Minisymposium 16: Cell walls

Abs # 31003: Deciphering the roles of xyloglucan galactosylation in the assembly of a functional plant cell wall

Presenter:	Li, Xuemei , xm0101@yahoo.com
Authors	Li, Xuemei  (A)   Peña, María  (B)   Cordero, Israel  (A)   Link, Bruce  (A)   Amin, Amy  (A)   Carpita, Nick  (B)   Reiter, Wolf-Dieter  (A)
Affiliations:	(A): Department of Molecular and Cell Biology, University of Connecticut (B): Department of Botany and Plant Pathology, Purdue University
Web Site:	http://www.sp.uconn.edu/~mcbstaff/reiter/index.html

Xyloglucans (XyGs) are abundant plant cell wall polysaccharides that bind to and cross-link cellulose microfibrils. The tight association between these two cell wall polymers leads to the establishment of a three-dimensionsal load-bearing network, which is embedded in a matrix of pectic components. Continuous remodeling of the XyG-cellulose network by wall-resident enzymes is believed to aid in the maintainence of a pliable but mechanically strong cell wall during expansion growth. XyGs consist of a β(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 are known to carry galactosyl and fucosyl-galactosyl side chains, respectively, but the functional significance of these modifications is poorly understood. XyGs of the mur3 mutant of Arabidopsis lack the fucosyl-galactosyl side chain because of a defect in XyG galactosyltransferase I, which converts XXXG to its galactosylated derivative XXLG. Although XyG binding to cellulose is unaffected, mur3 hypocotyls exhibit abnormal cell swelling and greatly reduced tensile strength. Shoots of chemically induced mur3 alleles do not show obvious visible phenotypes; however, three independent T-DNA insertion lines are slightly stunted with reduced cellulose content, decreased tensile strength of elongating inflorescence stems, wrinkled leaves, and short filaments. These phenotypes suggest defects in wall assembly and cell elongation. MUR3 belongs to glycosyltransferase family 47, which encompasses 39 members in Arabidopsis (http://afmb.cnrs-mrs.fr/CAZY/; Li et al. (2004) Plant Physiol. 134: 940-950). The recent isolation and characterization of a knockout mutant in the MUR3 paralog AtGT18 revealed only small amounts of galactose on the central xylose residue within the XyG repeat unit, suggesting that this gene encodes XyG galactosyltransferase II, which converts XXXG to XLXG. The atgt18 plants show a droopy phenotype with decreased cellulose content and abnormal patterns of lignin deposition in the inflorescence stems. Preliminary results on the F2 population from a mur3 x atgt18 cross indicate segregation for plants with severe growth defects, which appear to be double mutants. Comparative analysis of the various mutant lines should lead to interesting insights into the functional significance of xyloglucan galactosylation. Supported by grants from NSF-IBN and the Plant Genome Research Program.