Molecular cloning and characterization of a cDNA encoding poplar UDP-glucose dehydrogenase,a key gene of hemicellulose/pectin formation

Plant UDP-glucose dehydrogenase (UGDH) is an important enzyme in the formation of hemicellulose and pectin, the components of newly formed cell walls. A cDNA clone (Ugdh) corresponding to UGDH was isolated from a cDNA library prepared from cambial zone of poplar (Populus tremula x tremuloides). Within the 1824-nucleotide (nt)-long clone, an open reading frame encoded a protein of 481 amino acids (aa), with a calculated molecular weight of 53.1 kDa. The derived aa sequence showed 90% and 63% identity with UGDHs from soybean and bovine liver, respectively, and had highly conserved aa motifs believed to be of importance for nt binding and catalytic efficiency. In poplar, the Ugdh corresponds to one or two genes, as found by genomic Southern analysis. The gene was expressed predominantly in differentiating xylem and young leaves, with little expression in the phloem zone of the stem. The expression pattern matched that of UGDH protein, as found by immunoblotting. In leaves, the Ugdh expression was upregulated by a short-term feeding with sucrose, sorbitol and polyethylene glycol, and this effect was to some extent mimicked by light exposure. The data suggest that Ugdh is regulated via an osmoticum-dependent pathway, possibly related to the availability of osmotically active carbohydrate precursors to UDP-glucose, a substrate of UGDH.