Effect of Arabinogalactan Proteins from the Root Caps of Pea and Brassica napus on Aphanomyces euteiches Zoospore Chemotaxis and Germination      

Abstract

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.

Root exudate ofSolanum tuberosumis enriched in galactose-containing molecules and impacts the growth ofPectobacterium atrosepticum

“…Although we show that galactancontaining polymers such as RGI are constituents of potato root exudates, AGPs appear to be the most abundant molecules. This observation is in agreement with previous findings reporting the presence of AGPs in quite high amounts in root exudates of various plant species (Moody et al, 1988;Durand et al, 2009;Ma et al, 2010;Cannesan et al, 2012). However, AGPs were not the only HRGPs represented in the root exudates of potato.…”

“…Although less well studied, a higher defensive state seems to occur in roots compared with above-ground organs of the plants (Attard et al, 2010;Millet et al, 2010;Balmer et al, 2013). In addition, the root immune response seems to vary between different root tissues (Millet et al, 2010;Cannesan et al, 2011Cannesan et al, , 2012. Several studies have shown that root caps display enhanced local resistance to pathogen attacks compared with the elongation zone, which is often described as the primary site of infection (Gunawardena and Hawes, 2002;Gunawardena et al, 2005;Cannesan et al, 2011).…”

“…It contains high amounts of polysaccharides, which confer sticky and gel-like properties (Durand et al, 2009). In addition, the mucilage also contains many lowmolecular-weight molecules, proteins and extracellular DNA (Wen et al, 2007(Wen et al, , 2009Cannesan et al, 2012;York et al, 2016). Furthermore, by analogy with the 'neutrophil extracellular trap' (NET) described in mammalian blood cells, we have recently proposed that mucilage would constitute a 'root extracellular trap' (RET), involved in the protection of the root against soil-borne pathogens .…”

“…Based on the glycosidic linkage patterns established from exudates of different plants, it appears that polysaccharides are highly diverse and often unique within a plant species. Moreover, the nature of the polysaccharides and other glycan-containing molecules of root exudates plays a major role in mediating root-microbe interactions (Hinch and Clarke, 1980;Irving and Grant, 1984;Ray and Callow, 1988;Cannesan et al, 2012, Nguema-Ona et al, 2013. Although hydroxyproline has been reported to be a minor component in maize and cress exudates (Ray and Callow, 1988), glycoproteins such as arabinogalactan proteins (AGPs), which belong to the hydroxyproline-rich glycoprotein (HRGP) family, have been found to occur in root secretions of many plant species, such as pea, arabidopsis and Brassica napus (Knee et al, 2001;Durand et al, 2009;Cannesan et al, 2012).…”

Deciphering the Responses of Root Border-Like Cells of Arabidopsis and Flax to Pathogen-Derived Elicitors

“…During their detachment from the root cap, border cells of legumes export a large number of proteins, the secretome, containing antimicrobial enzymes, including chitinase, peptidase, and glucanase (Wen et al, 2007(Wen et al, , 2009. Furthermore, pea border cells secrete other compounds such as extracellular DNA, the phytoalexin pisatin, and arabinogalactan proteins that contribute to root protection against soil-borne pathogens (Wen et al, 2009;Cannesan et al, 2011Cannesan et al, , 2012Hawes et al, 2011).…”