integrity of chromosomes for proper segregation of the sister chromatids during cell division. Advances in the assembly of Triticeae genome sequences combined with the capacity to recover hybrid species derived from very distantly related species provides potential experimental systems for linking retrotransposon amplification and repositioning of centromeres via non-mendelian inheritance in partial amphiploid breeds. The decaploid tall wheatgrass (Thinopyrum ponticum) is one of the most successfully used perennial species in wheat breeding for generating translocation lines with valuable agronomic traits. We found that wheat centromere retrotransposons CRW and Quinta widely occur within the tall wheatgrass genome. In addition, one of the genome donors to Th. ponticum, Pseudoroegneria stipifolia (StSt), has been shown to have Abigail and a satellite repeat, CentSt. We also found two other centromeric retrotransposons, Abia and CL135 in Th. ponticum by ChIP-seq. Examination of partial amphiploid lines that were generated in the 1970s demonstrated extensive modification in centromere sequences using CentSt, Abigail and Abia as probes. We also detected that St-genome chromosomes were more enriched with Abigail and CentSt, whereas E-genome chromosomes were enriched with CRW and Quinta in tall wheatgrass and its closer relatives. It can be concluded that bursts of transposition of retrotransposons and repositioning of centromeres via non-mendelian segregation are common in partial amphiploids derived from interspecific hybrids. Practically speaking, our study reveals that the existence of homologous centromere functional sequences in both a donor and its receptor can substantially contribute to the successful transfer of alien genes into crop species. OPEN RESEARCH BADGES: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://www.ncbi.nlm.nih.gov/sra/SRR9089557; https://www.ncbi.nlm.nih.gov/sra/SRR9089558; https://www.ncbi.nlm.nih.gov/sra/SRR9089559; https://www.ncbi.nlm.nih.gov/sra/SRR9089560; https://www.ncbi.nlm.nih.gov/sra/SRR9089561; https://www.ncbi.nlm.nih.gov/sra/SRR9089562; https://www.ncbi.nlm.nih.gov/sra/SRR9089563; https://www.ncbi.nlm.nih.gov/sra/SRR9089564; https://www.ncbi.nlm.nih.gov/nuccore/MK999394; https://www.ncbi.nlm.nih.gov/nuccore/MK999395; https://www.ncbi.nlm.nih.gov/nuccore/MK999396.

Na+ extrusion from the cytosol and tissue-specific Na+ sequestration in roots confer differential salt stress tolerance between durum and bread wheat

The progress in plant breeding for salinity stress tolerance is handicapped by the lack of understanding of the specificity of salt stress signalling and adaptation at the cellular and tissue levels. In this study, we used electrophysiological, fluorescence imaging, and real-time quantitative PCR tools to elucidate the essentiality of the cytosolic Na+ extrusion in functionally different root zones (elongation, meristem, and mature) in a large number of bread and durum wheat accessions. We show that the difference in the root's ability for vacuolar Na+ sequestration in the mature zone may explain differential salinity stress tolerance between salt-sensitive durum and salt-tolerant bread wheat species. Bread wheat genotypes also had on average 30% higher capacity for net Na+ efflux from the root elongation zone, providing the first direct evidence for the essentiality of the root salt exclusion trait at the cellular level. At the same time, cytosolic Na+ accumulation in the root meristem was significantly higher in bread wheat, leading to the suggestion that this tissue may harbour a putative salt sensor. This hypothesis was then tested by investigating patterns of Na+ distribution and the relative expression level of several key genes related to Na+ transport in leaves in plants with intact roots and in those in which the root meristems were removed. We show that tampering with this sensing mechanism has resulted in a salt-sensitive phenotype, largely due to compromising the plant's ability to sequester Na+ in mesophyll cell vacuoles. The implications of these findings for plant breeding for salinity stress tolerance are discussed.

