Effect of Arabinogalactan Proteins from the Root Caps of Pea and Brassica napus on Aphanomyces euteiches Zoospore Chemotaxis and Germination
Résumé
Root tips of many plant species release a number of border, or border-like, cells that are thought to play a major role in the
protection of root meristem. However, little is currently known on the structure and function of the cell wall components of such
root cells. Here, we investigate the sugar composition of the cell wall of the root cap in two species: pea (Pisum sativum), which
makes border cells, and Brassica napus, which makes border-like cells. We find that the cell walls are highly enriched in arabinose
and galactose, two major residues of arabinogalactan proteins. We confirm the presence of arabinogalactan protein epitopes on
root cap cell walls using immunofluorescence microscopy. We then focused on these proteoglycans by analyzing their
carbohydrate moieties, linkages, and electrophoretic characteristics. The data reveal (1) significant structural differences
between B. napus and pea root cap arabinogalactan proteins and (2) a cross-link between these proteoglycans and pectic
polysaccharides. Finally, we assessed the impact of root cap arabinogalactan proteins on the behavior of zoospores of
Aphanomyces euteiches, an oomycetous pathogen of pea roots. We find that although the arabinogalactan proteins of both
species induce encystment and prevent germination, the effects of both species are similar. However, the arabinogalactan
protein fraction from pea attracts zoospores far more effectively than that from B. napus. This suggests that root arabinogalactan
proteins are involved in the control of early infection of roots and highlights a novel role for these proteoglycans in root-microbe
interactions.