Three-dimensional anatomy and dorsoventral asymmetry of the mature <i>Marchantia polymorpha</i> meristem develops from a symmetrical gemma meristem

Meristems are three-dimensional (3D) generative structures that contain stem cells and produce new organs and tissues. Meristems develop in all land plants; however we know little about the spatial and temporal regulation of meristem structure in lineages such as bryophytes. Here, we describe the 3D meristem anatomy during the development of the liverwort Marchantia polymorpha. We show that the apical stem cell of the mature meristem is sub-apical, ventral, and in the outer cell layer. Mature meristem anatomy is therefore asymmetrical in the dorsoventral axis, which is reflected by the domain-specific protein localisation of Class III and Class IV Homeodomain-Leucine-Zippers (MpC3HDZ and MpC4HDZ), and by the promoter activity of MpYUCCA2. The dorsoventral asymmetry that defines the mature meristem is absent in the juvenile meristems of asexual propagules known as gemmae. We discovered that anatomical dorsoventral asymmetry of the meristem forms after 1 to 2 days of gemmaling growth, and is accompanied by expression of the dorsal identity reporter MpC3HDZ. We conclude that the gemma meristem has arrested development and undergoes anatomical rearrangement to develop the 3D meristem structure of the mature plant.