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

Depolymerization of cell watt xyloglucan has been proposed to be involved in tomato fruit softening, along with the xyloglucan modifying enzymes. Xyloglucan endo-transgtucosylase/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 watt-bound xyloglucan, or restructuring the existing cell watt material by catalyzing transglucosylation between previously wall-bound xyloglucan molecules. Here, 10 tomato (Solanum lycopersicum) SIXTHs 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 SIXTHs contributed to the total XET and XEH activities. Our results showed that all of the SIXTHs studied were expressed during fruit growth and ripening, and that the expression of all. the SIXTHs in Group 1 was clearly related to fruit growth, as were SIXTH12 in Group 2 and SIXTH6 in Group 3-B. Only the expression of SIXTH5 and SIXTH8 from Group 3-A was clearly associated with fruit ripening, although all 10 of the different SIXTHs 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 tow and experienced a significant increase during fruit ripening, which was not correlated either with SIXTH 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 watt, 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. (C) 2008 Elsevier GmbH. All rights reserved.