FIGURE 2. Stomata diversity in bryophytes (bright field, fluorescence and confocal microscopy). A. Pohlia. B. Bartramia guard cells with chloroplasts (orange) in fluorescence microscopy. C. Pleurozium. D. Fluorescence image of Physcomitrella sporophyte with stomata. E. Hypnum. F. Fissidens. G. Funaria. H. Polytrichum stomata in fluorescence microscopy. H–I. Fluorescence images of sunken stomata of Orthotrichum at the epidermal level (H) and at pore (I). K–L. Pseudostomata of Sphagnum. M. Depth coded 3D reconstruction of epidermis and cortex of Sphagnum capsule, color represents cells at the same level (same as L). N. Phaeoceros confocal image of guard cells with chloroplasts (purple). Scale bars = 20μm.
Stomata are key innovations for the diversification of land plants. They consist of two differentiated epidermal cells or guard cells and a pore between that leads to an internal cavity.
Mosses and hornworts are the earliest among extant land plants to have stomata, but unlike those in all other plants, bryophyte stomata are located exclusively on the sporangium of the sporophyte.
Liverworts are the only group of plants that are entirely devoid of stomata.
Stomata on leaves and stems of tracheophytes are involved in gas exchange and water transport.
The function of stomata in bryophytes is highly debated and differs from that in tracheophytes in that they have been implicated in drying and dehiscence of the sporangium.
Over the past decade, anatomical, physiological, developmental, and molecular studies have provided new insights on the function of stomata in bryophytes.
In this review, we synthesize the contributions of these studies and provide new data on bryophyte stomata. We evaluate the potential role of stomata in moss and hornwort life histories and we identify areas that will provide valuable data in ascertaining the evolutionary history and function of stomata across land plants.
The development and structure of the guard cell walls of Funaria hygrometrica Hedw. (Musci) were studied with the light and electron microscopes.
The stoma consists of only one, binucleate guard cell as the pore wall does not extend to the ends of the cell. The guard cell wall is thinnest in the dorsal wall near the outer wall but during movement is most likely to flex at thin areas of the outer and ventral walls.
The mature wall contains a mottled layer sandwiched between two, more fibrillar layers. The internal wall layer has sublayers with fibrils in axial and radial orientations with respect to the pore.
During substomatal cavity formation, the middle lamella is stretched into an electron dense network and into strands and sheets.
After stomatal pore formation, the subsidiary cell walls close to the guard cell become strikingly thickened.
The functional implications of these results are discussed.
Key protoplasmic features of stomatal development in Funaria hygrometrica Hedw. (Musci) were characterized using light and electron microscopy.
Endoplasmic reticulum (ER) cisternae are initially rough and often arranged in parallel arrays. During pore formation, the cytoplasm becomes packed with tubular, smooth ER.
Older but still functional stomata contain small amounts of primarily cisternal ER. Lipid bodies decrease in electron density when tubular ER appears.
Preliminary observations indicate that two large vacuoles occupy the polar regions of open, but not closed, stomata.
Intact plasmodesmata occur in developing but not mature walls. Plastid structure, microtubule distribution, and other protoplasmic features are essentially similar to those described in the stomata of other genera.
Cuticle and pore development in the guard cells of Funaria were investigated with the electron microscope.
Pore cuticle formation is simultaneous with the creation of the pore itself. The morphology of the pore cuticle is unlike that of any cuticle described in the literature. It has many lamellae which are penetrated by electron dense fibrils.
Three different cuticular morphologies exist from the pore to the subsidiary cell walls. The cuticles on the pore and outer walls contain fibrils that sometimes reach to the surface.
The subsidiary cell cuticle lacks fibrils altogether. It is hypothesized that (1) cuticularization of the middle lamella contributes to ventral wall separation and (2) differences in extent of cuticular fibrils are related to greater water loss from stomata than from subsidiary cells (peristomatal transpiration).
The development of the one-celled condition in Funaria (Musci) stomata was investigated using light and electron microscopy. The guard cell parent cell is unusual in that it undergoes karyokinesis but incomplete cytokinesis. The septal wall, and the cell plate from which it forms, have incurved edges in contact with the polar cytoplasm.
No evidence was found to support Haberlandt’s claim that the stomate is initially two celled but undergoes wall resorption. Preprophase microtubule bands appear to be present in nonstomatal epidermal cells with normal cytokinesis, but the possibility is raised that they are absent in guard cell parent cells.
Stomatal pores evolved more than 410 million years ago [1, 2] and allowed vascular plants to regulate transpirational water loss during the uptake of CO(2) for photosynthesis .
Here, we show that stomata on the sporophytes of the moss Physcomitrella patens  respond to environmental signals in a similar way to those of flowering plants  and that a homolog of a key signaling component in the vascular plant drought hormone abscisic acid (ABA) response  is involved in stomatal control in mosses.
Cross-species complementation experiments reveal that the stomatal ABA response of a flowering plant (Arabidopsis thaliana) mutant, lacking the ABA-regulatory protein kinase OPEN STOMATA 1 (OST1) , is rescued by substitution with the moss P. patens homolog, PpOST1-1, which evolved more than 400 million years earlier.
We further demonstrate through the targeted knockout of the PpOST1-1 gene in P. patens that its role in guard cell closure is conserved, with stomata of mutant mosses exhibiting a significantly attenuated ABA response.
Our analyses indicate that core regulatory components involved in guard cell ABA signaling of flowering plants are operational in mosses and likely originated in the last common ancestor of these lineages more than 400 million years ago , prior to the evolution of ferns [8, 9].