CO2 Sensing and CO2 Regulation of Stomatal Conductance: Advances and Open Questions

Abstract

Guard cells form epidermal stomatal gas-exchange valves in plants and regulate the aperture of stomatal pores in response to changes in the carbon dioxide (CO2) concentration ([CO2]) in leaves. Moreover, the development of stomata is repressed by elevated CO2 in diverse plant species. Evidence suggests that plants can sense [CO2] changes via guard cells and via mesophyll tissues in mediating stomatal movements. We review new discoveries and open questions on mechanisms mediating CO2-regulated stomatal movements and CO2 modulation of stomatal development, which together function in the CO2 regulation of stomatal conductance and gas exchange in plants. Research in this area is timely in light of the necessity of selecting and developing crop cultivars that perform better in a shifting climate.

Keywords: agriculture; atmospheric carbon dioxide; climate; guard cell signaling; stomatal development; stomatal movements.

Figure 1. Change of leaf CO2 concentration (Ci) in response to light and darkness, atmospheric CO2 rise and stomatal gas exchange response to ambient [CO2] changes

(A) Effects of light on [CO₂] (C_i) in the Vicia faba substomatal cavity at an external ambient [CO₂] of 350 ppm. Sub-stomatal cavity carbon dioxide concentrations were measured with a potentiometric CO₂-biosensor microprobe inserted into leaves through open stomata [1] (Reprinted with permission from Elsevier publishing group). (B) Carbon dioxide concentrations in the earth’s atmosphere [2, 146]. (Reprinted with permission from: https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/co2_800k_zoom.png). (C–E) Effects of CO₂ on gas exchange in carbonic anhydrase mutant leaves. Raw (C and E) and normalized (D) stomatal conductance values for ca1 and ca4 single carbonic anhydrase mutants and wild type (WT). Individual ambient CO₂ treatments were each 30 minutes in duration. 1 leaf from each of 3 separate plants per genotype were analyzed over a period of 3 weeks and averaged. Errors represent s.e.m.

Figure 2. CO2 signaling pathway in stomatal movement regulation and putative convergence points with ABA signaling

(A) A simplified model for carbon dioxide and abscisic acid (ABA) signal transduction pathways in guard cells that mediate stomatal closure. The current model involves the enzymatic function of the carbonic anhydrases CA1 and CA4 with bicarbonate ions as intermediary signaling molecule. Downstream calcium, protein kinases, and ion channels are required for the stomatal closure response to CO₂. Note that amplifying signals from the mesophyll [60, 62] are not shown here for simplicity (see text for details). Different pathway components are color-coded for ease of viewing: ABA genes = green; CO₂ genes = brown; kinases = purple; channels = blue. Abbreviations: ABA = Abscisic acid; PYR/RCAR = ABA receptors; CO₂ = carbon dioxide; CA = carbonic anhydrase; HCO₃⁻ = bicarbonate Ca = calcium; ABI1 = protein phosphatase 2C. (B) The rate of cytosolic calcium transient production in guard cells of the Arabidopsis thaliana Col-0 ecotype are not significantly modulated by the CO₂ concentration, in contrast to the Ler ecotype. Cytosolic calcium transients in guard cells pre-exposed to low CO2, switched to high CO₂, then returned to low CO₂, at the indicated concentrations. Buffer composition was 10 mM KCl/50 mM CaCl2/10 mM Mes·Tris, pH 6.15. Data are the means of n=27 guard cells ±SE.

Figure 3. CO₂ control of stomatal development

A current framework [25, 120] includes EPF2, CRSP, the carbonic anhydrases CA1 and CA4 and the cell wall wax biosynthesis mutant hic [120]. The secreted extracellular protease CRSP cleaves and activates EPF2, which is involved in extracellular communication resulting in a modulatory repression of stomatal development in response to elevated CO₂. Experiments have shown that the ERECTA receptor kinase has specific EPF2 binding activity [128, 129]. Downstream signaling components include key stomatal fate transcriptional regulators (see: [113, 147]). Signaling components and mechanisms are indicated by question marks (see text for details). Abbreviations: CO₂ = carbon dioxide; CA = carbonic anhydrase; CRSP = CO₂-responsive secreted protease; EPF = epidermal patterning factor; HIC = high carbon dioxide mutant; HCO₃⁻ = bicarbonate.