Stomata on the earliest fossil land plants share remarkable similarities with hornwort stomata and sporophyte surfaces (Fig. Hornwort stomata open once and remain open throughout development. Bars = 0.5 μm except for E, where bar = 2 μm. It is estimated that there are about 9000 species of liverworts. While there are more than 300 published species names, the actual number could be as low as 100-150 species. The stomata are found only on the lower surface of leaves and lack subsidiary cells, and their guard cells contain densely arranged chloroplasts. S2). The role of stomata in facilitating sporangial drying/dehiscence is supported by experiments involving the moss Physcomitrella patens, in which mutation of basic helix-loop-helix transcription factors, orthologous to those governing stomata development in Arabidopsis, resulted in stomataless capsules that were delayed in dehiscence compared with wild-type capsules (Chater et al., 2016). 2D and 3, A–D). The loss of stomata in Notothylas, the sister taxon to Phaeoceros, can be explained by their highly reduced sporophytes that are often cleistocarpic and remain within the involucre throughout development (Renzaglia, 1978). This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Each cell of the thallus usually contains just one chloroplast. Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves, Guard cell size at developmental regions of an A. agrestis sporophyte, Stomatal function requires pectin de-methyl-esterification of the guard cell wall, Genome size increases in recently diverged hornwort clades, Genome size is a strong predictor of cell size and stomatal density in angiosperms, Stomata: key players in the earth system, past and present, Passive origins of stomatal control in vascular plants, The structure, function, and biosynthesis of plant cell wall pectic polysaccharides, Evolutionary conservation of ABA signaling for stomatal closure in ferns, Early evolutionary acquisition of stomatal control and development gene signalling networks, Origins and evolution of stomatal development, Regulatory mechanism controlling stomatal behavior conserved across 400 million years of land plant evolution, Origin and function of stomata in the moss, The Lower Devonian flora of the Senni Beds of Monmouthshire and Breconshire, Nuclear DNA content and genome size of trout and human, Exploding a myth: the capsule dehiscence mechanism and the function of pseudostomata in, Embryophytic sporophytes in the Rhynie and Windyfield cherts, Stomata in early land plants: an anatomical and ecophysiological approach, Cryptospores and cryptophytes reveal hidden diversity in early land floras. G, A. adscendens. E, L. dussii. 1C and 4, C and D). One control that excluded incubation of the primary antibody and two treatments were made for each antibody on three to five individual stomata. D, TEM cross section of a living, fully developed, open stoma with the pore leading to a substomatal cavity (asterisk). The 51- to 81-μm range of guard cell length in hornworts is on the higher end of lengths observed in fossil Devonian plants, which range from 21.5 to 85 μm (Lomax et al., 2014; Supplemental Table S2). The exceptions are the genera Notothylas and Megaceros, which do not have stomata. Thin sections (60–90 nm) were collected on nickel grids and dried for 1 to 3 h at room temperature. These two classes are divided further into five orders, each containing a single family. These pores superficially resemble the stomata of other plants. The stomata color does not necessarily coincide with the overall color of the sporophyte because stomata die and collapse while the sporophyte is actively photosynthesizing. Open pores of stomata provide a larger area for gas exchange and allow the assimilative tissue to be thicker, consequently increasing the self-sufficiency of sporophytes while developing spores. Following stomatal collapse, continued drying of the sporophyte results in the death of epidermal and assimilative cells and browning of the sporophyte (Fig. Mosses and hornworts, the most ancient extant lineages to possess stomata, possess orthologs of these Arabidopsis (Arabidopsis thaliana) stomatal toolbox genes, and manipulation in the model bryophyte Physcomitrella patens has shown that the bHLH and EPF components are also required for moss stomatal development and patterning. Microscopic studies focused on four hornwort genera with stomata. For consistency, all guard cell measurements that were correlated with genome sizes were made on green parts of sporophytes. Further phylogenetic analysis revealed that the Anthoceros ApSMF1 and ApSCRM1 are closely related to the respective genes of Physcomitrella, PpSMF1 and PpSCRM1, and that the peptide sequences share high degrees of homology across all plants. However, these are not sto… © 2017 American Society of Plant Biologists. Cross-section light micrograph of an L. dusii sporophyte with a large collapsed stoma over a substomatal cavity that connects to a system of intercellular air-filled spaces. These dried epidermal cells, together with the differentially thickened guard cell walls, hold the broad stomata in position over intercellular spaces (Figs. The life of a hornwort starts from a haploid spore. Small plastids (arrow) in epidermal cells contrast with large starch-filled plastids (p) in assimilative cells. Diagrammatic representation of a hornwort sporophyte with progressive development and color of stomata indicated from the base upward. B, L. dussii. The sporophyte of a hornwort is unusual in that it grows from a meristem near its base, instead of from its tip the way other plants do. The outer ledge (black arrow) and substomatal cavity (white arrow) are beginning to form. DOI: https://doi.org/10.1104/pp.17.00156. