Since a waxy cuticle covers outer leaf tissues, water vapor diffusion into the atmosphere 71 occurs mainly through the stomatal pores at the leaf surface. The size of the stomatal aperture is 72 tightly regulated to optimize gas exchanges between the leaf inner tissues and the atmosphere, 73 including CO 2 intake for photosynthesis and water loss by transpiration (Lawson and Blatt, 74 2014). This is achieved by fine tuning of the turgor pressure of the two guard cells which 75 surround the stomatal pore, and involves a complex coordinated activity of transport systems at 76 the guard cell plasma membrane and vacuolar membrane (Hedrich, 2012; Chen et al., 2012; Hills 77 et al., 2012;Kollist et al., 2014 4 nutrients from the roots, which take up these nutrients, to the aerial parts, to support plant growth 79 (Marschner et al., 1996). 80Potassium ion (K + ), as a major inorganic constituent of the plant cells and the most abundant 81 cation in the cytosol, is an essential macronutrient for growth and development. It is involved in 82 various functions including electrical neutralization of negative charges, control of cell 83 membrane polarization and osmoregulation (Clarkson and Hanson, 1980; Leigh and Wyn Jones, 84 1984). K + is thus the main cation absorbed by the roots and circulating within the plant at the 85 cellular or long distance levels. In guard cells, it is well known as a major contributor, with Cl -, 86 NO 3 -and malate, to the osmolarity (Raschke and Schnabl, 1978; Willner and Fricker, 1996). 87Stomatal opening is initiated by activation of plasma membrane proton pumps in guard cells, 88 which promotes K + influx through voltage-gated inward K + channels, as well as anion uptake 89 through H + -anion symporters (Blatt, 1987a; Schroeder et al., 1987;Roelfsema and Prins, 1997; 90 Talbott and Zeiger, 1998; Guo et al., 2003; Jezek and Blatt, 2017). Conversely, stomatal closure 91 requires inhibition of proton pumping at the guard cell membrane and activation of both anion 92 channels and voltage-gated outward K + channels. 93The molecular mechanisms responsible for inward and outward K + fluxes across the plasma 94 membrane have been extensively investigated in Arabidopsis. Shaker channel subunits, present as 95 a 9-member family in Arabidopsis, have been shown to form the major pathways for these fluxes 96 throughout the plant (Véry and Sentenac, 2003). In the Arabidopsis model species, four genes 97 encoding Shaker channel subunits have been identified as playing a major role in root to shoot K + 98 translocation and in stomatal movements. The SKOR subunit, which is expressed in root 99 pericycle and xylem parenchyma, forms outwardly-rectifying channels involved in K + secretion 100 into the xylem sap (Gaymard et al., 1998). In stomata, the inward Shaker channel subunits KAT1 101 and KAT2 are involved in guard cell K + uptake, and the outward Shaker channel GORK 102 mediates guard cell K + release (Ache et al., 2000;Pilot et al., 2001;Szyroki et al., 2001; Hosy et 103 al., 2003;Lebaudy...
Soil salinity constitutes a major environmental constraint to crop production worldwide. Leaf K + /Na + homoeostasis, which involves regulation of transpiration, and thus of the xylem sap flow, and control of the ionic composition of the ascending sap, is a key determinant of plant salt tolerance. Here, we show, using a reverse genetics approach, that the outwardly rectifying K + -selective channel OsK5.2, which is involved in both K + release from guard cells for stomatal closure in leaves and K + secretion into the xylem sap in roots, is a strong determinant of rice salt tolerance (plant biomass production and shoot phenotype under saline constraint). OsK5.2 expression was upregulated in shoots from the onset of the saline treatment, and OsK5.2 activity in guard cells led to a fast decrease in transpirational water flow and, therefore, reduced Na + translocation to shoots. In roots, upon saline treatment, OsK5.2 activity in xylem sap K + loading was maintained, and even transiently increased, outperforming the negative effect on K + translocation to shoots resulting from the reduction in xylem sap flow. Thus, the overall activity of OsK5.2 in shoots and roots, which both reduces Na + translocation to shoots and benefits shoot K + nutrition, strongly contributes to leaf K + /Na + homoeostasis.
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