Abstract
Recent studies have revealed that some responses of fern stomata to environmental signals differ from those of their relatives in seed plants. However, it is unknown whether the biophysical properties of guard cells differ fundamentally between species of both clades. Intracellular micro-electrodes and the fluorescent Ca2+ reporter FURA2 were used to study voltage-dependent cation channels and Ca2+ signals in guard cells of the ferns Polypodium vulgare and Asplenium scolopendrium. Voltage clamp experiments with fern guard cells revealed similar properties of voltage-dependent K+ channels as found in seed plants. However, fluorescent dyes moved within the fern stomata, from one guard cell to the other, which does not occur in most seed plants. Despite the presence of plasmodesmata, which interconnect fern guard cells, Ca2+ signals could be elicited in each of the cells individually. Based on the common properties of voltage-dependent channels in ferns and seed plants, it is likely that these key transport proteins are conserved in vascular plants. However, the symplastic connections between fern guard cells in mature stomata indicate that the biophysical mechanisms that control stomatal movements differ between ferns and seed plants.
Highlights
The development of stomata has been a major step in the evolution of land plants, as these adjustable pores in the leaf surface allow plants to restrict water loss (Berry et al, 2010; Raven & Edwards, 2014; Brodribb & McAdam, 2017)
The conductance of fern guard cells is dominated by voltage-dependent ion channels In seed plants, the uptake and release of K+ causes changes in the osmotic pressure of guard cells, which lead to stomatal movements (Roelfsema & Hedrich, 2005)
Little is known about voltage-dependent channels of ferns and we impaled doublebarreled electrodes filled with 300 mM KCl into guard cells of Polypodium vulgare and Asplenium scolopendrium
Summary
The development of stomata has been a major step in the evolution of land plants, as these adjustable pores in the leaf surface allow plants to restrict water loss (Berry et al, 2010; Raven & Edwards, 2014; Brodribb & McAdam, 2017). Stomata are of key importance in adaptive responses to changes in environmental conditions and they are a major topic of interest in plant biology. In accordance with their main function, stomata of seed plants open in response to light and low atmospheric CO2 levels and close in response to low air humidity (Shimazaki et al, 2007; Kim et al, 2010; Bauer et al, 2013; Kollist et al, 2014). Several studies found that the drought hormone abscisic acid (ABA) closes stomata of mosses, lycophytes and ferns (Chater et al, 2011, 2016; Ruszala et al, 2011; Horak et al, 2017), but other experiments on lycophytes and ferns revealed that stomata of these species are ABA insensitive (Brodribb & McAdam, 2011; McAdam & Brodribb, 2012; Duckett & Pressel, 2018)
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