Abstract

The cell wall is a dynamic structure that can be modified by different conditions during growth. Stem inclination induces biochemical changes and chemical modifications in the cell walls of conifers driving plant stem reorientation. Different enzymes take part in cell wall remodeling, including xyloglucan endotransglycosylase/hydrolase (XTH). This enzyme modifies xyloglucan, the main hemicellulose found in primary cell walls of pine trees. The full-length complementary DNA of PrXTH1 was isolated and characterized from radiata pine. PrXTH1 contains an open-reading frame of 882 bp encoding a protein of 294 amino acids, including an N-terminal signal peptide for secretion that targets the protein to the cell wall. Phylogenetic and gene structure analyses suggest that PrXTH1 belongs to group I of XTHs. The recombinant purified protein has only transglycosylase activity. A 3D model of PrXTH1 protein was built through comparative modeling, showing a beta-sandwich structure with the catalytic site in the middle of the protein structure, composed of residues Glu79, Asp81, and Glu83. One-year-old half-sib seedlings were inclined at 45A degrees and transcriptional analysis indicated that PrXTH1 transcripts are more abundant in the lower side of inclined stems. Additionally, along the stem, PrXTH1 transcript accumulation starts at the apex and moves down over time to the stem base. This gene shows regulation by ethylene, indicating that it could be a target in the hormonal cascade during the inclination response. Finally, in radiata pine seedlings PrXTH1 is induced during the inclination response, suggesting an active participation of this enzyme in the cell wall reorganization required during this phenomenon.