Evolutionary Conservation of ABA Signaling for Stomatal Closure

Abstract

Abscisic acid (ABA)-driven stomatal regulation reportedly evolved after the divergence of ferns, during the early evolution of seed plants approximately 360 million years ago. This hypothesis is based on the observation that the stomata of certain fern species are unresponsive to ABA, but exhibit passive hydraulic control. However, ABA-induced stomatal closure was detected in some mosses and lycophytes. Here, we observed that a number of ABA signaling and membrane transporter protein families diversified over the evolutionary history of land plants. The aquatic ferns Azolla filiculoides and Salvinia cucullata have representatives of 23 families of proteins orthologous to those of Arabidopsis (Arabidopsis thaliana) and all other land plant species studied. Phylogenetic analysis of the key ABA signaling proteins indicates an evolutionarily conserved stomatal response to ABA. Moreover, comparative transcriptomic analysis has identified a suite of ABAresponsive genes that differentially expressed in a terrestrial fern species, Polystichum proliferum. These genes encode proteins associated with ABA biosynthesis, transport, reception, transcription, signaling, and ion and sugar transport, which fit the general ABA signaling pathway constructed from Arabidopsis and Hordeum vulgare. The retention of these key ABA-responsive genes could have had a profound effect on the adaptation of ferns to dry conditions. Furthermore, stomatal assays have shown the primary evidence for ABA-induced closure of stomata in two terrestrial fern species P. proliferum and Nephrolepis exaltata. In summary, we report, to our knowledge, new molecular and physiological evidence for the presence of active stomatal control in ferns.

Evolution of the Stomatal Regulation of Plant Water Content

“…When combined with the fact that unlike angiosperms, fern and lycophyte stomata do not respond to endogenous levels of ABA (McAdam and Brodribb, 2012), the conclusion that ABA-mediated closure is not important in basal vascular plants seems robust. However, reports regularly emerge of small stomatal responses in fern and lycophyte guard cells artificially exposed to ABA levels hundreds of thousands to millions of times higher than endogenous levels (Ruszala et al, 2011;Cai et al, 2017;Hõrak et al 2017). The reasonable conclusion from these data is that fern and lycophyte guard cells react to exceedingly high levels of ABA, but the challenge remains to understand the adaptive relevance of such observations.…”

Fern Stomatal Responses to ABA and CO2 Depend on Species and Growth Conditions

“…1C; Brodribb and McAdam, 2011;Cai et al, 2017), suggesting that the ABA responsiveness may have been lost in some lineages of ferns. In addition to the direct effect on stomata, ABA reduces leaf hydraulic conductance in angiosperms (Shatil-Cohen et al, 2011;Pantin et al, 2013a).…”

“…This indicates that, while several ferns are capable of ABA perception and response, the kinetics and underlying molecular mechanisms of this response could be different in ferns and in angiosperms. The components of the core stomatal ABA signaling pathway have been shown to be present in mosses, lycophytes, ferns, and angiosperms (Hanada et al, 2011;Cai et al, 2017), indicating an early origin of ABA signal transduction. Nevertheless, it is possible that not all of these proteins have strictly stomata-related functions in ferns, as was shown recently for a homolog of the OST1 kinase that participates in sex determination in a fern (McAdam et al, 2016).…”

“…Recently, dose-dependent ABA-induced stomatal closure also was shown to be present in the ferns Polystichum proliferum and Nephrolepis exaltata (Cai et al, 2017). Several genes encoding proteins involved in ABA signal transduction are expressed in the stomata-bearing sporophyte of the moss P. patens (O'Donoghue et al, 2013) and in the epidermal fraction of the fern P. proliferum (Cai et al, 2017).…”

“…Recently, dose-dependent ABA-induced stomatal closure also was shown to be present in the ferns Polystichum proliferum and Nephrolepis exaltata (Cai et al, 2017). Several genes encoding proteins involved in ABA signal transduction are expressed in the stomata-bearing sporophyte of the moss P. patens (O'Donoghue et al, 2013) and in the epidermal fraction of the fern P. proliferum (Cai et al, 2017). Furthermore, the P. patens and Selaginella moellendorffii homologs of OPEN STOMATA1 (OST1), a SnRK-type kinase that participates in ABA-induced stomatal closure via phosphorylation of the central guard cell anion channel SLOW ANION CHANNEL1 (SLAC1) in Arabidopsis (Arabidopsis thaliana; Geiger et al, 2009;Lee et al, 2009;Vahisalu et al, 2010), complemented the ABA insensitivity of stomatal closure in the Arabidopsis ost1 mutant Ruszala et al, 2011).…”