Academic literature on the topic 'Plant salinity tolerance; sodium transport; Arabidopsis'

Duckweeds are a family of freshwater angiosperms with morphology reduced to fronds and propagation by vegetative budding. Unlike other angiosperm plants such as Arabidopsis and rice that have physical barriers between their photosynthetic organs and soils, the photosynthetic organs of duckweeds face directly to their nutrient suppliers (waters), therefore, their responses to salinity may be distinct. In this research, we found that the duckweed Spirodela polyrhiza L. accumulated high content of sodium and reduced potassium and calcium contents in large amounts under salt stress. Fresh weight, Rubisco and AGPase activities, and starch content were significantly decreaseded in the first day but recovered gradually in the following days and accumulated more starch than control from Day 3 to Day 5 when treated with 100 mM and 150 mM NaCl. A total of 2156 differentially expressed genes were identified. Overall, the genes related to ethylene metabolism, major CHO degradation, lipid degradation, N-metabolism, secondary metabolism of flavonoids, and abiotic stress were significantly increased, while those involved in cell cycle and organization, cell wall, mitochondrial electron transport of ATP synthesis, light reaction of photosynthesis, auxin metabolism, and tetrapyrrole synthesis were greatly inhibited. Moreover, salt stress also significantly influenced the expression of transcription factors that are mainly involved in abiotic stress and cell differentiation. However, most of the osmosensing calcium antiporters (OSCA) and the potassium inward channels were downregulated, Na+/H+ antiporters (SOS1 and NHX) and a Na+/Ca2+ exchanger were slightly upregulated, but most of them did not respond significantly to salt stress. These results indicated that the ion homeostasis was strongly disturbed. Finally, the shared and distinct regulatory networks of salt stress responses between duckweeds and other plants were intensively discussed. Taken together, these findings provide novel insights into the underlying mechanisms of salt stress response in duckweeds, and can be served as a useful foundation for salt tolerance improvement of duckweeds for the application in salinity conditions.

Salinity is a natural and anthropogenic process that plants overcome using various responses. Salinity imposes a two-phase effect, simplified into the initial osmotic challenges and subsequent salinity-specific ion toxicities from continual exposure to sodium and chloride ions. Plant responses to salinity encompass a complex gene network involving osmotic balance, ion transport, antioxidant response, and hormone signaling pathways typically mediated by transcription factors. One particular transcription factor mega family, WRKY, is a principal regulator of salinity responses. Here, we categorize a collection of known salinity-responding WRKYs and summarize their molecular pathways. WRKYs collectively play a part in regulating osmotic balance, ion transport response, antioxidant response, and hormone signaling pathways in plants. Particular attention is given to the hormone signaling pathway to illuminate the relationship between WRKYs and abscisic acid signaling. Observed trends among WRKYs are highlighted, including group II WRKYs as major regulators of the salinity response. We recommend renaming existing WRKYs and adopting a naming system to a standardized format based on protein structure.

Seawater intrusion in coastal regions and waterlogging in salinized lands are serious constraints that reduce crop productivity under changing climate scenarios. Under these conditions, plants encounter flooding and salinity concurrently or sequentially. Identification and characterization of genes and pathways associated with both flooding and salinity adaptation are critical steps for the simultaneous improvement of plant tolerance to these stresses. The PROTEOLYSIS 6 (PRT6) branch of the N-degron pathway is a well-characterized process that negatively regulates flooding tolerance in plants. Here, we determined the role of the PRT6/N-degron pathway in salinity tolerance in Arabidopsis. This study demonstrates that the prt6 mutation enhances salinity tolerance at the germination, seedling, and adult plant stages. Maintenance of chlorophyll content and root growth under high salt in the prt6 mutant was linked with the restricted accumulation of sodium ions (Na+) in shoots and roots of the mutant genotype. The prt6 mutation also stimulated mRNA accumulation of key transcription factors in ABA-dependent and independent pathways of osmotic/salinity tolerance, accompanied by the prominent expression of their downstream genes. Furthermore, the prt6 mutant displayed increased sensitivity to ethylene and brassinosteroids, which can suppress Na+ uptake and promote the expression of stress-responsive genes. This study provides genetic evidence that both salinity and flooding tolerance is coordinated through a common regulatory pathway in Arabidopsis.

