Editing Endosperm

{{Short description|Tissue inside seeds that is starchy in cereals and liquid in coconuts}}
[[File:Wheat seed.jpg|200px|thumb|Wheat seed]] 
The '''endosperm''' is a tissue produced inside the [[seed]]s of most of the [[flowering plant]]s following [[double fertilization]]. It is [[triploid]] (meaning three chromosome sets per nucleus) in most species,<ref>{{cite journal | vauthors = Stewart-Cox JA, Britton NF, Mogie M | title = Endosperm triploidy has a selective advantage during ongoing parental conflict by imprinting | journal = Proceedings. Biological Sciences | volume = 271 | issue = 1549 | pages = 1737–43 | date = August 2004 | pmid = 15306295 | pmc = 1691787 | doi = 10.1098/rspb.2004.2783 }}</ref> which may be [[auxin]]-driven.<ref name="Friedman-2009">{{cite journal | last=Friedman | first=W. E. | title=Auxin at the Evo-Devo Intersection | journal=[[Science (journal)|Science]] | publisher=[[American Association for the Advancement of Science]] | volume=324 | issue=5935 | date=2009-06-25 | issn=0036-8075 | doi=10.1126/science.1176526 | pages=1652–1653 | s2cid=206521265 | pmid=19556491| bibcode=2009Sci...324.1652F }}</ref> It surrounds the [[Embryo#Plant embryos|embryo]] and provides nutrition in the form of [[starch]], though it can also contain [[Vegetable oil|oil]]s and [[protein]]. This can make endosperm a source of nutrition in animal diet. For example, [[wheat]] endosperm is ground into flour for [[bread]] (the rest of the grain is included as well in [[whole wheat flour]]), while [[barley]] endosperm is the main source of sugars for [[beer]] production. Other examples of endosperm that forms the bulk of the edible portion are [[coconut]] "meat" and coconut "water",<ref>{{cite web | title = Edible Palm Fruits | url = http://waynesword.palomar.edu/ecoph10.htm | work = Wayne's Word: An Online Textbook of Natural History | access-date = 14 July 2010 | archive-date = 2 September 2017 | archive-url = https://web.archive.org/web/20170902214354/http://waynesword.palomar.edu/ecoph10.htm | url-status = dead }}</ref> and [[Maize|corn]]. Some plants, such as certain [[orchid]]s, lack endosperm in their [[seed]]s.

Ancestral flowering plants have seeds with small embryos and abundant endosperm. In some modern flowering plants the embryo occupies most of the seed and the endosperm is non-developed or consumed before the seed matures.<ref>{{cite web|url=http://www.seedbiology.de/dormancy.asp#evolution |title=The Seed Biology Place - Seed Dormancy |publisher=Seedbiology.de |access-date=2014-02-05}}</ref><ref name=Friedman1998>{{citation | last = Friedman | first = William E. | name-list-style = vanc | year = 1998 | title = The evolution of double fertilization and endosperm: an "historical" perspective | journal = Sexual Plant Reproduction | volume = 11 | page = 6 | doi = 10.1007/s004970050114| s2cid = 19785565 }}</ref> In other flowering plant taxa, the [[Poaceae]] for example, the endosperm is greatly developed.<ref>{{Cite journal |title=The Development of Endosperm in Grasses |journal=Plant Physiology|date=2009 |doi=10.1104/pp.108.129437 |last1=Sabelli |first1=Paolo A. |last2=Larkins |first2=Brian A. |volume=149 |issue=1 |pages=14–26 |pmid=19126691 |pmc=2613697 }}</ref> 

== Double fertilization ==
{{main|Double fertilization}}
An endosperm is formed after the two [[sperm]] nuclei inside a [[pollen]] grain reach the interior of a female gametophyte or megagametophyte, also called the [[embryonic sac]]. One sperm nucleus fertilizes the [[egg cell]], forming a [[zygote]], while the other sperm nucleus usually fuses with the binucleate central cell, forming a primary endosperm cell (its nucleus is often called the ''triple fusion nucleus''). That cell created in the process of [[double fertilisation|double fertilization]] develops into the endosperm.  Because it is formed by a separate fertilization event, the endosperm is a separate entity from the developing embryo, and some consider it to be a separate [[organism]]. 

