Reactive oxygen species
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.