Identification and characterization of evolutionarily conserved alternative splicing events in a mangrove genus Sonneratia

Alternative splicing (AS), which produces multiple mRNA transcripts from a single gene, plays crucial roles in plant growth, development and environmental stress responses. Functional significances of conserved AS events among congeneric species have not been well characterized. In this study, we performed transcriptome sequencing to characterize AS events in four common species of Sonneratia, a mangrove genus excellently adaptive to intertidal zones. 7,248 to 12,623 AS events were identified in approximately 25% to 35% expressed genes in the roots of the four species. The frequency of AS events in Sonneratia was associated with genomic features, including gene expression level and intron/exon number and length. Among the four species, 1,355 evolutionarily conserved AS (ECAS) events were identified from 1,170 genes. Compared with non-ECAS events, ECAS events are of shorter length and less possibility to introduce premature stop codons (PTCs) and frameshifts. Functional annotations of the genes containing ECAS events showed that four of the 26 enriched Gene Ontology (GO) terms are involved in proton transport, signal transduction and carbon metabolism, and 60 genes from another three GO terms are implicated in responses to osmotic, oxidative and heat stresses, which may contribute to the adaptation of Sonneratia species to harsh intertidal environments.

Improving Salt Tolerance of Chickpea Using Modern Genomics Tools and Molecular Breeding

INTRODUCTION

The high protein value, essential minerals, dietary fibre and notable ability to fix atmospheric nitrogen make chickpea a highly remunerative crop, particularly in low-input food production systems. Of the variety of constraints challenging chickpea productivity worldwide, salinity remains of prime concern owing to the intrinsic sensitivity of the crop. In view of the projected expansion of chickpea into arable and salt-stressed land by 2050, increasing attention is being placed on improving the salt tolerance of this crop. Considerable effort is currently underway to address salinity stress and substantial breeding progress is being made despite the seemingly highly-complex and environment-dependent nature of the tolerance trait.

CONCLUSION

This review aims to provide a holistic view of recent advances in breeding chickpea for salt tolerance. Initially, we focus on the identification of novel genetic resources for salt tolerance via extensive germplasm screening. We then expand on the use of genome-wide and cost-effective techniques to gain new insights into the genetic control of salt tolerance, including the responsive genes/QTL(s), gene(s) networks/cross talk and intricate signalling cascades.

An insight into salt stress tolerance mechanisms of Chenopodium album

Salt stress is one of the most dramatic abiotic stresses that induce oxidative and osmotic stress simultaneously. Salt stress is known to be more effective in reducing growth and yield of glycophytes; however, halophytes are able to withstand salt stress. Nonetheless, variability exists among different halophytic plants species from different plant families. Chenopodium album belongs to Chenopodiacea family and is known as weed in many regions of world; however, it is a very interesting halophytic plant. Little research has conducted so far by considering C. album as model plant to study salt stress tolerance mechanisms. This article attempts to compile current literature in order to explain C. album salt stress tolerance mechanism and to highlight the knowledge gap relating to salt stress tolerance mechanism in C. album. Briefly, C. album has remarkable ability of seed dimorphism, sodium exclusion, and potassium retention. C. album further tolerates salt stress by increasing redox potential associated with high production of osmolytes and antioxidants.

Nax loci affect SOS1-like Na+/H+ exchanger expression and activity in wheat

Salinity stress tolerance in durum wheat is strongly associated with a plant's ability to control Na(+) delivery to the shoot. Two loci, termed Nax1 and Nax2, were recently identified as being critical for this process and the sodium transporters HKT1;4 and HKT1;5 were identified as the respective candidate genes. These transporters retrieve Na(+) from the xylem, thus limiting the rates of Na(+) transport from the root to the shoot. In this work, we show that the Nax loci also affect activity and expression levels of the SOS1-like Na(+)/H(+) exchanger in both root cortical and stelar tissues. Net Na(+) efflux measured in isolated steles from salt-treated plants, using the non-invasive ion flux measuring MIFE technique, decreased in the sequence: Tamaroi (parental line)>Nax1=Nax2>Nax1:Nax2 lines. This efflux was sensitive to amiloride (a known inhibitor of the Na(+)/H(+) exchanger) and was mirrored by net H(+) flux changes. TdSOS1 relative transcript levels were 6-10-fold lower in Nax lines compared with Tamaroi. Thus, it appears that Nax loci confer two highly complementary mechanisms, both of which contribute towards reducing the xylem Na(+) content. One enhances the retrieval of Na(+) back into the root stele via HKT1;4 or HKT1;5, whilst the other reduces the rate of Na(+) loading into the xylem via SOS1. It is suggested that such duality plays an important adaptive role with greater versatility for responding to a changing environment and controlling Na(+) delivery to the shoot.