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Grids were observed unstained with a Hitachi H7650 transmission electron microscope at 60 kV. Pore opening is followed by the disappearance of liquid in the substomatal cavity and progressively inwardly in intercellular spaces. Semithin sections (250–750 nm) were mounted on glass slides and stained with 1.5% Toluidine Blue in distilled water to monitor for stomata using light microscopy. There are about 200 species known, but new species are still being discovered. This study identifies a developmental fate of guard cells in hornworts that involves pore development, early death, collapse, and increase in surface area and outer aperture width, all of which are associated with progressive drying of internal mucilage, spore differentiation, and sporophyte dehiscence. They argued that high levels of atmospheric CO2 as demonstrated in vitro for angiosperms (Edwards, 2003; Franks et al., 2012; Lomax et al., 2012) and paleopolyploidy may have resulted in exceptionally long guard cells. Stomata open directly above the involucre (Fig. Newly opened stomata are raised slightly above the epidermal surface (Fig. Grids were placed in 2% BSA in 0.02 mol L−1 PBS solution, pH 7.2 (PBS), overnight at 4°C in a humid chamber. What are the evolutionary origins of stomatal responses to abscisic acid in land plants? The gametophyte stage is the dominant stage in both liverworts and hornworts; however, liverwort sporophytes do not contain stomata, while hornwort sporophytes do. Do Pteridophytes have stomata? When the evolution of stomata is considered across land plants, several conclusions emerge (Fig. Thank you for your interest in spreading the word on Plant Physiology. I, P. carolinianus. 6, C and D). Stotler et al. ↵1 This work was supported by the National Science Foundation (grant no. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. [14], Recent studies of molecular, ultrastructural, and morphological data have yielded a new classification of hornworts. Once open, stomata never close, but the outer aperture increases slightly in width after guard cell collapse. In hornworts, the experimental treatments, based on measurements of > 9000 stomata, produced only a slight reduction in aperture dimensions after desiccation and plasmolysis, and no changes following ABA treatments and darkness. This open configuration is evident from a surface view, but the convoluted ventral guard cell walls surround an irregular inner pore (Fig. SEM images of hornwort sporophytes. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. www.plantphysiol.org/cgi/doi/10.1104/pp.17.00156. To our knowledge, there are no other stomata in extant plants that have the structure and developmental fate of those in hornworts. 4). Sporophytes of L. dussii were examined using immunogold labeling to identify pectin epitopes in guard cell walls and intercellular spaces. The aperture is wide open superficially, and the thin ventral guard cell walls are buckled. As one of the earliest plant groups to evolve stomata, hornworts are key to understanding the origin and function of stomata. The cells of hornworts usually have a single chloroplast and it is important during the energy production by photosynthesis. 3H) guard cell walls is very strong and homogenous throughout. At this time, the inner walls separate from cortical cells to form substomatal cavities (Fig. Hornwort genomes are among the smallest of all land plants (Renzaglia et al., 1995). Indeed, the thin outer walls and collapse of ventral walls in hornwort guard cells are strikingly similar to those of Sphagnum pseudostomata (Merced, 2015a). The earliest fossil stomata from the Silurian (yellow tag) exhibit the collapsed condition. A large substomatal cavity (asterisk) leads to internal air spaces. Average genome sizes (Bainard and Villarreal, 2013) and stomatal guard cell length from mature guard cells in sixteen hornwort species. Cell walls are thicker than those in A, and epidermal cells contain large amyloplasts (black arrow). The large collapsed hornwort stomata show similarities with the first fossil plants from rocks over 400 million years old. 2). The prominent plastids in guard cells are well developed with abundant starch and pyrenoids Phaeoceros carolinianus (Michx.) For TEM, sporophytes were harvested, cut into sections at 2-mm intervals from the gametophyte upward, and fixed in 2% glutaraldehyde in 0.05 m sodium phosphate buffer for 1 h at room temperature, then overnight at 20°C. Bars = 100 μm (A), 20 μm (B–D), and = 50 μm (E). D, Silurian stoma NMW97.37G.3 with no evidence of two guard cells as in A. Following senescence, guard cells collapse inwardly until the outer walls rest against the inner walls (Fig. Brodribb and McAdam (2011) suggested that the physiologically complex, regulatory role in water loss and