The chloroplast is potentially a major source of ROS. It
is the most important cellular player in production of 1O2,
and is also traditionally considered to be the major intracellular
producer of partially reduced oxygen species
such as O2
and H2O2. However, these latter molecules can
also be produced in substantial amounts by other organelles,
notably peroxisomes and mitochondria (Foyer and
Noctor, 2003), and a key outstanding question concerns the
importance of different subcellular compartments in ROS
production during plant responses to pathogens. Full
resolution of this issue has been hampered by the absence
of techniques able to generate quantitative information with
sufficient resolution.