Comparative proteomic analysis of early salt stress responsive proteins in roots and leaves of rice

Growth and productivity of rice (Oryza sativa L.) are severely affected by salinity. Understanding the mechanisms that protect rice and other important cereal crops from salt stress will help in the development of salt-stress-tolerant strains. In this study, rice seedlings of the same genetic species with various salt tolerances were studied. We first used 2DE to resolve the expressed proteome in rice roots and leaves and then used nanospray liquid chromatography/tandem mass spectrometry to identify the differentially expressed proteins in rice seedlings after salt treatment. The 2DE assays revealed that there were 104 differentially expressed protein spots in rice roots and 59 in leaves. Then, we identified 83 proteins in rice roots and 61 proteins in rice leaves by MS analysis. Functional classification analysis revealed that the differentially expressed proteins from roots could be classified into 18 functional categories while those from leaves could be classified into 11 functional categories. The proteins from rice seedlings that most significantly contributed to a protective effect against increased salinity were cysteine synthase, adenosine triphosphate synthase, quercetin 3-O-methyltransferase 1, and lipoxygenase 2. Further analysis demonstrated that the primary mechanisms underlying the ability of rice seedlings to tolerate salt stress were glycolysis, purine metabolism, and photosynthesis. Thus, we suggest that differentially expressed proteins may serve as marker group for the salt tolerance of rice.

Genotypic difference in salinity tolerance in quinoa is determined by differential control of xylem Na(+) loading and stomatal density

Quinoa is regarded as a highly salt tolerant halophyte crop, of great potential for cultivation on saline areas around the world. Fourteen quinoa genotypes of different geographical origin, differing in salinity tolerance, were grown under greenhouse conditions. Salinity treatment started on 10 day old seedlings. Six weeks after the treatment commenced, leaf sap Na and K content and osmolality, stomatal density, chlorophyll fluorescence characteristics, and xylem sap Na and K composition were measured. Responses to salinity differed greatly among the varieties. All cultivars had substantially increased K(+) concentrations in the leaf sap, but the most tolerant cultivars had lower xylem Na(+) content at the time of sampling. Most tolerant cultivars had lowest leaf sap osmolality. All varieties reduced stomata density when grown under saline conditions. All varieties clustered into two groups (includers and excluders) depending on their strategy of handling Na(+) under saline conditions. Under control (non-saline) conditions, a strong positive correlation was observed between salinity tolerance and plants ability to accumulate Na(+) in the shoot. Increased leaf sap K(+), controlled Na(+) loading to the xylem, and reduced stomata density are important physiological traits contributing to genotypic differences in salinity tolerance in quinoa, a halophyte species from Chenopodium family.

Proteomic analysis of salt-responsive ubiquitin-related proteins in rice roots

RATIONALE

Ubiquitination of proteins plays an important role in regulating a myriad of physiological functions in plants such as xylogenesis, senescence, cell cycle control, and stress response. However, only a limited number of proteins in plants have been identified as being ubiquitinated in response to salt stress. The relationships between ubiquitination and salt-stress responses in plants are not clear.

METHODS

Rice (Oryza sativa) seedlings from the same genetic background with various salt tolerances exposed to salt stress were studied. The proteins of roots were extracted then analyzed using western blotting against ubiquitin. Differentially expressed ubiquitinated proteins were identified by nanospray liquid chromatography/tandem mass spectrometry (nano-LC/MS/MS) and quantified by label-free methods based on the Exponentially Modified Protein Abundance Index (emPAI) and on the peak areas of XIC spectra derived from ubiquitinated peptides. In addition, we performed a gel-based shotgun proteomic analysis to detect the ubiquitinated proteome that may be involved in response to salt stress.

