Isolation of metabolic mutants. Physcomitrella plants derived from transformation with the gene-disruption library and regenerated on supplemented medium were split in two parts, which were transferred to minimal Knop medium with and without supplements (see Materials) and cultured for 8 weeks. The picture shows four independent Physcomitrella transformants cultured on minimal medium, one of which (bottom right) displays a clear growth defect. All four plants grew equally well on supplemented medium (data not shown). The scale bar at the top indicates size in centimetres.