The moss Physcomitrella patens performs efficient homologous recombination in both the nucleus and plastid enabling the study of individual gene function by generating precise inactivation or modification of genes. Polyethylene glycol (PEG)-mediated transformation of protoplasts is routinely used to study the nuclear gene function of P. patens. PEG-mediated protoplast transformation is also applied for plastid transformation of this moss. The efficiency of plastid transformation is quite reliable, and one or two homoplasmic transplastomic lines are obtained in a plastid transformation experiment (5 × 10(5) protoplasts) by selection for spectinomycin resistance.