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Chloroplast Delivery by UPS

Identification of a membrane-anchored E3 ligase in plants reveals a role for the ubiquitin proteasome system in chloroplast development. Chloroplasts are the organelles of photosynthesis in plants and are responsible for much of the food and biomass production on our planet. But chloroplasts are only the best-known members of an extended family of organelles termed plastids. Their name suggests plasticity and, indeed, plastids exist in various incarnations depending on developmental cues (e.g., nonphotosynthetic etioplasts in dark-grown leaves, colored chromoplasts in petals and fruit, and starch-storing amyloplasts in roots). Yet, the mechanisms underlying the transformation from one plastid type to another are largely unknown. On page 655 in this issue, Ling et al. (1) show that the ubiquitin-26S proteasome system (UPS) directly targets plastids and promotes chloroplast biogenesis, controlling yet another important facet of cell biology.