Rice tolerance to salinity stress involves diverse and complementary mechanisms, such as the regulation of genome expression, activation of specific ion-transport systems to manage excess sodium at the cell or plant level, and anatomical changes that avoid sodium penetration into the inner tissues of the plant. These complementary mechanisms can act synergistically to improve salinity tolerance in the plant, which is then interesting in breeding programs to pyramidize complementary QTLs (quantitative trait loci), to improve salinity stress tolerance of the plant at different developmental stages and in different environments. This approach presupposes the identification of salinity tolerance QTLs associated with different mechanisms involved in salinity tolerance, which requires the greatest possible genetic diversity to be explored. To contribute to this goal, we screened an original panel of 179 Vietnamese rice landraces genotyped with 21,623 SNP markers for salinity stress tolerance under 100 mM NaCl treatment, at the seedling stage, with the aim of identifying new QTLs involved in the salinity stress tolerance via a genome-wide association study (GWAS). Nine salinity tolerance-related traits, including the salt injury score, chlorophyll and water content, and K+ and Na+ contents were measured in leaves. GWAS analysis allowed the identification of 26 QTLs. Interestingly, ten of them were associated with several different traits, which indicates that these QTLs act pleiotropically to control the different levels of plant responses to salinity stress. Twenty-one identified QTLs colocalized with known QTLs. Several genes within these QTLs have functions related to salinity stress tolerance and are mainly involved in gene regulation, signal transduction or hormone signaling. Our study provides promising QTLs for breeding programs to enhance salinity tolerance and identifies candidate genes that should be further functionally studied to better understand salinity tolerance mechanisms in rice.

In this report, we test the hypothesis that Na+ accumulation in the shoot in maize is negatively correlated with salt tolerance. Salt tolerance is defined as a percentage of the control on a dry weight basis. Two hybrids (Pioneer hybrid 3578 and Pioneer hybrid 3772) differing widely in Na+ accumulation were compared. Plants were treated with two types of salinity for 15 days (80 mol m-3 NaCl or 80 mol m-3 NaCl plus 8.75 mol m-3 CaCl2). Ion concentrations (Na+, K+, Ca2+ and Cl-) were measured in the roots, stalks, sheaths and leaves of plants harvested every third day. Ion concentrations were significantly affected by the treatments. Na+ and Cl- concentrations increased with salinity treatments; K+ and Ca2+ concentrations decreased. Supplemental Ca2+ increased Ca2+ and decreased Na+ concentrations. Hybrid 3772 maintained very low Na+ concentrations in the shoots, whereas 3578 did not. The largest distinction between the hybrids was in the ability to transport Na+ to the shoot; hybrid 3578 transported Na+ at twice the rate of hybrid 3772. In general, ion transport to the shoot appeared to be a function of root ion concentration. This model could account for the effects of NaCl salinity and supplemental Ca2+ on ion transport, although Na+ transport was complicated by an apparent reabsorption mechanism in the root and mesocotyl. The lack of correlation of Na+ accumulation in the shoot and other ion parameters with growth indicated that the mineral nutrition of the plants was not correlated with salt tolerance. It was concluded that the growth response of maize to salinity was primarily affected by osmotic factors.

