The capsule dehiscence mechanism and the function of pseudostomata in Sphagnum

Exploding a myth: the capsule dehiscence mechanism and the function of pseudostomata in Sphagnum

by Duckett J. G., Pressel S., P’ng K. M. Y., Renzaglia K. S. (2009)

Jeffrey G. Duckett, Silvia Pressel, Ken M. Y. P’ng, Karen S. Renzaglia,

In New Phytologist 183: 1053– 1063 – https://doi.org/10.1111/j.1469-8137.2009.02905.x

https://nph.onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2009.02905.x

Abstract

• The nineteenth century air‐gun explanation for explosive spore discharge in Sphagnum has never been tested experimentally. Similarly, the function of the numerous stomata ubiquitous in the capsule walls has never been investigated.

• Both intact and pricked Sphagnum capsules, that were allowed to dry out, all dehisced over an 8–12 h period during which time the stomatal guard cells gradually collapsed and their potassium content, measured by X‐ray microanalysis in a cryoscanning electron microscope, gradually increased. By contrast, guard cell potassium fell in water‐stressed Arabidopsis.

• The pricking experiments demonstrate that the air‐gun notion for explosive spore discharge in Sphagnum is inaccurate; differential shrinkage of the capsule walls causes popping off the rigid operculum. The absence of evidence for a potassium‐regulating mechanism in the stomatal guard cells and their gradual collapse before spore discharge indicates that their sole role is facilitation of sporophyte desiccation that ultimately leads to capsule dehiscence.

• Our novel functional data on Sphagnum, when considered in relation to bryophyte phylogeny, suggest the possibility that stomata first appeared in land plants as structures that facilitated sporophyte drying out before spore discharge and only subsequently acquired their role in the regulation of gaseous exchange.

Advertisements

Variations in guard cell wall composition reflect different physiological activity of stomata in land plants

 

 

Contrasting pectin polymers in guard cell walls of Arabidopsis and the hornwort Phaeoceros reflect physiological differences

by Merced A., Renzaglia K. S. (2018)

Amelia Merced, Karen S. Renzaglia,

===

in Annals of Botany, mcy168, https://doi.org/10.1093/aob/mcy168 –

https://academic.oup.com/aob/advance-article-abstract/doi/10.1093/aob/mcy168/5092734

Abstract

Background and Aims

In seed plants, stomata regulate CO2 acquisition and water relations via transpiration, while minimizing water loss. Walls of guard cells are strong yet flexible because they open and close the pore by changing shape over the substomatal cavity. Pectins are necessary for wall flexibility and proper stomata functioning. This study investigates the differences in pectin composition in guard cells of two taxa that represent key lineages of plants with stomata: Arabidopsis, an angiosperm with diurnal stomatal activity, and Phaeoceros, a bryophyte that lacks active stomatal movement.

Methods

Using immunolocalization techniques in transmission electron microscopy, this study describes and compares the localization of pectin molecule epitopes essential to stomata function in guard cell walls of Arabidopsis and Phaeoceros.

Key Results

In Arabidopsis, unesterified homogalacturonans very strongly localize throughout guard cell walls and are interspersed with arabinan pectins, while methyl-esterified homogalacturonans are restricted to the exterior of the wall, the ledges and the junction with adjacent epidermal cells. In contrast, arabinans are absent in Phaeoceros, and both unesterified and methyl-esterified homogalacturonans localize throughout guard cell walls.

Conclusions

Arabinans and unesterified homogalacturonans are required for wall flexibility, which is consistent with active regulation of pore opening in Arabidopsis stomata. In contrast, the lack of arabinans and high levels of methyl-esterified homogalacturonans in guard cell walls of Phaeoceros are congruent with the inability of hornwort stomata to open and close with environmental change. Comparisons across groups demonstrate that variations in guard cell wall composition reflect different physiological activity of stomata in land plants.

Variations in guard cell wall composition reflect different physiological activity of stomata in land plants

 

 

Contrasting pectin polymers in guard cell walls of Arabidopsis and the hornwort Phaeoceros reflect physiological differences

by Merced A., Renzaglia K. S. (2018)

Amelia Merced,  Karen S. Renzaglia,

===

in Annals of Botany, mcy168 – https://doi.org/10.1093/aob/mcy168 –

https://academic.oup.com/aob/advance-article-abstract/doi/10.1093/aob/mcy168/5092734?redirectedFrom=fulltext

Abstract

Background and Aims

In seed plants, stomata regulate CO2 acquisition and water relations via transpiration, while minimizing water loss. Walls of guard cells are strong yet flexible because they open and close the pore by changing shape over the substomatal cavity. Pectins are necessary for wall flexibility and proper stomata functioning. This study investigates the differences in pectin composition in guard cells of two taxa that represent key lineages of plants with stomata: Arabidopsis, an angiosperm with diurnal stomatal activity, and Phaeoceros, a bryophyte that lacks active stomatal movement.

