Phytochromes and Functions: Studies Using Gene Targeting in Physcomitrella

The lowly moss Physcomitrella patens is an excellent model organism for modern molecular physiology. The protonemal filaments are haploid, displaying the phenotype of a genetic lesion immediately after mutagenesis (selfing is possible but unnecessary). Filament cells are amenable to a wide range of cell biological methods including microinjection and, uniquely amongst plants, gene targeting via homologous recombination. Thus a Physcomitrella gene can be cloned, disrupted in situ and the mutant filament-together with its phenotype—regenerated within a couple of weeks. Protonemata are especially interesting photobiologically as they use phytochrome to steer their direction of growth in relation to light (phototropism). As we shall see, according to the fashionable view of phytochrome molecular action, this is simply not possible. We hope to resolve this self-contradictory situation using the power of the Physcomitrella system.