Alphaexpansins

Alpha-expansins, or alpha-expansins (EXPA), are a family of plant cell wall proteins that promote cell expansion by loosening wall structure. They are secreted into the apoplast and accumulate in growing tissues where turgor pressure drives enlargement. The EXPA gene family is widespread in land plants, with multiple members in species such as Arabidopsis and rice. EXPAs are distinct from beta-expansins and expansin-like proteins, forming a key subgroup within the wider expansin superfamily.

Structurally, alpha-expansins are characterized by a signal peptide for secretion and a two-domain organization typical of expansins. The proteins act nonenzymatically to modify the physical interactions within the cell wall rather than hydrolyze polysaccharides. By weakening noncovalent linkages between cellulose microfibrils and matrix polysaccharides, they facilitate wall extensibility in response to growth signals. Expansin activity is often associated with acidic conditions in the cell wall, consistent with the acid-growth hypothesis for hormone-stimulated growth.

Expression and function of EXPA genes are dynamically regulated during development and in response to environmental cues. They are commonly expressed in expanding tissues such as young leaves, roots, stems, developing fruits, and pollen tubes. Hormonal regulation (for example by auxin) and environmental factors influence their transcription, contributing to processes like organ growth, fruit softening, seed germination, and stress adaptation.

Evolution and applications of alpha-expansins reflect their central role in plant growth. The expansion of EXPA gene families across species underpins diverse growth strategies, and targeted manipulation of EXPAs has potential for affecting crop size, fruit texture, and stress resilience.