Expansin

Expansin refers to a family of closely-related nonenzymatic proteins, found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, and other developmental processes where cell wall loosening occurs [Cosgrove, DJ (2000) Loosening of plant cell walls by expansins. Nature 407: 321-326] . Expansins were originally discovered as mediators of acid growth, which refers to the widespread characteristic of growing plant cell walls to expand faster at low (acid) pH than at neutral pH. They are thus linked to auxin action. They are also linked to growth and cell wall changes induced by other plant hormones such as gibberellin, cytokinin, ethylene, and brassinosteroids.

Families

Thus far, two large families of expansin genes have been discovered in plants, named alpha-expansins (given the gene symbol EXPA] and beta-expansins (EXPB). A subset of beta-expansins has evolved a special role in grass pollen, where they known as group 1 grass pollen allergens. Plants also have a small set of expansin-like genes (named EXLA and EXLB) whose function has not been established [J. Sampedro and D. J. Cosgrove. The expansin superfamily. Genome Biol. 6 (12):242, 2005] .

Actions

Expansins characteristically cause wall stress relaxation and irreversible wall extension (wall creep). No enzymatic activity has been found for expansin. The only assay for expansin activity is to measure wall stress relaxation or wall extension.

Mechanism

In cell growth: Because the plant cell wall is quite high in tensile strength, it must be 'loosened' to enable the cell to grow, that is, to enlarge irreversibly. Within the cell wall, this expansion of surface area involves slippage or movement of cellulose microfibrils, which normally is coupled to simultaneous uptake of water. In physical terms, this mode of wall expansion requires cell turgor pressure to stretch the cell wall and to put the network of interlinked cellulose microfibrils in tension. By loosening the linkages between cellulose microfibrils, expansins allow the wall to yield to the tensile stresses created in the wall by turgor pressure [D. J. Cosgrove. Growth of the plant cell wall. Nat.Rev.Mol.Cell Biol. 6 (11):850-861, 2005] .

The molecular mechanism by which expansin loosens the cellulosic network within the cell wall is not yet established in molecular detail. Expansin is hypothesized to disrupt the noncovalent adhesion or entrapment of hemicellulose on the surface of cellulose microfibrils. Hemicellulose can tether cellulose microfibrils together, forming a strong load-bearing network. Expansin is thought to disrupt the cellulose:hemicellulose association transiently, allowing slippage or movement of cell wall polymers before association reforms and the integrity of the cell wall network is reestablished [Yennawar,N.H., Li, L-C., Dudsinski,D.M., Tabuchi,A., Cosgrove, D.J. (2006). Crystal structure and activities of EXPB1 (Zea m 1), a {beta}-expansin and group-1 pollen allergen from maize. PNAS 103:14664-14671] .

ources

* [http://www.bio.psu.edu/expansins/about_expansins.htm]
* [http://www.plantphysiol.org/cgi/content/full/128/3/854]
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References

ee also

*Cellulose
*plant cell wall