RESULTS

The expressions of ubiquitination on pyruvate phosphate dikinase 1, heat shock protein 81-1, probable aldehyde oxidase 3, plasma membrane ATPase, cellulose synthase A catalytic subunit 4 [UDP-forming] and cyclin-C1-1 were identified and compared before and after salt treatment. The functions of those ubiquitinated proteins were further discussed for defence against salt stress. In addition, a large number of ubiquitinated proteins were successfully identified as well in this study.

CONCLUSIONS

The ubiquitination of proteins affected the protective mechanisms in rice seedlings to resist the salt stress during the initial phase. The findings in the present study also demonstrate that the regulated mechanisms through protein ubiquitination are important for rice seedlings against salt stress.

Genetic behaviour of physiological traits conferring cytosolic K+/Na+ homeostasis in wheat

A plant's ability to maintain an optimal cytosolic K(+)/Na(+) ratio has long been cited as a key feature of salinity tolerance. As traditional whole-leaf nutrient analysis does not account for tissue and organelle-specific ion sequestration, the predictive value of this index at the whole-plant level is not always satisfactory. Consequently, suitable in situ methods for functionally assessing the activity of the key membrane transporters contributing to this trait at the cellular level need to be developed. The aim of this work was to investigate the extent to which plasma membrane transporter-mediated Na(+) exclusion and KOR-mediated K(+) retention traits, measured with the microelectrode ion flux measuring (MIFE) technique, are inheritable in wheat, and whether the MIFE technique has the potential to be used in combination with molecular markers to determine QTLs for these transporter proteins. Experiments involved two bread (Triticum aestivum) and two durum (Triticum turgidum) wheat lines contrasting in their salinity tolerance. Net Na(+), K(+) and H(+) fluxes were measured from 6-day-old roots of parental lines and their F(1) hybrids upon addition and removal of NaCl. These results were complemented by assessment of whole-plant physiological and agronomic characteristics. We show evidence for a strong heritability of plasma membrane transporter-mediated Na(+) exclusion and K(+) retention traits in wheat at the cellular level. This opens the prospect of using the MIFE technique to map the position of these transporters on particular loci of wheat chromosomes. The next obvious step would be to pyramid these traits in one ideotype with superior salinity tolerance.

Comparative phosphoproteomic analysis of microsomal fractions of Arabidopsis thaliana and Oryza sativa subjected to high salinity

Plants respond to salt stress by initiating phosphorylation cascades in their cells. Many key phosphorylation events take place at membranes. Microsomal fractions from 400 mM salt-treated Arabidopsis suspension plants were isolated, followed by trypsin shaving, enrichment using Zirconium ion-charged or TiO(2) magnetic beads, and tandem mass spectrometry analyses for site mapping. A total of 27 phosphorylation sites from 20 Arabidopsis proteins including photosystem II reaction center protein H PsbH were identified. In addition to Arabidopsis, microsomal fractions from shoots of 200 mM salt-treated rice was carried out, followed by trypsin digestion using shaving or tube-gel, and enrichment using Zirconium ion-charged or TiO(2) magnetic beads. This yielded identification of 13 phosphorylation sites from 8 proteins including photosystem II reaction center protein H PsbH. Label-free quantitative analysis suggests that the phosphorylation sites of PsbH were regulated by salt stress in Arabidopsis and rice. Sequence alignment of PsbH phosphorylation sites indicates that Thr-2 and Thr-4 are evolutionarily conserved in plants. Four conserved phosphorylation motifs were predicted, and these suggest that a specific unknown kinase or phosphatase is involved in high-salt stress responses in plants.

