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Abstract
<p class="first" id="d868667e59">Salt stress is a major environmental factor limiting
plant growth and productivity.
A better understanding of the mechanisms mediating salt resistance will help researchers
design ways to improve crop performance under adverse environmental conditions. Salt
stress can lead to ionic stress, osmotic stress and secondary stresses, particularly
oxidative stress, in plants. Therefore, to adapt to salt stress, plants rely on signals
and pathways that re-establish cellular ionic, osmotic, and reactive oxygen species
(ROS) homeostasis. Over the past two decades, genetic and biochemical analyses have
revealed several core stress signaling pathways that participate in salt resistance.
The Salt Overly Sensitive signaling pathway plays a key role in maintaining ionic
homeostasis, via extruding sodium ions into the apoplast. Mitogen-activated protein
kinase cascades mediate ionic, osmotic, and ROS homeostasis. SnRK2 (sucrose nonfermenting
1-related protein kinase 2) proteins are involved in maintaining osmotic homeostasis.
In this review, we discuss recent progress in identifying the components and pathways
involved in the plant's response to salt stress and their regulatory mechanisms. We
also review progress in identifying sensors involved in salt-induced stress signaling
in plants.
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Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mm NaCl or more, constitute about 1% of the world's flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na+, K+ and Cl- and the synthesis of organic 'compatible' solutes, even where salt glands are operative. Although there is evidence that different species may utilize different transporters in their accumulation of Na+, in general little is known of the proteins and regulatory networks involved. Consequently, it is not yet possible to assign molecular mechanisms to apparent differences in rates of Na+ and Cl- uptake, in root-to-shoot transport (xylem loading and retrieval), or in net selectivity for K+ over Na+. At the cellular level, H+-ATPases in the plasma membrane and tonoplast, as well as the tonoplast H+-PPiase, provide the trans-membrane proton motive force used by various secondary transporters. The widespread occurrence, taxonomically, of halophytes and the general paucity of information on the molecular regulation of tolerance mechanisms persuade us that research should be concentrated on a number of 'model' species that are representative of the various mechanisms that might be involved in tolerance.
Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.
In Arabidopsis thaliana, the SOS1 (Salt Overly Sensitive 1) locus is essential for Na(+) and K(+) homeostasis, and sos1 mutations render plants more sensitive to growth inhibition by high Na(+) and low K(+) environments. SOS1 is cloned and predicted to encode a 127-kDa protein with 12 transmembrane domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the C-terminal part. The transmembrane region of SOS1 has significant sequence similarities to plasma membrane Na(+)/H(+) antiporters from bacteria and fungi. Sequence analysis of various sos1 mutant alleles reveals several residues and regions in the transmembrane as well as the tail parts that are critical for SOS1 function in plant salt tolerance. SOS1 gene expression in plants is up-regulated in response to NaCl stress. This up-regulation is abated in sos3 or sos2 mutant plants, suggesting that it is controlled by the SOS3/SOS2 regulatory pathway.
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