DSSS - Development of the Marchantia polymorpha sporeling

The multicellular bodies of plants develop from single cells; the multicellular diploid phase of the plant life cycle develops from a polarised zygote, and the multicellular haploid phase develops from a spore without polarity. Each of these cells produces a mass of cells on which meristems – the generative centres that form the plant body – develop. The aim of our research is to define the molecular mechanisms that control the morphogenetic transitions that occur between spore germination and the formation of the first functional meristem in the haploid phase of the Marchantia polymorpha life cycle.

Development starts with imbibition of the spore that swells continuously until it divides divides 28-30 h later. Cryptochrome-mediated blue light signalling promotes cell division and acts by negatively regulating transcription factors that repress division. The nucleus is located at the cell centroid until 27-29 h after imbibition when phototropin-mediated blue light signalling directs the migration of the nucleus to the basal pole. There, the nuclear envelope disappears and mitosis occurs. The basal location of the mitotic spindle and associated phragmoplast results in the formation of a relatively large cell at the apical pole that divides to form an early cell mass, and a smaller cell that terminally differentiates as a rhizoid cell at the basal pole. This asymmetry establishes the apical-basal body axis that is propagated during the subsequent devleopment of the plant.

A single meristem develops at the apical pole of the apical-basal axis after approxiamtely 10 days. Using a combination of approaches – quantitative imaging, single nucleus sequencing, single sporeling sequencing, marker expression – we defined the stages in the formation of an active meristem from a population of non-meristematic cells. A single cell on the surface of the early cell mass divides asymmetrically to form a flat structure – prothallus – on which a meristem forms. Between 7 and 10 days after imbibition genes with meristem functions express. We are defining gene regulatory programs that are expressed before this meristem program. We hypothesize that among these pre-meristem programs, is a set of genes required for the initiation of the meristem. We will report an the latest data on the nature of the mechanisms that promotes the de novo generation of the first meristem in the Marchantia sporeling.