Auxin regulation of axial growth in bryophyte sporophytes: its potential significance for the evolution of early land plants

To identify developmental mechanisms that might have been involved in the evolution of axial sporophytes in early land plants, we examined the effects of auxin‐regulatory compounds in the sporophytes of the hornwort Phaeoceros personii , the liverwort Pellia epiphylla , and the moss Polytrichum ohioense , members of the three divisions of extant bryophytes. The altered growth of isolated young sporophytes exposed to applied auxin (indole‐3‐acetic acid) or an auxin antagonist ( p ‐chlorophenoxyisobutyric acid) suggests that endogenous auxin acts to regulate the rates of axial growth in all bryophyte divisions. Auxin in young hornwort sporophytes moved at very low fluxes, was insensitive to an auxin‐transport inhibitor (N‐[1‐naphthyl]phthalamic acid), and exhibited a polarity ratio close to 1.0, implying that auxin moves by simple diffusion in these structures. Emerging liverwort sporophytes had somewhat higher auxin fluxes, which were sensitive to transport inhibitors but lacked any measurable polarity. Thus, auxin movement in liverwort sporophytes appears to result from a unique type of apolar facilitated diffusion. In young Polytrichum sporophytes, auxin movement was predominantly basipetal and occurred at high fluxes exceeding those measured in maize coleoptiles. In older Polytrichum sporophytes, acropetal auxin flux had increased beyond the level measured for basipetal flux. Insofar as acropetal and basipetal fluxes had different inhibitor sensitivities, these results suggested that moss sporophytes carry out bidirectional polar transport in different cellular pathways, which resembles the transport in certain angiosperm structures. Therefore, the three lineages of extant bryophytes appear to have evolved independent innovations for auxin regulation of axial growth, with similar mechanisms operating in moss sporophytes and vascular plants.