Unexpected Connections between Humidity and Ion Transport Discovered Using a Model to Bridge Guard Cell-to-Leaf Scales

Abstract

Stomatal movements depend on the transport and metabolism of osmotic solutes that drive reversible changes in guard cell volume and turgor. These processes are defined by a deep knowledge of the identities of the key transporters and of their biophysical and regulatory properties, and have been modeled successfully with quantitative kinetic detail at the cellular level. Transpiration of the leaf and canopy, by contrast, is described by quasilinear, empirical relations for the inputs of atmospheric humidity, CO 2 , and light, but without connection to guard cell mechanics. Until now, no framework has been available to bridge this gap and provide an understanding of their connections. Here, we introduce OnGuard2, a quantitative systems platform that utilizes the molecular mechanics of ion transport, metabolism, and signaling of the guard cell to define the water relations and transpiration of the leaf. We show that OnGuard2 faithfully reproduces the kinetics of stomatal conductance in Arabidopsis thaliana and its dependence on vapor pressure difference (VPD) and on water feed to the leaf. OnGuard2 also predicted with VPD unexpected alterations in K + channel activities and changes in stomatal conductance of the slac1 Cl 2 channel and ost2 H + -ATPase mutants, which we verified experimentally. OnGuard2 thus bridges the micro-macro divide, offering a powerful tool with which to explore the links between guard cell homeostasis, stomatal dynamics, and foliar transpiration. INTRODUCTIONStomata provide the main pathway for CO 2 entry for photosynthesis and for transpirational water loss across the leaf epidermis. Pairs of guard cells surround each stoma, regulating the aperture to balance the often conflicting demands for CO 2 and for water conservation. Guard cells open and close the pore, driven by osmotic solute uptake and loss, notably of K + and Cl 2 , and by the synthesis and metabolism of organic solutes, especially sucrose (Suc) and malate (Mal) (Willmer and Fricker, 1996;Kim et al., 2010;Roelfsema and Hedrich, 2010;Lawson and Blatt, 2014;Jezek and Blatt, 2017). A number of well-defined signals, including light, CO 2 , and the water stress hormone abscisic acid (ABA), modulate transport and solute accumulation to alter cell volume, turgor, and stomatal aperture. Much research at the cellular level has focused on these inputs and their connection to stomatal movements, especially stomatal closing. Studies have highlighted both Ca 2+ -independent and Ca 2+ -dependent signaling, including elevated free cytosolic Ca 2+ concentration ([Ca 2+ ] i ), cytosolic pH (pH i ), protein kinases, and phosphatases, that inactivate inward-rectifying K + channels and activate Cl 2 channels and outward-rectifying K + channels to bias the membrane for solute loss (Blatt et al., 1990;Lemtiri-Chlieh and MacRobbie, 1994; Blatt, 1998, 1999;Marten et al., 2007;Assmann and Jegla, 2016;Jezek and Blatt, 2017).At the tissue and whole-plant levels, by contrast, attention has been drawn to inputs closely tied to photosynthesis, including tra...

Stomatal Response to Humidity: Blurring the Boundary between Active and Passive Movement

“…Thus, ABA is unlikely to contribute in any short-term response to VPD. In summary, the studies of Merilo et al (2018) and Wang et al (2017) are consistent with an alternative scheme (Fig. 1) in which neither OST1 nor ABA couple the VPD stimulus to stomatal response.…”

“…Perhaps the logic is simply misguided, and it is time to reframe the questions around solute and water flux. In a recent paper, Wang et al (2017) take this approach and, in the process, unify the macroscopic water relations and evapotranspiration of the whole plant with the microscopic events of subcellular transport in the guard cells. They extend the quantitative OnGuard systems platform by incorporating the concept of a VPD gradient that extends through the stomatal pore and into the substomatal cavity adjacent to the guard cells (Peak and Mott, 2011).…”

“…Using OnGuard2, Wang et al (2017) accurately predict changes in transport, stomatal aperture, and g s dynamics in response to VPD in wild-type Arabidopsis and in the slac1 and ost2 mutants that they subsequently validated experimentally. Both mutations affect solute flux rather than passive water flux per se.…”

“…OnGuard is a proven computational platform for modeling stomatal physiology that encompasses guard cell transport, signaling, and homeostasis, and has predicted stomatal behavior across species (Chen et al, 2012;Wang et al, 2012). In OnGuard2, Wang et al (2017) connect these processes, largely ignored in past models of g s (Dewar, 2002;Peak and Mott, 2011), with water in the guard cell wall. They allow the wall to equilibrate with the water vapor pressure in the substomatal cavity adjacent to the guard cells.…”

Communication between the Plasma Membrane and Tonoplast Is an Emergent Property of Ion Transport

“…The examples are for guard cells and the ensuing stomatal phenotypes, but the concepts apply equally to transport in other plant cell types. We use two studies (Wang et al, 2012(Wang et al, , 2017 to 1 This work was supported by the National Research Council of Thailand (Ph.D. studentship to W.H. ), the Biotechnology and Biological Sciences Research Council (grant nos.…”