Origin and function of stomata in the moss Physcomitrella patens.

Abstract

Stomata are microscopic valves on plant surfaces that originated over 400 million years ago and facilitated the greening of Earth’s continents by permitting efficient shoot-atmosphere gas exchange and plant hydration1. However, the core genetic machinery regulating stomatal development in non-vascular land plants is poorly understood2–4 and their function has remained a matter of debate for a century5. Here, we show that genes encoding the two basic helix-loop-helix proteins PpSMF1 and PpSCRM1 in the moss Physcomitrella patens are orthologous to transcriptional regulators of stomatal development in the flowering plant Arabidopsis thaliana and essential for stomata formation in moss. Targeted knock-out P. patens mutants lacking either PpSMF1 or PpSCRM1 develop gametophytes indistinguishable from wild-type plants but mutant sporophytes lacking stomata. Protein-protein interaction assays reveal heterodimerisation between PpSMF1 and PpSCRM1 which, together with moss-angiosperm gene complementations6, suggests deep functional conservation of the heterodimeric SMF1 and SCRM1 unit required to activate transcription for moss stomatal development, as in A. thaliana7. Moreover, stomata-less sporophytes of ΔPpSMF1 and ΔPpSCRM1 mutants exhibited delayed dehiscence, implying stomata might have promoted dehiscence in the first complex land plant sporophytes.