Methods

Using immunolocalization techniques in transmission electron microscopy, this study describes and compares the localization of pectin molecule epitopes essential to stomata function in guard cell walls of Arabidopsis and Phaeoceros.

Key Results

In Arabidopsis, unesterified homogalacturonans very strongly localize throughout guard cell walls and are interspersed with arabinan pectins, while methyl-esterified homogalacturonans are restricted to the exterior of the wall, the ledges and the junction with adjacent epidermal cells. In contrast, arabinans are absent in Phaeoceros, and both unesterified and methyl-esterified homogalacturonans localize throughout guard cell walls.

Conclusions

Arabinans and unesterified homogalacturonans are required for wall flexibility, which is consistent with active regulation of pore opening in Arabidopsis stomata. In contrast, the lack of arabinans and high levels of methyl-esterified homogalacturonans in guard cell walls of Phaeoceros are congruent with the inability of hornwort stomata to open and close with environmental change. Comparisons across groups demonstrate that variations in guard cell wall composition reflect different physiological activity of stomata in land plants.

A lack of physiological regulation of stomatal movements in hornworts compared with tracheophytes

hornwortyoung
Introduction to the Anthocerotophyta (UCMP), Berkeley Above, you can see pictures of the hornwort Phaeoceros. On the left is a plant with young sporophytes beginning to elongate from the top of the gametophyte. (http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html)

 

 

Hornwort stomata do not respond actively to exogenous and environmental cues

by Pressel S., Renzaglia K. S., Clymo R. S., Duckett, J. G. (2018) 

Silvia Pressel, Karen S. Renzaglia, Richard S. (Dicky) Clymo, Jeffrey G. Duckett

 

in Annals of Botany, mcy045 –  https://doi.org/10.1093/aob/mcy045

https://academic.oup.com/aob/advance-article-abstract/doi/10.1093/aob/mcy045/4979633?redirectedFrom=fulltext

Abstract

Backgrounds and Aims

Because stomata in bryophytes occur on sporangia, they are subject to different developmental and evolutionary constraints from those on leaves of tracheophytes. No conclusive experimental evidence exists on the responses of hornwort stomata to exogenous stimulation.

 

Methods

Responses of hornwort stomata to abscisic acid (ABA), desiccation, darkness and plasmolysis were compared with those in tracheophyte leaves. Potassium ion concentrations in the guard cells and adjacent cells were analysed by X-ray microanalysis, and the ontogeny of the sporophytic intercellular spaces was compared with those of tracheophytes by cryo-scanning electron microscopy.

 

Key Results

The apertures in hornwort stomata open early in development and thereafter remain open. 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. In tracheophytes, all these treatments resulted in complete stomatal closure. Potassium concentrations are similar in hornwort guard cells and epidermal cells under all treatments at all times. The small changes in hornwort stomatal dimensions in response to desiccation and plasmolysis are probably mechanical and/or stress responses of all the epidermal and spongy chlorophyllose cells, affecting the guard cells. In contrast to their nascent gas-filled counterparts across tracheophytes, sporophytic intercellular spaces in hornworts are initially liquid filled.

 

Conclusions

Our experiments demonstrate a lack of physiological regulation of opening and closing of stomata in hornworts compared with tracheophytes, and support accumulating developmental and structural evidence that stomata in hornworts are primarily involved in sporophyte desiccation and spore discharge rather than the regulation of photosynthesis-related gaseous exchange. Our results run counter to the notion of the early acquisition of active control of stomatal movements in bryophytes as proposed from previous experiments on mosses.

==================

https://www.botany.one/2018/07/hornwort-stomata-are-not-actively-regulated/

Hornwort stomata are not actively regulated

Stomata, pores in the plant epidermis that regulate gas exchange, are a key innovation that enabled freshwater algae to colonize Earth’s landmasses some 500 Mya. Because stomata in bryophytes occur on sporangia, they are subject to different developmental and evolutionary constraints from those on leaves of tracheophytes. No conclusive experimental evidence exists on the responses of hornwort stomata to exogenous stimulation.

Hornwort stomata

Pressel and colleagues investigate stomatal behaviour in hornworts. They investigated responses of hornwort stomata to abscisic acid (ABA), desiccation, darkness and plasmolysis and compared these with those in tracheophyte leaves. Potassium ion concentrations in the guard cells and adjacent cells were analysed by X-ray microanalysis, and the ontogeny of the sporophytic intercellular spaces was compared with those of tracheophytes by cryo-scanning electron microscopy.

