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Plant Physiol, November 2001, Vol. 127, pp. 1180-1192
Characterization of a Tomato Xyloglucan Endotransglycosylase Gene
That Is Down-Regulated by Auxin in Etiolated
Hypocotyls1
Carmen
Catalá,2*
Jocelyn K.C.
Rose,2
William S.
York,
Peter
Albersheim,
Alan G.
Darvill, and
Alan B.
Bennett
Complex Carbohydrate Research Center and Department of Biochemistry
and Molecular Biology, University of Georgia, 220 Riverbend Road,
Athens, Georgia 30602-4712 (C.C., A.G.D., J.K.C.R., P.A., W.S.Y.); and
Mann Laboratory, Department of Vegetable Crops, University of
California, Davis, California 95616 (A.B.B.)
The reorganization of the cellulose-xyloglucan matrix is
proposed to serve as an important mechanism in the control of strength and extensibility of the plant primary cell wall. One of the key enzymes associated with xyloglucan metabolism is xyloglucan
endotransglycosylase (XET), which catalyzes the endocleavage and
religation of xyloglucan molecules. As with other plant species,
XETs are encoded by a gene family in tomato (Lycopersicon
esculentum cv T5). In a previous study, we demonstrated that
the tomato XET gene LeEXT was abundantly expressed in
the rapidly expanding region of the etiolated 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 spatial
expression and diametrically opposite pattern of auxin regulation from
LeEXT. LeXET2 was expressed more
abundantly in the mature nonelongating regions of the hypocotyl, and
its mRNA abundance decreased dramatically following auxin treatment of
etiolated hypocotyl segments. Analysis of the effect of several plant
hormones on LeXET2 expression revealed that the
inhibition of LeXET2 mRNA accumulation also occurred
with cytokinin treatment. LeXET2 mRNA levels increased
significantly in hypocotyl segments treated with gibberellin, but this
increase could be prevented by adding auxin or cytokinin to the
incubation media. Recombinant LeXET2 protein obtained by heterologous
expression in Pichia pastoris exhibited greater XET
activity against xyloglucan from tomato than that from three other
species. The opposite patterns of expression and differential auxin
regulation of LeXET2 and LeEXT suggest that they encode XETs with distinct roles during plant growth and development.
1
This work was supported in part by the
U.S. Department of Energy (grant nos. DE-FG02-96ER20220 and
DE-FG05-93ER20097).
2
Present address: Department of Plant Biology, 214 Plant
Science Building, Cornell University, Ithaca, NY 14853.
*
Corresponding author; e-mail cc283{at}cornell.edu; fax
607-255-5407.
© 2001 American Society of Plant Physiologists
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