Plant peroxisomes perform multiple vital metabolic processes including lipid mobilization in oil-storing seeds, photorespiration, and hormone biosynthesis. Peroxisome biogenesis requires the function of peroxin (PEX) proteins, including PEX10, a C(3)HC(4) Zn RING finger peroxisomal membrane protein. Loss of function of PEX10 causes embryo lethality at the heart stage. We investigated the function of PEX10 with conditional sublethal mutants. Four T-DNA insertion lines expressing pex10 with a dysfunctional RING finger were created in an Arabidopsis WT background (DeltaZn plants). They could be normalized by growth in an atmosphere of high CO(2) partial pressure, indicating a defect in photorespiration. beta-Oxidation in mutant glyoxysomes was not affected. However, an abnormal accumulation of the photorespiratory metabolite glyoxylate, a lowered content of carotenoids and chlorophyll a and b, and a decreased quantum yield of photosystem II were detected under normal atmosphere, suggesting impaired leaf peroxisomes. Light and transmission electron microscopy demonstrated leaf peroxisomes of the DeltaZn plants to be more numerous, multilobed, clustered, and not appressed to the chloroplast envelope as in WT. We suggest that inactivation of the RING finger domain in PEX10 has eliminated protein interaction required for attachment of peroxisomes to chloroplasts and movement of metabolites between peroxisomes and chloroplasts.