Stressors such as soil salinity and dehydration are major constraints on plant growth, causing worldwide crop losses. Compounding these insults, increasing climate volatility requires adaptation to fluctuating conditions. Salinity stress responses are relatively well understood in Arabidopsis thaliana, making this system suited for the rapid molecular dissection of evolutionary mechanisms. In a large-scale genomic analysis of Catalonian A. thaliana, we resequenced 77 individuals from multiple salinity gradients along the coast and integrated these data with 1,135 worldwide A. thaliana genomes for a detailed understanding of the demographic and evolutionary dynamics of naturally evolved salinity tolerance. This revealed that Catalonian varieties adapted to highly fluctuating soil salinity are not Iberian relicts but instead have immigrated to this region more recently. De novo genome assembly of three allelic variants of the high-affinity K+ transporter (HKT1;1) locus resolved structural variation between functionally distinct alleles undergoing fluctuating selection in response to seasonal changes in soil salinity. Plants harboring alleles responsible for low root expression of HKT1;1 and consequently high leaf sodium (HKT1;1HLS) were migrants that have moved specifically into areas where soil sodium levels fluctuate widely due to geography and rainfall variation. We demonstrate that the proportion of plants harboring HKT1;1HLS alleles correlates with soil sodium level over time, HKT1;1HLS-harboring plants are better adapted to intermediate levels of salinity, and the HKT1;1HLS allele clusters with high-sodium accumulator accessions worldwide. Together, our evidence suggests that HKT1;1 is under fluctuating selection in response to climate volatility and is a worldwide determinant in adaptation to saline conditions.

Salinity stress is one of the most important and common abiotic stress factors that cause significant physiological and metabolic changes in plants, negatively affecting plant growth and development, and causing decrease in product quality and quantity. The elucidation of the molecular control mechanisms associated with salt stress tolerance is based on the activation and /or inactivation of various stress-related genes. Salt Overly Sensitive (SOS) tolerance mechanism under salt stress is of great importance in terms of salt tolerance of the plants. Although this mechanism has been studied for many years, the physiological changes that the plants give as a result of mutation of the genes in the pathway under different levels of sodium chloride (NaCl) during development have not been examined comparatively. In this study, we found that the Arabidopsis thaliana sos1-1 mutant plant showed sensitivity to 10 mM NaCl while the sos3-1 and hkt1-1 mutants showed tolerance. The sos1-1, sos3-1 and hkt1-1 mutants showed increasing sensitivity when NaCl was applied beyon 50 mM of concentration. In addition, plants did not show significant sensitivity for 1 day of stress application, while significant effects were observed in plant root length when exposed to salinity for 3 to 4 days. Col-0, hkt1-1 and sos3-1 roots treated with low levels of NaCl for a short term were positively affected in length. In the light of these results, the amount and duration of salt stress is very critical in Arabidopsis thaliana's responses to the stress and determination of molecular tolerance pathways.

Plasma membrane intrinsic proteins (PIPs) are a subfamily of aquaporin proteins located on plasma membranes where they facilitate the transport of water and small uncharged solutes. PIPs play an important role throughout plant development, and in response to abiotic stresses. Jojoba (Simmondsia chinensis (Link) Schneider), as a typical desert plant, tolerates drought, salinity and nutrient-poor soils. In this study, a PIP1 gene (ScPIP1) was cloned from jojoba and overexpressed in Arabidopsis thaliana. The expression of ScPIP1 at the transcriptional level was induced by polyethylene glycol (PEG) treatment. ScPIP1 overexpressed Arabidopsis plants exhibited higher germination rates, longer roots and higher survival rates compared to the wild-type plants under drought and salt stresses. The results of malonaldehyde (MDA), ion leakage (IL) and proline content measurements indicated that the improved drought and salt tolerance conferred by ScPIP1 was correlated with decreased membrane damage and improved osmotic adjustment. We assume that ScPIP1 may be applied to genetic engineering to improve plant tolerance based on the resistance effect in transgenic Arabidopsis overexpressing ScPIP1.

