Aleurone (from

Greek Greek may refer to: 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 of all kno ...

''aleuron'', flour) is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

found in protein granules of maturing

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

s and

tuber Tubers are a type of enlarged structure that plants use as storage organs for nutrients, derived from stems or roots. Tubers help plants perennate (survive winter or dry months), provide energy and nutrients, and are a means of asexual reproduc ...

s. The term also describes one of the two major cell types of the

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...

, the aleurone layer. The aleurone layer is the outermost layer of the endosperm, followed by the inner

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...

y endosperm. This layer of cells is sometimes referred to as the peripheral endosperm. It lies between the

pericarp 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. Fruitlike structures may develop directly from the seed itself rather th ...

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) s a result of double fertilization">double_fertilization.html" ;"title="s a result of double fertilization">s a result of double fertilization

Description

The aleurone layer surrounds the

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">wheat">wheat species, rye, oats">rye.html" ;"title="wheat">wheat species, rye">wheat">wheat species, rye, oats, rice and maize) the aleurone is single-layered, whereas barley 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 Anthocyanins (), also called anthocyans, are solubility, water-soluble vacuole, vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart named a chemical compou ...

pigments in the aleurone layer give the kernels a dark, bluish-black color. Aleurone proteins can have two different morphological features, homogenous and heterogeneous. The homogenous aleurone consists of similar protein bodies (e.g. ''

Phaseolus vulgaris ''Phaseolus vulgaris'', the common bean,, is a herbaceous annual plant grown worldwide for its edible dry seeds or green, unripe pods. Its leaf is also occasionally used as a vegetable and the straw as fodder. Its botanical classification, alo ...

'') 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 ca ...

'').

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), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome. The discovery of mobile genetic elements earned Barbara McClinto ...

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 Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...

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 (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...

abscisic acid Abscisic acid (ABA or abscisin II) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to ...

(ABA), and

gibberellin Gibberellins (GAs) are plant hormones that regulate various Biological process, developmental processes, including Plant stem, stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. They are one of th ...

(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.

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 The ''apoplast '' is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extr ...

. 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: *Adhesive#Anaerobic, Anaerobic ad ...

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 Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, ...

, the plant

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

produces the hormone

which triggers the aleurone cells to release α-amylase 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 reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of starch, proteases, and storage proteins into the

. 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

, which keeps the seed dormant. After completing this function, the aleurone cells in the developing seed undergo

apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...

. 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 Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and #Fermenting, fermenting the resulting sweet liquid with Yeast#Beer, yeast. It may be done in a brewery ...

, specifically in the production of barley

malt Malt is any cereal grain that has been made to germinate by soaking in water and then stopped from germinating further by drying with hot air, a process known as "malting". Malted grain is used to make beer, whisky, malted milk, malt vinegar, ...

where treatment ensures that a batch of barley seeds will germinate evenly.

References

External links

* {{Botany Plant proteins