Extracellular Proteins in Pea Root Tip and Border Cell Exudates

Abstract

Newly generated plant tissue is inherently sensitive to infection. Yet, when pea (Pisum sativum) roots are inoculated with the pea pathogen, Nectria haematococca, most newly generated root tips remain uninfected even though most roots develop lesions just behind the tip in the region of elongation. The resistance mechanism is unknown but is correlated spatially with the presence of border cells on the cap periphery. Previously, an array of .100 extracellular proteins was found to be released while border cell separation proceeds. Here we report that protein secretion from pea root caps is induced in correlation with border cell separation. When this root cap secretome was proteolytically degraded during inoculation of pea roots with N. haematococca, the percentage of infected root tips increased from 4% 6 3% to 100%. In control experiments, protease treatment of conidia or roots had no effect on growth and development of the fungus or the plant. A complex of .100 extracellular proteins was confirmed, by multidimensional protein identification technology, to comprise the root cap secretome. In addition to defense-related and signaling enzymes known to be present in the plant apoplast were ribosomal proteins, 14-3-3 proteins, and others typically associated with intracellular localization but recently shown to be extracellular components of microbial biofilms. We conclude that the root cap, long known to release a high molecular weight polysaccharide mucilage and thousands of living cells into the incipient rhizosphere, also secretes a complex mixture of proteins that appear to function in protection of the root tip from infection.

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

“…These cells play a fundamental role in controlling the interaction of plant roots with neighboring organisms within the soil (Hawes et al, 2000;Gunawardena and Hawes, 2002;Cannesan et al, 2011). Upon their separation from the root cap, border cells become uniquely differentiated, producing proteins and metabolites that are distinct from those made by the root cap cells (Brigham et al, 1995;Wen et al, 2007).…”

“…Border cells have also been shown to repel pathogenic bacteria by means of their secreted mucilage (Hawes et al, 2000). 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).…”

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

“…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 .…”

“…All plant species produce populations of border cells that vary in number, shape, size and activity (Hawes et al, 2003;Driouich et al, 2007Driouich et al, , 2012Endo et al, 2011;Plancot et al, 2013). Border cells synthesize and secrete a large variety of substances, among which are many defence-related molecules that contribute to root protection (Wen et al, 2007(Wen et al, , 2009Cannesan et al, 2011;Plancot et al, 2013;Driouich et al, 2013). These molecules also contribute to the composition of the mucilage that often encloses border cells and influences microbial dynamic and activity within the rhizosphere.…”

“…Root exudates comprise lowmolecular-weight compounds such as amino acids, organic acids and phenolics as well as high-molecular-weight compounds, mainly present in the mucilage (Walker et al, 2003b;Liu et al, 2014). The term 'mucilage' refers to the slimy 'halo' that is visible at the microscopic level surrounding the root tip (Wen et al, 2007;York et al, 2016). This mucilage is mainly produced by root cap cells.…”