Deciphering the roles of xyloglucan galactosylation in the assembly of a functional plant cell wall

Xuemei Li¹, Nick Carpita², Michael Madson², Israel Cordero¹ and Wolf-Dieter Reiter¹

¹University of Connecticut, Storrs, CT; ²Purdue University, West Lafayette, IN

Xyloglucans are the principal hemicellulose in the primary walls of the most flowering plants. These polysaccharides consist of a b-(1,4)-linked glucan backbone, which is substituted by xylose residues in an “XXXG” repeat pattern. Some of the second and third xylosyl residues within this core structure carry galactosyl and fucosyl-galactosyl side chain, respectively. The main function of xyloglucan is the binding and cross-linking of cellulose microfibrils, which leads to the establishment of a load-bearing three-dimensional network believed to play a key role in cell wall assembly and remodeling during expansion growth.

We recently found that the xyloglucan of mur3 mutant of Arabidopsis lacks the fucosyl-galactosyl side chain because of a defect in xyloglucan galactosyltransferase I, which converts XXXG to its galatosylated derivative XXLG (Madson, et al. Plant Cell 2003). While chemically induced mur3 alleles do not lead to obvious visible phenotypes, a T-DNA insertion line is slightly stunted suggesting a defect in cell wall assembly.

The recent isolation and characterization of a knockout mutant in the MUR3 paralog AtGT18 revealed that it contains only small amounts of galactose on the central xylose residue within the xyloglucan repeat unit. This suggests that the AtGT18 gene encodes xyloglucan galactosyltransferase II, which converts XXXG to XLXG. The atgt18 plants show a “droopy” phenotype and abnormal patterns of lignin deposition presumably reflecting a defect in xyloglucan-cellulose interactions.

To obtain plants with a very low degree of xyloglucan galactosylation, we conducted crosses between mur3 and atgt18 lines. Preliminary results on the F2 population indicate segregation for plants with very severe growth defects, which could be double mutants. We hope that a comparative analysis of the various mutant lines will lead to meaningful insights into the functional significance of xyloglucan galactosylation.