Identification, characterization and interpretation of single-nucleotide sequence variation in allopolyploid crop species

An understanding of nature and extent of nucleotide sequence variation is required for programmes of discovery and characterization of single nucleotide polymorphisms (SNPs), which provide the most versatile class of molecular genetic marker. A majority of higher plant species are polyploids, and allopolyploidy, because of hybrid formation between closely related taxa, is very common. Mutational variation may arise both between allelic (homologous) sequences within individual subgenomes and between homoeologous sequences among subgenomes, in addition to paralogous variation between duplicated gene copies. Successful SNP validation in allopolyploids depends on differentiation of the sequence variation classes. A number of biological factors influence the feasibility of discrimination, including degree of gene family complexity, inbreeding or outbreeding reproductive habit, and the level of knowledge concerning progenitor diploid species. In addition, developments in high-throughput DNA sequencing and associated computational analysis provide general solutions for the genetic analysis of allopolyploids. These issues are explored in the context of experience from a range of allopolyploid species, representing grain (wheat and canola), forage (pasture legumes and grasses), and horticultural (strawberry) crop. Following SNP discovery, detection in routine genotyping applications also presents challenges for allopolyploids. Strategies based on either design of subgenome-specific SNP assays through homoeolocus-targeted polymerase chain reaction (PCR) amplification, or detection of incremental changes in nucleotide variant dosage, are described.

Involvement of calcium-mediated effects on ROS metabolism in the regulation of growth improvement under salinity

Salinity reduces Ca(2+) availability, transport, and mobility to growing regions of the plant and supplemental Ca(2+) is known to reduce salinity damages. This study was undertaken to unravel some of the ameliorative mechanisms of Ca(2+) on salt stress at the cellular and tissue levels. Zea mays L. plants were grown in nutrient solution containing 1 or 80 mM NaCl with various Ca(2+) levels. Measurements of growth and physiological parameters, such as ion imbalance, indicated that the Ca(2+)-induced alleviation mechanisms differed between plant organs. Under salinity, H(2)O(2) levels increased in the leaf-growing tissue with increasing levels of supplemental Ca(2+) and reached the levels of control plants, whereas superoxide levels remained low at all Ca(2+) levels, indicating that Ca(2+) affected growth by increasing H(2)O(2) but not superoxide levels. Salinity completely abolished apoplastic peroxidase activity. Supplemental Ca(2+) increased its activity only slightly. However, under salinity, polyamine oxidase (PAO) activity was shifted toward the leaf base probably as an adaptive mechanism aimed at restoring normal levels of reactive oxygen species (ROS) at the expansion zone where NADPH oxidase could no longer provide the required ROS for growth. Interestingly, addition of Ca(2+) shifted the PAO-activity peak back to its original location in addition to its enhancement. The increase in PAO activity in conjunction with low levels of apoplastic peroxidase is supportive of cellular growth via nonenzymatic wall loosening derived by the increase in H(2)O(2) and less supportive of the peroxidase-mediated cross-linking of wall material. Thus extracellular Ca(2+) can modulate ROS levels at specific tissue localization and developmental stages thereby affecting cellular extension.

Regulation of durum wheat Na+/H + exchanger TdSOS1 by phosphorylation

We have identified a plasma membrane Na(+)/H(+) exchanger from durum wheat, designated TdSOS1. Heterologous expression of TdSOS1 in a yeast strain lacking endogenous Na(+) efflux proteins showed complementation of the Na(+)- and Li(+)-sensitive phenotype by a mechanism involving cation efflux. Salt tolerance conferred by TdSOS1 was maximal when co-expressed with the Arabidopsis protein kinase complex SOS2/SOS3. In vitro phosphorylation of TdSOS1 with a hyperactive form of the Arabidopsis SOS2 kinase (T/DSOS2∆308) showed the importance of two essential serine residues at the C-terminal hydrophilic tail (S1126, S1128). Mutation of these two serine residues to alanine decreased the phosphorylation of TdSOS1 by T/DSOS2∆308 and prevented the activation of TdSOS1. In addition, deletion of the C-terminal domain of TdSOS1 encompassing serine residues at position 1126 and 1128 generated a hyperactive form that had maximal sodium exclusion activity independent from the regulatory SOS2/SOS3 complex. These results are consistent with the presence of an auto-inhibitory domain at the C-terminus of TdSOS1 that mediates the activation of TdSOS1 by the protein kinase SOS2. Expression of TdSOS1 mRNA in young seedlings of the durum wheat variety Om Rabia3, using different abiotic stresses (ionic and oxidative stress) at different times of exposure, was monitored by RT-PCR.

