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PLANTS INTERACTING WITH OTHER ORGANISMS

A Novel Type of Thioredoxin Dedicated to Symbiosis in Legumes^1,[W],[OA]

Physiologie Moléculaire des Semences, UMR 1191 Université d'Angers-Institut National d'Horticulture-INRA, IFR 149 QUASAV, ARES, 49045 Angers cedex 01, France (F.A., M.R., F.M.); Interactions Biotiques et Santé Végétale, UMR INRA 1301/CNRS 6243 Université de Nice-Sophia Antipolis, Centre de Recherche INRA de Sophia Antipolis, 06903 Sophia Antipolis cedex, France (P.F.); Institut de Biologie Moléculaire des Plantes, laboratoire propre du CNRS (UPR 2357) conventionné avec l'Université Louis Pasteur (Strasbourg 1), 67084 Strasbourg cedex, France (C.K., C.R.); Génome et Développement des Plantes, UMR 5096 Université de Perpignan-CNRS, 66860 Perpignan, France (Y.M.); Interaction Arbres/Micro-organismes, UMR 1136 Université Nancy I-INRA, Faculté des Sciences, 54506 Vandoeuvre, France (E.G.); and Department of Chemistry and Biochemistry and Center for Biotechnology and Genomics, Texas Tech University, Lubbock, Texas 79409–1061 (M.H., D.B.K.)

Thioredoxins (Trxs) constitute a family of small proteins in plants. This family has been extensively characterized in Arabidopsis (Arabidopsis thaliana), which contains six different Trx types: f, m, x, and y in chloroplasts, o in mitochondria, and h mainly in cytosol. A detailed study of this family in the model legume Medicago truncatula, realized here, has established the existence of two isoforms that do not belong to any of the types previously described. As no possible orthologs were further found in either rice (Oryza sativa) or poplar (Populus spp.), these novel isoforms may be specific for legumes. Nevertheless, on the basis of protein sequence and gene structure, they are both related to Trxs m and probably have evolved from Trxs m after the divergence of the higher plant families. They have redox potential values similar to those of the classical Trxs, and one of them can act as a substrate for the M. truncatula NADP-Trx reductase A. However, they differ from classical Trxs in that they possess an atypical putative catalytic site and lack disulfide reductase activity with insulin. Another important feature is the presence in both proteins of an N-terminal extension containing a putative signal peptide that targets them to the endoplasmic reticulum, as demonstrated by their transient expression in fusion with the green fluorescent protein in M. truncatula or Nicotiana benthamiana leaves. According to their pattern of expression, these novel isoforms function specifically in symbiotic interactions in legumes. They were therefore given the name of Trxs s, s for symbiosis.

² Present address: LC1, UMR 7175, Institut Gilbert Laustriat, Pôle API, boulevard Brant, BP 10413, 67412 Illkirch cedex, France.

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: Françoise Montrichard (francoise.montrichard{at}univ-angers.fr).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.123778

^* Corresponding author; e-mail francoise.montrichard{at}univ-angers.fr.

Received May 30, 2008; accepted July 3, 2008; published July 9, 2008.

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