Theendosperm is a tissue produced inside theseeds of most of theflowering plants followingdouble fertilization. It istriploid (meaning three chromosome sets per nucleus) in most species,[1] which may beauxin-driven.[2] It surrounds theembryo and provides nutrition in the form ofstarch, though it can also containoils andprotein. This can make endosperm a source of nutrition in animal diet. For example,wheat endosperm is ground into flour forbread (the rest of the grain is included as well inwhole wheat flour), whilebarley endosperm is the main source of sugars forbeer production. Other examples of endosperm that forms the bulk of the edible portion arecoconut "meat" and coconut "water",[3] andcorn. Some plants, such as certainorchids, lack endosperm in theirseeds.
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.[4][5] In other flowering plant taxa, thePoaceae for example, the endosperm is greatly developed.[6]
An endosperm is formed after the twosperm nuclei inside apollen grain reach the interior of a female gametophyte or megagametophyte, also called theembryonic sac. One sperm nucleus fertilizes theegg cell, forming azygote, while the other sperm nucleus usually fuses with the binucleate central cell, forming a primary endosperm cell (its nucleus is often called thetriple fusion nucleus). That cell created in the process ofdouble 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 separateorganism.
About 70% of angiosperm species have endosperm cells that arepolyploid.[7] These are typicallytriploid (containing three sets ofchromosomes), but can vary widely fromdiploid (2n) to 15n.[8]
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.[9] 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.[10]
Double fertilisation is a characteristic feature ofangiosperms.
There are three types of endosperm development:
Nuclear endosperm development – where repeated free-nuclear divisions take place by suppression of cell wall formation;[11] 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 acell-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.
The evolutionary origins of double fertilization and endosperm are unclear, attracting researcher attention for over a century. There are the two major hypotheses:[8]
The triploid transition - and the production ofantipodal cells - 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.[2]
In some groups (e.g. grains of the familyPoaceae), 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 familyFabaceae, including thecommon bean,Phaseolus vulgaris), in which case the seeds are called "exalbuminous" or "cotyledonous" and the function of storage tissue is performed by enlargedcotyledons ("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 oforchids have no endosperm. Orchid seedlings aremycoheterotrophic in their early development. In some other species, such ascoffee, the endosperm also does not develop.[13] Instead, thenucellus produces a nutritive tissue termed "perisperm". The endosperm of some species is responsible forseed dormancy.[14] 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.[7] In angiosperms, the endosperm contain hormones such ascytokinins, which regulate cellular differentiation and embryonic organ formation.[15]
Cereal crops are grown for their ediblefruit (grains orcaryopses), 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 calledwhite flour), and the embryo (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.
Thealeurone 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 theamylase enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.[16][17]
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