Editing Chloroplast (section)

=== Division ===

{{expand section|functions, Z-ring dynamic assembly, regulators such as Giant Chloroplast 1 <!--- Sorry, just had to mention it.--->|date=February 2013}}

Most chloroplasts in a photosynthetic cell do not develop directly from proplastids or etioplasts. In fact, a typical [[shoot meristematic]] plant cell contains only 7–20 [[proplastids]]. These proplastids differentiate into chloroplasts, which divide to create the 30–70 chloroplasts found in a mature photosynthetic plant cell. If the cell [[Mitosis|divides]], chloroplast division provides the additional chloroplasts to partition between the two daughter cells.<ref name="Burgess-1989c">{{cite book|last=Burgess|first=Jeremy| name-list-style=vanc |title=An introduction to plant cell development|year=1989|publisher=Cambridge university press|location=Cambridge|isbn=0-521-31611-1|page=57|url=https://books.google.com/books?id=r808AAAAIAAJ&pg=PA57|edition=Pbk.}}</ref>

In single-celled [[algae]], chloroplast division is the only way new chloroplasts are formed. There is no proplastid differentiation—when an algal cell divides, its chloroplast divides along with it, and each [[daughter cell]] receives a mature chloroplast.<ref name="Burgess-1989d" />

Almost all chloroplasts in a cell divide, rather than a small group of rapidly dividing chloroplasts.<ref name="Possingham-1976">{{cite journal |doi=10.1098/rspb.1976.0047 |title=Chloroplast Replication and Chloroplast DNA Synthesis in Spinach Leaves |year=1976 | vauthors=Possingham JV, Rose RJ |journal=Proceedings of the Royal Society B: Biological Sciences |volume=193 |issue=1112 |pages=295–305 |bibcode=1976RSPSB.193..295P|s2cid=2691108 }}</ref> Chloroplasts have no definite [[S-phase]]—their DNA replication is not synchronized or limited to that of their host cells.<ref name="Cannon-1993">{{cite journal | vauthors=Cannon GC, Heinhorst S |title=DNA replication in chloroplasts |journal=Journal of Cell Science |date=1 January 1993 |volume=104 |issue=1 |pages=1–9 |doi=10.1242/jcs.104.1.1 |url=http://jcs.biologists.org/content/104/1/1 |citeseerx=10.1.1.1026.3732 }}</ref>
Much of what we know about chloroplast division comes from studying organisms like ''[[Arabidopsis]]'' and the red alga ''[[Cyanidioschyzon merolæ]]''.<ref name="Glynn-2007" />

{{plain image with caption|Chloroplast division.svg|Most chloroplasts in plant cells, and all chloroplasts in [[algae]] arise from chloroplast division.<ref name="Burgess-1989d" /> ''Picture references,<ref name="Glynn-2007" /><ref name="Miyagishima-2003" />''|800px|center|bottom|triangle|#00cd4c}}

The division process starts when the proteins [[FtsZ1]] and [[FtsZ2]] assemble into filaments, and with the help of a protein [[ARC6]], form a structure called a Z-ring within the chloroplast's stroma.<ref name="Glynn-2007">{{cite journal | vauthors=Glynn JM, Miyagishima SY, Yoder DW, Osteryoung KW, Vitha S | title=Chloroplast division | journal=Traffic | volume=8 | issue=5 | pages=451–61 | date=May 2007 | pmid=17451550 | doi=10.1111/j.1600-0854.2007.00545.x | s2cid=2808844 | doi-access=free }}</ref><ref name="Miyagishima-2003">{{cite journal | vauthors=Miyagishima SY, Nishida K, Mori T, Matsuzaki M, Higashiyama T, Kuroiwa H, Kuroiwa T | title=A plant-specific dynamin-related protein forms a ring at the chloroplast division site | journal=The Plant Cell | volume=15 | issue=3 | pages=655–65 | date=March 2003 | pmid=12615939 | pmc=150020 | doi=10.1105/tpc.009373 | bibcode=2003PlanC..15..655M }}</ref> The [[Min system]] manages the placement of the Z-ring, ensuring that the chloroplast is cleaved more or less evenly. The protein [[MinD]] prevents FtsZ from linking up and forming filaments. Another protein [[ARC3]] may also be involved, but it is not very well understood. These proteins are active at the poles of the chloroplast, preventing Z-ring formation there, but near the center of the chloroplast, [[MinE]] inhibits them, allowing the Z-ring to form.<ref name="Glynn-2007" />

