Aleurone (fromGreekaleuron, flour) is aprotein found in protein granules of maturingseeds andtubers.[clarification needed] The term also describes one of the two major cell types of theendosperm, thealeurone layer. The aleurone layer is the outermost layer of the endosperm, followed by the innerstarchy endosperm.[1] This layer of cells is sometimes referred to as the peripheral endosperm. It lies between thepericarp 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 ofdouble fertilization].[2]
The aleurone layer surrounds theendosperm 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.[3] In most cultivated cereals (wheat species,rye,oats,rice andmaize) the aleurone is single-layered, whereasbarley has a multicellular aleurone layer.[4][5] Thick primary cell walls enclose and protect the aleurone cells.[6]
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.[7] 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 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).[8]
The development of the aleurone layer involves severalpericlinal, andanticlinal cell divisions and several steps of genetic regulation. Thedek1 gene andcrinkly4 (cr4) kinase both function as positive regulators of aleurone cell fate.[9] The normaldek1 gene is needed in order to receive and respond to positional cues that determine the fate of aleurone cells during development.[10]
Mutants of thedek1 gene block the formation of aleurone and cause the cells to develop as starchy endosperm cells instead of aleurone cells.[11] This causes the seed to lack an aleurone layer. This mutation is caused by the insertion of aMutransposon into thedek1 gene, causing it to function incorrectly. However, this transposon may sometimes remove itself from the gene, restoring the function ofdek1. 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.[12]
Similar to thedek1 mutation, genes with a mutation in thecr4 gene also cause a switch in the fate of aleurone cells. Thecr4 gene codes for a receptor kinase and so is involved in signal transduction pathways involving the fate of aleurone cells. Plants with a mutatedcr4 gene are shorter than normal and produce crinkled leaves.[13]
In addition, several hormones influence the development of the aleurone layer, includingauxin,cytokinin,abscisic acid (ABA), andgibberellin (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.
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 theapoplast. 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 underanaerobic conditions.[14] 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.[15]
During seedgermination, the plantembryo produces the hormonegibberellin which triggers the aleurone cells to releaseα-amylase for thehydrolysis of starch, proteases, and storage proteins into theendosperm. Evidence that G-proteins play a role in the gibberellin signaling events has been obtained.[16] The breakdown of the starchy endosperm supplies sugars to drive the growth of roots and theacrospire. 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 hormoneabscisic acid, which keeps the seed dormant. After completing this function, the aleurone cells in the developing seed undergoapoptosis.
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.[17]
The gibberellin effect on the aleurone is used inbrewing, specifically in the production of barleymalt where treatment ensures that a batch of barley seeds will germinate evenly.
| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
