Heterogeneous distribution of pectin and hemicellulose epitopes in the phloem of four hardwood species

Abstract

Key message: Distributional patterns of pectin and hemicellulose epitopes in the phloem of four hardwoods vary between cell types including sieve tube elements, companion cells, parenchyma and sclerenchyma and between tree species. Abstract: Using immunolocalization methods combined with monoclonal antibodies, the distribution of pectin and hemicellulose epitopes was examined in the secondary phloem of two diffuse porous (birch, aspen)- and two ring porous (oak, ash) hardwoods with a focus on sieve tube elements (SEs), companion cells (CCs), axial/ray parenchyma cells, and sclerenchyma cells (sclereids and phloem fibers). In all tree species, rhamnogalacturonan-I (RG-I), homogalacturonan (HG), and xyloglucan epitopes were common in cell walls of SEs, CCs, and axial/ray parenchyma cells. However, the amount of these epitopes varied greatly between cell types and between hardwood species. Apart from aspen, heteroxylan or/and heteromannan epitopes were detected in SEs, but were not detected in CCs and parenchyma cells. With sclerenchyma cells, RG-I, HG, and xyloglucan epitopes were common in compound middle lamellae (CML) of sclereids and phloem fibers. Except for oak, heteromannan epitopes were also detected in CML of sclereids. Distributional patterns of epitopes in CML of birch and ash sclereids varied greatly depending on anatomical structure of CML. Secondary cell walls of sclereids and phloem fibers revealed abundant heteroxylan epitopes, but showed no heteromannan epitopes. Some phloem fibers also showed sparse xyloglucan epitopes in secondary cell walls. Together, results suggest that there are great variations in distributional patterns of pectin and hemicellulose epitopes in hardwood phloem between cell types and between tree species.