Plant development is directed by auxin, a phytohormone which acts as a contextual cue that informs growth and organogenesis throughout a plant’s life, with many developmental decisions relying on plasma-membrane-localised PIN-FORMED (PIN) auxin transporters to control the flow of auxin out of cells. In angiosperms PINs are involved in the initiation of floral organ primordia, the arrangement of floral organs, root meristem formation, apical-basal embryo polarity establishment, vascular formation, and both photo- and gravitropism.
We provide the first characterization of PIN proteins in liverworts using Marchantia polymorpha as a model system. M. polymorpha possesses a single PIN gene, whose protein product is predicted to be plasma-membrane-localised, MpPIN1. One of our findings was that MpPIN1’s position on a plasma-membrane can be polarised, a key feature shared with flowering plant PINs. This indicates that PIN function has been largely conserved across more than 450 million years of independent evolution, although liverworts don’t have flowers, roots, seeds, or vasculature, so what is the function of MpPIN1 in M. polymorpha?
We found that, like for PINs in Arabidopsis, MpPIN1 influences the development of M. polymorpha in numerous ways throughout its life cycle. One recurrent theme was meristem support, and another was photo- and gravitropism, with PIN polarisation being most striking in the latter. We propose that involvement in orthotropic growth is a fundamental function of PIN polarisation.