They show that there are no potassium fluxes associated with hornwort stomata, and that these do not respond to external factors (abscisic acid, desiccation, darkness and plasmolysis), which cause stomatal closure in other land plants. Their results run counter to the notion that active stomatal control was acquired early in the evolution of land plants, and support the alternative hypothesis of gradual acquisition of active control mechanisms.

Structure and function of hornwort stomata.

Photo credit: Google

The hornwort Phaeoceros

Structure and function of hornwort stomata.

by Lucas J. R., Renzaglia K. S. (2002)

Jessica R. Lucas, Karen S. Renzaglia, Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901

in Microscopy and Microanalysis 8, Supplement 2: 10901. – DOI 10.1017.S143192760210701X –

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/86FDD369AEC9E488CBBE7529D424D8A7/S143192760210701Xa.pdf/div-class-title-structure-and-function-of-hornwort-stomata-div.pdf

As minute structures that open and close in response to environmental cues, stomata are key adaptive innovations of land plants [1]. In vascular plants, stomata function in gas exchange in photosynthetic tissue and in facilitating nutrient transport through the transpirational stream. In bryophytes, the basal most land plants, stomata are restricted in occurrence and are poorly characterized in regards to structure-function relationships.

This study was undertaken to evaluate the microanatomy and physiology of stomata in hornworts, the oldest living lineage of plants that possess stomata. Protocols for SEM and TEM follow Renzaglia et al. [2]. To assess the osmotic regulation of stomatal functioning, we performed cytochemical localizations of potassium with Macullum’s reagent [3] and organic ions with Fast Violet B solution [4].

In hornworts, stomata occur scattered among elongated epidermal cells in the sporophyte. Mature stomata each consist of two bean-shaped guard cells subtended by an air-filled cavity (Figs. 1-3). In cross section, guard cells are larger than epidermal cells and they each contain a large starchfilled chloroplast (Figs. 1, 2).

Aside from prominent ledges on the outer and inner walls where the cells meet, the guard cell wall is thin throughout (Fig. 1). Extensive fibrous and flocculent material is associated with the outer ledge where the pore forms (Figs. 1, 3). The large chloroplasts in the underlying photosynthetic tissue are invariably positioned adjacent to air spaces, indicating a role of stomata in gas exchange (Fig. 4).

In contrast, the lack of an internal conducting system in hornworts suggests that stomata are not involved in nutrient transport or in regulating the transpirational flow as they are in plants with vascular tissue. Stomata examined in immature regions of the sporophyte are closed, while in mature regions, the stomata are typically opened.

Examination of sporophytes collected during dark and light periods indicates that there are no diurnal movements, i.e., guard cells in hornworts do not open and close in response to light. Further experiments with abscisic acid, a hormone that results in stomatal closure in vascular plants, resulted in no change in the pore (not illustrated).

In immature regions, both K+ and malate exhibit general localization in all epidermal cells (Figs. 5, 7). In mature tissue, these ions are localized in guard cells and are interpreted as functioning as osmoticum that is responsible for water influx and the creation and maintenance of stomatal pores (Figs. 6, 8).

These results coupled with the structural observations above support the concept that hornwort stomata open once through osmotic changes and remain open during the duration of physiological activity of the sporophyte. It appears that unlike vascular plants, the primary role of stomata in hornworts is in providing a passageway for gas exchange.

New data on bryophyte stomata

Photo credit: Bry. Div. Evo. 39 (1) © 2017 Magnolia Press

FIGURE 3. Stomata across model species. A. hornwort Anthoceros, B. moss Physcomitrella, C. Lycophyte Selaginella and D. flowering plant Arabidopsis.

Scale bars = 20μm.

 

Screen Shot 2017-11-16 at 21.45.01
FIGURE 1. Phylogenetic tree of stomata evolution in land plants.

 

Structure, function and evolution of stomata from a bryological perspective

by Merced A., Renzaglia  K. S. (2017)

AMELIA MERCED, KAREN S. RENZAGLIA

1 Institute of Neurobiology, University of Puerto Rico, San Juan, PR 00901,

2 Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901-6509.


 

in Bryophyte Diversity and Evolution 39(1): 7-20 – DOI: http://dx.doi.org/10.11646/bde.39.1.4 –

http://www.mapress.com/j/bde

Screen Shot 2017-11-16 at 21.47.01

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.

 

Abstract

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.

