Editing Chloroplast (section)

=== Plant innate immunity ===

[[Plant]]s lack specialized [[immune cells]]—all plant cells participate in the plant [[plant innate immunity|immune response]]. Chloroplasts, along with the [[Cell nucleus|nucleus]], [[cell membrane]], and [[endoplasmic reticulum]],<ref name="Padmanabhan-2010">{{cite journal | vauthors=Padmanabhan MS, Dinesh-Kumar SP | title=All hands on deck—the role of chloroplasts, endoplasmic reticulum, and the nucleus in driving plant innate immunity | journal=Molecular Plant-Microbe Interactions | volume=23 | issue=11 | pages=1368–80 | date=November 2010 | pmid=20923348 | doi=10.1094/MPMI-05-10-0113 | doi-access=free }}</ref> are key players in [[pathogen]] defense. Due to its role in a plant cell's immune response, pathogens frequently target the chloroplast.<ref name="Padmanabhan-2010" />

Plants have two main immune responses—the [[hypersensitive response]], in which infected cells seal themselves off and undergo [[programmed cell death]], and [[systemic acquired resistance]], where infected cells release signals warning the rest of the plant of a pathogen's presence.
Chloroplasts stimulate both responses by purposely damaging their photosynthetic system, producing [[reactive oxygen species]]. High levels of reactive oxygen species will cause the [[hypersensitive response]]. The reactive oxygen species also directly kill any pathogens within the cell. Lower levels of reactive oxygen species initiate [[systemic acquired resistance]], triggering defense-molecule production in the rest of the plant.<ref name="Padmanabhan-2010" />

In some plants, chloroplasts are known to move closer to the infection site and the [[Cell nucleus|nucleus]] during an infection.<ref name="Padmanabhan-2010" />

Chloroplasts can serve as cellular sensors. After detecting stress in a cell, which might be due to a pathogen, chloroplasts begin producing molecules like [[salicylic acid]], [[jasmonic acid]], [[nitric oxide]] and [[reactive oxygen species]] which can serve as defense-signals. As cellular signals, reactive oxygen species are unstable molecules, so they probably don't leave the chloroplast, but instead pass on their signal to an unknown second messenger molecule. All these molecules initiate [[retrograde signaling (cell biology)|retrograde signaling]]—signals from the chloroplast that regulate [[gene expression]] in the nucleus.<ref name="Padmanabhan-2010" />

In addition to defense signaling, chloroplasts, with the help of the [[peroxisome]]s,<ref name="Katsir-2008">{{cite journal | vauthors=Katsir L, Chung HS, Koo AJ, Howe GA | title=Jasmonate signaling: a conserved mechanism of hormone sensing | journal=Current Opinion in Plant Biology | volume=11 | issue=4 | pages=428–35 | date=August 2008 | pmid=18583180 | pmc=2560989 | doi=10.1016/j.pbi.2008.05.004 | bibcode=2008COPB...11..428K }}</ref> help synthesize an important defense molecule, [[jasmonate]]. Chloroplasts synthesize all the [[fatty acid]]s in a plant cell<ref name="Padmanabhan-2010" /><ref name="Schnurr-2002">{{cite journal | vauthors=Schnurr JA, Shockey JM, de Boer GJ, Browse JA | title=Fatty acid export from the chloroplast. Molecular characterization of a major plastidial acyl-coenzyme A synthetase from Arabidopsis | journal=Plant Physiology | volume=129 | issue=4 | pages=1700–9 | date=August 2002 | pmid=12177483 | pmc=166758 | doi=10.1104/pp.003251 }}</ref>—[[linoleic acid]], a fatty acid, is a precursor to jasmonate.<ref name="Padmanabhan-2010" />