* The development of photosynthetic competence is a key requirement for all land plants and many aquatic algae. Previous work has demonstrated that a pair of Golden2-like (GLK) transcription factors regulates chloroplast development in diverse land plants, in that recessive glk1;glk2 mutants are pale green and fail to accumulate components of the light-harvesting machinery. To determine the extent to which the GLK pathway has diverged in land plants, we compared GLK gene function in the flowering plant Arabidopsis thaliana and the moss Physcomitrella patens. * Cross-species complementation experiments were carried out to assess the ability of AtGLK1 to activate downstream targets in P. patens Ppglk1;glk2 double mutants, and the ability of upstream components in A. thaliana to activate PpGLK promoter::AtGLK1 coding region fusions in Atglk1;glk2 double mutants. * The results demonstrate that expression of the A. thaliana AtGLK1 gene cannot rescue the Ppglk1;glk2 mutant phenotype and that P. patens GLK promoter sequences are not activated in A. thaliana. * In combination with previous work which demonstrated partial complementation of A. thaliana double mutants by PpGlk1, this work provides an example of unidirectional complementation between the two species. This situation infers specialization of the GLK pathway during land plant evolution.