Eukaryotic ribosome biogenesis proceeds from nucleolus to cytosol assisted by various assembly factors. The process is evolutionarily conserved across eukaryotes but differences between kingdoms are emerging. Here, we describe how the OPENER (OPNR) protein complex is required for 60S ribosome assembly in the model plant Arabidopsis thaliana. The complex was observed on both nuclear envelope and mitochondria, and contains OPNR, OPENER ASSOCIATED PROTEIN 1 (OAP1), OAP2, Cell Division Cycle 48 D (CDC48D) and Calmodulin-interacting protein 111 (CIP111). Depletion of the OPNR complex components resulted in reproductive lethality and cytoplasmic retention of assembly factors on 60S ribosomes. Subsequent biochemical analyses and structural modelling suggested that OPNR, OAP1 and OAP2 form a claw-like trimer which grabs the ribosome assembly factor RPL24C on the pre-60S ribosome. Our results reveal previously unrecognised subcellular complexity of ribosome biogenesis in plants, and point to mitochondria association as a feature to ensure sufficient translational capacity.