The Arabidopsis gene, PARVUS/AtGATL1, belongs to the Galacturonosyl-transferase-Like (GATL) gene family, which is part of CAZy Family GT8. Five GATL sequences of the moss Physcomitrella patens (PpGATL-A,B,C,D,E) have been identified through phylogenetic analyses. The five PpGATLs form a sister group that lies basal to all of the vascular plant GATLs. Homozygous parvus/gatl1 mutants in Arabidopsis thaliana are semi-sterile and exhibit a dwarf growth phenotype with collapsed xylem. Cell walls of parvus/gatl1 show significant reductions in xylose content compared with wild-type cell walls. Constitutive expression of PpGATL-A in the Arabidopsis parvus mutant rescues the defects conferred by the parvus mutation, including changes in plant growth and tissue morphology, collapsed xylem, decreased xylose content based on cell wall monosaccharide composition and the altered overall cell wall structure as indicated by glycome profiling. These findings suggest that PpGATL-A is a functional ortholog of PARVUS/GATL1 and at least one basic function of GATL1 is conserved in highly divergent land plants. Sub-cellular localization experiments showed that PpGATL-A is a Golgi-localized protein, as is the Arabidopsis AtGATL1. Promoter::-glucuronidase expression studies show that PpGATL-A is expressed at the base of leaves in the apical region of leafy gametophores. The role of PpGATL-A in the development of Physcomitrella was investigated by targeted gene knock-out experiments. The ppgatl-a knockout mutants produce significantly reduced numbers of leafy gametophores compared to wild type plants. The mutation of PpGATL-A gene blocked the developmental stage transition from protenema to leafy gametophores, resulting in a protonema-rich mutant phenotype. Analysis of the ppgatl-a knockout phenotype indicates that PpGATL-A is required for leafy gametophore development in Physcomitrella.