The evolution of special types of cells requires the acquisition of new gene regulatory networks controlled by transcription factors (TFs). In stomatous plants, a TF module formed by subfamilies Ia and IIIb basic helix–loop–helix TFs (Ia-IIIb bHLH) regulates stomatal formation; however, how this module evolved during land plant diversification remains unclear. Here we show that, in the astomatous liverwort Marchantia polymorpha, a Ia-IIIb bHLH module regulates the development of a unique sporophyte tissue, the seta, which is found in mosses and liverworts. The sole Ia bHLH gene, MpSETA, and a IIIb bHLH gene, MpICE2, regulate the cell division and/or differentiation of seta lineage cells. MpSETA can partially replace the stomatal function of Ia bHLH TFs in Arabidopsis thaliana, suggesting that a common regulatory mechanism underlies setal and stomatal formation. Our findings reveal the co-option of a Ia-IIIb bHLH TF module for regulating cell fate determination and/or cell division of distinct types of cells during land plant evolution.
In higher plants the phytohormone ABA is involved in processes that are connected to water deficit, like stomatal closure or desiccation tolerance.
In bryophytes, also containing ABA in their tissues, physiological functions remained uncertain for a long time. Quite recently, several papers have shown different effects of exogenously applied ABA: stomatal closure in Anthoceros, drought hardening in Funaria and production of the landform in Riccia. In all these cases the relevant conditions (water deficit) enhance the endogenous ABA level significantly. For induced desiccation tolerance, ABA serves as a mediator to induce specific proteins (dehydrins) strongly connected with this tolerance.
Therefore, it can be concluded that in bryophytes ABA has the same function as in higher plants. It acts as a mediator in stress conditions.
Stomata are present across all plants excluding liverworts and are very similar, consisting of a pore surrounded by two guard cells. In all extant plants, stomata are found in the sporophyte. The origin and evolution of stomata in land plants is controversial. Moss guard cells have similar wall architecture and are less variable than tracheophytes guard cells. In reduced sporophytes (such as Ephemerum and Physcomitrella) capsule anatomy is modified and some stomata features are also reduced (Merced & Renzaglia 2013). We described the developmental pattern and distribution of stomata in the moss Funaria (Merced & Renzaglia 2016) and changes in pectin composition during guard cell development. We found that guard cell walls are thinner and rich in pectins during the short period where stomata can open and close (Merced & Renzaglia 2014). We hypothesize that during development of the sporophyte, stomata serves as passages for gas exchange and bringing up water into the expanding capsule, later stomata helps to dry the capsule and influence the release of spores. The single origin of stomata is complicated by the absence of true stomata in early-divergent mosses, but Sphagnum has specialized epidermal cells, pseudostomata, that partially separate but do not form a pore. Pseudostomata may be related to stomata and share a common function to moss stomata (Merced 2015), with wall architecture and behavior specialized to facilitate capsule dehydration, shape change, and dehiscence. To have a better picture of stomata evolution we studied the ultrastructure, anatomy and composition of stomata of hornworts and proposed that they share a common architecture and fate to stomata of ancient plants (Renzaglia et al. 2017). It turns out that guard cell walls of hornworts lack some of the pectin components necessary for stomata movement that are present in angiosperms (Merced & Renzaglia 2019). In a review article we summarize and synthesize the knowledge acquire in the last few years about bryophyte stomata and future directions of study (Merced & Renzaglia 2017).
Bryophyte Diversity And Ecology
Bryophytes are usually a neglected group of plants, being small they can be bypass without notice, but once you stop to look at them or better yet get ahold of a hand-lens or a microscope you will be able to see their beauty. Bryophytes is the collective name given to three groups of plants: mosses, liverworts and hornworts. I have been studying bryophytes since 2001.
I am working with the bryophytes of Puerto Rico, collecting and identifying bryophytes around the islands. In particular, focusing on the role of bryophytes in Puerto Rican forests and how they respond to anthropogenic and non-anthropogenic disturbances. I am also interested in urban and community forests that sustain bryophytes to understand how they are different to non-urban vegetations.
To better understand the distribution of bryophytes in a tropical forest, we are studying the presence and abundance of a common moss and liverwort in El Verde LTER. We are interested in learning what ecological factors influence the presence and size of these bryophytes, and how it compares to other plants.
OTHER PROJECTS Southern Illinois was a great area for bryophytes. In collaboration with S. Jesselson, a SIU Plant Biology undergraduate student in the Renzaglia lab, we collected, identify and image mosses of the area. Here is a link to the project of some common mosses of southern Illinois and the poster.
As a research assistant in Dr. Shaw’s Bryology Lab at Duke University I worked on the virtual flora of the mosses of North Carolina. The key to the mosses of NE United States, created by Lewis Anderson and others, is illustrated with pictures of some of the species and important characters for the identification. Here is the link to the Mosses of North Carolina.
