Expansins: expanding importance in plant growth and development

Expansins were originally identified as cell wall-loosening proteins. The existence and various roles of expansins have been discovered in many plants. Expansins are encoded by a superfamily of genes comprised of subfamilies that evolved from a common ancestor and encode the alpha-expansins (EXPAs), the beta-expansins (EXPBs), the expansin-like A (EXLA), and expansin-like B (EXLB) proteins. Several expansin-like genes have also been identified in non-plant organisms (e.g. a slime mold, fungi, nematodes, and a mollusk). Localization of EXPA and EXPB in the cell wall was confirmed by immunogold electron microscopy. Studies using transgenic plants provided evidence for a broad range of biological roles of expansins in diverse aspects of plant growth and development, such as cell wall extension, fruit softening, abscission, floral organ development, symbiosis, and the response to environmental stresses.