Harden the chloroplast to protect the plant.

The chloroplast is the central switch of the plant's response to cold and light stress. The ability of many plant species to develop a cold tolerant phenotype is dependent on the presence of light and photosynthetic activity during low-temperature growth. Light exposure at low temperature stimulates an over-reduction of the plastoquinone pool as well as the accumulation of reactive oxygen species, and both metabolic conditions generate a retrograde signal controlling nuclear gene expression. At the same time the chloroplast is the target of many cold acclimation processes which are the results of the chloroplast-nucleus cross-talk. Often, the extent of cold acclimation of the chloroplast is tightly correlated with the overall plant tolerance to chilling and freezing temperatures, a finding suggesting that the chloroplast cold acclimation could be the rate limiting factor in the adaptation to low temperature.