From 2005 to 2008 I worked at the UPRRP Herbarium (San Juan PR), where I was in charge of the database activities of the herbarium and supervision of students. There I collected and identify specimens for the bryophyte collection. Here is the link to the UPRRP herbarium database.
During the time I was at UPR Río Piedras I worked with two undergraduate students doing research in bryology. With S. Galva we collected and identified bryophytes of the Carite Forest Reserve, PR and prepared a preliminary list of bryophyte species with new records for the area. Here is a poster with our findings (in Spanish). Working with orchid expert Dr. J. Ackerman I learn to see orchids everywhere. Did you know that small orchids, like Lepanthes, are often found between bryophyte matts? As an undergrad student J.G. García Cancel, advised by Dr. Melendez-Ackerman, looked at the relationship between orchid distribution and bryophyte cover, this study found that in thick bryophyte cover adult orchids are more frequent than younger plants but that interactions between bryophytes and this orchid are dynamic during different life stages. This research resulted in a publication in the Caribbean Naturalist.
Stomatal development in Funaria hygrometrica Hedw. was studied with light and electron microscopy. The stoma is one-celled at all stages. Nuclear division in the guard cell parent cell is followed by incomplete cytokinesis; the cell plate, and thus the newly formed septum, never reach the ends of the cell as they do in the stomata of most other plants. Preprophase bands of microtubules were absent in guard cell parent cells but present in some pre-division non-stomatal epidermal cells.
Throughout development, the guard cell wall is thinnest in areas of the outer and dorsal walls near the subsidiary cell and at the mid-depth of the ventral (pore) wall. 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.
The rear and forechambers of the pore generally form before the central aperture and ledges do. Material present between the separating interfaces of the ventral wall is involved in cuticle assembly i.e. cuticle formation is simultaneous with the creation of the pore. This material includes rods, globules and a granular-fibrillar matrix. The contents of the globules form the bulk of the pore cuticle and the electron dense borders of the globules become the cuticular fibrils. Both the ventral and outer wall cuticles contain fibrils that sometimes reach the surface but the fibril arrangement is roughly perpendicular to the surface in the pore cuticle and reticulate in the outer wall cuticle. Fibrils are absent in the thinner, subsidiary cell cuticle.
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 the tubular ER appears. Preliminary observations indicate that two large vacuoles occupy the polar regions of open but not closed stomata.
Colonization of the terrestrial environment by land plants (embryophytes), a monophyletic clade that evolved from freshwater streptophyte algae, forever changed Earth by transforming biogeochemical cycles. The evolution of stomata was a key adaptation that allowed the colonization of terra firma. Present in most land plants, stomata control the passage of carbon dioxide and water vapor, thus providing plants with a coping mechanism against arid and fluctuating conditions. The origin and ancestral function of stomata are obscure because of stomata-bearing and -lacking lineages and diversity in their morphology and function. By making use of new plant genomes and data from the 1KP project, Harris et al. sought to unravel the phylogeny of land plant and through it resolve the evolutionary history of stomatal development and function. By analyzing single-copy orthologs of 162 Viridiplantae genomes the authors determined that bryophytes were monophyletic and sister to tracheophytes, suggesting that the common ancestor of land plants already possessed stomata, with secondary loss of these in liverworts and some mosses. Subsequent presence/absence analyses of genes involved in stomata development and function confirmed this, as several genes essential for these processes were present in the common ancestor of embryophytes, whereas many of them were later lost in bryophytes. These results confirm bryophyte monophyly and shed light on stomata origin, suggesting that the ancestor of land plants possessed stomata, while bryophytes underwent reductive evolution probably due to loss of key developmental and functional regulators, with liverworts entirely losing stomata and later evolving liverwort-specific air pores. (Summary by Jesus Leon@jesussaur) Curr. Biol. 10.1016/j.cub.2020.03.048 [altmetric doi=”10.1016/j.cub.2020.03.048″ details=”right” float=”right”]
The occurrence of stomata in 29 tropical African moss species representing 12 families is reported. The stomata (22-51 μm × 22-29 μm) are mostly round-pored with two guard-cells each, ranging from 2 to more than 200 per capsule. In Wijkia trichocoleoides (C. Muell.) Crum, Trichosteleum microcalyx Ren. & Card., Stereophyllum radiculosum (Hook.) Mitt. and Stereophyllum virens Card. stomata are raised above the level of epidermis but are sunken in Brachymenium leptophyllum C. Muell.) Jaeg. and Bryum coronatum Schwaegr. Significant correlations have been obtained between stoma number and seta length, and stoma size and epidermal cell size.
The stomatal complex in Bryophyta is morphologically more uniform than in vascular plants. Nevertheless there are many species without data about the morphology, physiology and ontogeny of their stomata. We present here a morphological and histochemical study of the stomatal complex in ten species of Crossidium, Didymodon, Pottia and Tortula (Pottiaceae).The number of stomata per capsule, their size, orientation, location, neighbouring cells, morphological type and the results of the histochemical tests for the guard and adjacent cells for starch, callose, cellulose and pectin are described. The results suggest that the morphological features of the stomata in these ten species have not any taxonomical value.
