Roles of the XTH protein family in the expanding cell

Since xyloglucan is believed to be an important structural polysaccharide in the cell wall, possibly interconnecting adjacent cellulose microfibrils, enzymes that modify xyloglucan during the cell expansion process receive much attention. One of the enzymes is xyloglucan endotransglucosylase/hydrolase (XTH), a subgroup of glycoside hydrolase family 16. XTH proteins examined so far display endotransglycosylase (XET), hydrolase (XEH), or both activities towards xyloglucans. Genome sequencing of several model plant species has revealed that XTH proteins are encoded by large multigene families. Comprehensive analyses of XTH gene expression, together with functional analyses based on loss-of-function mutants, have provided evidence in support of the hypothesis that each member of these multigene families has its own role. This is reflected by different substrate specificities and pH dependencies of several individual members. Expression of each of these genes is precisely regulated by various hormones as well as by environmental signals. Some members appear to be critical in promoting cell wall expansion and are therefore essential for cell expansion, whereas others are required for construction of cell walls in cells that have completed the expansion process.