Aleurone (from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''aleuron'', flour) is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

found in protein granules of maturing

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 ...

s and

tuber Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growin ...

s. The term also describes one of the two major cell types of the endosperm, the aleurone layer. The aleurone layer is the outermost layer of the endosperm, followed by the inner starchy endosperm. This layer of cells is sometimes referred to as the peripheral endosperm. It lies between the

pericarp Fruit anatomy is the plant anatomy of the internal structure of fruit. Fruits are the mature ovary or ovaries of one or more flowers. They are found in three main anatomical categories: aggregate fruits, multiple fruits, and simple fruits. Aggr ...

and the hyaline layer of the endosperm. Unlike the cells of the starchy endosperm, aleurone cells remain alive at maturity. The ploidy of the aleurone is (3n) as a result of

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 a ...

Aleurone protein

Aleurone proteins can have two different morphological features, homogenous and heterogeneous. The homogenous aleurone consists of similar protein bodies (e.g. '' Phaseolus vulgaris'') while the heterogeneous aleurone consists of granules of different shapes and types of proteins covered with a membrane (e.g. ''

Ricinus communis ''Ricinus communis'', the castor bean or castor oil plant, is a species of perennial flowering plant in the spurge family, Euphorbiaceae. It is the sole species in the monotypic genus, ''Ricinus'', and subtribe, Ricininae. The evolution of cas ...

'').

Aleurone tissue

The aleurone layer surrounds the endosperm tissue of grass seeds and is morphologically and biochemically distinct from it. Starchy endosperm cells are large, irregularly shaped cells and contain starch grains while aleurone cells are cuboidal in shape and contain aleurone grains. In most cultivated cereals ( wheat species, rye,

oats The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural, unlike other cereals and pseudocereals). While oats are suitable for human co ...

rice Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly ''Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and '' Porteresia'', both wild and domesticat ...

and

maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...

) the aleurone is single-layered, whereas

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...

has a multicellular aleurone layer. Thick primary cell walls enclose and protect the aleurone cells. The aleurone layer is important for both the developing seed and the mature plant. The aleurone tissue accumulates large quantities of oils and lipids that are useful during seed development. It is also a site of mineral storage and in some species, functions in seed dormancy. The aleurone may also express several pathogen-protective proteins including PR-4. Aleurone also serves as the most dietarily beneficial fraction in many brans. In addition, the aleurone tissue contains many protein-storing vacuoles known as protein bodies. In cereals with starchy endosperm, the aleurone contains about 30% of the kernel's proteins. In multicolored corn, anthocyanin pigments in the aleurone layer give the kernels a dark, bluish-black color.

Aleurone development

The development of the aleurone layer involves several periclinal, and anticlinal cell divisions and several steps of genetic regulation. The ''dek1'' gene and ''crinkly4 (cr4)'' kinase both function as positive regulators of aleurone cell fate. The normal ''dek1'' gene is needed in order to receive and respond to positional cues that determine the fate of aleurone cells during development. Mutants of the ''dek1'' gene block the formation of aleurone and cause the cells to develop as starchy endosperm cells instead of aleurone cells. This causes the seed to lack an aleurone layer. This mutation is caused by the insertion of a ''Mu''

transposon A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Tra ...

into the ''dek1'' gene, causing it to function incorrectly. However, this transposon may sometimes remove itself from the gene, restoring the function of ''dek1''. Experiments in this area have helped demonstrate that the cues that determine aleurone positioning are still present in the later stages of development, and the aleurone cells still respond to these cues. Similar to the ''dek1'' mutation, genes with a mutation in the ''cr4'' gene also cause a switch in the fate of aleurone cells. The ''cr4'' gene codes for a receptor kinase and so is involved in signal transduction pathways involving the fate of aleurone cells. Plants with a mutated ''cr4'' gene are shorter than normal and produce crinkled leaves. In addition, several hormones influence the development of the aleurone layer, including auxin,

cytokinin Cytokinins (CK) are a class of plant hormones 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 le ...

, abscisic acid (ABA), and

gibberellin Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plan ...

(GA). Auxin and cytokinin play a role in the earlier stages of aleurone development. The maturation of aleurone is promoted by ABA while germination is promoted by GA.

Aleurone function

The aleurone layer performs a variety of functions to help maintain proper development of the seed. One example of this is maintaining a low pH in the

apoplast Inside a plant, the apoplast can mean the space outside of cell membranes, where material can diffuse freely; that is, the extracellular spaces. ''Apoplast '' can also refer especially to the continuum of cell walls of adjacent cells; fluid and ...

. In cereals, the aleurone layer releases organic and phosphoric acids in order to keep the pH of the endosperm between a pH of 3.5 and 4. In barley, the aleurone layer also releases nitrite into the starchy endosperm and apoplast under

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

conditions. In addition, although the function is unclear, a certain class of hemoglobins is present in the outer layer of living cells including the aleurone tissue in barley and rice seeds. During seed germination, the plant embryo produces the hormone

which triggers the aleurone cells to release

α-amylase α-Amylase is an enzyme (EC 3.2.1.1; systematic name 4-α-D-glucan glucanohydrolase) that hydrolyses α bonds of large, α-linked polysaccharides, such as starch and glycogen, yielding shorter chains thereof, dextrins, and maltose: :Endohyd ...

for the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

of starch, proteases, and storage proteins into the endosperm. Evidence that G-proteins play a role in the gibberellin signaling events has been obtained. The breakdown of the starchy endosperm supplies sugars to drive the growth of roots and the acrospire. This release of amylase is considered to be the most important and sole function of the aleurone layer. This effect is inhibited by the plant hormone abscisic acid, which keeps the seed dormant. After completing this function, the aleurone cells in the developing seed undergo apoptosis. Experiments conducted in the 1960s confirmed that in order for the aleurone layer to secrete starch-degrading enzymes, the embryo must be present. Following removal of the embryo, starch-degrading enzymes were not released and no degradation of the starch tissue occurred.Taiz, L., & Zeiger, E. (2002). Plant physiology. (3 ed., p. 484). Sunderland, MA: Sinauer Associates, Inc., Publishers. The gibberellin effect on the aleurone is used in brewing, specifically in the production of barley malt where treatment ensures that a batch of barley seeds will germinate evenly.

References

External links

* {{Botany Plant proteins