A model of the role of atABC1 and PPO in chlorophyll biosynthesis. PPO oxidizes protogen IX to proto IX in the chloroplast envelope (dark gray), whereupon it is imported back into the stroma by atABC1 and an integral membrane protein. In the laf6 mutant (shown in blue), proto IX import is inhibited, allowing its accumulation in the cytoplasm. In the presence of flumioxazin (shown in green), PPO is inhibited, allowing protogen IX into the cytoplasm where it is oxidized non-specifically to proto IX, thus mimicking the laf6 phenotype. Adapted from ; for abbreviations, see Figure 1 legend.