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Published on July 9, 2008; 10.1104/pp.108.123778

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Received May 30, 2008
Accepted July 3, 2008

A novel type of thioredoxin dedicated to symbiosis in legumes

Fatima Alkhalfioui , Michelle Renard , Pierre Frendo , Corinne Keichinger , Yves Meyer , Eric Gelhaye , Masakazu Hirasawa , David B. Knaff , Christophe Ritzenthaler , and Francoise Montrichard *

Physiologie Moleculaire des Semences, UMR 1191 Universite d'Angers - Institut National d'Horticulture - INRA, ARES, 16 boulevard Lavoisier, 49045 Angers Cedex 01, France; LC1, UMR 7175, Institut Gilbert Laustriat, Pole API, boulevard Brant, BP 10413, 67412 Illkirch Cedex, France (current address); Interactions Biotiques et Sante Vegetale UMR INRA 1301 –CNRS 6243-Universite de Nice-Sophia Antipolis, Centre de Recherche INRA de Sophia Antipolis, 400, Route des Chappes, BP167 06903 Sophia Antipolis Cedex, France; Institut de Biologie Moleculaire des Plantes, laboratoire propre du CNRS (UPR 2357) conventionne avec l'Universite Louis Pasteur (Strasbourg 1), 12 rue du General Zimmer, 67084 Strasbourg Cedex, France; Genome et Developpement des Plantes, UMR 5096 Universite de Perpignan - CNRS, 52 avenue de Villeneuve, 66860 Perpignan, France; Interaction Arbres/Micro-organismes, UMR 1136 Universite Nancy I - INRA, Faculte des Sciences, 54506 Vandoeuvre, France; Department of Chemistry and Biochemistry and Center for Biotechnology and Genomics, Texas Tech University, Lubbock, Texas 79409-1061, U.S.A

* Corresponding author; email: francoise.montrichard{at}univ-angers.fr.

Thioredoxins (Trxs) constitute a family of small proteins in plants. This family has been extensively characterized in 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 or poplar, 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-thioredoxin 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.






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