Mg-protoporphyrin IX signaling in Cyanidioschyzon merolae: multiple pathways may involve the retrograde signaling in plant cells.

Plastids and mitochondria are organelles in plant cells, which are considered to have evolved from endosymbiotic associations of bacteria. These organelles have their own genomes descended from their ancestors, and the organelle DNA replications (ODR) of plant cells are coordinated with the nuclear DNA replication (NDR) as ODR precedes NDR during a cell cycle progression. However, the underlying mechanism for this coordination remains largely to be determined. Recently, we identified that a tetrapyrrole compound, Mg-Protoporphyrin IX (Mg-Proto), is a cell cycle coordinator from organelle to NDR in plant cells.1 While Mg-Proto has been suggested to be a retrograde plastid signal to modulate transcription of nuclear genes for plastid proteins, our results indicated that nuclear transcriptional regulation is not involved in the NDR induction in C. merolae. Thus, our finding for the NDR control is likely to represent a novel signaling mechanism independent of the conventional plastid signal, which we named "parasitic signal", and suggests multiple Mg-Protoinvolved pathways for the plastid-nucleus retrograde signaling.