The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Abstract

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Figure 1.

Guard cells expressing the GORK K⁺ channel in the Arabidopsis leaf epidermis. A to D, Confocal images of the leaf epidermal surface of Arabidopsis stably transformed with GORK-GFP under the control of the Ubiquitin-10 promoter (Grefen et al., 2010b), showing the distribution of GORK-GFP (A), chloroplast autofluorescence (B), and the overlay of these images (C) with the corresponding bright-field image (D). Bar = 20 μm. E, Single optical section from a Z-stack through two kidney-shaped guard cells surrounding one stoma (center), showing the punctate distribution of GORK-GFP around the periphery of the two guard cells. Z-plane transects taken along the x axis at positions 1 to 5 are shown below. F, The full three-dimensional projection of the Z-stack clearly shows the punctate character of GORK localization and the prevalence of the channel at the junctions between the two guard cells. Bar = 5 μm. Data are from C. Eisenach, Ph.D. thesis. See Eisenach et al. (2014) for further details.

Figure 2.

The syp121 SNARE mutation slows stomatal reopening and shows a strong growth phenotype at moderate relative humidities. A, Stomatal apertures normalized to values at time zero for stomata from the wild type and the SYP121-complemented syp121 mutant (black circles) and the mutants syp121 (white circles) and syp122 (black triangles) before, during, and after the closing stimulus of elevated CaCl₂ outside (gray bar). B, Arabidopsis wild-type, syp121, and syp122 plants grown for 3 weeks under 150 µmol m⁻² s⁻¹ light and relative humidities (RH) of 95% and 55%. (This figure was modified from Eisenach et al., 2012.)

Figure 3.

Voltage-dependent conformation of a K⁺ channel VSD regulates secretion. Coexpression of the VSD (VSD^wt) of the K⁺ channel KC1 and its mutant VSD^D132E rescues secretory traffic block by the SYP121 soluble domain SYP121^ΔC. Coexpression with the mutant VSD^D132E, which locks the VSD in the open-channel configuration, also rescues secretory traffic. Traffic is not rescued by coexpression with VSD^F129W, which locks the VSD in the closed-channel configuration, nor with VSD^wt in 50 mm KCl, which depolarizes the plasma membrane. Images are projections of Arabidopsis roots transiently transformed using the tetracistronic vector pTecG-2in1-CC (Karnik et al., 2013) carrying secretory marker secYFP, GFP-HDEL as a transformation marker and ratiometric reference, SYP121^ΔC, and VSD^wt, VSD^F129W, or VSD^D132E in 1 and 50 mm KCl (left). Bright-field images are single medial plane images with fluorescence overlaid. VSD structures are shown in the closed, open, and again closed conformations (right, top to bottom) corresponding to the conditions and VSD constructs used. For clarity, only water molecules (light blue) on either side (in and out) of the membrane are shown. VSD transmembrane α-helices are color coded in green (S1), black (S2), red (S3), and yellow (S4). The RYxxWE motif that forms the binding site for SYP121 is shown with stick representations. Bar = 100 μm. (This figure was modified from Grefen et al. 2015, and Karnik et al. 2017.)