gas exchange evolved in the Mid-Devonian, well after stomata first appeared in the fossil record. Drying and dehiscence in this system are essential for spore maturation, sporophyte dehiscence, and spore release. Scarce labeling is shown for LM6 (I) and LM13 (J), both localized toward the inside of the wall at the plasmalemma. Species examined were Leiosporoceros dussii (Steph.) C, Tetrad with spore mother cell wall drying down on the papillate distal wall ornamentation. Do bryophytes have stomata? Stomata on hornworts are larger in width and depth than the surrounding epidermal cells, which is an unusual character in plants. Stomata are expendable in hornworts, as they have been lost twice in derived taxa. The hornworts have stomata and it is also capable of carrying out a photosynthesis reaction. Meristem cells at the base of the plant keep dividing and adding to the height of the sporophyte. TEM immunogold localization of LM19 pectin epitopes in mucilage in spore sacs of Leiosporoceros sporophytes. S1; Macquet et al., 2007). Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees. Almost exclusively terrestrial and have a cuticle over mush of their body and many have stomata The numerous plate-like structures are gametophytes of a hornwort, … Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. Small plastids (arrow) occur in epidermal cells, and a substomatal cavity (asterisk) leads to intercellular spaces in the assimilative (cortical) tissue. These seemingly disparate approaches to the study of guard cells come together with the oldest fossil stomata to provide an understanding of the role and evolution of stomata in hornworts and the first land plants. All tracheophytes (blue tags) have green, living stomata. ); Enter multiple addresses on separate lines or separate them with commas. Stages of senescence and collapse of stomata in three genera of hornworts. Moreover, a low frequency of stomata and large size of guard cells (Supplemental Table S2) are shared by hornworts and early fossil plants. TEM images showing wall ultrastructure in guard cell walls of Leiosporoceros dussii. The smallest guard cells are those in Anthoceros fusiformis Aust. The rate of mucilage drying is governed by seasonal conditions and is completed where the sporophyte dehisces. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Karen S. Renzaglia (renzaglia{at}siu.edu). [clarification needed] The tip continues to divide new cells, which produces a thalloid protonema. [9] However, the same form of columella is also characteristic of basal moss groups, such as the Sphagnopsida and Andreaeopsida, and has been interpreted as a character common to all early land plants with stomata. The development and wall architecture of hornwort stomata are intricately associated with spore and sporophyte differentiation. Thickening of guard cell walls begins at the juncture of outer and ventral walls, where outer ledges will form (Fig. Guard cells of hornworts are similar in length to those of Psilotum (72.7 μm) and Ophioglossum (65.6 μm; Obermayer et al., 2002), both of which have genome sizes 300 times that of the largest hornwort genome. In most true mosses, stomata are restricted to the apophysis, where they are hypothesized to function in drying and dehiscence, as they are in Sphagnum and hornworts. Hornwort stomata are large and scattered on sporangia that grow from their bases and release spores at their tips. The epidermal cells are fully compressed in width at this location (Fig. During cell collapse, the ventral guard cell walls fold onto each other, forming a convoluted inner pore (Fig. 4, D and G). There are usually two chloroplasts in each guard cell that are substantially bigger and with more starch and thylakoids than chloroplasts in epidermal cells. Cross section light micrograph of a newly opened stoma shows large starch-filled plastids in guard cells and differentially thickened epidermal and guard cell walls. Stomata occur in all major groups of extant land plants except liverworts, but they are found on sporangia (capsules) only in hornworts and mosses. Collapsed stomata remain broad and prominent throughout the drying process (Fig. A, Developing stoma. First, stomata on sporangia, as occur in hornworts and mosses, are expendable. Due to differential wall thickenings on epidermal and guard cell walls, guard cells remain perched in position over the substomatal cavity, expanding the surface area in contact with the environment, including the width of the outer aperture. E, Large collapsed stoma (arrow) in dried epidermis of a dehiscing Anthoceros cristatus Stephani. Many hornworts establish symbiotic relationships with … B, Transmission electron microscopy (TEM) cross section of young guard cells before forming the pore. D to F, Fossil stomata reproduced with permission from Edwards et al. A to C, Extant hornwort stomata. B, Mature, living, and open stoma. Hence, it provides much of the energy needed for the growth and also for reproduction. Anatomy, development, and classification of hornworts. Stomata form at the base of the sporophyte in the green region, where they develop differential wall thickenings, form a pore, and die. 2B). 