About 70% of angiosperm species have endosperm cells that are [[polyploid]].<ref name=Olsen2007>{{cite book | first = Odd-Arne | last = Olsen | name-list-style = vanc | year = 2007 | title = Endosperm: Developmental and Molecular Biology | publisher = Springer | url=https://books.google.com/books?id=TFEA8Gg5_hAC| isbn = 9783540712350 }}</ref> These are typically [[triploid]] (containing three sets of [[Chromosome|chromosomes]]), but can vary widely from [[diploid]] (2n) to 15n.<ref name="Baroux_2002">{{cite journal | vauthors = Baroux C, Spillane C, Grossniklaus U | title = Evolutionary origins of the endosperm in flowering plants | journal = Genome Biology | volume = 3 | issue = 9 | pages = reviews1026 | date = August 2002 | pmid = 12225592 | pmc = 139410 | doi = 10.1186/gb-2002-3-9-reviews1026 | doi-access = free }}</ref>

One flowering plant, ''[[Nuphar polysepala]]'', has diploid endosperm, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei. The same is supposed for some other basal angiosperms.<ref>{{cite journal | vauthors = Williams JH, Friedman WE | title = Identification of diploid endosperm in an early angiosperm lineage | journal = Nature | volume = 415 | issue = 6871 | pages = 522–6 | date = January 2002 | pmid = 11823859 | doi = 10.1038/415522a | bibcode = 2002Natur.415..522W | s2cid = 4396197 }}</ref> It is believed that early in the development of angiosperm lineages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endosperms with a 2:1 maternal to paternal genome ratio.<ref name=Friedman2003>{{cite journal | vauthors = Friedman WE, Williams JH | title = Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants | journal = Evolution; International Journal of Organic Evolution | volume = 57 | issue = 2 | pages = 216–30 | date = February 2003 | pmid = 12683519 | doi = 10.1111/j.0014-3820.2003.tb00257.x | s2cid = 24303275 | doi-access = free }}</ref>

Double fertilisation is a characteristic feature of [[angiosperms]].

== Endosperm development ==
There are three types of endosperm development:

'''Nuclear endosperm development''' – where repeated free-nuclear divisions take place; if a cell wall is formed it will form after free-nuclear divisions. Commonly referred to as liquid endosperm. Coconut water is an example of this.

'''Cellular endosperm development''' – where a [[Cell wall|cell-wall]] formation is coincident with nuclear divisions. Coconut meat is cellular endosperm. Acoraceae has cellular endosperm development while other monocots are helobial.

'''Helobial endosperm development''' – where a cell wall is laid down between the first two nuclei, after which one half develops endosperm along the cellular pattern and the other half along the nuclear pattern.

== Evolutionary origins ==
The evolutionary origins of double fertilization and endosperm are unclear, attracting researcher attention for over a century. There are the two major hypotheses:<ref name="Baroux_2002"/>
* The double fertilization initially used to produce two identical, independent embryos ("twins"). Later these embryos acquired different roles, one growing into the mature organism, and another merely supporting it. Thus, the early endosperm was probably diploid, like the embryo. Some [[gymnosperm]]s, such as ''[[Ephedra (plant)|Ephedra]]'', may produce twin embryos by double fertilization. Either of these two embryos is capable of filling in the seed, but normally only one develops further (the other eventually aborts). Also, most basal angiosperms still contain the four-cell embryo sac and produce diploid endosperms.
* Endosperm is the evolutionary remnant of the actual [[gametophyte]], similar to the complex multicellular gametophytes found in gymnosperms. In this case, acquisition of the additional nucleus from the sperm cell is a later evolutionary step. This nucleus may provide the parental (not only maternal) organism with some control over endosperm development. Becoming triploid or polyploid are later evolutionary steps of this "primary gametophyte". Nonflowering seed plants (conifers, cycads, ''Ginkgo'', ''Ephedra'') form a large homozygous female gametophyte to nourish the embryo within a seed.<ref name="pmid11607532">{{cite journal | vauthors = Friedman WE | title = Organismal duplication, inclusive fitness theory, and altruism: understanding the evolution of endosperm and the angiosperm reproductive syndrome | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 92 | issue = 9 | pages = 3913–7 | date = April 1995 | pmid = 11607532 | pmc = 42072 | doi = 10.1073/pnas.92.9.3913 | bibcode = 1995PNAS...92.3913F | doi-access = free }}</ref>
The triploid transition - and the production of [[antipodal cell]]s - may have occurred due to a shift in gametophyte development which produced a new interaction with an auxin-dependent mechanism originating in the earliest angiosperms.<ref name="Friedman-2009" />

