Deep dive into CO2-dependent molecular mechanisms driving stomatal responses in plants

Abstract

Recent advances are revealing mechanisms mediating CO₂-regulated stomatal movements in Arabidopsis, stomatal architecture and stomatal movements in grasses, and the long-term impact of CO₂ on growth.

Figure 1

Simplified model for CO₂-induced stomatal movements in Arabidopsis. CO₂ enters guard cells through PIP2;1, and likely other aquaporins and perhaps also passively. It is then converted into bicarbonate (${\text{HCO}}_3^{-}$) via the action of ßCA4 at the plasma membrane and ßCA1. MPK4/12, HT1, and CBC1/2 are proposed to function in a signaling pathway, but the detailed mechanisms and whether they act in the same pathway remain unknown (dashed lines indicate need for more research and possible intermediate steps). CBC1 and CBC2 function at the convergence of blue light signaling and low CO₂-induced stomatal opening pathways in guard cells. By interacting and phosphorylating the VK channel TPK1 at the tonoplast, KIN7 favors the exit of K⁺ cations that are crucial for stomatal closure. GHR1 interacts with SLAC1 at the plasma membrane, thereby positively triggering the export of anions. It is hypothesized that GHR1 could act as a scaffolding protein, potentially bringing important components around SLAC1. Anion export via the action of both S-type (SLAC1) and R-type (QUAC1) anion channels is required for stomatal closure in response to elevated CO₂. Red arrows = inhibition; blue arrows = activation.

Figure 2

Stomatal movements in Brachypodium and Arabidopsis are faster in the former due to the presence of subsidiary cells. In both B. distachyon (top) and A. thaliana (bottom), stomatal closure is induced by elevated CO₂ and ABA while stomatal opening is triggered by low CO₂ and blue and red-light signals. The presence of subsidiary cells surrounding dumbbell-shaped guard cells allows for faster stomatal movements in Brachypodium and other grasses. In both Arabidopsis and Brachypodium, the turgor pressure associated with vacuole swelling/shrinking plays a crucial part in stomatal opening/closure, respectively. In Brachypodium, the regulation of the turgor pressure in subsidiary cells enhances the speed of stomatal movements.

Figure 3

Brachypodium distachyon plants grown under distinct CO₂ concentrations have similar stomatal densities and indices. Brachypodium distachyon plants do not regulate stomatal development in response to long-term CO₂ exposure (A–B). Brachypodium distachyon plants grown under distinct CO₂ concentrations have similar stomatal indices (A) and densities (B). Plants were grown under 530 ppm, low (150 ppm), or high (900 ppm) CO₂ concentrations for 4 weeks and stomatal indices and densities within the same area (18 μm²) were calculated. Bars represent the average of three leaves from independent plants with three images per leaf per treatment ± sd.