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      Plant Salinity Stress: Many Unanswered Questions Remain


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          Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na + and Cl uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na + and Cl uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K + in plant salinity stress are discussed.

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          Regulation of SOS1, a plasma membrane Na+/H+ exchanger in Arabidopsis thaliana, by SOS2 and SOS3.

          Maintaining low levels of sodium ions in the cell cytosol is critical for plant growth and development. Biochemical studies suggest that Na(+)/H(+) exchangers in the plasma membrane of plant cells contribute to cellular sodium homeostasis by transporting sodium ions out of the cell; however, these exchangers have not been identified at the molecular level. Genetic analysis has linked components of the salt overly sensitive pathway (SOS1-3) to salt tolerance in Arabidopsis thaliana. The predicted SOS1 protein sequence and comparisons of sodium ion accumulation in wild-type and sos1 plants suggest that SOS1 is involved directly in the transport of sodium ions across the plasma membrane. To demonstrate the transport capability of SOS1, we studied Na(+)/H(+)-exchange activity in wild-type and sos plants using highly purified plasma membrane vesicles. The results showed that plasma membrane Na(+)/H(+)-exchange activity was present in wild-type plants treated with 250 mM NaCl, but this transport activity was reduced by 80% in similarly treated sos1 plants. In vitro addition of activated SOS2 protein (a protein kinase) increased Na(+)/H(+)-exchange activity in salt-treated wild-type plants 2-fold relative to transport without added protein. However, the addition of activated SOS2 did not have any stimulatory effect on the exchange activity in sos1 plants. Although vesicles of sos2 and sos3 plants had reduced plasma membrane Na(+)/H(+)-exchange activity, transport activity in both increased with the addition of activated SOS2 protein. These results demonstrate that SOS1 contributes to plasma membrane Na(+)/H(+) exchange and that SOS2 and SOS3 regulate SOS1 transport activity.
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            Potassium transport and plant salt tolerance.

            Salinity is a major abiotic stress affecting approximately 7% of the world's total land area resulting in billion dollar losses in crop production around the globe. Recent progress in molecular genetics and plant electrophysiology suggests that the ability of a plant to maintain a high cytosolic K+/Na+ ratio appears to be critical to plant salt tolerance. So far, the major efforts of plant breeders have been aimed at improving this ratio by minimizing Na+ uptake and transport to shoot. In this paper, we discuss an alternative approach, reviewing the molecular and ionic mechanisms contributing to potassium homeostasis in salinized plant tissues and discussing prospects for breeding for salt tolerance by targeting this trait. Major K+ transporters and their functional expression under saline conditions are reviewed and the multiple modes of their control are evaluated, including ameliorative effects of compatible solutes, polyamines and supplemental calcium. Subsequently, the genetic aspects of inheritance of K+ transport 'markers' are discussed in the general context of salt tolerance as a polygenic trait. The molecular identity of 'salt tolerance' genes is analysed, and prospects for future research and breeding are examined.
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              Overexpression of a plasma membrane Na+/H+ antiporter gene improves salt tolerance in Arabidopsis thaliana.

              High concentrations of Na+ in saline soils inhibit plant growth and reduce agricultural productivity. We report here that CaMV 35S promoter driven overexpression of the Arabidopsis thaliana SOS1 gene, which encodes a plasma membrane Na+/H+ antiporter, improves plant salt tolerance in A. thaliana. Transgenic plants showed substantial upregulation of SOS1 transcript levels upon NaCl treatment, suggesting post-transcriptional control of SOS1 transcript accumulation. In response to NaCl treatment, transgenic plants overexpressing SOS1 accumulated less Na+ in the xylem transpirational stream and in the shoot. Undifferentiated callus cultures regenerated from the transgenic plants were also more tolerant of salt stress, which was correlated with reduced Na+ content in the transgenic cells. These results show that improved salt tolerance could be achieved by limiting Na+ accumulation in plant cells.

                Author and article information

                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                15 February 2019
                : 10
                [1] 1 Department of Plant Food Products and Biofortification, Institute of Food Biotechnology and Genomics NAS of Ukraine , Kyiv, Ukraine
                [2] 2 Department of Biology , University of York , York, United Kingdom
                Author notes

                Edited by: Meixue Zhou, University of Tasmania, Australia

                Reviewed by: Jayakumar Bose, University of Adelaide, Australia; Francisco Rubio, Centro de Edafología y Biología Aplicada del Segura (CEBAS), Spain

                *Correspondence: Stanislav V. Isayenkov, stan.isayenkov@ 123456gmail.com

                This article was submitted to Plant Abiotic Stress, a section of the journal Frontiers in Plant Science

                Copyright © 2019 Isayenkov and Maathuis.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 2, Tables: 0, Equations: 0, References: 146, Pages: 11, Words: 10809
                Plant Science

                Plant science & Botany
                salt stress,role of k+,transport of na+ and cl−,mechanisms of salt tolerance,membrane transporters,ion uptake,symplastic and apoplastic pathway


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