Move over endosymbionts, peroxisomes pass electrons too

The importance of the peroxisome as a site of oxidative metabolism in plants is well recognised, but the consequences of peroxisomal biochemistry for the broader metabolic network of plant cells are somewhat overlooked. In this review, we place a spotlight on the peroxisome as a redox-active organelle which mediates substantial flows of electrons. These electron flows not only have consequences within the peroxisome, but they also flow to and from the cytosol and at least two other major redox-active organelles, chloroplasts and mitochondria, with broad implications for metabolism and redox balance of electron carriers such as NADPH and NADH. We will outline the nature of these peroxisome-mediated electron flows and discuss the new appreciation of their quantitative significance derived from metabolic network flux analysis. We emphasise that the flows of reducing equivalents into and out of the peroxisome can be substantial - in some tissues equivalent to that to and from mitochondria. We also highlight key areas of uncertainty around specific redox reactions in the peroxisome and open questions about how redox state is balanced. Finally, we also consider the implications of peroxisomal electron flows in the context of re-engineering key metabolic processes such as photorespiration and lipid accumulation.