Pexophagy helps plant leaves cope with stress

Peroxisomes are essential organelles that contain enzymes that produce hydrogen peroxide as part of their normal catalytic cycle; in plant leaves, they have an important role in photorespiration. However, proteins in peroxisomes are inevitably damaged by reactive oxygen species (ROS) that are generated during normal peroxisome function and impaired peroxisome function has been shown to lead to defects in seedling development and growth. So how is the basal activity of peroxisomes maintained? Autophagy is known to be involved in the clearance of other damaged organelles but its role in the quality control of peroxisomes remains unclear. On page 1161, Kohki Yoshimoto, Yoshinori Ohsumi and colleagues now analyse the behaviour of peroxisomes in Arabidopsis thaliana mutants, in which core autophagy-related (ATG) genes have been knocked out. They find that peroxisomes accumulate in the leaves but not the roots of these mutant plants. Importantly, when the mutants are complemented with ATG5, the accumulated peroxisomes are delivered to the vacuole for degradation via the autophagy pathway, indicating that organ-specific peroxisome autophagy (pexophagy) is responsible for maintaining normal plant leaf function. The authors then addressed the mechanism underlying the selectivity of pexophagy and found that autophagosomal membranes specifically recognise protein aggregates that appear to be segregated from the whole peroxisome; these are subsequently enwrapped for degradation. Taken together these results provide the first evidence that autophagy is essential for the quality control of peroxisomes in photosynthetic tissues and suggest that photosynthetic organisms have evolved pexophagy as a unique strategy to survive conditions of environmental stress.