Acquiring Control: The Evolution of Stomatal Signalling Pathways

In vascular plants, stomata balance two opposing functions: they open to facilitate CO2 uptake and close to prevent excessive water loss. Here, we discuss the evolution of three major signalling pathways that are known to control stomatal movements in angiosperms in response to light, CO2, and abscisic acid (ABA). We examine the evolutionary origins of key signalling genes involved in these pathways, and compare their expression patterns between an angiosperm and moss. We propose that variation in stomatal sensitivity to stimuli between plant groups are rooted in differences in: (i) gene presence/absence, (ii) specificity of gene spatial expression pattern, and (iii) protein characteristics and functional interactions. Recent findings reveal that stomata function differently in mosses and hornworts than in vascular plants, with bryophyte stomata promoting rather than preventing water loss. Important signalling genes that control stomatal opening and closure in response to changes in a plant’s environment have been characterised in angiosperms. Less is known about the evolutionary origins of these signalling pathways, and whether or not they are also present in bryophytes. Here, we review recent findings in this field, and further examine the evolutionary origins and expression patterns of key signalling genes, using newly available plant genomic and transcriptomic resources.