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Soil salinity is responsible for significant reductions in crop yield. The salinity tolerance of crops can be improved by minimising the amount of sodium ions (Na⁺) accumulating in the shoot. One hypothesis for reducing shoot Na⁺ accumulation is to minimise Na⁺ entering the plant via the root. Previous studies indicate that in most plants, the majority of Na⁺ entry into root cells is through voltage-insensitive non-selective cation channels (vi-NSCCs), however, the molecular identities of these channels are unclear. Recently two genes that belong to the PQL family were identified as putative vi-NSCCs in yeast. This project aims to functionally characterise three orthologous PQL genes from Arabidopsis thaliana (AtPQL1, AtPQL2 and AtPQL3) and investigate their role in Na⁺ entry into cells and into roots. Bioinformatic tools and in planta techniques were used to determine gene expression profiles, analyse protein sequences and determine the cellular and subcellular localisations of AtPQL1-3. The plasma membrane localisation of AtPQL1 and 2 agrees with the proposed function of vi-NSCCs as ion transport channels. Furthermore, the suggested role of vi-NSCCs in facilitating initial Na⁺ entry into the roots was supported by in silico expression profiles of AtPQL2 and 3 and by observations of reporter proteins driven by PQL promoters in root tissues. Heterologous expression of AtPQL1 in yeast resulted in yeast which were more salt sensitive than controls, suggesting a role in Na⁺ influx into cells. Furthermore, this sensitivity could be ameliorated by the addition of CaCl₂, (indicating Ca²⁺ inhibited the movement of Na⁺), an attribute which corresponds with known properties of vi-NSCCs. A number of transgenic Arabidopsis lines were generated to have altered expression of AtPQL1 to 3 and were then phenotypically analysed in hydroponics under a range of salt treatments. Results of these experiments proved largely inconclusive primarily because individual plants with significantly altered expression of AtPQL1, 2 and/or 3 could not be obtained.
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2013

Chloride (Cl⁻) is an essential plant micronutrient, but is toxic when accumulated to high concentrations within the cytoplasm, especially in the shoot. Exclusion of Cl⁻ from the shoot is an important trait contributing to salinity tolerance of plants, particularly for Cl⁻ sensitive woody perennials (e.g. grapevine, citrus and avocado) and legumes (e.g. soybean and lotus), where Cl⁻ is considered to be more toxic than the sodium ion (Na⁺). To enhance plant salinity tolerance, it is necessary to understand the mechanisms of Cl⁻ transport through the plant and how it is regulated in response to salinity stress. However, when compared with Na⁺, much less is known about the transport processes involved in controlling Cl⁻ accumulation in the shoot. Two candidate genes encoding putative Cl⁻ transporters in Arabidopsis, proton dependent oligo-peptide transporter 1 (AtPOT1) and AtPOT2 were investigated to examine their role in controlling the loading of Cl⁻ into the apoplastic vessels of root xylem, and therefore Cl⁻ accumulation in the shoot. Transient expression of yellow fluorescent protein (YFP)::AtPOT1 or YFP::AtPOT2 in Arabidopsis mesophyll protoplasts, along with stable expression of green fluorescent protein (GFP)::AtPOT1 or GFP::AtPOT2 determined that both AtPOT1 and AtPOT2 are targeted to the plasma membrane, a location necessary for both POTs to be involved in facilitating Cl⁻ efflux from a cell. Promoter:UidA fusions showed that pAtPOT1 drives expression of the AtPOT1 predominantly in the root stelar cells, suggesting the involvement of AtPOT1 in long distance transport in vasculature tissue. In contrast, AtPOT2 was shown to be located in the cortex of the mature root. Use of quantitative real-time PCR to determine the levels of mRNA transcripts in response to salt stress demonstrated that AtPOT1 transcripts are significantly reduced by both salt and ABA treatments, whereas AtPOT2 transcripts are increased by salt stress. As AtPOT1 transcripts are reduced by ABA and as AtPOT1 encodes an anion transporter located at the plasma membrane of the cells bordering root xylem vessels, it is hypothesised that AtPOT1 is responsible, at least partially, for loading of Cl⁻ into the conductive cells of xylem in roots. Electrophysiological characterisation of AtPOT1 in Xenopus laevis oocytes showed that AtPOT1 is able to facilitate Cl⁻ efflux across the cell membrane at negative membrane potentials, suggesting a role of AtPOT1 in the efflux of Cl⁻ across the plasma membrane of xylem parenchyma cells into the apoplastic xylem transpiration stream. This flux was not affected by the changes in external pH, consistent with the Cl⁻ transport being a uniport, independent of the movement of H⁺. There were no knockout mutants of AtPOT1 available. Therefore, in order to test the effect of alterations of AtPOT1 expression on Cl⁻ accumulation in the shoot, artificial microRNA knockdown constructs were designed and used to transform Arabidopsis Col-0 plants. AtPOT1 transcripts were shown to be reduced by up to 80% in the knockdown lines when compared with nulls, which resulted in a reduction in shoot Cl⁻ concentration by up to 60%. AtPOT1 expression was found to be negatively correlated with shoot Cl⁻ concentration (R² = 0.77). Conversely, constitutive over expression of AtPOT1 increased shoot Cl⁻ accumulation, indicating the important role that AtPOT1 plays in facilitating Cl⁻ xylem loading in Arabidopsis. It is concluded that AtPOT1 mediates Cl⁻ flux into the conductive cells of root xylem in Arabidopsis and the expression of the AtPOT1 is down-regulated during salinity stress. Manipulations of AtPOT1 transcript levels altered shoot Cl⁻ concentrations, which could be utilised for enhancing shoot exclusion of Cl⁻, and hence plant salinity tolerance. Although more functional data is required, AtPOT2 might be involved in the efflux of Cl⁻ from the root to the soil. Therefore, reducing AtPOT1 expression and increasing AtPOT2 expression, may be two strategies for excluding Cl⁻ from the shoot under saline conditions.
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2013

