PLANT-CELL ENLARGEMENT AND THE ACTION OF EXPANSINS

Plant cells are caged within a distended polymeric network (the cell w all), which enlarges by a process of stress relaxation and slippage (c reep) of the polysaccharides that make up the load-bearing network of the wall. Protein mediators of wall creep have recently been isolated and characterized. These proteins, called expansins, appear to disrupt the noncovalent adhesion of matrix polysaccharides to cellulose micro fibrils, thereby permitting turgor-driven wall enlargement. Expansin a ctivity is specifically expressed in the growing tissues of dicotyledo ns and monocotyledons. Sequence analysis of cDNAs indicates that expan sins are novel proteins, without previously known functional motifs. C omparison of expansin cDNAs from cucumber, pea, Arabidopsis and rice s hows that the proteins are highly conserved in size and amino acid seq uence. Phylogenetic analysis of expansin sequences suggests that this multigene family diverged before the evolution of angiosperms. Specula tion is presented about the role of this gene family in plant developm ent and evolution.