Physcomitrella patens

Physcomitrella patens is a moss (Bryophyta) used as a model organism for studies on plant evolution, development and physiology.

Model organism
Mosses share fundamental genetic and physiological processes with vascular plants, although the two lineages diverged early in land plant evolution. A comparative study between modern representatives of the two lines can give an insight into the evolution of the mechanisms behind the complexity of modern plants. It is in this context that Physcomitrella patens is used as a model organism.

Physcomitrella patens is one of a few known multicellular organisms with highly efficient homologous recombination. Basically, this means that researchers are able to target an exogenous DNA sequence to a specific genomic position (a technique called gene targeting) to create knockout mosses. This approach is called reverse genetics and it is a powerful and sensitive tool to study the function of genes and, when combined with studies in plants like Arabidopsis thaliana, can help to unravel major molecular trends of plant evolution.

In addition, P. patens is increasingly used in biotechnology. Prominent examples are the identification of moss genes with implications for crop improvement or human health and the safe production of complex biopharmaceuticals in the moss bioreactor, developed by Ralf Reski and his co-workers.

The genome of Physcomitrella patens, with about 500 megabase pairs organized into 27 chromosomes, was completely sequenced in 2006.

Physcomitrella ecotypes, mutants, and transgenics are stored and made freely available to the scientific community by the International Moss Stock Center (IMSC). The accession numbers given by the IMSC can be used for publications to ensure safe deposit of newly described moss materials.

Life cycle
Like all mosses, the life cycle of Physcomitrella patens is characterized by an alternation of two generations: 1) a haploid gametophyte that produces gametes and 2) a diploid sporophyte where haploid spores are produced.

A spore develops into a filamentous structure called protonema, composed of two types of cells – chloronema with large and numerous chloroplasts and caulonema with very fast growth. Protonema filaments grow exclusively by tip growth of their apical cells and can originate side branches from subapical cells. Some side branch initial cells can differentiate into buds rather than side branches. These buds give rise to gametophores (0.5 – 5 mm ), more complex structures bearing leaf-like structures, rhizoids and the sexual organs: female archegonia and male antheridia. Physcomitrella patens is monoicous, meaning that male and female organs are produced in the same plant. If water is available flagellate sperm cells can swim from the antheridia to an archegonium and fertilize the egg within. The resulting diploid zygote originates a sporophyte composed of a foot, seta and capsule, where thousands of haploid spores are produced by meiosis.

Distribution
Physcomitrella patens is widely distributed in the northern hemisphere.