Physcomitrella patens is a species of moss that has been studied experimentally for over half a century. The techniques that have been developed for the study of this model organism allow us to answer biological questions relating to plant growth and development, adaptation to environments and plant evolution. The recent completion of the P. patens genome sequence as the first non-angiosperm land plant allows comparative genomic analyses and identification of conserved genes as well as genes with novel functions. P. patens has a special feature among plants, that of an efficient gene-targeting mechanism, enabling gene functions to be identified by targeted mutations. Such comparative and experimental approaches allowed the determination of similarities and differences between mosses and angiosperms in their response to light and hormones. Finally, we may be able to learn how plants deal with environmental stresses by studying these processes in P. patens.

Key concepts:

• P. patens is a model organism because it can be studied simply within a controlled environment to address biological questions.
  
• P. patens is easy to culture in vitro because the cells are totipotent.
  
• P. patens undergoes highly efficient gene targeting that allows in vivo analysis of a gene's function by mutating the endogenous gene after introduction of a cloned homologue.
  
• It is easy to identify mutant phenotypes in P. patens because the gametophytic cells are haploid.
  
• P. patens is the first nonangiosperm land plant to have its genome sequence published.
  
• Genomic comparisons and other studies suggest that there are many similarities in the ways in which mosses and angiosperms use environmental cues to develop, indicating conserved evolution; however, mosses also possess some unique sequences which might present novel gene functions.