Abstract
Thioredoxins (Trxs) h, small disulfide reductases, and NADP-thioredoxin reductases (NTRs) have been shown to accumulate in seeds of different plant species and play important roles in seed physiology. However, little is known about the identity, properties, and subcellular location of Trx h isoforms that are abundant in legume seeds. To fill this gap, in this work, we characterized the Trx h family of Medicago truncatula, a model legume, and then explored the activity and localization of Trx h isoforms accumulating in seeds. Twelve Trx h isoforms were identified in M. truncatula. They belong to the groups previously described: h1 to h3 (group I), h4 to h7 (group II), and h8 to h12 (group III). Isoforms of groups I and II were found to be reduced by M. truncatula NTRA, but with different efficiencies, Trxs of group II being more efficiently reduced than Trxs of group I. In contrast, their insulin disulfide-reducing activity varies greatly and independently of the group to which they belong. Furthermore, Trxs h1, h2, and h6 were found to be present in dry and germinating seeds. Trxs h1 and, to a lesser extent, h2 are abundant in both embryonic axes and cotyledons, while Trx h6 is mainly present in cotyledons. Thus, M. truncatula seeds contain distinct isoforms of Trx h that differ in spatial distribution and kinetic properties, suggesting that they play different roles. Because we show that Trx h6 is targeted to the tonoplast, the possible role of this isoform during germination is finally discussed.
Highlights
Thioredoxins (Trxs) h, small disulfide reductases, and NADP-thioredoxin reductases (NTRs) have been shown to accumulate in seeds of different plant species and play important roles in seed physiology
From EST libraries, we were able to identify by homology searches about 80 sequences, tentative consensus (TC) or singletons, related to Trx, from which 11 TCs of putative Trxs h were identified (Table I)
It should be noted that two supplementary Trx sequences (TC167868 and TC169151) related to Trxs of h type were found in MtGI
Summary
Thioredoxins (Trxs) h, small disulfide reductases, and NADP-thioredoxin reductases (NTRs) have been shown to accumulate in seeds of different plant species and play important roles in seed physiology. Thioredoxins (Trxs) are small and powerful disulfide reductases with two close and reactive Cys residues in the conserved motif: WCG/PPC (Holmgren, 1985) In their dithiol form, they act as hydrogen donors for redox enzymes like ribonucleotide reductase, the enzyme that led to their discovery (Laurent et al, 1964). Trx h isoforms constitute the largest type in the Trx family They were further subdivided into three groups according to both their primary sequences and their activities (Juttner et al, 2000; Gelhaye et al, 2003a, 2003b, 2004b). Poplar isoforms were further shown to be reduced either by glutathione (GSH) or glutaredoxins (Grxs)
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