Increasing salinity is a worldwide problem, but the knowledge on how salt enters the roots of plants remains largely unknown. Non-selective cation channels (NSCCs) have been suggested to be the major pathway for the entry of sodium ions (Na+) in several species. The hypothesis tested in this research is that PQ loop (PQL) proteins could form NSCCs, mediate some of the Na+ influx into the root and contribute to ion accumulation and the inhibition of growth in saline conditions. This is based on previous preliminary evidence indicating similarities in the properties of NSCC currents and currents mediated by PQL proteins, such as the inhibition of an inward cation current mediated by PQL proteins by high external calcium and pH acidification. PQL family members belonging to clade one in Arabidopsis and barley were characterized using a reverse genetics approach, electrophysiology and high-throughput phenotyping. Expression of AtPQL1a and HvPQL1 in HEK293 cells increased Na+ and K+ inward currents in whole cell membranes. However, when GFP-tagged PQL proteins were transiently overexpressed in tobacco leaf cells, the proteins appeared to localize to intracellular membrane structures. Based on q-RT-PCR, the levels of mRNA of AtPQL1a, AtPQL1b and AtPQL1c is higher in salt stressed plants compared to control plants in the shoot tissue, while the mRNA levels in the root tissue did not change in response to stress. Salt stress responses of lines with altered expression of AtPQL1a, AtPQL1b and AtPQL1c were examined using RGB and chlorophyll fluorescence imaging of plants growing in soil in a controlled environment chamber. Decreases in the levels of expression of AtPQL1a, AtPQL1b and AtPQL1c resulted in larger rosettes, when measured seven days after salt stress imposition. Interestingly, overexpression of AtPQL1a also resulted in plants having larger rosettes in salt stress conditions. Differences between the mutants and the wild-type plants were not observed at earlier stages, suggesting that PQLs might be involved in long-term responses to salt stress. These results contribute towards a better understanding of the role of PQLs in salinity tolerance and provide new targets for crop improvement.