Plant development varies depending on whether it occurs in the light (photomorphogenesis) or in the dark (skotomorphogenesis). This phenomenon has been studied in depth in flowering plants such as Arabidopsis thaliana but remains largely unexplored in more ancient plant groups such as the bryophytes, which include the model moss, Physcomitrella patens, despite decades-old awareness of the markedly different gametophytic morphologies that result from photomorphogenesis and skotomorphogenesis in this moss. Research undertaken with Arabidopsis has elucidated in detail roles for various phytohormones, including auxin, in regulating the reversible developmental interconversion of photomorphogenesis and skotomorphogenesis. By contrast, although auxin has previously been shown to be active at many stages of Physcomitrella development, its specific involvement in photomorphogenesis and skotomorphogenesis and their interconversion, especially in relation to gametophore development, is unknown. Through the use of transgenic auxin-responsive GUS reporter strains in conjunction with auxin signalling and polar transport inhibitors it was shown that auxin is critically important for the development of gametophore stems and leaves in Physcomitrella during photomorphogenesis but it has a diminished role in leaf development and is not required for stem elongation during skotomorphogenesis. However, auxin is required for the gametophore bud to leafy gametophore transition in a dark-dependent manner and for protonemal development both in the dark and in the light. The roles of auxin in development of the moss gametophore are very similar to those in hypocotyl elongation in Arabidopsis, implying that they are ancient and have been highly conserved during land plant evolution.