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.

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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.

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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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