Simple plant systems as testbeds for engineering

Marchantia polymorpha sporelings, 1 week after spore gemination (image: Jim Haseloff)

Marchantia polymorpha sporelings, 1 week after spore gemination
(image: Jim Haseloff)

The liverworts (or Marchantiophyta) are descendants of the earliest terrestrial plants. The group is characterised by morphological simplicity. Liverworts have been a largely neglected area of plant biology, but show great promise as model plant systems after recent developments in transformation methods, genome characterisation and biotechnology.

Marchantia polymorpha is the best characterised liverwort. It is a thalloid liverwort, forming a body of sheet-like tissues that possess distinct upper and lower surfaces. The upper surface has a modular structure, with repeated cellular units that form simple cell complexes adapted for photosynthesis and gas exchange. Like other Bryophytes, the gametophyte or haploid generation is dominant phase of the life cycle. Marchantia has a global distribution, and is often found as a weed in horticulture. The plants grow vigorously on soil or artificial media. Marchantia plants spontaneously produce clonal vegetative propagules, or gametogenesis can be induced by exposure to far red light. Male and female plants can be sexually crossed to produce spores. The plants are extraordinarily prolific. A single cross can produce millions of propagules in the form of single-cell spores. Spores can be harvested in huge numbers and stored indefinitely in a cold, desiccated state. Each spore can germinate to produce a new plant, and, unlike higher plants, can undergo the entire developmental sequence to produce an adult plant under direct microscopic observation. Sequencing efforts have provided a draft of the ~280Mbp genome. Most of the major gene families present in more advanced plants are represented by a single or few orthologues in Marchantia, meaning that there is low genetic redundancy. The apparent simplicity of genetic networks in liverworts, combined with the growing set of techniques for genetic manipulation, culture and microscopy, are set to make this primitive plant a major new system for analysis and engineering. OpenPlant has adopted Marchantia as a simple testbed for plant synthetic biology.