The development of hornwort stomata is very simple. This is indicated by the single longitudinal division of the guard cell precursor, pectinous ledges, lack of subsidiary cells, and lack of radial micellation. Gas exchange seems to be a likely function of hornwort stomata, but the absence of vascular tissue makes water transport improbable. Histochemical stains for malate and potassium indicate that guard cells localize ions for a short time- after the differentiation of the epidermis and before spore dispersal.
Diurnal guard cell movements do not occur in hornworts. Neither dehydration or ABA treatment effects the guard cells in respect to movement. It is still unclear whether or not hornwort stomata are homologous to stomata of vascular plants. The prominent chloroplast, the localization of ions, and the role of stomata in gas exchange suggest that anthocerote stomata are related to those of other embryophytes.
However, the lack of vascular tissue and stomatal movement counter the homologous theory. Also in opposition to this paradigm is the distinct wall structure of hornwort guard cells. A multilayered wall and ledges of pectin have only been reported for a few other plants. In the future to elucidate the homology of these structures, the effect of ABA should further be studied. Also the guard cells’ ability to transport ions, which is essential for movement, should be determined.
As one of the earliest plant groups to evolve stomata, hornworts are key to understanding stomata origin and function.
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. 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 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.