Ion Transport at the Vacuole during Stomatal Movements.

Plant gas exchange with the environment is facilitated by stomata, small pores found on most aerial surfaces of land plants. Stomatal pores are formed between a pair of specialized guard cells. In C3 plants, open stomata allow the uptake of CO2 for photosynthesis during the day and at the same time the loss of water vapor, maintaining the transpiration stream. At night and during drought, plants close their stomata to conserve water, while they open them in response to low CO2 and at high temperatures to allow for evaporative cooling. The opening and closure of stomata depends on guard cell turgor, which, in turn, relies on fluxes of osmotically active solutes in and out of the guard cell. During stomatal opening, osmotically active solutes enter guard cells via the plasma membrane or are produced inside the cell and ultimately are stored in the vacuole (Roelfsema and Hedrich, 2005; Kollist et al., 2014; Santelia and Lawson, 2016). This causes water influx, a concomitant increase in turgor and swelling of the guard cell pair, resulting in the opening of the stomatal pore. K and its charge-balancing anions Cl, NO3 , and malate (Mal), as well as sugars, are the osmotica accumulating in guard cell vacuoles for stomatal opening. The ions are transported across the vacuolar membrane (also, the tonoplast) by ion channels and secondary active transporters, while vacuolar proton pumps generate the necessary proton motive force and acidify the vacuolar lumen. With the dawn of the patch-clamp technique, many ion channels were characterized in the 1980s and 1990s. A lot of these early studies were conducted with the broad bean Vicia faba and the monocotyledon dayflower plant Commelina communis because of the large size of their guard cells. It was only with the era of molecular genetics that the proteins responsible for the characterized transport were identified in Arabidopsis (Arabidopsis thaliana), and this review focuses almost exclusively on vacuolar transport proteins of this species with a role in stomatal physiology (Table I; Fig. 1). Nevertheless, many discoveries about guard cell function were made using V. faba, C. communis, and other species but were carried over into Arabidopsis guard cell research and now shape our reasoning. For example, Mal is often listed as an important osmoticum. This is true for some species such as V. faba (Outlaw and Lowry, 1977; Outlaw and Kennedy, 1978; Van Kirk and Raschke, 1978a, 1978b) and in certain growth conditions (Raschke and Schnabl, 1978; Van Kirk and Raschke, 1978a). But in Arabidopsis guard cells, on average, K is charge balanced to 50% by Cl and only to 5% by Mal, with Mal concentrations in the range of 1 to 2 mM (Negi et al., 2008; Monda et al., 2011, 2016; Takahashi et al., 2015). Nevertheless,