The origin and evolution of vascular tissue regulators TMO5 and LHW

The emergence of vascular plants (Tracheophytes) has had great impacts on our planet and now make up the majority of the biomass on Earth. The innovation of vascular tissues in plants allowed the colonization of a larger habitats and subsequently created the opportunity for new ecosystems to arise. Vascular tissues can thus be regarded as one of the key evolutionary innovations in the plant lineage. Yet, the molecular innovations that led to the evolution of these conductive tissues are unknown. Here, we reveal the evolutionary trajectory for the heterodimeric TMO5/LHW transcription factor complex, which is rate-limiting for vascular cell proliferation in Arabidopsis thaliana. We show that both regulators have origins predating vascular tissue emergence, and even terrestrialization. We further show that TMO5 evolved its modern function at the origin of land plants, by mutations in the DNA-binding region of the bHLH domain. An innovation in LHW, coinciding with vascular plant emergence, conditioned obligate heterodimerization and generated the critical function in vascular development. In summary, our results suggest that division potential of vascular cells may have been an important factor contributing to the evolution of vascular plants.