Bending of protonema cells in a plastid glycolate/glycerate transporter knockout line of physcomitrella patens

Arabidopsis LrgB (synonym PLGG1) is a plastid glycolate/glycerate transporter associated with recycling of 2-phosphoglycolate generated via the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). We isolated two homologous genes (PpLrgB1 and B2) from the moss Physcomitrella patens. Phylogenetic tree analysis showed that PpLrgB1 was monophyletic with LrgB proteins of land plants, whereas PpLrgB2 was divergent from the green plant lineage. Experiments with PpLrgB-GFP fusion proteins suggested that both PpLrgB1 and B2 proteins were located in chloroplasts. We generated PpLrgB single (ΔB1 andΔB2) and double (ΔB1/ΔB2)-knockout lines using gene targeting of P. patens. TheΔB1 plants showed decreases in growth and photosynthetic activity, and their protonema cells were bent and accumulated glycolate. However, because ΔB2 and ΔB1/ΔB2 plants showed no obvious phenotypic change relative to the wild-type or Δ;B1 plants, respectively, the function of PpLrgB2 remains unclear. Arabidopsis LrgB could complement the ΔB1 phenotype, suggesting that the function of PpLrgB1 is the same as that of AtLrgB. WhenΔB1 was grown under high-CO₂ conditions, all novel phenotypes were suppressed. Moreover, protonema cells of wild-type plants exhibited a bending phenotype when cultured on media containing glycolate or glycerate, suggesting that accumulation of photorespiratory metabolites caused P. patens cells to bend.