Extracellular proteins in pea root tip and border cell exudates.

Fushi Wen,George Tsaprailis,Martha C Hawes,Hans D Vanetten

doi:10.1104/pp.106.091637

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% +/- 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.

Highlights

Generated plant tissue is inherently sensitive to infection
Considering the absence of infection within root tips, was the discovery that when viewed with a microscope, a conspicuous mantle of fungal hyphae was present on the cap periphery of most inoculated roots (Hawes et al, 1998; Gunawardena and Hawes, 2002)
Such hyphae, whose identity and pathogenicity were established as the inoculating strain of N. haematococca by expression of specific marker genes, spontaneously detached from the tip along with border cells when the root was immersed in water

Summary

Introduction

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 apical meristem within the root apex adds new cells to the growing root but it is the root cap that senses water, gravity, touch, and other signals and controls the direction of root growth toward positive stimuli such as nutrients and away from deleterious stimuli such as toxins (Darwin, 1896; Feldman, 1984; Aiken and Smucker, 1996) Synthesized tissue such as that found in the root tip normally is inherently sensitive to physical and biotic injury. Considering the absence of infection within root tips, was the discovery that when viewed with a microscope, a conspicuous mantle of fungal hyphae was present on the cap periphery of most inoculated roots (Hawes et al, 1998; Gunawardena and Hawes, 2002) Such hyphae, whose identity and pathogenicity were established as the inoculating strain of N. haematococca by expression of specific marker genes, spontaneously detached from the tip along with border cells when the root was immersed in water. A similar mechanism of parasite expulsion has been reported to occur in mammalian intestinal cells induced to undergo turnover in response to microbial infection (Cliffe et al, 2005)

Methods

Results

Conclusion