reviews Mosses as model systems oov,, covo, Celia D. Knight and TiHman Lamparter Mosses hold many attractions as mode] organisms for research in plant science. Their position as the simplest of land plants makes them central to the study of plant evo- lution, particularly in shedding light on how their aquatic predecessors evolved to survive on land. The use of mosses for developmental studies hinges on the ability to observe development in living material at the level of the individual cell. However, more recently techniques for the molecular analysis of mosses have provided tools for new approaches for determining the mechanisms controlling plant development, incorporating both cell and molecular biology. T he use of mosses as model systems for the study of plant genetics and development is not new. The research of von Wettstein and his co-workers on mosses, from 1920-1945, was at the forefront of genetic research at that time. These studies, which put mosses alongside Drosophila and maize as important model sys- tems for genetic studies, contributed to our understanding of tetrad formation and both X-ray and chemical mutagen- esis. The potential of mosses for the study of plant develop- ment, using a combination of genetic and physiological procedures, was already recognized over 70 years ago 1, and was beginning to be realized by von Wettstein's research group by 1940 (Ref. 2). However, von Wettstein's death in 1945 led to a hiatus in moss research, and only in recent years has their use as model organisms, particularly for the study of plant development, become reestablished. Although there is an extensive literature on the develop- mental physiology of many moss species3, this review will concentrate principally on three species- Ceratodon pur- pureus, Funaria hygrometrica and Physcomitrella patens - for which there are also recent genetic studies. These species may be cultured without difficulty under con- trolled conditions, using microbiological techniques in- cluding axenic culture - not only in petri dishes, but also, for example, in fermenters 4 (see Fig. 1). Moss development The moss life cycle comprises a free-living haploid ga- metophyte stage and a diploid sporophyte stage, with the sporophyte partly dependent for nutrition and support on the gametophyte. Gametes are produced by the gameto- phyte via mitosis, and fuse to produce zygotes that develop into the sporophyte. The sporophyte produces spores by meiosis that germinate to produce further gametophytes. Most developmental studies, and certainly those employing genetic analysis, have concentrated on the gametophyte stage. Its haploid condition alone gives it an advantage, allowing the effects of recessive mutations to be observed directly. Spore germination in almost all moss species results in the production of a filamentous stage, the protonema. This stage is short-lived in some species; however, in other species, including the three considered here, the protonemal stage can be long-lived, especially under lab culture. The protonemal stage of Funaria com- prises branching cell filaments extending by the serial division of the filament apical cell, with subapical cells dividing to produce side filaments. Protonemata contain Fig. 1. Air-lift fermenter culture of Physcomitrella patens. The green moss cells in suspension in the central cylinder are illuminated by banks of fluorescent tubes on either side. Protonemal tissue, cultured in a 6 1 air-lift fermenter incor- porating a tissue chopper, grows exponentially with a doubling time of approximately 26 h to give yields in excess of 1 g (dry weight) 1 =1. two distinct cell types, chloronemata and caulonemata. Chloronemal filaments have cells densely packed, with large chloroplasts and cross walls between adjacent cells that are perpendicular to the filament axis. Caulonemal © 1997 Elsevier Science Ltd PII $1360-1385(96)10056-X March t997, Vol.2, No, 3 99