Overexpression of a Cell Wall Enzyme Reduces Xyloglucan Depolymerization and Softening of Transgenic Tomato Fruits

Xyloglucan xyloglucosyltransferase/endohydrolase (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151) has been proposed to have a dual role integrating newly secreted xyloglucan chains into an existing wall-bound xyloglucan and restructuring existing cell wall material by catalyzing transglucosylation between previously wall bound xyloglucan molecules. In this work we generated transgenic tomatoes with altered levels of an XTH gene. These transgenic fruits showed significant overexpression of the XTH proteins in comparison with the wild type. Specific XET activity was approximately 4.33 fold higher in the transgenic fruits compared with the wild type fruits, although in both cases the activity decreased during fruit development. Cell wall hemicelluloses extracted with 24% KOH showed a depolymerization of total sugar and xyloglucan during ripening, although this depolymerization was much lower in the transgenic than in the wild type tomatoes. These results suggest that the increased XET activity in the transgenic plants was responsible for the lower xyloglucan depolymerization. Fruit softening, during ripening, was lower in the transgenic tomatoes, indicating that the xyloglucan structure is related with the softening mechanism and that XET is one of the enzymes involved in the process. We suggest that the role of XET during fruit growth and ripening could be related to the maintenance of the structural integrity of the cell wall and the decrease in activity during ripening might contribute to the fruit softening.