Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake

Abstract

Stomatal movements rely on alterations in guard cell turgor. This requires massive K + bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K + into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K + / H + exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K + and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K + accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K + is a requirement for stomatal opening and a critical component in the overall K + homeostasis essential for stomatal closure, and suggest that vacuolar K + fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.stomata | luminal pH control T he rapid accumulation and release of K + and of organic and inorganic anions by guard cells controls the opening and closing of stomata and thereby gas exchange and transpiration of plants. The intracellular events that underlie stomatal opening start with plasma membrane hyperpolarization caused by the activation of H + -ATPases, which induces K + uptake through voltage-gated inwardly rectifying K + in channels (1). Potassium uptake is accompanied by the electrophoretic entry of the counterions chloride, nitrate, and sulfate, and by the synthesis of malate. These osmolytes, together with sucrose accumulation, increase the turgor in guard cells and thereby drive stomatal opening. Stomatal closure is initiated by activation of the plasma membrane localized chloride and nitrate efflux channels SLAC1 and SLAH3 that are regulated by the SnRK2 protein kinase OST1 and the Ca 2+ -dependent protein kinases CPK21 and 23 (2, 3). CPK6 also activates SLAC1 and coordinately inhibits rectifying K + in channels to hinder stomatal opening (4, 5). Sulfate and organic acids exit the guard cell through R-type anion channels. The accompanying reduction in guard cell turgor results in stomatal closure (1).Despite the established role of plasma membrane transport in guard cell function and stomatal movement, ion influx into the cytosol represents only a transit step to the vacuole, as more than 90% of the solutes released from guard cells originate from vacuoles (6). In contrast to the plasma membrane, knowledge of the transport processes occurring in intracellular compartments of guard cells during stomatal movements is less advanced (7). Only recently, AtALMT9 has been shown to act as a malateinduced chloride channel at the tonoplast that is required for stomatal opening (8). Vacuoles govern turgor-driven cha...

Pea lectin receptor-like kinase functions in salinity adaptation without yield penalty, by alleviating osmotic and ionic stresses and upregulating stress-responsive genes

“…At this point, increased activity of HKT1 could help to remove excess Na + from xylem stream by diverting it to the leaves (Apse and Blumwald 2007;Davenport et al 2007) where NHX1 and NHX2 could compartmentalize the Na + from cytoplasm to vacuole (Apse et al 1999;Venema et al 2002) thus preventing ion toxicity in cytosol of photosynthetic tissues (Munns and Tester 2008;Zhu 2001). Recent publications also indicate that K + , and not Na + , is the main substrate for NHXs (Bassil et al 2011;Barragán et al 2012;Andrés et al 2014). This proves added advantage due to higher accumulation of K + which would further lower Na + /K + ratio in plant tissues.…”

The Arabidopsis Vacuolar Sorting Receptor1 Is Required for Osmotic Stress-Induced Abscisic Acid Biosynthesis

“…There has been some other evidence to suggest that vacuolar trafficking machinery not only delivers cargo proteins to the vacuole, but also involves signal transduction and environmental stress responses (Surpin and Raikhel, 2004;Yamada et al, 2010;Andrés et al, 2014). For example, VSR3 is involved in ABAmediated stomatal guard cell movement and has an important role in responses to plant stress (Avila et al, 2008).…”

Rethinking Guard Cell Metabolism

“…Most of the ions taken up, or synthesized by guard cells, are sequestered into the vacuole (Barbier-Brygoo et al, 2011). As a result, the guard cell vacuoles undergo dynamic changes in volume and structure, which are crucial for achieving the full amplitude of stomatal movements (Gao et al, 2005;Tanaka et al, 2007;Andrés et al, 2014). During stomatal closure, guard cells reduce their volume through the release of ions into the cell wall and the consequent efflux of water.…”