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PLANTS INTERACTING WITH OTHER ORGANISMS

Extracellular Proteins in Pea Root Tip and Border Cell Exudates^1,[OA]

Department of Plant Sciences, Division of Plant Pathology and Microbiology (F.W., H.D.V., M.C.H.) and Center for Toxicology, Pharmacy 205 (G.T.), University of Arizona, Tucson, Arizona 85721

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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Martha C. Hawes (mhawes{at}u.arizona.edu).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.091637

^* Corresponding author; e-mail mhawes{at}u.arizona.edu; fax 520–621–9290.

Received October 19, 2006; accepted November 17, 2006; published December 1, 2006.

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