Isolation and characterization of a chlorophyll degradation regulatory gene from tall fescue

The non-yellowing gene (NYE1), initially identified from Arabidopsis, is a key regulatory gene responsible for chlorophyll degradation during senescence. Here, FaNYE1, an orthologue of AtNYE1, was further identified from a major type of cool-season turf grass, tall fescue (Festuca arundinacea Schreb.), by RACE-PCR. It consists of 1,441 bp, with an open reading frame of 834 bp, encoding a predicted polypeptide of 278 amino acids. Sequence similarity as well as exon and intron characteristics clearly suggested that FaNYE1 encoded an AtNYE1-like chloroplast protein. FaNYE1 could be strongly induced by dark treatment and natural senescence. FaNYE1, driven by a 1.5 kb upstream fragment of AtNYE1, could rescue the stay-green phenotype of nye1-1. Constitutive overexpression of FaNYE1 in Arabidopsis resulted in the whole spectrum of leaf yellowing phenotypes, the severity of which correlates with its transcript level. These results collectively indicate that FaNYE1 might play an important regulatory role in chlorophyll degradation during senescence in tall fescue, and therefore is a valuable gene for improving the green period or lawn color of turf grasses by genetic engineering.

Assessing the role of root plasma membrane and tonoplast Na+/H+ exchangers in salinity tolerance in wheat: in planta quantification methods

This work investigates the role of cytosolic Na+ exclusion in roots as a means of salinity tolerance in wheat, and offers in planta methods for the functional assessment of major transporters contributing to this trait. An electrophysiological protocol was developed to quantify the activity of plasma membrane Na+ efflux systems in roots, using the microelectrode ion flux estimation (MIFE) technique. We show that active efflux of Na+ from wheat root epidermal cells is mediated by a SOS1-like homolog, energized by the plasma membrane H+-ATPase. SOS1-like efflux activity was highest in Kharchia 65, a salt-tolerant bread wheat cultivar. Kharchia 65 also had an enhanced ability to sequester large quantities of Na+ into the vacuoles of root cells, as revealed by confocal microscopy using Sodium Green. These findings were consistent with the highest level of expression of both SOS1 and NHX1 transcripts in plant roots in this variety. In the sensitive wheat varieties, a greater proportion of Na+ was located in the root cell cytosol. Overall, our findings suggest a critical role of cytosolic Na+ exclusion for salinity tolerance in wheat and offer convenient protocols to quantify the contribution of the major transporters conferring this trait, to screen plants for salinity tolerance.

Variation in salinity tolerance and shoot sodium accumulation in Arabidopsis ecotypes linked to differences in the natural expression levels of transporters involved in sodium transport

Salinity tolerance can be attributed to three different mechanisms: Na+ exclusion from the shoot, Na+ tissue tolerance and osmotic tolerance. Although several key ion channels and transporters involved in these processes are known, the variation in expression profiles and the effects of these proteins on Na+ transport in different accessions of the same species are unknown. Here, expression profiles of the genes AtHKT1;1, AtSOS1, AtNHX1 and AtAVP1 are determined in four ecotypes of Arabidopsis thaliana. Not only are these genes differentially regulated between ecotypes, the expression levels of the genes can be linked to the concentration of Na+ in the plant. An inverse relationship was found between AtSOS1 expression in the root and total plant Na+ accumulation, supporting a role for AtSOS1 in Na+ efflux from the plant. Similarly, ecotypes with high expression levels of AtHKT1;1 in the root had lower shoot Na+ concentrations, due to the hypothesized role of AtHKT1;1 in retrieval of Na+ from the transpiration stream. The inverse relationship between shoot Na+ concentration and salinity tolerance typical of most cereal crop plants was not demonstrated, but a positive relationship was found between salt tolerance and levels of AtAVP1 expression, which may be related to tissue tolerance.

