Conditional stomatal closure in a fern shares molecular features with flowering plant active stomatal responses

Stomata evolved as plants transitioned from water to land, enabling carbon dioxide uptake and water loss to be controlled. In flowering plants, the most recently divergent land plant lineage, stomatal pores actively close in response to drought. In this response, the phytohormone abscisic acid (ABA) triggers signaling cascades that lead to ion and water loss in the guard cells of the stomatal complex, causing a reduction in turgor and pore closure. Whether this stimulus-response coupling pathway acts in other major land plant lineages is unclear, with some investigations reporting that stomatal closure involves ABA but others concluding that closure is passive. Here, we show that in the model fern Ceratopteris richardii active stomatal closure is conditional on sensitization by pre-exposure to either low humidity or exogenous ABA and is promoted by ABA. RNA-seq analysis and de novo transcriptome assembly reconstructed the protein-coding complement of the C. richardii genome, with coverage comparable to other plant models, enabling transcriptional signatures of stomatal sensitization and closure to be inferred. In both cases, changes in abundance of homologs of ABA, Ca²⁺, and ROS-related signaling components were observed, suggesting that the closure-response pathway is conserved in ferns and flowering plants. These signatures further suggested that sensitization is achieved by lowering the threshold required for a subsequent closure-inducing signal to trigger a response. We conclude that the canonical signaling network for active stomatal closure functioned in at least a rudimentary form in the stomata of the last common ancestor of ferns and flowering plants.