A Novel Type of Thioredoxin Dedicated to Symbiosis in Legumes  

Abstract

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.

Mitochondrial and Nuclear Localization of a Novel Pea Thioredoxin: Identification of Its Mitochondrial Target Proteins

“…Plants contain several nuclear genes encoding Trx proteins (Gelhaye et al, 2005). In Arabidopsis (Arabidopsis thaliana), at least 20 Trx genes have been reported (Meyer et al, 2002), classified into six families, Trx f, Trx h, Trx o, Trx m, Trx x, and Trx y, and a novel Trx s, recently described in Medicago truncatula that expands this family of proteins in legumes (Alkhalfioui et al, 2008). The subcellular location of the different Trxs showed that the chloroplast contains Trx f, m, x, and y and the atypical form CDSP32 (Collin et al, 2003).…”

Peroxiredoxins and NADPH-Dependent Thioredoxin Systems in the Model Legume Lotus japonicus

“…Knowledge of the redox regulatory network of nodules is only slowly emerging and is still in a fragmentary state. In pea (Pisum sativum), the content of PrxIIF in nodules is similar to that in roots and remains constant during nodule development (Groten et al, 2006), whereas in soybean (Glycine max), a Trxh isoform is essential for nodulation (Lee et al, 2005), and in M. truncatula, two isoforms of a new type of Trx, designated Trxs ("s" for symbiosis), are highly expressed in nodules (Alkhalfioui et al, 2008).…”

Cadmium affects the NADP-thioredoxin reductase/thioredoxin system in germinating pea seeds

“…The Arabidopsis cytosolic system consists of eight different Trx h and two homodimeric NADPH-dependent thioredoxin reductases (NTR, EC 1.6.4.5) (Laloi et al 2001;Meyer et al 2002), while the mitochondrial and nuclei ones include a Trx o and NTR (Laloi et al 2001;Martıé t al. 2009) and endoplasmic reticulum contain a Trx s (Alkhalfioui et al 2008). However, the chloroplastic Trx system is even more complex than the cytosolic one.…”