Functional phosphoproteomic profiling of phosphorylation sites in membrane fractions of salt-stressed Arabidopsis thaliana

Background

Under conditions of salt stress, plants respond by initiating phosphorylation cascades. Many key phosphorylation events occur at the membrane. However, to date only limited sites have been identified that are phosphorylated in response to salt stress in plants.

Results

Membrane fractions from three-day and 200 mM salt-treated Arabidopsis suspension plants were isolated, followed by protease shaving and enrichment using Zirconium ion-charged magnetic beads, and tandem mass spectrometry analyses. From this isolation, 18 phosphorylation sites from 15 Arabidopsis proteins were identified. A unique phosphorylation site in 14-3-3-interacting protein AHA1 was predominately identified in 200 mM salt-treated plants. We also identified some phosphorylation sites in aquaporins. A doubly phosphorylated peptide of PIP2;1 as well as a phosphopeptide containing a single phosphorylation site (Ser-283) and a phosphopeptide containing another site (Ser-286) of aquaporin PIP2;4 were identified respectively. These two sites appeared to be novel of which were not reported before. In addition, quantitative analyses of protein phosphorylation with either label-free or stable-isotope labeling were also employed in this study. The results indicated that level of phosphopeptides on five membrane proteins such as AHA1, STP1, Patellin-2, probable inactive receptor kinase (At3g02880), and probable purine permease 18 showed at least two-fold increase in comparison to control in response to 200 mM salt-stress.

Conclusion

In this study, we successfully identified novel salt stress-responsive protein phosphorylation sites from membrane isolates of abiotic-stressed plants by membrane shaving followed by Zr⁴⁺-IMAC enrichment. The identified phosphorylation sites can be important in the salt stress response in plants.

Development of wheat-Lophopyrum elongatum recombinant lines for enhanced sodium 'exclusion' during salinity stress

Lophopyrum elongatum (tall wheatgrass), a wild relative of wheat, can be used as a source of novel genes for improving salt tolerance of bread wheat. Sodium 'exclusion' is a major physiological mechanism for salt tolerance in a wheat-tall wheatgrass amphiploid, and a large proportion ( approximately 50%) for reduced Na(+) accumulation in the Xag leaf, as compared to wheat, was earlier shown to be contributed by genetic effects from substitution of chromosome 3E from tall wheatgrass for wheat chromosomes 3A and 3D. Homoeologous recombination between 3E and wheat chromosomes 3A and 3D was induced using the ph1b mutant, and putative recombinants were identified as having SSR markers specific for tall wheatgrass loci. As many as 14 recombinants with smaller segments of tall wheatgrass chromatin were identified and low-resolution breakpoint analysis was achieved using wheat SSR loci. Seven recombinants were identified to have leaf Na+ concentrations similar to those in 3E(3A) or 3E(3D) substitution lines, when grown in 200 mM NaCl in nutrient solution. Phenotypic analysis identified recombinants with introgressions at the distal end on the long arm of homoeologous group 3 chromosomes being responsible for Na(+) 'exclusion'. A total of 55 wheat SSR markers mapped to the long arm of homoeologous group 3 markers by genetic and deletion bin mapping were used for high resolution of wheat-tall wheatgrass chromosomal breakpoints in selected recombinants. Molecular marker analysis and genomic in situ hybridisation confirmed the 524-568 recombinant line as containing the smallest introgression of tall wheatgrass chromatin on the distal end of the long arm of wheat chromosome 3A and identified this line as suitable for developing wheat germplasm with Na(+) 'exclusion'.

Genetic technologies for the identification of plant genes controlling environmental stress responses

Abiotic conditions such as light, temperature, water availability and soil parameters determine plant growth and development. The adaptation of plants to extreme environments or to sudden changes in their growth conditions is controlled by a well balanced, genetically determined signalling system, which is still far from being understood. The identification and characterisation of plant genes which control responses to environmental stresses is an essential step to elucidate the complex regulatory network, which determines stress tolerance. Here, we review the genetic approaches, which have been used with success to identify plant genes which control responses to different abiotic stress factors. We describe strategies and concepts for forward and reverse genetic screens, conventional and insertion mutagenesis, TILLING, gene tagging, promoter trapping, activation mutagenesis and cDNA library transfer. The utility of the various genetic approaches in plant stress research we review is illustrated by several published examples.