1B), where spore mother cells undergo meiosis and tetrads initiate spore wall development. (Fig. H, L. dusii. C, Dead (dying) guard cells at the onset of collapse of the outer walls. A, Differential interference contrast image showing two new guard cells, each with a large amyloplast and an aperture beginning to form in ventral walls (arrow). By using inorganic c… Stomata on leaves and stems of tracheophytes are involved in gas exchange and water transport. However, stomata in ferns are found on leaves, organs that are not found in bryophyte sporophytes and that did not exist in the earliest fossil plants. We examined the composition of guard cell walls for the occurrence of arabinan-containing polysaccharides that allow for flexibility and resilience in actively moving stomata (Jones et al., 2003, 2005; Merced and Renzaglia, 2014). The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines. Rhizoids: thin, rootlike structures which they use to attach to substrate. Department of Plant Biology, Southern Illinois University, Carbondale, Illinois 62901-6509 (K.S.R., J.R.L. Guard cells in opened stomata (Fig. (66.8 μm), Leiosporoceros dussii (76.5 μm), and Phaeoceros engelii Cargill and Fuhrer. The sporophyte is green in this region above the involucre due to chloroplasts in the assimilative region, and the intercellular spaces may have some fluid (Fig. The exceptions are the genera Notothylas and Megaceros, which do not have stomata. In half of the roughly 200 hornwort species, this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food. This third region is the capsule. They have helical thickenings that change shape in response to drying out; they twist and thereby help to disperse the spores. and Lindenb. Esau’s Plant Anatomy: Meristems, Cells and Tissues of the plant Body—Their Structure, Function and Development. Fossil stomata occur on sporangia as they do in bryophytes, and many occur on leafless axes that bear terminal sporangia (Edwards et al., 1998). Hässel abundantly label for unesterified homogalacturonan (LM19; Fig. (Table I). Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils. (2014) noted the inconsistency in guard cell length of fossil stomata vis-a-vis a predicted increase in genome size from the earliest plants through geologic time. [10] Chromosome-scale genome sequencing of three hornwort species corroborate that stomata evolved only once during land plant evolution. 4, F and H). In tracheophytes, all these treatments resulted in complete stomatal closure. DUE 1136414) and National Institutes of Health (grant no. 5). 1. These structures are morphologically similar to stomata in the rest of land plants, consisting of a pair of guard cells surrounding a pore. Outer guard cell walls, in comparison, are thin, smooth, and covered by a thin cuticle on the outer ledge. Chloroplast ultrastructure and sporophyte anatomy in hornworts support an early role of stomata in gas exchange, including CO2 acquisition for photosynthesis and water evaporation as the fluid disappears from intercellular spaces (Villarreal and Renzaglia, 2015). C, Thin fibrillar ventral wall with scattered cuticle/waxes (arrow). In half of the roughly 200 hornwort species, this chloroplast is fused with other organelles to form a large pyrenoid that both enables more efficient photosynthesis and stores food. Particularly notable are the guard cells of fossils from the Early Devonian, which are over 80 μm in length in Horneophyton lignieri and Aglaophyton major. We thank Nicholas Flowers for technical assistance. 1B) are differentially thickened and consist of loose radial fibrils (Figs. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. First, a major finding in the stomatal development of hornworts is that pore formation is followed by the production of differentially thickened cell walls, then the death and collapse of guard cells. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Black dots in images are secondary gold labels attached to specific antibodies. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. Bars = 10 μm (A and C) and 5 μm (B and D). LM19 labeling in ventral (Fig. [8] In either case, the protonema is a transitory stage in the life of a hornwort. sporophyte that contains pseudoelaters separating dried tetrads not surrounded by mucilage or spore mother cell wall. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. 1D and 4, H and I). Sporophyte structure in the neotropical hornwort, The hornworts: important advancements in early land plant evolution, RBR-Type E3 Ligases and the Ubiquitin-Conjugating Enzyme UBC26 Regulate Abscisic Acid Receptor Levels and Signaling, CLE40 Signaling Regulates Root Stem Cell Fate, Design Principle for Decoding Calcium Signals to Generate Specific Gene Expression Via Transcription, by The American Society of Plant Biologists. SEM cross section shows the epidermis and a stoma with dead collapsing guard cells that contain degenerated protoplasm (arrow). Chloroplasts have been shown to perceive abiotic and biotic stimuli to bring about a range of responses, including the initiation of senescence and programmed cell death (Spetea et al., 2014). Stomata collapse at the base where mucilage surrounds tetrads, and through progressive drying of mucilage upwardly, spores and pseudoelaters separate. A pair of guard cells die and collapse still being discovered we stomatal... 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