Because stomata in bryophytes are uniquely located on sporangia, the physiological and evolutionary constraints placed on bryophyte stomata are fundamentally different from those on leaves of tracheophytes. Although losses of stomata have been documented in mosses, the extent to which this evolutionary process occurred remains relatively unexplored. We initiated this study by plotting the known occurrences of stomata loss and numbers per capsule on the most recent moss phylogeny. From this, we identified 40 families and 74 genera that lack stomata, of which at least 63 are independent losses. No trends in stomata losses or numbers are evident in any direction across moss diversity. Extant taxa in early divergent moss lineages either lack stomata or produce pseudostomata that do not form pores. The earliest land plant macrofossils from 400 ma exhibit similar sporangial morphologies and stomatal distribution to extant mosses, suggesting that the earliest mosses may have possessed and lost stomata as is common in the group. To understand why stomata are expendable in mosses, we conducted comparative anatomical studies on a range of mosses with and without stomata. We compared the anatomy of stomate and astomate taxa and the development of intercellular spaces, including substomatal cavities, across mosses. Two types of intercellular spaces that develop differently are seen in peristomate mosses, those associated with stomata and those that surround the spore sac. Capsule architecture in astomate mosses ranges from solid in the taxa in early divergent lineages to containing an internal space that is directly connected to the conducing tissue and is involved in capsule expansion and the nourishment, hydration and development of spores. This anatomy reveals there are different architectural arrangements of tissues within moss capsules that are equally effective in accomplishing the essential processes of sporogenesis and spore dispersal. Stomata are not foundational to these processes.
Mature capsules of Funaria. (A) Scanning electron micrograph of expanded capsulewith stomata in irregular rows and files on apophysis (arrowheads). (B) Drawing of stomata distribution in the apophysis of mature capsule. (C, D) Scanning electron micrographs of spongy tissue inside the capsule. (E) Scanning electron micrograph of apophysis showing slightly raised stomata covered by smooth cuticle that is thickened around the pore (arrow). Scale bars: (A, C) = 500 µm; (B) = 35 µm; (D) = 100 µm; (E) = 10 µm.
Patterning of stomata in the moss Funaria: a simple way to space guard cells
Early capsule expansion at the same stage as in Fig. 2D. (A) Predominantly longitudinal cell divisions of epidermal cells in the expanding apophysis as visualized by bright turquoise fluorescence of callose in new cell walls, detected by aniline blue. (B) Differential interference contrast image of capsule during expansion. Most stomata have differentiated and have ventral walls but no pore. Line drawing overlay of part of the capsule shows the arrangement of stomata. (C) Guard cells have abundant chloroplasts and are bigger than epidermal cells; stomata are arranged in files and rows. Distal round cells in the same files as stomata appear to be arrested stomata (arrowheads). (D) Aniline blue fluorescence of the same area as (C), identifying callose (bright turquoise) in newly formed walls of epidermal cells (arrows); divisions are mostly parallel to the sporophyte axis and are consistent with expansion in width of the capsule. Asterisks in C and D indicate the same stoma. (E) Light micrograph of capsule expansion with fully formed stomata with prominent peripheral chloroplasts. (F) Aniline blue fluorescence of the same area as in (E), showing callose (bright turquoise) in newly formed walls of epidermal cells in various planes around stomata (arrows). No callose is found in guard cell walls. Asterisks in (E) and (F) indicate the same stoma. Scale bars: (A) = 75 µm; (B, E, F) = 35µm; (C, D) = 10 µm.
Background and Aims Studies on stomatal development and the molecular mechanisms controlling patterning have provided new insights into cell signalling, cell fate determination and the evolution of these processes in plants. To fill a major gap in knowledge of stomatal patterning, this study describes the pattern of cell divisions that give rise to stomata and the underlying anatomical changes that occur during sporophyte development in the moss Funaria.
Methods Developing sporophytes at different stages were examined using light, fluorescence and electron microscopy; immunogold labelling was used to investigate the presence of pectin in the newly formed cavities.
Key Results Substomatal cavities are liquid-filled when formed and drying of spaces is synchronous with pore opening and capsule expansion. Stomata in mosses do not develop from a self-generating meristemoid as in Arabidopsis, but instead they originate from a protodermal cell that differentiates directly into a guard mother cell. Epidermal cells develop from protodermal or other epidermal cells, i.e. there are no stomatal lineage ground cells.
Conclusions Development of stomata in moss occurs by differentiation of guard mother cells arranged in files and spaced away from each other, and epidermal cells that continue to divide after stomata are formed. This research provides evidence for a less elaborated but effective mechanism for stomata spacing in plants, and we hypothesize that this operates by using some of the same core molecular signalling mechanism as angiosperms.
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