Expansins are 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, meristem function, and other developmental processes where cell wall loosening occurs. 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 (acidic) pH than at neutral pH. Expansins are thus linked to auxin action. They are also linked to cell enlargement and cell wall changes induced by other plant hormones such as gibberellin, cytokinin, ethylene and brassinosteroids.
A subset of the β-expansins are also the major group-1 allergens of grass pollens.
Families
So far, two large families of expansin genes have been discovered in plants, named alpha-expansins (given the gene symbol EXPA) and beta-expansins (EXPB). Both families of expansins have been identified in a wide range of land plants, from angiosperms and gymnosperms to ferns and mosses. The model plant Arabidopsis thaliana contains around 26 different α-expansin genes and 6 β-expansin genes. A subset of β-expansins has evolved a special role in grass pollen, where they are 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. Some proteins in bacteria and fungi are known to have distant sequence similarity to plant expansins. Strong evidence that at least some of these sequences are indeed expansins came in 2008 when the crystal structure of the YOAJ protein from a bacterium (Bacillus subtilis) was shown to be very similar to the structure of plant expansins, despite the low sequence similarity. This study also noted that proteins related to YOAJ were found in diverse species of plant pathogenic bacteria, but not in related bacteria that did not attack or colonize plants, thus suggesting that these bacterial expansins have a role in plant-microbe interactions. Some animals, such as Globodera rostochiensis, a plant-parasitic nematode, can produce a functional expansin which uses it to loosen cell walls when invading its host plant.
To be designated as expansin or expansin-like, genes and their protein products must contain both domain I (N-terminal, catalytic, GH45-like - GH meaning glycoside-hydrolase) and domain II (C-terminal, distantly related to group-2 grass pollen allergens). Non-plant expansins can be designated with the symbol EXLX (expansin-like X), but they do not constitute a monophyletic group; distantly similar to plant expansins, they could have diverged prior to the origin of land plants, or else could have been acquired by horizontal transfer.
Nomenclature of genes and proteins of expansins and expansin-like: e.g., Arabidopsis thaliana EXPANSIN A1 is named "AtEXPA1" as for the gene, and "AtEXPA1" as for the protein; one adds "-1" for mutant allele 1.
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Expansins characteristically cause wall stress relaxation and irreversible wall extension (wall creep). This process is essential for cell enlargement. Expansins are also expressed in ripening fruit where they function in fruit softening, and in grass pollen, where they loosen stigmatic cell walls and aid pollen tube penetration of the stigmain germinating seeds for cell wall disassembly, in floral organs for their patterning, in developing nitrogen-fixing nodules in legumes, in abscessing leaves, in parasitic plants, and in ‘resurrection’ plants during their rehydration. No enzymatic activity has been found for expansin and in particular, no glucanase activity: they don't hydrolyze the matrix polysaccharides; the only definitive assay for expansin activity is thus to measure wall stress relaxation or wall extension.