The EgMUR3 xyloglucan galactosyltransferase from Eucalyptus grandis complements the mur3 cell wall phenotype in Arabidopsis thaliana

Xyloglucan is the major hemicellulosic polymer found in the primary cell walls of dicots. Xyloglucan tethers cellulose microfibrils conferring rigidity and strength for maintenance of cell integrity, and it is thought that its metabolism contributes to cell elongation and thus plant growth. Here, we have cloned and characterized a Eucalyptus grandis gene ortholog of the Arabidopsis thaliana MUR3 gene (xyloglucan galactosyltransferase), thus termed EgMUR3. EgMUR3 represents an intronless sequence of 1,854 bp predicted to encode a protein of 617 amino acid residues. It exhibits 73% identity and 82% similarity to the A. thaliana MUR3 gene. To demonstrate that this gene encodes a functional enzyme, the putative ORF was cloned into a binary vector under the control of a constitutive promoter and transformed into the A. thaliana mur3 mutant. The effect of the genetic complementation was investigated by xyloglucan oligosaccharide fingerprinting of wall material. The results confirmed that EgMUR3 represents indeed a xyloglucan galactosyltransferase of E. grandis able to use endogenous substrate(s) in A. thaliana, suggesting that both species share common steps in xyloglucan biosynthesis.