The enigmatic pseudostomata of Sphagnum

Photo credit: Research Gate

Fig. 1. Sphagnum sporophytes. (A) SEM of close capsule covered by the calyptra (arrowhead) and attached to the pseudopodium (arrow). (B) SEM of pseudostomata in closed capsule. (C) Fluorescence light microscopy of chloroplasts autofl uorescing (arrows) in pseudostoma and epidermal cells. (D) SEM of open capsule. (E) SEM of collapsed pseudostoma in open capsule. (F–I) Light micrographs of semithin cross sections of capsules. (F) Capsule with several layers of capsule wall (arrow) and mature spores. (G) Pseudostoma in close capsule. (H) Pseudostoma in close capsule with ventral walls that begun to separate. (I) Collapse pseudostoma in open capsule. Scale bars: A, D = 1 mm; B, E = 50 μm; C = 10 μm; F = 75 μm; G, H, I = 20 μm.

Highlights: Clues on the evolution of stomata: The enigmatic pseudostomata of Sphagnum

by Merced A. (2015)

in Am. J. Bot. 102(3): 329, 2015 doi:10.3732/ajb.1400564

Abstract

Stomata are ubiquitous in green aerial parts of vascular plants and are critical for plant

survival. Multiple lines of evidence support a single evolutionary origin of stomata in land plants yet the absence of true stomata in early-divergent mosses challenges this hypothesis.

Sphagnum are mosses that have specialized epidermal cells located in the midsection of the capsule surface, known as pseudostomata, which partially separate but never open to the inside. Merced (pp. 329–335) describes the anatomy and structure of the enigmatic pseudostomata of Sphagnum to evaluate affiliation with true stomata.

Pseudostomata are structurally unique but similarities to true stomata suggest that pseudostomata may be related to or precursors of stomata

https://www.researchgate.net/publication/280288657_Highlights_Clues_on_the_evolution_of_stomata_The_enigmatic_pseudostomata_of_Sphagnum [accessed May 6, 2016].

Changes in pectin composition during stomatal development

 

Photo credit: Annals of Botany

Fig. 1.

Diagram of a Funaria stoma, which consists of guard cells with continuous cytoplasm. (A) Transverse section though polar end. (B) Transverse section through pore. DW, dorsal wall; gc, guard cell; IW, inner wall; OW, outer wall; VW, ventral wall.

Developmental changes in guard cell wall structure and pectin composition in the moss Funaria: implications for function and evolution of stomata

by Merced A.,

 Renzaglia  K. S.

(2014)

in Annals of Botany 114:1001-1010. – doi: 10.1093/aob/mcu165 –

Fig. 4. Mature stomata. (A) Guard cells with layered walls. (B) Guard cell outer wall with differential LM19 label in external, middle and internal wall layers. (C) More homogeneous LM6 label in wall layers of outer wall. Asterisk denotes the cuticle. (D) LM13 label in guard cell wall but not in epidermal cell wall. (E) LM19 localized in outer ledge. (F) LM13 did not localize in epidermal cells. (G, H) LM20 (G) and LM6 (H) did not localize in the outer ledge except very sparsely (arrows). Scale bars: (A) = 2 µm; (B, C) = 100 nm; (D, F) = 500 nm; (E, G, H) = 100 nm. Abbreviations: c, chloroplast; DW, dorsal wall; ecw, epidermal cell wall; EWL, external wall layer; gcw, guard cell wall; IWL, inner wall layer; m, mitochondrion; MWL, middle wall layer; n, nucleus; OW, outer wall; v, vacuole. – http://aob.oxfordjournals.org/content/114/5/1001/F4.medium.gif

Abstract

Background and Aims

In seed plants, the ability of guard cell walls to move is imparted by pectins. Arabinan rhamnogalacturonan I (RG1) pectins confer flexibility while unesterified homogalacturonan (HG) pectins impart rigidity. Recognized as the first extant plants with stomata, mosses are key to understanding guard cell function and evolution. Moss stomata open and close for only a short period during capsule expansion. This study examines the ultrastructure and pectin composition of guard cell walls during development in Funaria hygrometrica and relates these features to the limited movement of stomata.

 

Methods

Developing stomata were examined and immunogold-labelled in transmission electron microscopy using monoclonal antibodies to five pectin epitopes: LM19 (unesterified HG), LM20 (esterified HG), LM5 (galactan RG1), LM6 (arabinan RG1) and LM13 (linear arabinan RG1). Labels for pectin type were quantitated and compared across walls and stages on replicated, independent samples.

 

Key Results

Walls were four times thinner before pore formation than in mature stomata. When stomata opened and closed, guard cell walls were thin and pectinaceous before the striated internal and thickest layer was deposited. Unesterified HG localized strongly in early layers but weakly in the thick internal layer. Labelling was weak for esterified HG, absent for galactan RG1 and strong for arabinan RG1. Linear arabinan RG1 is the only pectin that exclusively labelled guard cell walls. Pectin content decreased but the proportion of HG to arabinans changed only slightly.

