Immunocytochemical Study of the Distribution of Pectic Polysaccharides during Secondary Xylem Development of Normal, Opposite and Compression Woods of Pinus radiata

Abstract

The occurrence and distribution of different pectic polysaccharide domains in the walls of developing tracheids were examined using indirect immunofluorescence microscopy with seven monoclonal antibodies in normal, opposite and compression woods of seedling stems of Pinus radiata. The developing wood was divided into three zones: the cambium and secondary xylem derivatives (Zone 1); differentiating tracheids (Zone 2); and mature tracheids (Zone 3). Walls of cambial cells in Zone 1 of all wood types labelled strongly with the monoclonal antibody LM19, but only weakly with LM20, indicating that galacturonic acid residues in the homogalacturonan (HG) domain were only slightly methyl esterified. These walls also labelled strongly with LM6, indicating the presence of large amounts of (1→5)-α-L-arabinans, which form side chains to rhamnogalacturonan 1 (RG-1). Strong labelling with LM5 was also found in these walls in normal and opposite wood, but weak labelling in compression wood, indicating large amounts and small amounts of (1→4)-β-D-galactans, which also form side chains of RG-1. The walls of the xylem derivative cells of all wood types showed similar LM6 and LM5 labelling patterns to the walls of the cambial cells. In contrast, the walls of the xylem derivative cells of all wood types labelled strongly with LM20, but only weakly with LM19, indicating that the galacturonic acid residues in HG were highly methyl esterified. The walls of differentiating tracheids of NW and OW labelled very weakly with LM19 but moderately with LM20, LM6 and LM5. In contrast, the walls of the differentiating tracheids of CW labelled strongly with LM5, moderately with LM6, but not with LM19 or LM20. Walls of mature tracheids of NW and OW in Zone 3 labelled weakly with LM20, LM5 and LM6, but not with LM19. In contrast, the walls of mature tracheids of CW labelled strongly with LM5, weakly with LM6, but not with LM19 or LM20. Lignin in walls in Zones 2 and 3 may have masked the epitopes of the different antibodies used. No labelling was detected with LM8 which binds to xylogalacturonan domains, PAM1, which binds to long blocks of ~30 contiguous unesterified GalA residues in HG, and 2F4, which detects egg-box structures.