Cambridge iGEM 2014

Introduction to Marchantia polymorpha

Marchantia polymorpha is a member of one of the most primitive divisions of land plants, the liverworts. It grows as flat sheets of tissue called thalli (a singular one is called a thallus), and once spotted is a common sight by the sides of ponds and streams, and in shaded gaps in masonry throughout the world. Each plant grows to be between two and ten centimetres across, and is quite unlike more familiar plants in a number of ways.

Some Marchantia plants near a shoe for scale

Liverworts are defined by their simplicity. Marchantia, like other liverworts, has no vascular tissue and anchors itself not with true roots but with very long single cells called rhizoids, which become a matted array of filaments extending only ten or fifteen millimetres into the substrate. This simplicity is due to the retention of primitive characteristics evolved in early plants. One of these is the absence of flowers or seeds and the propagation of the plant in two old-fashioned ways.

One of these ways is through the splash cups (or gemma cups) which pockmark the upper surface of the plant. Inside these cups grow tiny sproutlings of Marchantia called gemmae, with undeveloped rhizoids and thalli. They are halted from growing large inside their cups by the presence of a compound called lunularic acid in the space they inhabit, placed there by the tissue of the splash cup. When it rains, drops which fall into these cups rebound out and take these tiny gemmae with them, carrying them to the soil. Once on solid ground, they start to grow into a new individual.

A gemma cup with gemmae visible inside

Marchantiacan reproduce sexually as well as through this ‘fragmentation’, and is in fact dioecious, meaning that an individual plant is either a male or a female. When induced by a shift in ambient light to far red, which signals that other plants are looming overhead, strange mushroom-like structures begin to sprout from the top of each plant. In males, these eventually take the form of a flat polygonal table, in females, the sprouts become archegonia, which look like miniature palm trees and when mature will bear eggs.

The antheridia, as the male table-like structures are called, produce two-tailed sperm instead of the pollen you might expect. These are spread by rain to the waiting eggs of the archegonia , which they fertilise and induce to become hardy spores. These spores number in the thousands in each archegonium, which when twinned with the dispersal of gemma makes Marchantia polymorpha very prolific.

An archegonium, removed from a female plant (left) and an antheridium from a male plant (right)

The plants also have the ability to regenerate a whole new plant from a mere scrap of tissue, without the necessity of adding any hormones or other growth factors. Like in other primitive plants such as ferns, every cell contains a haploid genome except for a small amount of sexual tissue during the plant’s spore production stage.

These characteristics and the simple structure of Marchantia’s genome, which often contains just one gene where in higher plants (like Arabidopsis thaliana) are found complex clusters of genes of similar and overlapping functions, make it a fascinating plant to study. They also make it a chassis perfect for synthetic biology.