Expansins: Molecular Mediators of Cell Wall Extensibility in Plant Development and Stress Adaptation.

Expansins are fundamental agents of plant growth and adaptation, enabling cell walls to expand and remodel through unique non-enzymatic mechanisms. This review elucidates the structural biology, evolutionary history, and physiological versatility of these essential proteins. Characterised by a conserved two-domain architecture, a GH45-like domain (D1) and a CBM63-like domain (D2), expansins facilitate "acid growth" by physically disrupting hydrogen bonds at the cellulose-matrix interface without polymer hydrolysis. This article extends the indispensable roles of expansins throughout the plant life cycle, from sculpting root and shoot architecture to regulating reproduction in pollen tube growth, fruit softening, and seed germination. Furthermore, the review emphasises their function as critical modulators of plant resilience, illustrating how expansin-mediated cell wall remodelling drives adaptation to abiotic stresses as well as to biotic challenges.