Architecture and fate of stomata in hornworts

Figure 4.Stages of senescence and collapse of stomata in three genera of hornworts. The outer aperture remains open and increases in diameter during the drying process. A, P. carolinianus. Scanning electron microscopy (SEM) shows newly opened, slightly raised stoma directly above the involucre. B, L. dussii. 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. Small plastids (arrow) occur in epidermal cells, and a substomatal cavity (asterisk) leads to intercellular spaces in the assimilative (cortical) tissue. C, A adscendens Lehm. and Lindenb. SEM cross section shows the epidermis and a stoma with dead collapsing guard cells that contain degenerated protoplasm (arrow). Adjacent epidermal cells have thickened radial walls and are beginning to collapse in the opposite direction from the guard cells. The aperture is wide open superficially, and the thin ventral guard cell walls are buckled. A large substomatal cavity (asterisk) leads to internal air spaces. D, A. adscendens. SEM of stoma shows the onset of guard cell collapse before epidermal cells dry. A thicker cuticle covers epidermal cells compared with guard cells. E, L. dussii. Cross section light micrograph shows dead guard cells with degenerated protoplasm at the onset of collapse of the outer cell wall and while fluid is still within the substomatal cavity (asterisk) and intercellular spaces (double asterisks). Small plastids (arrow) in epidermal cells contrast with large starch-filled plastids (p) in assimilative cells. F, L. dussii. TEM of dead, collapsed stoma shows the coordinated folding of the thin ventral walls of guard cells. The aperture is open from the outside due to the rigid outer ledges. G, A. adscendens. SEM shows completely collapsed guard cells surrounded by hydrated epidermal cells. Stoma diameter is greater than in the precollapsed guard cell in E. The outer aperture is open, and folded ventral walls of guard cells are visible internally (arrow). H, L. dusii. Cross section light micrograph shows collapsed guard cells. The adjacent epidermal cell contains degenerated cytoplasm and has begun to collapse like an accordion in the opposite direction from the guard cells. Assimilative cells begin to die around the substomatal cavity (asterisk) and intercellular space (double asterisks). I, P. carolinianus. SEM shows the epidermis in desiccated and dehisced sporophyte with ridges of collapsed epidermal cell surrounding an enlarged stoma that has a broadened outer aperture. Bars = 5 μm except for F, where bar = 2 μm.

 

Hornwort Stomata: Architecture and Fate Shared with 400-Million-Year-Old Fossil Plants without Leaves

by Renzaglia K. S., Villarreal J. C., Piatkowski B. T., Lucas J. R., Merced A. (2017)

Karen S. Renzaglia, Juan Carlos Villarreal, Bryan T. Piatkowski, Jessica R. Lucas, and Amelia Merced

Department of Plant Biology, Southern Illinois University, Carbondale, Illinois 62901-6509 (K.S.R., J.R.L.);

Département de Biologie, Université Laval, Quebec, Quebec, Canada G1V 0A6 (J.C.V.);

Smithsonian Tropical Research Institute, Ancon, 0843-03092 Panama, Republic of Panama (J.C.V.);

Department of Biology, Duke University, Durham, North Carolina 27708 (B.T.P.);

Institute of Neurobiology, University of Puerto Rico, San Juan, Puerto Rico 00901 (A.M.)

 

 

in Plant Physiology, June 2017, Vol. 174, pp. 788–797

Screen Shot 2017-06-07 at 21.07.21

Abstract

As one of the earliest plant groups to evolve stomata, hornworts are key to understanding the origin and function of stomata.

Hornwort stomata are large and scattered on sporangia that grow from their bases and release spores at their tips. We present data from development and immunocytochemistry that identify a role for hornwort stomata that is correlated with sporangial and spore maturation.

We measured guard cells across the genera with stomata to assess developmental changes in size and to analyze any correlation with genome size. Stomata form at the base of the sporophyte in the green region, where they develop differential wall thickenings, form a pore, and die.

Screen Shot 2017-06-07 at 21.09.21

Guard cells collapse inwardly, increase in surface area, and remain perched over a substomatal cavity and network of intercellular spaces that is initially fluid filled. Following pore formation, the sporophyte dries from the outside inwardly and continues to do so after guard cells die and collapse.

Spore tetrads develop in spore mother cell walls within a mucilaginous matrix, both of which progressively dry before sporophyte dehiscence.

A lack of correlation between guard cell size and DNA content, lack of arabinans in cell walls, and perpetually open pores are consistent with the inactivity of hornwort stomata.

Stomata are expendable in hornworts, as they have been lost twice in derived taxa. Guard cells and epidermal cells of hornworts show striking similarities with the earliest plant fossils.

Our findings identify an architecture and fate of stomata in hornworts that is ancient and common to plants without sporophytic leaves.