Plant organelles emit signals that regulate the expression of nuclear genes, a process known as retrograde signaling. The chlorophyll intermediate Mg-protoporphyrin IX (Mg-ProtoIX) is one such signal that accumulates under stress conditions and acts as a negative regulator of photosynthetic gene expression. Using confocal laser microscopy, Ankele et al. (pages 1964–1979) show that Mg-ProtoIX is a signaling metabolite that is exported from the chloroplast to the cytosol under certain conditions. The accumulation of Mg-ProtoIX under stress conditions in both the chloroplast and the cytosol is also shown to inhibit the expression of both chloroplastic and nuclear photosynthesis genes.
Plastid-encoded photosynthesis-related genes are transcribed by plastid-encoded RNA polymerase (PEP), whereas other plastid genes tend to be transcribed by a nuclear-encoded plastid RNA polymerase. The authors show that, in addition to regulating the expression of nuclear-encoded photosynthesis genes, stress-induced accumulation of Mg-ProtoIX also affects the expression of plastid-encoded photosynthesis genes, specifically by regulating the expression of the nuclear-encoded sigma factors necessary for the function of the multisubunit PEP enzyme. Thus, retrograde signaling of Mg-ProtoIX from plastid to nucleus constitutes a mechanism of coordinating gene expression in the two compartments.
Figure 1.
Mg-ProtoIX.