expansin

Expansin is a family of plant cell wall–loosening proteins that promote cell expansion by altering the physical properties of the cell wall. First described in 1992 by McQueen-Mason and Cosgrove, expansins were named for their ability to induce growth in plant tissues such as cucumber hypocotyls. These are secreted proteins that accumulate in the cell wall (apoplast) and possess a signal peptide and two conserved domains (D1 and D2). They are thought to act without enzymatic hydrolysis, instead loosening non-covalent bonds between cellulose microfibrils and matrix polysaccharides to allow turgor-driven wall extension.

Expansin activity is pH-responsive, being most active in mildly acidic conditions typical of the expanding apoplast, consistent with the expansin hypothesis that wall loosening is regulated by the apoplastic pH.

The expansin superfamily includes four subfamilies: EXPA (alpha-expansins) and EXPB (beta-expansins) that predominate in many angiosperms, and EXLA and EXLB (expansin-like) that are more divergent. In most species, expansin genes form large, multigene families with distinct tissue- and developmental‑specific expression patterns.

Expansin expression correlates with cell elongation in stems, roots, and leaves, pollen tube growth, and fruit softening during ripening. Hormones such as auxin, brassinosteroids, and gibberellins modulate expansin expression and activity; environmental factors including drought, salinity, and mechanical stress can also influence expansin-mediated wall loosening.

Because wall loosening controls plant growth and mechanical properties, expansins have been explored in crop improvement for fiber quality, root architecture, and fruit texture, though their precise roles are context-dependent and subject to regulation by other wall-modifying enzymes and signaling pathways.