Interorganellar communication.

Signals originating from chloroplasts and mitochondria modulate nuclear gene expression (retrograde signalling). Relevant signals are derived from the pool of reactive oxygen species or generated by changes in redox state, flux through the tetrapyrrole biosynthetic pathway, or rates of organelle protein synthesis. In addition, multiple interactions of these four pathways with sugar and hormone signalling occur. Although the nature of the molecules that relay information through the cytosol to the nucleus is still unknown, the first putative signalling components in the chloroplast have recently been identified, and give tentative hints of overlaps between the different pathways. Retrograde signalling-dependent modulation of nuclear gene expression seems to involve multilayered transcriptional control and the transcription factor ABI4.