Guard Cells Elongate: Relationship of Volume and Surface Area during Stomatal Movement

Abstract

Stomata in the epidermis of photosynthetically active plant organs are formed by pairs of guard cells, which create a pore, to facilitate CO2 and water exchange with the environment. To control this gas exchange, guard cells actively change their volume and, consequently, surface area to alter the aperture of the stomatal pore. Due to the limited elasticity of the plasma membrane, such changes in surface area require an exocytic addition or endocytic retrieval of membrane during stomatal movement. Using confocal microscopic data, we have reconstructed detailed three-dimensional models of open and closed stomata to precisely quantify the necessary area to be exo- and endocytosed by the guard cells. Images were obtained under a strong emphasis on a precise calibration of the method and by avoiding unphysiological osmotical imbalance, and hence osmocytosis. The data reveal that guard cells of Vicia faba L., whose aperture increases by 111.89+/-22.39%, increase in volume and surface area by 24.82+/-6.26% and 14.99+/-2.62%, respectively. In addition, the precise volume to surface area relationship allows quantitative modeling of the three-dimensional changes. While the major volume change is caused by a slight increase in the cross section of the cells, an elongation of the guard cells achieves the main aperture change.

Critical role of actin bundling in plant cell morphogenesis

“…8 Furthermore, GFP-mTn-expressing guard cells displayed heavily bundled actin microfilaments and their stomatal opening was suppressed during the diurnal cycles. 8 Three-dimensional morphometrical analysis further showed that guard cells elongated when stomata opened, 26 thus also clearly demonstrating the importance of dynamic actin bundling in guard cell elongation. Indeed, the involvement of actin dynamics during guard cell movement has been suggested through various pharmacological, imaging and molecular genetic studies (for details see the excellent review by Galatis and Apostolakos (2004)).…”

Functional Analysis of Cellulose and Xyloglucan in the Walls of Stomatal Guard Cells of Arabidopsis

“…Polarized light microscopy and field emission scanning electron microscopy have revealed that cellulose microfibrils are arranged radially relative to the stomatal pore in mature guard cells so as to constrain radial cell expansion (Palevitz and Hepler, 1976;Fujita and Wasteneys, 2014), instead favoring longitudinal cell expansion during stomatal opening (Wu et al, 1985;Meckel et al, 2007). However, fundamental questions remain to be addressed, such as whether there are any differences in cellulose organization between open and closed stomatal guard cells, and if so, how cellulose reorientation and/or reorganization facilitate stomatal movements.…”

Integrating cell biology, image analysis, and computational mechanical modeling to analyze the contributions of cellulose and xyloglucan to stomatal function

“…1 Cellulose in guard cells is arranged radially, 11,12 and xyloglucan is a hemicellulose that modulates wall mechanics 13 and is present in guard cell walls. 4 To test whether cellulose and xyloglucan regulate the anisotropic deformation of guard cells during stomatal movements, 14,15 we analyzed mutants deficient in cellulose or lacking detectable xyloglucan, finding that cellulose restricts, whereas xyloglucan facilitates longitudinal expansion and contraction in guard cells, thus regulating stomatal aperture. 1 Cellulose and xyloglucan are also required for the reorganization of cellulose in guard cells between a diffuse pattern in the open state and extensive bundles in the close state.…”

“…2) that integrates several aspects of guard cell geometry and dynamics, including aperture, 1 pore area, surface area, and volume changes. 15,25 In this model, the guard cell pair is constructed with anisotropic shell elements, including mechanical interactions with neighboring cells. This model enables quantitative investigation of changes in turgor and of interactions between guard cells and surrounding cells during stomatal movements.…”