Physcomitrella patens

Physcomitrella patens, the spreading earthmoss, is a moss (bryophyte) used as a model organism for studies on plant evolution, development, and physiology. Physcomitrella patens is an early colonist of exposed mud and earth around the edges of pools of water. P. patens has a disjunct distribution in temperate parts of the world, with the exception of South America. The standard laboratory strain is the 'Gransden' isolate, collected by H. Whitehouse from Gransden Wood, in Cambridgeshire in 1962. 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 may give insight into the evolution of mechanisms that contribute to the complexity of modern plants. In this context, P. patens is used as a model organism. P. patens is one of a few known multicellular organisms with highly efficient homologous recombination. meaning that an exogenous DNA sequence can be targeted 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 higher plants such as Arabidopsis thaliana, can be used to study molecular plant evolution. The targeted deletion or alteration of moss genes relies on the integration of a short DNA strand at a defined position in the genome of the host cell. Both ends of this DNA strand are engineered to be identical to this specific gene locus. The DNA construct is then incubated with moss protoplasts in the presence of polyethylene glycol. As mosses are haploid organisms, the regenerating moss filaments (protonemata) can be directly assayed for gene targeting within 6 weeks using PCR methods. The first study using knockout moss appeared in 1998 and functionally identified ftsZ as a pivotal gene for the division of an organelle in a eukaryote. In addition, P. patens is increasingly used in biotechnology. Examples are the identification of moss genes with implications for crop improvement or human health and the safe production of complex biopharmaceuticals in moss bioreactors. By multiple gene knockout Physcomitrella plants were engineered that lack plant-specific post-translational protein glycosylation. These knockout mosses are used to produce complex biopharmaceuticals in a process called molecular farming. The genome of P. patens, with about 500 megabase pairs organized into 27 chromosomes, was completely sequenced in 2008. 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.