Exploring the ultrastructural localization and biosynthesis of β(l,4)-galactan in pinus radiata compression wood

Abstract

Softwood species such as pines react to gravitropic stimuli by producing compression wood, which unlike normal wood contains significant amounts of β(l,4)-galactan. Currently, little is known regarding the biosynthesis or physiological function of this polymer or the regulation of its deposition. The subcellular location of β(l,4)-galactan in developing tracheids was investigated in Pinus radiata D. Don using anti-β(l,4)-galactan antibodies to gain insight into its possible physiological role in compression wood. β(1,4)-Galactan was prominent and evenly distributed throughout the S2 layer of developing tracheid cell walls in P. radiata compression wood. In contrast, β(l,4)-galactan was not detected in normal wood. Greatly reduced antibody labeling was observed in fully lignified compression wood tracheids, implying that lignification results in masking of the epitope. To begin to understand the biosynthesis of galactan and its regulation, an assay was developed to monitor the enzyme that elongates the β(l,4)-galactan backbone in pine. A β(l,4)-galactosyltransferase (GalT) activity capable of extending 2-aminopyridine-labeled galacto-oligosaccharides was found to be associated with microsomes. Digestion of the enzymatic products using a β(l,4)-specific endogalactanase confirmed the production of β(l,4)-galactan by this enzyme. This GalT activity was substantially higher in compression wood relative to normal wood. Characterization of the identified pine GalT enzyme activity revealed pH and temperature optima of 7.0 and 20°C, respectively. The β(l,4)-galactan produced by the pine GalT had a higher degree of polymerization than most pectic galactans found in angiosperms. This observation is consistent with the high degree of polymerization of the naturally occurring β(l,4)-galactan in pine. © 2009 American Society of Plant Biologists.