The three-dimensional anatomy and dorsoventral asymmetry of the mature Marchantia polymorpha meristem develops from a symmetrical gemma meristem

Meristems are three-dimensional generative structures that maintain a population of stem cells whilst producing new organs and tissues. Meristems develop in all land plants, however we know relatively little about the spatial and temporal regulation of meristem structure in lineages such as the bryophytes. Here we describe the three-dimensional anatomy of the meristem during the development of the liverwort, Marchantia polymorpha. Using optical reconstructions of the frontal, sagittal and transverse planes through the mature meristem, we show that the apical stem cell is sub-apical, ventral, and located in the outer cell layer. The anatomy of the mature meristem is therefore asymmetrical in the dorsoventral axis, which is reflected by the domain specific protein localisation of Marchantia Class III and Class IV Homeodomain-Leucine-Zippers (MpC3HDZ and MpC4HDZ) and promoter activity of MpYUCCA2. The dorsoventral asymmetry that defines the mature meristem is absent in the juvenile meristems of the asexual propagules known as gemmae. We discovered that anatomical dorsoventral asymmetry of the meristem forms after two days of gemmaling growth and is accompanied by expression of the dorsal identity reporter, MpC3HDZ. We conclude that the gemma meristem is in a state of arrested development and undergoes anatomical rearrangement to develop the three-dimensional meristem structure of the mature plant.