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Retrograde Signaling in biology is the process where a signal travels backwards from a target source to its original source. For example, under normal conditions the nucleus of a cell is the original source for creating signaling proteins. During retrograde signaling, instead of signals leaving the nucleus, they are sent to the nucleus. In cell biology, this type of signaling typically occurs between the mitochondria or chloroplast and the nucleus. Signaling molecules from the mitochondria or chloroplast act on the nucleus to affect nuclear gene expression. In this regard, the chloroplast or mitochondria act as a a sensor for internal external stimuli which activate a signaling pathway.

Cell Biology
Retrograde signals are transmitted from plastids to the nucleus in plants and eukaryotic algae, and from mitochondria to the nucleus in most eukaryotes. Retrograde signals are generally considered to convey intracellular signals related to stress and environmental sensing. Many of the molecules associated with retrograde signaling act on modifying the transcription or by directly binding and acting as a transcription factor. The outcomes of these signaling pathways vary by organism and by stimuli or stress.

Evolution
Retrograde signaling is believe to have arisen after endocytosis of the mitochondria billions of years ago. Originally believed to be photosynthetic bacteria, the mitochondria transferred some of its DNA to the membrane protected nucleus. This transfer of DNA further required an establish network of communication to properly respond to external and internal and produce required proteins.

In Yeast
The first retrograde signaling pathways discovered is the RTG pathway. The RTG pathway play an important role in maintain the metabolic homeostasis of yeast. Under limited resources the mitochondria must maintain a balance of glutamate for the Citric acid cycle. Retrograde signaling form the mitochondria initiates production precursor molecules of glutamate to properly balance supplies within the mitochondria. Retrograde signaling can also act to arrest growth if problems are encountered. In Saccharomyces cerevisiae, if the mitochondria fails to develop properly, they will stop growing until the issue is addressed or cell death is induced. These mechanism are vital to maintain homeostasis of the cell and ensure proper function of the mitochondria and nucleus.

In Plants
One of the most studied retrograde signaling molecules in plants are reactive oxygen species (ROS). These compounds, previously believed to be damaging to the cell, have since been discovered to act as a signaling molecule. Reactive oxygen species are created as a by-product of aerobic respiration and act on genes involved in the stress response. Depending on the stress, reactive oxygen species can act on neighboring cells to initiate a local signal. By doing this, surrounding cells are "primed" to react to the stress because genes involved in stress response are initiated prior to encountering the stress. The chloroplast can also act as a sensor for pathogen response and drought. Detection of these stresses in the cell will induce the formation of compounds that can then act on the nucleus to produce pathogen resistance genes or drought tolerance. For instance, an isoprene precusor MEcPP is produced in response to pathgoen attack. This