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JXB Advance Access published online on March 21, 2005
Journal of Experimental Botany, doi:10.1093/jxb/eri132
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received September 7, 2004
Accepted February 3, 2005

RESEARCH PAPER

Apoplastic oxidation of L-asparagine is involved in the control of the green algal endophyte Acrochaete operculata Correa & Nielsen by the red seaweed Chondrus crispus Stackhouse

Florian Weinberger 1*, Georg Pohnert 2, Mary-Lynn Berndt 1, Kamal Bouarab 1, Bernard Kloareg 1, and Philippe Potin 1

1 UMR 7139 CNRS, Université Pierre et Marie Curie and Laboratoires Goëmar, Station Biologique, Place Georges Teissier, F-29680 Roscoff, France
2 Max-Planck-Institut für Chemische Ökologie, Hans-Knöll-Str. 8, D-07745 Jena, Germany

* To whom correspondence should be addressed.
Florian Weinberger, E-mail: weinberg{at}sb-roscoff.fr


  Abstract

Gametophytes of the marine alga Chondrus crispus are more resistant than tetrasporophytes to infection by the filamentous endophytic alga Acrochaete operculata. It has been shown recently that carrageenan oligosaccharides from the resistant gametophytic generation of C. crispus stimulate the secretion of L-asparagine (L-Asn) by the endophyte and that the host generates hydrogen peroxide and 2-oxo-succinamic acid after contact with this amino acid. Here the response of C. crispus to L-Asn and its effect on the pathogen is investigated. Chondrus crispus released hydrogen peroxide, ammonium ions, and a carbonyl compound into the medium when exposed to L-Asn. This response was correlated with an increase in oxygen consumption. Inhibitor studies indicated the involvement of a flavoenzyme in the reaction, which was sensitive to high concentrations of the reaction product, ammonium, and to chlorpromazine, quinacrine, and cyanide, inhibitors of L-amino acid oxidase. Cell wall macerate of C. crispus also responded to L-Asn, while protoplasts were inactive. Uptake of L-Asn into the cell was not necessary for the response, suggesting that the involved L-amino acid oxidase is apoplastic. Acrochaete operculata was more sensitive to hydrogen peroxide than C. crispus and settlement of A. operculata zoospores on C. crispus was reduced by 86% in the presence of L-Asn. This reduced settlement could be prevented with catalase. Chondrus crispus thus features an apoplastic amino acid oxidase, which is involved in the control of its endophytic pathogen. The modulation of the amino acid secretion in A. operculata by carrageenan oligosaccharides is therefore a key issue in the etiology of the association.

Keywords: Acrochaete; asparagine; Chondrus; endophytism; host-pathogen interactions; L-amino acid oxidase.
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