Excretion and folding of plasmalemma function to accommodate alterations in guard cell volume during stomatal closure in Vicia faba L.

Abstract

Stomatal movement results in large and repetitive changes in cell volume and consequently surface area. While endocytosis has been extensively studied and is thought to be a major mechanism for accommodating the volume changes as evidenced mainly by fluorescent labelling and confocal imaging, studies at the ultrastructural level in intact guard cells of stomata regulated by natural factors have never been reported. Here, it is reported that excretion and folding of the plasmalemma are critical for accommodating the volume alterations in intact guard cells in Vicia faba L. Using transmission electron microscopy in combination with laser confocal microscopy, it was observed that in fully opened stomata the plasmalemma was smooth and tightly adhered to the cell walls while a whole large vacuole appeared in the cell. In the closed stomata, besides vacuole fragmentation, endocytosis of the tonoplast rather than the plasmalemma commonly occurred. Importantly, in stomata where pore closure was induced by circadian rhythm or CO2, numerous tiny vesicles were found outside the plasmalemma and, moreover, extensive folding of the plasmalemma could also be found in some regions of the cells. Additionally, an unknown structure was found at the interface between the plasmalemma and cell walls, especially in those areas of the cell where extensive folding occurred, suggesting that plasmalemma turnover is possibly associated with an interaction between the plasmalemma and cell walls. Collectively, the results strongly indicate that excretion and folding of the plasmalemma are critical for the accommodation of the cell volume alterations during stomatal movement.

Mechanics of surface area regulation in cells examined with confined lipid membranes

“…The retraction phase is followed by a slower phase of gradual absorption of the spherical lipid formations into the expanding bilayer. Because of insufficient microscopic resolution, we are unable to say whether these formations are vesicles or lipid aggregates but we note that similar lipid aggregates have been observed by transmission electron microscopy in shrinking plant guard cells (13). Finally, we observe that during cyclic expansion and compression, tubes form and retract recurrently at the same location on the bilayer.…”

“…The fusion of vesicles with the expanding bilayer is equivalent to the exocytosis of cytoplasmic vesicles, which is documented in expanding cells (3,6,8). The tubes formed by compressing the supported bilayers strikingly resemble the microtubular invaginations of the membrane observed in shrinking neurons, and renal and plant cells, because such invaginations occur at sites of membrane adhesion to a solid substrate or the cell wall (3,5,13). Next, we present the quantitative dynamics of bilayer transformations …”

“…The confinement restricts the modes of the membrane deformation and so influences the mechanisms for surface area regulation. For example, endocytosis has been observed in shrinking protoplasts but not in intact plant cells, which are surrounded by a rigid cell wall (13).…”

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

“…In contrast, darkness, high CO 2 levels, and abscisic acid (ABA) trigger stomatal closure, which is dependent on the efflux of solutes from guard cells and a resultant drop in turgor (Kim et al, 2010). During stomatal movements, guard cell volume can fluctuate by up to 40%, which is accompanied by alterations of cell surface area through membrane turnover rather than physical deformation of the plasma membrane (Franks et al, 2001;Shope et al, 2003;Li et al, 2010). It has been proposed that the coordination between guard cells and neighboring epidermal cells and the unique mechanical properties of guard cells, especially their walls, facilitate stomatal movements (Franks et al, 1998).…”

The role of the membrane confinement in the surface area regulation of cells

“…17 The adhesion forces influence the bending and stretching dynamics of the membrane and may induce local variations in the membrane tension; adhesive interactions also restrict the available Our in vitro findings closely reproduce observations on shrinking neurons, T-tubules in skeletal muscles, renal cells, plant cells, etc. 3,5,23,24 For example, it has been proposed that the tubular membrane invaginations in these cells serve as a tension/surface area buffer in the demand for rapid surface area variations. 23 Because of their reversibility under cycles of area expansion and compression, the membrane tubes are energetically less demanding than the well-known mechanisms of vesicle endo-and exocytosis.…”