Plants contain double-stranded circular DNA that does not associate with histones in their chloroplasts - organelles within each plant cell that enable photosynthesis. This DNA helps compose the plant genome along with DNA in the nucleus and mitochondria. The genes found within the chloroplast code for a majority of the enzymes that are used in photosynthesis, although some of the larger polypepetides that are required (ATPase and Ribulose biphosphate carboxylase ) are coded by genes found both in the chloroplast and the nucleus.
The theory behind the evolution of chloroplast DNA is that the chloroplast was once a free-living, one-celled organism that "infected" a plant cell. The relationship between the chloroplast and the plant cell then evolved into a symbiotic relationship, that provided a benefit or gain to each participant.