The moss Physcomitrella patens, now and then.

Mosses (Musci, Bryophytaea) are one of the oldest groups of land plants present among the earth’s flora. They originated 500 million years ago (for a recent discussion of the time scale involved, see Heckman et al., 2001) and are currently represented by approximately 10,000 species that colonize diverse habitats that range from high mountains to deep forests and from Antarctica to deserts. Evolutionary studies support the monophyletic origin of land plants and indicate that bryophytes may form a sister clade with tracheophytes, although the exact relationship between bryophytes (hornworts, liverworts, and mosses) and vascular plants is still a matter of debate (Kenrick and Crane, 1997; Nickrent et al., 2000). This nevertheless places mosses at an evolutionary position that is ideal for comparative studies of the evolution of biological processes in land plants. Their life cycle is dominated by a photoautotrophic haploid gametophytic generation that supports a relatively simple and mainly heterotrophic diploid sporophyte. The haploid gametophyte itself is characterized by two distinct developmental stages: the protonema, a filamentous network of chloronemal and caulonemal cells, which develop by apical growth and cell division of apical and subapical cells; and the gametophore or leafy shoot, which differentiates by caulinary growth from a simple apical meristem (the bud). The latter is made up of a photosynthetic non-vascularized stem, which carries the leaves and the reproductive organs and of filamentous rhizoids that arise from the base of the stem (for review, see Reski, 1998). The potential of mosses as model systems to study plant biological processes was already recognized in the forties and reflects their relatively simple developmental pattern, their suitability for cell lineage analysis, their similar responses to plant growth factors and environmental stimuli as those observed in other land plants, and the facilitated genetic approaches resulting from the dominance of the gametophyte in their life cycle (Cove et al., 1997). Plant physiologists have focused their studies on species like Funaria hygrometrica, Ceratodon purpureus, and Physcomitrella patens, but it was the possibility to realize crosses in vitro that led the latter to be chosen for genetic approaches. P. patens is a monoecious moss (i.e. both sex organs are present on the same individual) that requires very simple growth conditions for the completion of its life cycle (Fig. 1A; Cove, 1992). P. patens is the first moss to be successfully transformed (Schaefer et al., 1991) and has recently been singled-out as the first land-plant, and perhaps more interestingly the first multicellular eukaryote, in which gene targeting occurs with an efficiency similar to that observed in the yeast Saccharomyces cerevisiae (Schaefer and Zrÿd, 1997; Schaefer, 2001).