Genetic Transformation of the Moss Physcomitrella patens

Abstract

Physcomitrella patens is a widely spread moss species which colonizes open habitat in cold temperate zones. For the developmental geneticist, haplobiontic plants, such as mosses, present distinct advantages over the diplobiontic angiosperms: the haploid gametophytic generation dominates over the diploid sporophytic generation during most of their life cycle. This facilitates mutant isolation and genetic analysis (Cove 1983; Ashton et al. 1988; Wang and Cove 1989). Since the first report of the successful isolation of biochemical and morphological mutants in P.patens (Engel 1968), this organism has been used as a model genetic system for physiological and developmental studies. In vitro propagation conditions have been defined which allow the complete life cycle to be achieved on a simple mineral medium in less than 3 months (Ashton and Cove 1977). Physiological studies have demonstrated that the main factors controlling development in P. patens were similar to those involved in angiosperms, i.e., light, gravity, and growth substances such as cytokinins and auxins (Cove et al. 1978; Cove and Ashton 1984, 1988). The simple morphology and the well-defined cell lineage in the development of the gametophyte (Cove 1992) offer the opportunity to study morphogenetic processes at the single cell level, permitting the efficient combination of genetic, cellular, and physiological approaches. An efficient protoplast system has been developed which allows regeneration of up to 80% of protoplasts into fertile plants (Grimsley, et al. 1977). The production of new chloronemata from these protoplasts is strictly light-dependent (Jenkins and Cove 1983), does not require an exogenous hormone supply, and does not involve the dedifferentiation stages that are typical in angiosperms. Somatic fusion of these protoplasts has been used for complementation analysis and dominance studies in fertile diploid plants (Grimsley et al. 1980; Ashton et al. 1988). Finally, chemical mutagenesis has been achieved using spores or somatic tissue to induce mutants of the following classes (review and references in Knight et al. 1988):

(1) auxotrophy and analog resistance, (2) alteration of the gravitropic or phototropic responses (Knight et al. 1991), (3) alteration of the overall morphology, (4) cytokinin and auxin resistance.