Salinity tolerance in plants. Quantitative approach to ion transport starting from halophytes and stepping to genetic and protein engineering for manipulating ion fluxes

Abstract

Ion transport is the fundamental factor determining salinity tolerance in plants. The Review starts from differences in ion transport between salt tolerant halophytes and salt-sensitive plants with an emphasis on transport of potassium and sodium via plasma membranes. The comparison provides introductory information for increasing salinity tolerance. Effects of salt stress on ion transport properties of membranes show huge opportunities for manipulating ion fluxes. Further steps require knowledge about mechanisms of ion transport and individual genes of ion transport proteins. Initially, the Review describes methods to measure ion fluxes, the independent set of techniques ensures robust and reliable basement for quantitative approach. The Review briefly summarizes current data concerning Na+ and K+ concentrations in cells, refers to primary thermodynamics of ion transport and gives special attention to individual ion channels and transporters. Simplified scheme of a plant cell with known transport systems at the plasma membrane and tonoplast helps to imagine the complexity of ion transport and allows choosing specific transporters for modulating ion transport. The complexity is enhanced by the influence of cell size and cell wall on ion transport. Special attention is given to ion transporters and to potassium and sodium transport by HKT, HAK, NHX, and SOS1 proteins. Comparison between non-selective cation channels and ion transporters reveals potential importance of ion transporters and the balance between the two pathways of ion transport. Further on the Review describes in detail several successful attempts to overexpress or knockout ion transporters for changing salinity tolerance. Future perspectives are questioned with more attention given to promising candidate ion channels and transporters for altered expression. Potential direction of increasing salinity tolerance by modifying ion channels and transporters using single point mutations is discussed and questioned. An alternative approach from synthetic biology is to create new regulation networks using novel transport proteins with desired properties for transforming agricultural crops. The approach had not been widely used earlier; it leads also to theoretical and pure scientific aspects of protein chemistry, structure-function relations of membrane proteins, systems biology and physiology of stress and ion homeostasis. Summarizing, several potential ways are aimed at required increase in salinity tolerance of plants of interest.

Making Plants Break a Sweat: the Structure, Function, and Evolution of Plant Salt Glands

“…The vacuolar H + -ATPase is encoded by multiple genes coding for distinct essential subunits while the vacuolar H + -pyrophosphatase can be generated by a single gene. Additionally, both of these may have variable gene copy numbers for each subunit or protein in different species (Silva and Gerós, 2009; Fuglsang et al, 2011; Volkov, 2015). For example, salt gland bladder cells in Mesembryanthemum crystallinum in response to salt stress showed significantly higher expression for 10 transcripts coding for different subunits of the vacuolar H + -ATPase, while two transcripts likely encoding two copies for the vacuolar H + -pyrophosphatase showed downregulation (Oh et al, 2015).…”

“…The use of halophytes to study these processes is rare (see reviews Flowers et al, 2010; Shabala et al, 2015; Volkov, 2015), and the targeted use of specialized structures such as salt glands to study salt exclusion in a molecular genetic framework is even less common. The scarcity of genetic, cellular, or biochemical research on salt glands could be due to their occurrence on diverse taxa in plant families that are ecologically important, but not economically valued as crops.…”

Assessment of Stress Tolerance, Productivity, and Forage Quality in T1 Transgenic Alfalfa Co-overexpressing ZxNHX and ZxVP1-1 from Zygophyllum xanthoxylum

“…Because vacuolar compartmentation of cations (such as, Na + and K + ) is mediated by NHXs and H + -PPase provides the proton motive force for this process as a tonoplast H + pump (Apse et al, 1999; Zhang et al, 2001; Gaxiola et al, 2002; Kronzucker and Britto, 2011; Yamaguchi et al, 2013). This mechanism may contribute to alleviating the toxicity of excessive Na + in the cytosol, maintaining intracellular K + /Na + homeostasis, and enhancing vacuolar osmoregulatory capacity (Apse et al, 1999; Blumwald, 2000; Gaxiola et al, 2007; Flowers et al, 2015; Volkov, 2015; Yuan et al, 2015). In our previous study, the co-overexpression of ZxNHX and ZxVP1-1 genes resulted in higher Na + , K + , and Ca 2+ accumulation in leaves and roots of T 0 generation transgenic alfalfa (Bao et al, 2016).…”

Comparative study of salt stress-induced physiological and molecular responses in tomato (Solanum lycopersicum L.)

“…For example, wheat can survive with 25% reduction of growth on 120 mM NaCl with internal 20 mM Na + and high K + while saltbush maintained normal ion homeostasis on 400 mM NaCl (Munns 2002). But the third strategy is individual for halophytes classified as includers and Na + is actively transported within the plant (Flowers et al, 1977;Volkov, 2015).…”