Characterization of a tomato xyloglucan endotransglycosylase gene that is down-regulated by auxin in etiolated hypocotyls

The reorganization of the cellulose-xyloglucan matrix is proposed to serve as an important mechanism in the control of strength and extensibility of t he plant primary cell wall. One of the key enzymes associated with xylogluc an metabolism is xyloglucan endotransglycosylase (XET), which catalyzes the endocleavage and religation of xyloglucan molecules. As with other plant s pecies, XETs are encoded by a gene family in tomato (Lycopersicon esculentu m cv T5). In a previous study, we demonstrated that the tomato XET gene LeE XT was abundantly expressed in the rapidly expanding region of the etiolate d hypocotyl and was induced to higher levels by auxin. Here, we report the identification of a new tomato XET gene, LeXET2, that shows a different spa tial expression and diametrically opposite pattern of auxin regulation from LcEXT. LeXET2 was expressed more abundantly in the mature nonelongating re gions of the hypocotyl, and its mRNA abundance decreased dramatically follo wing auxin treatment of etiolated hypocotyl segments. Analysis of the effec t of several plant hormones on LeXET2 expression revealed that the inhibiti on of LeXET2 mRNA accumulation also occurred with cytokinin treatment. LeXE T2 mRNA levels increased significantly in hypocotyl segments treated with g ibberellin, but this increase could be prevented by adding auxin or cytokin in to the incubation media. Recombinant LeXET2 protein obtained by heterolo gous expression in Pichia pastoris exhibited greater XET activity against x yloglucan from tomato than that from three other species. The opposite patt erns of expression and differential auxin regulation of LeXET2 and LcEXT su ggest that they encode XETs with distinct roles during plant growth and dev elopment.