The fossil record suggests stomata-like pores were present on the surfaces of land plants over 400 million years ago.
Whether stomata arose once or whether they arose independently across newly evolving land plant lineages has long been
a matter of debate. In Arabidopsis, a genetic toolbox has been identified that tightly controls stomatal development and
patterning. This includes the basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, FAMA, and
ICE/SCREAMs (SCRMs), which promote stomatal formation. These factors are regulated via a signaling cascade, which
includes mobile EPIDERMAL PATTERNING FACTOR (EPF) peptides to enforce stomatal spacing. Mosses and hornworts,
the most ancient extant lineages to possess stomata, possess orthologs of these Arabidopsis (Arabidopsis thaliana) stomatal
toolbox genes, and manipulation in the model bryophyte Physcomitrella patens has shown that the bHLH and EPF
components are also required for moss stomatal development and patterning. This supports an ancient and tightly
conserved genetic origin of stomata. Here, we review recent discoveries and, by interrogating newly available plant
genomes, we advance the story of stomatal development and patterning across land plant evolution. Furthermore, we
identify potential orthologs of the key toolbox genes in a hornwort, further supporting a single ancient genetic origin of
stomata in the ancestor to all stomatous land plants