Next, the two [[plastid-dividing rings]], or PD rings form. The inner plastid-dividing ring is located in the inner side of the chloroplast's inner membrane, and is formed first.<ref name="Glynn-2007" /> The outer plastid-dividing ring is found wrapped around the outer chloroplast membrane. It consists of filaments about 5 nanometers across,<ref name="Glynn-2007" /> arranged in rows 6.4 nanometers apart, and shrinks to squeeze the chloroplast. This is when chloroplast constriction begins.<ref name="Miyagishima-2003" /> <br />In a few species like ''[[Cyanidioschyzon merolæ]]'', chloroplasts have a third plastid-dividing ring located in the chloroplast's intermembrane space.<ref name="Glynn-2007" /><ref name="Miyagishima-2003" />

Late into the constriction phase, [[dynamin]] proteins assemble around the outer plastid-dividing ring,<ref name="Miyagishima-2003" /> helping provide force to squeeze the chloroplast.<ref name="Glynn-2007" /> Meanwhile, the Z-ring and the inner plastid-dividing ring break down.<ref name="Miyagishima-2003" /> During this stage, the many chloroplast DNA plasmids floating around in the stroma are partitioned and distributed to the two forming daughter chloroplasts.<ref name="Hashimoto-1989">{{cite journal | vauthors=Hashimoto H, Possingham JV | title=Effect of light on the chloroplast division cycle and DNA synthesis in cultured leaf discs of spinach | journal=Plant Physiology | volume=89 | issue=4 | pages=1178–83 | date=April 1989 | pmid=16666681 | pmc=1055993 | doi=10.1104/pp.89.4.1178 }}</ref>

Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane,<ref name="Miyagishima-2003" /> to cleave the chloroplast in two daughter chloroplasts.<ref name="Glynn-2007" />

A remnant of the outer plastid dividing ring remains floating between the two daughter chloroplasts, and a remnant of the dynamin ring remains attached to one of the daughter chloroplasts.<ref name="Miyagishima-2003" />

Of the five or six rings involved in chloroplast division, only the outer plastid-dividing ring is present for the entire constriction and division phase—while the Z-ring forms first, constriction does not begin until the outer plastid-dividing ring forms.<ref name="Miyagishima-2003" />

{{plain image with caption|File:Moss chloroplasts 100× objective oblique.jpg|'''Chloroplast division''' In this [[light micrograph]] of some [[moss]] chloroplasts, many dumbbell-shaped chloroplasts can be seen dividing. Grana are also just barely visible as small granules.|330px|right|bottom|triangle|#7ee232|image override={{Stacked image|File:Moss chloroplasts 100× objective oblique.jpg|File:Moss chloroplasts 100× objective oblique text.svg|330px|In this light micrograph of some [[moss]] chloroplasts, some dumbbell-shaped chloroplasts can be seen dividing. Grana are also just barely visible as small granules.|top=155px|left=8px}}}}

==== Regulation ====

In species of [[algae]] that contain a single chloroplast, regulation of chloroplast division is extremely important to ensure that each daughter cell receives a chloroplast—chloroplasts can't be made from scratch.<ref name="Alberts-2002b">{{cite book |last=Alberts |first=Bruce | name-list-style=vanc |title=Molecular biology of the cell|year=2002|publisher=Garland|location=New York [u.a.]|isbn=0-8153-4072-9|url=https://www.ncbi.nlm.nih.gov/books/NBK26924/|edition=4.}}</ref><ref name="Glynn-2007" /> In organisms like plants, whose cells contain multiple chloroplasts, coordination is looser and less important. It is likely that chloroplast and cell division are somewhat synchronized, though the mechanisms for it are mostly unknown.<ref name="Glynn-2007" />

Light has been shown to be a requirement for chloroplast division. Chloroplasts can grow and progress through some of the constriction stages under [[Chlorophyll#Spectrophotometry|poor quality green light]], but are slow to complete division—they require exposure to bright white light to complete division. Spinach leaves grown under green light have been observed to contain many large dumbbell-shaped chloroplasts. Exposure to white light can stimulate these chloroplasts to divide and reduce the population of dumbbell-shaped chloroplasts.<ref name="Possingham-1976" /><ref name="Hashimoto-1989" />