Characterization and Immunolocalization of Arabinogalactan-Proteins in Roots of Echinacea purpurea

From the high molecular weight fraction of an aqueous extract from roots of Echinacea purpurea L. Moench, arabinogalactan-proteins (AGPs), a class of proteoglycans proposed to be involved in cell differentiation and plant growth, were purified and characterized with regard to amino acid composition and structure of the polysaccharide moiety. The protein content of the AGP was 5.0% (w/w) with the dominating amino acids Glx, Hyp, Asx, Set, Thr and Ala. The highly branched polysaccharide moiety shows a linkage composition typical of AGPs with 1,3-, 1,6- and 1,3,6-linked galactopyranosyl residues and arabinofuranosyl residues predominantly as terminal and 1,5-linked residues. Terminal units Of glucuronopyranose acid were also detected. Furthermore, a new method for the localization of AGPs in plant tissue has been developed. The synthetic (beta-D-Glc)(3) Yariv phenylgycoside (beta GlcY) is known to specifically bind to AGPs. For immunolocalization, polyclonal beta GlcY-antibodies have been generated and were used to label Yariv-treated thin sections of roots from E. purpurea. After addition of the FITC-conjugated secondary antibody, the sections were analyzed by confocal laser scanning microscopy. AGPs are detected mainly in the central cylinder in the area of the xylem. Cell walls of vessels and tracheids are strongly labelled, especially at the inner area of the wall. Furthermore, there is intense labelling of the pit canals.