A Dual Role for the OsK5.2 Ion Channel in Stomatal Movements and K⁺ Loading into Xylem Sap

Abstract

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...

The Arabidopsis AtUNC-93 Acts as a Positive Regulator of Abiotic Stress Tolerance and Plant Growth via Modulation of ABA Signaling and K+ Homeostasis

“…The expression of AtUNC-93 was detected mainly in the vascular tissues ( Figure 1D ), suggesting that AtUNC-93 may have the similar role as SKOR in K + translocation toward the shoots. This regulation of K + translocation from roots to shoots was recently reported in Arabidopsis by transporter NRT1.5 and in rice by outward Shaker K + channel OsK5.2 ( Li et al, 2017 ; Nguyen et al, 2017 ). However, the AtUNC-93 -overexpressing plants did not show significant phenotype differences compared with wild type plants ( Figures 7A,B ).…”

Potassium in the Grape (Vitis vinifera L.) Berry: Transport and Function

“…K + in guard cells is important for regulating the aperture of stomata. K + is accumulated into the guard cell and this draws in water from the surrounding cells, opening the stomata ( Nguyen et al, 2017 ). K + channel activity is known to be regulated by ABA in guard cells in a Ca 2+ dependent ( Köhler et al, 2003 ) or independent ( Blatt, 2000 ) manner.…”

Xylem Ion Loading and Its Implications for Plant Abiotic Stress Tolerance

“…While OsK5.1 is mainly expressed in root vasculature (Kim et al, 2015), like AtSKOR. OsK5.2 is expressed in both roots and shoots (Kim et al, 2015) Similar to AtSKOR, OsK5.2 mediates K + translocation into the xylem sap in the root, and like the guard cell-localised K + outward rectifying channel AtGORK (Ache et al, 2000;Ivashikina et al, 2001), it is involved in K + release from guard cells and stomatal movements (Nguyen et al, 2017). Thus, one channel in rice, AsK5.2, combines the functions of two Arabidopsis homologs, AtSKOR and AtGORK.…”