Endosperm

Endosperm

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Encyclopedia
Endosperm is the tissue produced inside the seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...

s of most flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

s around the time of fertilization. It surrounds the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 and provides nutrition in the form of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

, though it can also contain oils and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

. This makes endosperm an important source of nutrition in human diet. For example, wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

 endosperm is ground into flour for bread
Bread
Bread is a staple food prepared by cooking a dough of flour and water and often additional ingredients. Doughs are usually baked, but in some cuisines breads are steamed , fried , or baked on an unoiled frying pan . It may be leavened or unleavened...

 (the rest of the grain is included as well in whole wheat flour
Whole wheat flour
Whole wheat flour is a powdery substance derived by grinding or mashing the wheat's whole grain. It is used in baking but typically added to other "white" flours to provide nutrients , texture, and body to the finished product....

), while barley
Barley
Barley is a major cereal grain, a member of the grass family. It serves as a major animal fodder, as a base malt for beer and certain distilled beverages, and as a component of various health foods...

 endosperm is the main source for beer
Beer
Beer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...

 production. Other examples of endosperm that forms the bulk of the edible portion are coconut
Coconut
The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word...

 "meat" and coconut "water", and corn
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

, including popcorn
Popcorn
Popcorn, or popping corn, is corn which expands from the kernel and puffs up when heated. Corn is able to pop because, like sorghum, quinoa and millet, its kernels have a hard moisture-sealed hull and a dense starchy interior. This allows pressure to build inside the kernel until an explosive...

. Some plants, like the orchid, lack endosperm in their seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...

s.

Origin of endosperm


Primitive flowering plants have seeds that have small embryos and abundant endosperm, and the evolutionary development of flowering plants tends to show a trend towards plants with mature seeds with little or no endosperm. In more derived flowering plants the embryo occupies most of the seed and the endosperm is non developed or consumed before the seed matures.

Double fertilization


Endosperm is formed when the two sperm nuclei
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

 inside a pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

 grain reach the interior of an embryo sac
Ovule
Ovule means "small egg". 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 , and the megaspore-derived female gametophyte in its center...

 or female gametophyte. One sperm nucleus fertilizes the egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

, forming a zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

, while the other sperm nucleus usually fuses with the two polar nuclei at the center of the embryo sac, forming a primary endosperm cell (its nucleus is often called the triple fusion nucleus). This cell created in the process of double fertilization develops into the endosperm. Because it is formed by a separate fertilization, the endosperm constitutes an organism separate from the embryo.

About 70% of angiosperm species have endosperm cells that are polyploid. These are typically triploid (containing three sets of chromosomes) but can vary widely from diploid (2n) to 15n.

One primitive species of flowering plant, Nuphar polysepala
Nuphar
Nuphar is genus of aquatic plants in the family Nymphaeaceae, with a temperate to subarctic Northern Hemisphere distribution. Common names include water-lily , pond-lily, and spatterdock .A total of eight species and three hybrids are currently accepted in the genus...

, has endosperm that is diploid, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei. 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.

Endosperm formation


There are three types of Endosperm development:

Nuclear endosperm formation - 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 juice is an example of this.

Cellular endosperm formation - where a 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 formation - 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 role of endosperm in seed development


In some groups (e.g. grains of the family Poaceae
Poaceae
The Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...

) 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
Fabaceae
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

, including the common bean
Common bean
Phaseolus vulgaris, the common bean, is an herbaceous annual plant domesticated independently in ancient Mesoamerica and the Andes, and now grown worldwide for its edible bean, popular both dry and as a green bean. The leaf is occasionally used as a leaf vegetable, and the straw is used for fodder...

, Phaseolus vulgaris), in which case the seeds are called "exalbuminous" or "cotyledonous" and the function of storage tissue is performed by enlarged cotyledons ("seed leaves"). In certain species (e.g. corn, Zea mays); the storage function is distributed between both endosperm and the embryo. Some mature endosperm tissues store fats (e.g. castor bean, Ricinis communis) and others (including grains, such as wheat and corn) store mainly starches.

The dust-like seeds of orchids have no endosperm. Orchid seedlings are mycoheterotrophic in their early development. In some other species, such as coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

, the endosperm also does not develop. Instead the nucellus produces a nutritive tissue termed "perisperm". The endosperm of some species is responsible for seed dormancy
Seed dormancy
Seed dormancy is a condition of plant seeds that prevents germination when the seeds are under optimal environmental conditions for germination. Living, non dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division; dormant seeds do...

. Endosperm 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. In angiosperms, the endosperm contain hormones such as cytokinin
Cytokinin
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...

s, which regulate cellular differentiation and embryonic organ formation.

Cereal grains


Cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...

 crops are grown for their palatable fruit
Fruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...

 (grains
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...

 or caryopses
Caryopsis
In botany, a caryopsis is a type of simple dry fruit — one that is monocarpelate and indehiscent and resembles an achene, except that in a caryopsis the pericarp is fused with the thin seed coat....

), 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 white flour), and the embryo (germ) and seed coat (bran) removed. Endosperm thus has an important role within the human diet, worldwide.

The aleurone
Aleurone
Aleurone is a protein found in protein granules of maturing seeds and tubers. The term is also used for the outermost cell layer of the endosperm, the aleurone layer.-Aleurone protein:...

 is a maternal tissue that is retained as part of the seed in many small grains. The aleurone functions for both storage and digestion. During germination it secretes the amylase
Amylase
Amylase is an enzyme that catalyses the breakdown of starch into sugars. Amylase is present in human saliva, where it begins the chemical process of digestion. Food that contains much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns...

enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.