Changes in the developmental or metabolic state of chloroplasts or mitochondria can have profound effects on other cellular compartments and involve massive changes in the transcript profiles of nuclear genes. The concept of retrograde signalling posits that signals originating from chloroplasts or mitochondria modulate nuclear gene expression. Put simply, it claims that signalling factors are generated in the organelles, exported from the organelles, traverse the cytosol, and act in the nucleus. Pertinent signals are thought to derive from various sources, including among others the tetrapyrrole pathway, organellar gene expression, reactive oxygen species, or the redox state of the organelle. Recent studies have cast doubt on whether each of the candidate signalling molecules is genuine. Moreover, the unambiguous experimental verification of a signalling molecule is principally a difficult task to undertake. Therefore, despite some decades of research our understanding of organellar signalling mechanisms remains limited. Thus, we want to discuss in this Frontiers Research Topic collection how future research should enable us to provide an integrated view of organellar-nucleus signalling and how this phenomenon can be addressed under physiologically relevant conditions and at multiple levels. Moreover, we want to provide an overview on the set of tentative signalling pathways and molecules and their potential interaction with each other, including “established” signals as well as very recent findings. In particular, we welcome critical views on organelle-to-nucleus signalling and want to stimulate a discussion what experimental prerequisites are necessary and sufficient to define a plastid signal and what role different physiological conditions play in organelle-to-nucleus signalling and vice versa.
Because of the diversity and controversy of the field, we encourage besides ‘Original Research’ and ‘Mini Reviews’ also ‘Hypothesis and Theory’, ‘Perspectives’, ‘Opinion’ and ‘Methods‘ papers. All aspects of organelle-to-nucleus signalling can be addressed, including the source of signals in the organelles, tentative signalling molecules and the responses in the nucleus and beyond.