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 (Delta B1 and Delta B2) and double (Delta B1/Delta B2)-knockout lines using gene targeting of P. patens. The Delta B1 plants showed decreases in growth and photosynthetic activity, and their protonema cells were bent and accumulated glycolate. However, because Delta B2 and Delta B1/Delta B2 plants showed no obvious phenotypic change relative to the wild-type or Delta B1 plants, respectively, the function of PpLrgB2 remains unclear. Arabidopsis LrgB could complement the Delta B1 phenotype, suggesting that the function of PpLrgB1 is the same as that of AtLrgB. When Delta B1 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.