 

Conclusions

This is the first study to demonstrate changes in pectin composition during stomatal development in any plant. Movement of Funaria stomata coincides with capsule expansion before layering of guard cell walls is complete. Changes in wall architecture coupled with a decrease in total pectin may be responsible for the inability of mature stomata to move. Specialization of guard cells in mosses involves the addition of linear arabinans.

Development of stomata in Funaria

Photo credit: Annals of Botany

FIG. 1.

Mature capsules of Funaria. (A) Scanning electron micrograph of expanded capsule with 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

by Merced A.,

 Renzaglia  K. S.

(2016)

in Ann Bot (2016) – 117(6):mcw029 · April 2016

doi: 10.1093/aob/mcw029 – First published online: April 23, 2016

Abstract

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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Ann Bot-2016-ContentSnapshoot

AnnalsofBotany

Volume 117 Number 6 2016

Stomata in the moss Funaria are regularly spaced at the base of the capsule.

Merced and Renzaglia (pp. 985–994) describe the patterning and distribution of stomata and how they are coordinated with the formation of substomatal cavities and spaces in the

sporophyte. Unlike Arabidopsis,stomata in moss do not develop from a meristem-like cell that continuously divides. Instead guard mother cells differentiate into guard cells from non-contiguous protodermal cells arranged in files.

The surrounding epidermal cells divide after stomata are formed in synchrony with capsule expansion. Differentiation of guard cells before the rest of the epidermis ensures that stomata are spaced apart from each other.

Ann Bot-2016-ContentSnapshoot. Available from: https://www.researchgate.net/publication/303715177_Ann_Bot-2016-ContentSnapshoot [accessed Jun 2, 2016].

 

Stomata in sporophytes of mosses

Photo credit: Am. J. Bot.

(A–C) Oedipodium griffithianum. (A) SEM of capsule and part of apophysis (arrow to bottom of image). Bar = 300 µm. (B) SEM of open long-pored stoma. Bar = 10 µm. (C) SEM of closed pore filled with cuticular waxes. Bar = 10 µm. (D–G)Ephemerum spinulosum. (D) SEM of mature sporophyte attached to gametophyte. Bar = 200 µm. (E) SEM of base of capsule with stomata. Bar = 100 µm. (F) Stoma in fluorescent light treated with DAPI showing blue nuclei and autofluorescing red chloroplasts; the round pore is flanked by wall ledges (arrow), and bacteria are abundant around guard cells (b). Bar = 10 µm. (G) SEM of stoma partially filled with cuticular waxes. Bar = 10 µm.

Moss stomata in highly elaborated Oedipodium (Oedipodiaceae) and highly reduced Ephemerum (Pottiaceae) sporophytes are remarkably similar

by Merced A.,

 Renzaglia K. S.

(2013)

in American Journal of Botany 100(12): 2318-2327. 2013.

(http://www.amjbot.org/cgi/doi/10.3732/ajb.1300214)

Diagram of stoma in (A) tangential and (B) transverse section. gc, guard cell; pe, polar end; ow, outer wall; dw, dorsal wall; iw, inner wall; vw, ventral wall. –
http://www.amjbot.org/content/100/12/2318/F1.small.gif

ABSTRACT

• Premise of the study: Mosses are central in understanding the origin, diversification, and early function of stomata in land plants. Oedipodium, the first extant moss with true stomata, has an elaborated capsule with numerous long-pored stomata; in contrast, the reduced and short-lived Ephemerum has few round-pored stomata. Here we present a comparative study of sporophyte anatomy and ultrastructure of stomata in two divergent mosses and its implications for stomata diversity and function.

Oedipodium griffithianum –
http://www.biosphere-images.net/Atlantic-European%20Album%20(Bryophytes)/pictures/picture-38.jpg

• Methods: Mature sporophytes of two moss species were studied using light, fluorescence, and scanning and transmission electron microscopy. Immunolocalization of pectin was conducted on Oedipodium using the LM19 antibody.

Ephemerum serratum –
http://www.cisfbr.org.uk/images/Ephemerum_serratum_007C.JPG

• Key results: Oedipodium capsules have extensive spongy tissue along the apophysis, whereas those of Ephemerum have minimal substomatal cavities. Stomatal ultrastructure and wall thickenings are highly similar. Sporophytes are covered by a cuticle that is thicker on guard cells and extends along walls surrounding the pore. Epicuticular waxes and pectin clog pores in old capsules.

• Conclusions: Ultrastructure of stomata in these mosses is similar to each other and less variable than that of tracheophytes. Anatomical features such as the presence of a cuticle, water-conducting cells, and spongy tissues with large areas for gas exchange are more pronounced in Oedipodium sporophytes and support the role of stomata in gas exchange and water transport during development and maturation. These features are modified in the reduced sporophytes of Ephemerum. Capsule anatomy coupled with the exclusive existence of stomata on capsules supports the concept that stomata in moss may also facilitate drying and dispersal of spores.

Read the full article: Am. J. Bot.