Vacuolar calcium channels

Abstract

The central vacuole is the largest Ca2+ store in a mature plant cell. Ca2+ release from this store contributes to Ca2+-mediated intracellular signalling in a variety of physiological responses. However, the routes for vacuolar Ca2+ release are not well characterized. To date, at least two voltage-dependent and two ligand-gated Ca2+-permeable channels have been reported in plant vacuoles. However, the so-called VVCa (vacuolar voltage-gated Ca2+) channel most probably is not a separate channel but is identical to another voltage-dependent channel-the so-called SV (slow vacuolar) channel. Studies in the last few years have added a new dimension to our knowledge of SV channel-mediated ion transport and the mechanisms of its regulation by multiple natural factors. Recently, the SV channel was identified as the product of the TPC1 gene in Arabidopsis. In contrast, the TPC1 channel from other species was thought to be localized in the plasma membrane. A re-evaluation of this work under the assumption that the TPC1 channel is generally a vacuolar channel provides interesting insights into the physiological function of the TPC1/SV channel. Considerably less is known about vacuolar Ca2+ channels that are supposed to be activated by inositol 1,4,5-trisphosphate or cADP ribose. The major problems are controversial reports about functional characteristics, and a remarkable lack of homologues of animal ligand-gated Ca2+ channels in higher plants. To help understand Ca2+-mediated intracellular signalling in plant cells, a critical update of existing experimental evidence for vacuolar Ca2+ channels is presented.

A Novel Calcium Binding Site in the Slow Vacuolar Cation Channel TPC1 Senses Luminal Calcium Levels

“…With submillimolar to low millimolar free Mg 2+ concentrations inside the vacuole (Shaul, 2002), a background of 2 mM vacuolar Mg 2+ resembles physiological conditions. Variation of vacuolar Ca 2+ concentrations from 0.1 to 10 mM covers the extremes of reported vacuolar free Ca 2+ concentrations (Pottosin and Schö nknecht, 2007).…”

“…Among different cations, potassium, sodium, and calcium can accumulate in large quantities herein (Taiz, 1992;Bethmann et al, 1995). Potassium, as well as sodium content (under salt stress), can reach levels greater than 100 mM (Beyhl et al, 2009;Rienmü ller et al, 2010), whereas luminal free calcium concentrations can get to 1 mM (Pottosin and Schö nknecht, 2007). While the vacuolar Ca 2+ concentration is more than 1000-fold higher compared with the cytosol, fluctuations of the extracytosolic Ca 2+ concentration may exert biological actions, as has been demonstrated for extracellular calcium fluctuations (Han et al, 2003;Hofer, 2005).…”

Stomatal Size, Speed, and Responsiveness Impact on Photosynthesis and Water Use Efficiency

“…Instead, solute uptake and loss, and stomatal movements, arise through a dynamic balance in solute flux (Blatt and Armstrong, 1993;Gradmann et al, 1993;Chen et al, 2012c). A second point arises from the general finding that the ion transport underpinning stomatal movements reflects a small fraction only of the maximal capacity of the several transporters mediating these fluxes (Blatt et al, 1990;Thiel et al, 1992;Hamilton et al, 2000;Pottosin and Schönknecht, 2007;DeAngeli et al, 2009). The activities of these transporters are kinetically limited by their inherent gating or other kinetic properties within the range of voltages typical for the plasma membrane and tonoplast.…”

Ion Transport at the Vacuole during Stomatal Movements

“…Details of these intracellular Ca 2+ signals are reviewed elsewhere (Pottosin and Schönknecht, 2007;Dodd et al, 2010;Roelfsema and Hedrich, 2010;Jezek and Blatt). Suffice it to say that cytosolic free Ca 2+ oscillations rely on Ca 2+ release from and rapid reuptake into the vacuole and/or other endomembrane stores, where the concentration of free Ca 2+ is about 1,000 times higher than in the cytosol (see refs.…”

“…As is the case for the plasma membrane, our knowledge of the molecular identity of guard cell Ca 2+ transporters in the tonoplast is poor. There are two Ca 2+ uptake systems in plant vacuoles: P-type Ca 2+ pumps of the ACA (Autoinhibited Ca 2+ ATPases) family, with a high affinity, and H + /Ca 2+ antiporters of the CAX (Cation Exchanger) family, with a lower Ca 2+ affinity (Pottosin and Schönknecht, 2007). However, to date, a clear role for these transporters in guard cell Ca 2+ signaling has not been established.…”