Plastid signalling to the nucleus: messengers still lost in the mists?

The concept of plastid signalling posits that signals originating from chloroplasts modulate nuclear gene expression (NGE). Put simply, it claims that signalling factors are exported from the chloroplast, traverse the cytosol, and act in the nucleus. Pertinent signals are thought to derive from various sources, including the tetrapyrrole pathway, protein synthesis, reactive oxygen species, or the redox state of the organelle. Recent studies have cast doubt on the most popular candidate signalling molecule, the tetrapyrrole pathway intermediate Mg-protoporphyrin IX, indicating that chloroplast activity might control NGE indirectly by affecting cytosolic metabolite levels or redox states (metabolic signalling). Here, we focus on recent developments and confusions in the field of plastid signalling research and highlight alternative scenarios of plastid–nucleus signal transduction. Future analyses of chloroplast–nucleus communication should focus on providing an integrated view of plastid signalling under physiologically relevant conditions.