Xyloglucan endotransglucosylase/hydrolases (XTHs) during tomato fruit growth and ripening.

Depolymerization of cell wall xyloglucan has been proposed to be involved in tomato fruit softening, along with the xyloglucan modifying enzymes. Xyloglucan endotransglucosylase/hydrolases (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151) have been proposed to have a dual role integrating newly secreted xyloglucan chains into an existing wall-bound xyloglucan, or restructuring the existing cell wall material by catalyzing transglucosylation between previously wall-bound xyloglucan molecules. Here, 10 tomato (Solanum lycopersicum) SlXTHs were studied and grouped into three phylogenetic groups to determine which members of each family were expressed during fruit growth and fruit ripening, and the ways in which the expression of different SlXTHs contributed to the total XET and XEH activities. Our results showed that all of the SlXTHs studied were expressed during fruit growth and ripening, and that the expression of all the SlXTHs in Group 1 was clearly related to fruit growth, as were SlXTH12 in Group 2 and SlXTH6 in Group 3-B. Only the expression of SlXTH5 and SlXTH8 from Group 3-A was clearly associated with fruit ripening, although all 10 of the different SlXTHs were expressed at the red ripe stage. Both total XET and XEH activities were higher during fruit growth, and decreased during fruit ripening. Ethylene production during tomato fruit growth was low and experienced a significant increase during fruit ripening, which was not correlated either with SlXTH expression or with XET and XEH activities. We suggest that the role of XTH during fruit development could be related to the maintenance of the structural integrity of the cell wall, and the decrease in XTHs expression, and the subsequent decrease in activity during ripening may contribute to fruit softening, with this process being regulated through different XTH genes.