The phosphoinositide signalling pathway is important in plant responses to extracellular and intracellular signals. To elucidate the physiological functions of phosphoinositide-specific phopspholipase C, PI-PLC, targeted knockout mutants of PpPLC1, a gene encoding a PI-PLC from the moss Physcomitrella patens, were generated via homologous recombination. Protonemal filaments of the plc1 lines show a dramatic reduction in gametophore formation relative to wild type: this was accompanied by a loss of sensitivity to cytokinin. Moreover, plc1 appeared paler than the wild type, the result of an altered differentiation of chloroplasts and reduced chlorophyll levels compared with wild type filaments. In addition, the protonemal filaments of plc1 have a strongly reduced ability to grow negatively gravitropically in the dark. These effects imply a significant role for PpPLC1 in cytokinin signalling and gravitropism.