Expansins in Plant Development

Expansins are a class of cell wall proteins that mediate pH-dependent wall loosening, probably by disrupting hydrogen bonds between cellulose and matrix glycans. Expansins are distributed primarily over the expanding cell wall and bind to the cell wall. Expansin proteins share the common structural features of protein: a signal peptide for secretion to the cell wall and two distinct domains. Domain 1 contains the conserved Cys residues and HFD (His-Phe-Asp) motif while domain 2 has the conserved Trp residues. Crystal structure of an expansin B (EXPB) protein suggests that the latter domain may participate in binding to polysaccharides. Expansins are encoded by the four gene families in vascular plants: expansin A (EXPA), expansin B (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB). It appears that the moss Physcomitrella patens lacks expansin genes that belong to EXLA and EXLB families. Based on intron position and gene clusters, all the families appear to originate from a common ancestor. Biological functions of expansins are considered in diverse aspects of plant growth and development: shoot and root development, shoot and root elongation, leaf morphogenesis, floral organ development, fruit development, embryogenesis, pollen development, and pollen tube growth. Expression of expansin genes in the processes is regulated by hormones, partly contributing to adaptation of plants in response to environmental stimuli and partly to morphological development of plants.