Proteomic analysis on a high salt tolerance introgression strain of Triticum aestivum/Thinopyrum ponticum

Soil salinity is a major abiotic constraint to agricultural productivity. We successfully bred a new common wheat (Triticum aestivum L.) introgression variety (Shanrong No. 3) with high salt-tolerance via asymmetric somatic hybridization between common wheat cultivar (Jinan 177) and UV-irradiated Agropyron elongatum (Thinopyrum ponticum Podp). We report here a comparative proteomic analysis to investigate variety-specific and salt-responsive proteins between seedling-roots of Shanrong No. 3 and Jinan 177. In total, 114 spots reproducibly presented differential expression patterns on 2-DE maps. Of them, 34 were variety-specific and 49 were salt-responsive. We identified 110 spots by MALDI-TOF MS and partially confirmed by MALDI-TOF-TOF MS, and functionally classified them into signal transduction, transcription and translation, transporting, chaperones, proteolysis and detoxification, etc. Meanwhile, we also found the alteration of protein expression of Shanrong No. 3 through inhibition of old proteins and production of novel ones, change in abundance and sensitivity of some nonsalt-responsive and salt-responsive proteins, as well as PTMs. Furthermore, comparison between proteome and transcripteome using cDNA microarray showed that there were only 20 proteins with abundances correlative to signal densities of corresponding EST probes. This study gives us a global insight into proteomic difference between Shanrong No. 3 and Jinan 177 in constitute and to salt-response.

Calcium- and salt-stress signaling in plants: shedding light on SOS pathway

As salt stress imposes a major environmental threat to agriculture, understanding the basic physiology and genetics of cell under salt stress is crucial for developing any transgenic strategy. Salt Overly Sensitive (SOS) genes (SOS1-SOS3) were isolated through positional cloning. Since sos mutants are hypersensitive to salt, their characterization resulted in the discovery of a novel pathway, which has helped in our understanding the mechanism of salt-stress tolerance in plants. Genetic analysis confirmed that SOS1-SOS3 function in a common pathway of salt tolerance. This pathway also emphasizes the significance of Ca2+ signal in reinstating cellular ion homeostasis. SOS3, a Ca2+ sensor, transduces the signal downstream after activating and interacting with SOS2 protein kinase. This SOS3-SOS2 complex activates the Na+/H+ antiporter activity of SOS1 thereby reestablish cellular ion homeostasis. Recently, SOS4 and SOS5 have also been characterized. SOS4 encodes a pyridoxal (PL) kinase that is involved in the biosynthesis of pyridoxal-5-phosphate (PLP), an active form of vitamin B6. SOS5 has been shown to be a putative cell surface adhesion protein that is required for normal cell expansion. Under salt stress, the normal growth and expansion of a plant cell becomes even more important and SOS5 helps in the maintenance of cell wall integrity and architecture. In this review we focus on the recent advances in salt stress and SOS signaling pathway. A broad coverage of the discovery of SOS mutants, structural aspect of these genes and the latest developments in the field of SOS1-SOS5 has been described.

Na+ transport in plants

The ability of plants to grow in high NaCl concentrations is associated with the ability of the plants to transport, compartmentalize, extrude, and mobilize Na(+) ions. While the influx and efflux at the roots establish the steady state rate of entry of Na(+) into the plant, the compartmentation of Na(+) into the cell vacuoles and the radial transport of Na(+) to the stele and its loading into the xylem establish the homeostatic control of Na(+) in the cytosol of the root cells. Removal of Na(+) from the transpirational stream, its distribution within the plant and its progressive accumulation in the leaf vacuoles, will determine the ability to deal with the toxic effects of Na(+). The aim of this review is to highlight and discuss the recent progress in understanding of Na(+) transport in plants.