nucellar embryony
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Nucellar embryony (notated Nu+) is a form of
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
that occurs in certain plant species, including many
citrus ''Citrus'' is a genus of flowering plant, flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as Orange (fruit), oranges, Lemon, lemons, grapefruits, pomelos, and lim ...
varieties. Nucellar embryony is a type of
apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...
, where eventually nucellar embryos from the
nucellus In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fe ...
tissue of the ovule are formed, independent of
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
and
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
. During the development of seeds in plants that possess this genetic trait, the nucellus tissue which surrounds the
megagametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the ...
can produce nucellar cells, also termed initial cells. These additional embryos (
polyembryony Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic differ ...
) are genetically identical to the parent plant, rendering them as clones. By contrast,
zygotic A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
seedlings are sexually produced and inherit
genetic material Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cla ...
from both parents. Most angiosperms reproduce sexually through
double fertilization Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte ( megagametophyte, also called the embryo sac) with two male gametes (sperm). It begins when ...
. Different from nucellar embryony, double fertilization occurs via the syngamy of sperm and egg cells, producing a triploid endosperm and a diploid zygotic embryo. In nucellar embryony, embryos are formed asexually from the nucellus tissue. Zygotic and nucellar embryos can occur in the same seed (monoembryony), and a zygotic embryo can divide to produce multiple embryos. The nucellar embryonic initial cells form, divide, and expand. Once the zygotic embryo becomes dominant, the initial cells stop dividing and expanding. Following this stage, the zygotic embryo continues to develop and the initial cells continue to develop as well, forming nucellar embryos. The nucellar embryos generally end up outcompeting the zygotic embryo, rending the zygotic embryo dormant. The polyembryonic seed is then formed by the many adventitious embryos within the ovule (to picture this process, refer to Figure 1). The nucellar embryos produced via apomixis inherit its mother’s genetics, making them desirable for citrus propagation, research, and breeding.


Nucellar embryony outside of citrus varieties

Nucellar embryos have also been found in polyembryonic Mango varieties, where generally one of the embryos is zygotic and the rest are nucellar. However, there is little research on Mangos undergoing nucellar embryo development as there has on varieties of citrus.


Conditions

Nucellar embryony is able to occur within both fertilized and unfertilized ovules. Furthermore, instead of using the endosperm as nutritive tissue, it will utilize the surrounding nucellus tissue for nutrition. For example, the ‘Valencia’ orange undergoes nucellar embryony in both fertilized and unfertilized conditions. But, it has been found that nucellar embryo development, under fertilized or unfertilized conditions, can take place in different positions.


Features

An important component of nucellar embryo development is its changing cell wall thickness. Between nucellar embryo’s initial cell stage and its dividing and expanding stage, the cells’ wall thickens. This most likely occurs due to callose deposition; callose deposition reduces the permeability of a cell and is usually found in the initial cells about to undergo embryogenesis. The initial cells become enlarged, rounded, and divided. During this stage, the initial cell’s cell walls thin out, leaving room for the nucleus to become distinguished.


Seedless fruits and influence by the citrus industry

Many seed plants, including citrus fruits, are self-compatible, meaning that they are able to fertilize themselves. Self-compatibility produces a seedy fruit which may be deemed as undesirable to the citrus industry. Seedless fruits have been made popular as they are sought-after in the citrus industry. To be seedless, a citrus must exhibit
self-incompatibility Self-incompatibility (SI) is a general name for several genetic mechanisms that prevent self-fertilization in sexually reproducing organisms, and thus encourage outcrossing and allogamy. It is contrasted with separation of sexes among individuals ...
, another reproductive trait within citrus fruits and many seed plants. Self incompatibility is the phenomena where hermaphroditic plants are not able to produce fertile embryos after self-pollination. Self-incompatibility is regulated by the S-loci; if pollen is rendered incompatible, it is determined by its haploid S genotype, or if its sporophyte is rendered incompatible, it would be determined by its diploid S genotype. This is also termed and associated with
parthenocarpy In botany and horticulture, parthenocarpy is the natural or artificially induced production of fruit without fertilisation of ovules, which makes the fruit seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are ac ...
, the production of fruit without fertilization. Self-incompatible fruits are able to undergo parthenocarpy to yield seedless fruits. In citrus specifically, there have been other modes developed to reduce seeding as well: gibberellic acid enhances ovule abortion and copper sulfate has been shown to reduce seed number in fruit. An example is the ‘Afourer’ mandarin that contains a haploid self-incompatibility system and parthenocarpy. Under conditions where cross-pollination is not present, the ‘Afourer’ mandarin produces a seedless fruit by undergoing parthenocarpy. Where cross-pollination is present, gibberellic acid is applied and produces a decreased seeding fruit. Nucellar embryony is important to the citrus industry, as it allows for the production of uniform
rootstock A rootstock is part of a plant, often an underground part, from which new above-ground growth can be produced. It could also be described as a stem with a well developed root system, to which a bud from another plant is grafted. It can refer to a ...
which yields consistent results in fruit production. However, this trait can interfere with progress in cross-breeding; most commercial
scion Scion may refer to: Arts, entertainment, and media Fictional entities *Scion, a playable class in the game ''Path of Exile'' (2013) *Atlantean Scion, a device in the ''Tomb Raider'' video game series *Scions, an alien race in the video game ''Ba ...
varieties produce mainly nucellar seedlings which do not inherit any of the traits of the "father" plant.


See also

*
Apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...


References


External links

* *{{cite conference , first = Joseph L. , last = Kepiro , author2 = Mikeal L. Roose , title = Molecular Genetic Analysis of Nucellar Embryony (Apomixis) in ''Citrus Maximus'' x ''Poncirus Trifoliata'' Using AFLP , book-title = Plant & Animal Genomes XI Conference. January 11–15, 2003, Town & Country Convention Center, San Diego, CA , url = http://www.intl-pag.org/11/abstracts/W23_W161_XI.html , access-date = 2006-10-26 , archive-url = https://web.archive.org/web/20061010014927/http://www.intl-pag.org/11/abstracts/W23_W161_XI.html , archive-date = 2006-10-10 , url-status = dead Tropical agriculture Citrus Plant reproduction Asexual reproduction