==Role in seed development==
In some groups (e.g. grains of the family [[Poaceae]]), the endosperm persists to the mature seed stage as a storage tissue, in which case the seeds are called "albuminous" or "endospermous", and in others it is absorbed during embryo development (e.g., most members of the family [[Fabaceae]], including the [[common bean]], ''Phaseolus vulgaris''), in which case the seeds are called "exalbuminous" or "cotyledonous" and the function of storage tissue is performed by enlarged [[cotyledon]]s ("seed leaves"). In certain species (e.g. corn, ''Zea mays''); the storage function is distributed between both endosperm and the embryo. Some mature endosperm tissue stores fats (e.g. [[castor bean]], ''Ricinus communis'') and others (including grains, such as wheat and corn) store mainly starches.

The dust-like seeds of [[orchid]]s have no endosperm. Orchid seedlings are [[mycoheterotrophy|mycoheterotrophic]] in their early development. In some other species, such as [[coffee]], the endosperm also does not develop.<ref>{{Cite journal | vauthors = Houk WG | title = Endosperm and Perisperm of Coffee with Notes on the Morphology of the Ovule and Seed Development | journal = American Journal of Botany | volume = 25 | issue = 1 | pages = 56–61 | doi = 10.2307/2436631 | year = 1938 | jstor = 2436631 }}</ref> Instead, the [[nucellus]] produces a nutritive tissue termed "perisperm". The endosperm of some species is responsible for [[seed dormancy]].<ref>{{cite book | first = Amarjit S. | last = Basra | name-list-style = vanc | title = Mechanisms of plant growth and improved productivity: modern approaches | date = 1994 | location =  New York | publisher = M. Dekker | isbn = 978-0-8247-9192-6 }}</ref> Endosperm tissue also mediates the transfer of nutrients from the mother plant to the embryo, it acts as a location for gene imprinting, and is responsible for aborting seeds produced from genetically mismatched parents.<ref name="Olsen2007"/> In angiosperms, the endosperm contain hormones such as [[cytokinin]]s, which regulate cellular differentiation and embryonic organ formation.<ref>{{cite book | last = Pearson | first = Lorentz C. | name-list-style = vanc | title=The diversity and evolution of plants | year=1995 | publisher=CRC Press | location=Boca Raton | isbn=978-0-8493-2483-3 | page=547}}</ref>

==Cereal grains==
[[Image:Wheat-kernel nutrition.png|300px|right]]
[[Cereal]] crops are grown for their edible [[fruit]] (grains or [[caryopsis|caryopses]]), which are primarily endosperm. In the caryopsis, the thin fruit wall is fused to the seed coat. Therefore, the nutritious part of the grain is the seed and its endosperm. In some cases (e.g. wheat, rice) the endosperm is selectively retained in food processing (commonly called [[Flour|white flour]]), and the embryo ([[Cereal germ|germ]]) and seed coat ([[bran]]) removed. The processed grain has a lower quality of nutrition. Endosperm thus has an important role within the human diet worldwide.

The [[aleurone]] is the outer layer of endosperm cells, present in all small grains and retained in many dicots with transient endosperm. The cereal aleurone functions for both storage and digestion. During germination, it secretes the [[amylase]] enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.<ref name="pmid21109580">{{cite journal | vauthors = Becraft PW, Yi G | title = Regulation of aleurone development in cereal grains | journal = Journal of Experimental Botany | volume = 62 | issue = 5 | pages = 1669–75 | date = March 2011 | pmid = 21109580 | doi = 10.1093/jxb/erq372 | doi-access = free }}</ref><ref name="pmid23801534">{{cite journal | vauthors = Becraft PW, Gutierrez-Marcos J | title = Endosperm development: dynamic processes and cellular innovations underlying sibling altruism | journal = Wiley Interdisciplinary Reviews: Developmental Biology | volume = 1 | issue = 4 | pages = 579–93 | date = 2012 | pmid = 23801534 | doi = 10.1002/wdev.31 | s2cid = 5752973 }}</ref>

== See also ==
* [[Ovule]]

== References ==
{{reflist|30em}}

== External links ==
*{{Cite NSRW|wstitle=Endosperm}}
*[https://earthlingnature.wordpress.com/2016/01/10/endosperm-the-pivot-of-the-sexual-conflict-in-flowering-plants/ Endosperm: the pivot of the sexual conflict in flowering plants] at Earthling Nature

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[[Category:Plant morphology]]
[[Category:Plant physiology]]