Physcomitrella patens

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

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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.^[1] Basically, this means that researchers are able to target an exogenous DNA sequence to a specific genomic position; 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.

The genome of Physcomitrella patens, with about 500MB organized in 27 chromosomes, was completely sequenced in 2006.^[2]

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, 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.

References

1. ^ Schaefer, D. (2002) A new moss genetics: Mutagenesis in Physcomitrella patens. Annu. Rev. Plant Biol. 53: 477–501.
2. ^ JGI