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

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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.

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Published by

Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

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