Stomatal Spacing Safeguards Stomatal Dynamics by Facilitating Guard Cell Ion Transport Independent of the Epidermal Solute Reservoir

Abstract

Stomata enable gaseous exchange between the interior of the leaf and the atmosphere through the stomatal pore. Control of the pore aperture depends on osmotic solute accumulation by, and its loss from the guard cells surrounding the pore. Stomata in most plants are separated by at least one epidermal cell, and this spacing is thought to enhance stomatal function, although there are several genera that exhibit stomata in clusters. We made use of Arabidopsis (Arabidopsis thaliana) stomatal patterning mutants to explore the impact of clustering on guard cell dynamics, gas exchange, and ion transport of guard cells. These studies showed that stomatal clustering in the Arabidopsis too many mouths (tmm1) mutant suppressed stomatal movements and affected CO 2 assimilation and transpiration differentially between dark and light conditions and were associated with alterations in K + channel gating. These changes were consistent with the impaired dynamics of tmm1 stomata and were accompanied by a reduced accumulation of K + ions in the guard cells. Our findings underline the significance of spacing for stomatal dynamics. While stomatal spacing may be important as a reservoir for K + and other ions to facilitate stomatal movements, the effects on channel gating, and by inference on K + accumulation, cannot be explained on the basis of a reduced number of epidermal cells facilitating ion supply to the guard cells.Stomata are pores found in the epidermis of most aerial parts of plants and are formed between a specialized pair of cells, the guard cells. Stomata facilitate the uptake of CO 2 at the expense of water vapor release via transpiration (Hetherington and Woodward, 2003). Hence, stomata play a crucial role in the physiology of plants. They permit gaseous exchange between the environment and the inside of the leaf for photosynthesis and, in turn, they influence the water use efficiency and growth of the plant. Mathematical models have suggested that historical changes in the freshwater resources can be attributed to stomatal transpiration, and it has been argued that the manipulation of stomata will be an important factor in ensuring water availability over the next 20 to 30 years (UNESCO World Water Development Report, 2015). Efforts to develop crops with higher water use efficiency through conventional breeding strategies have led to some successes, including the Drysdale wheat (Triticum aestivum) variety (Condon et al., 2002). It is likely that further advances will be possible as we gain insights into the physiology of stomata in situ.The regulation of gas exchange is achieved by controlling the stomatal pore. Stomata respond dynamically to environmental changes, including light quality and intensity, ambient CO 2 concentration, and humidity (Aphalo and Jarvis, 1991;Hetherington and Woodward, 2003;Shimazaki et al., 2007). Stomatal movements result from changes in guard cell volume and turgor and are driven by solute and water fluxes across the plasma membrane and tonoplast of guard cells. In particular, fluxes of ...

Diurnal Variation in Gas Exchange: The Balance between Carbon Fixation and Water Loss

“…The reduced stomatal opening in the genotypes with patterning defects was reportedly due to mechanical failure of the guard cells of one or more of the following: (1) impaired guard cell function due to a lack of ions from epidermal cells for osmotic function (Outlaw, 1989); (2) competition between adjacent guard cells through increasing turgor pressure, creating opposing forces between the two guard cells; and (3) disruption to the signaling mechanism that determines the structure of the guard cells . Papanatsiou et al (2016) confirmed that incorrect stomatal patterning impacted guard cell dynamics in the cluster mutant too many mouths and showed that this was accompanied by a reduction in K + accumulation in Figure 1. Interspecific diversity of (A) g s , (B) A, and (C) intrinsic water use efficiency (W i ) of Phaseolus vulgaris, Vicia faba, Triticum aestivum, and Nicotiana tabacum in response to a diurnal (8-h) sinusoidal variation of light intensity (from 0 to 2,000 mmol m 22 s 21 ).…”

Leaf Hydraulic Architecture and Stomatal Conductance: A Functional Perspective

“…The one-cell-spacing rule between any two stomata, which may derive from requirements for solute reservoirs or potassium ion channel function, as well as space requirements for other cell types such as epidermal hairs (Bergmann and Sack, 2007;Papanatsiou et al, 2016) explain why stomatal index is more highly conserved between different environments than stomatal density (the number per unit area). As with increases in vein density, increasing stomatal density while holding stomatal size constant has a diminishing effect on total conductance, as the concentration gradients around each pore increasingly overlap (Ting and Loomis, 1965;Lehmann and Or, 2015).…”

Origins and Evolution of Stomatal Development

“…In Arabidopsis, the control of stomatal patterning has been shown to directly influence plant gas exchange, photosynthetic function, and productivity Franks and Casson, 2014;Franks et al, 2015;Lehmann and Or, 2015;Papanatsiou et al, 2016). In particular, correct spacing via alterations to stomatal size and density ensures optimal guard cell pore control and faster responses to environmental cues .…”