The TOPLESS corepressor regulates developmental switches in the bryophyte Physcomitrium patens that were critical for plant terrestrialisation.

The plant-specific TOPLESS (TPL) family of transcriptional corepressors is integral to multiple angiosperm developmental processes. Despite this, we know little about TPL function in other plants. To address this gap, we investigated the roles TPL plays in the bryophyte Physcomitrium patens, which diverged from angiosperms approximately 0.5 billion years ago. Although complete loss of PpTPL function is lethal, transgenic lines with reduced PpTPL activity revealed that PpTPLs are essential for two fundamental developmental switches in this plant: the transitions from basal photosynthetic filaments (chloronemata) to specialised foraging filaments (caulonemata) and from 2-dimensional to 3-dimensional growth. Using a transcriptomics approach, we integrate PpTPL into the regulatory network governing 3D growth and propose that PpTPLs represent another important class of regulators that are essential for the 2D to 3D developmental switch. Transcriptomics also revealed a previously unknown role for PpTPL in the regulation of flavonoids. Intriguingly, 3D growth and formation of caulonemata were crucial innovations that facilitated the colonization of land by plants; a major transformative event in the history of life on Earth. We conclude that TPL, which existed before the land plants, was co-opted into new developmental pathways, enabling phytoterrestrialisation and the evolution of land plants.