DG Schaefer 2001.pdfVIP

Update on Physcomitrella patens The Moss Physcomitrella patens, Now and Then Didier G. Schaefer* and Jean-Pierre Zry?d Laboratoire de Phytoge?ne?tique Cellulaire, Institut d’Ecologie, Universite? de Lausanne, CH 1015 Lausanne, Switzerland 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 rela- tionship 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 evolu- tionary position that is ideal for comparative studies of the evolution of biological processes in land plants. Their life cycle is dominated by a photoau- totrophic haploid gametophytic generation that sup- ports a relatively simple and mainly heterotrophic diploid sporophyte. The haploid gametophyte it- self is characterized by two distinct developmental stages: the protonema, a filamentous network of chlor- onemal and caulonemal cells, which develop by api- cal growth and cell division of apical and subapical cells; and the gametophore or leafy shoot, which differentiates by caulinary growth from a simple api- cal meristem (the bud). The latter is made up of a photosynthetic non-vascularized stem, which carries the leaves and the reproductive organs and of fila- mentous 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