|Hurkman Ii, William|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2007
Publication Date: 5/18/2007
Citation: Alkhalfiour, F., Renard, M., Wong, J., Tanaka, C.K., Vensel, W.H., Hurkman, W.J., Buchanan, B.B., Montichard, F. 2007. Thioredoxin-linked proteins are reduced during germination of medicago truncatula seeds. Plant Physiology. 144: 1559-1579. Interpretive Summary: Germination of cereals is accompanied by extensive changes in the redox state of seed proteins. Proteins present in the oxidized form (S-S) in dry seeds are converted to the reduced form (SH) following imbibition. Thioredoxin (Trx), a small regulatory protein that interacts with many other proteins to regulate a range of cellular activities, appears to play a role in this transition in cereals. It is not known, however, whether Trx-linked redox changes are restricted to cereals or a more widespread phenomenon in germinating seeds. Trx targets in seeds of Medicago truncatula, a model legume, were identified by mass spectroscopy following in vitro labelling with a fluorescent probe or isolation by affinity chromatography. The results of this study advance our understanding of the role of redox in plant biology in two ways. First, thioredoxin was linked to a group of 111 proteins, 59 of which are functional in processes not previously known to involve redox changes. Second, the Trx targets in seeds of Medicago, like those of cereals, were confirmed to undergo redox changes during germination. Together, these findings underscore the significance of redox and thioredoxin in seed biology by extending their role to the germination of dicotyledons, a major plant group.
Technical Abstract: Germination of cereals is accompanied by extensive change in the redox state of seed proteins. Proteins present in oxidized (S-S) form in dry seeds are converted to the reduced (SH) state following imbibition. Thioredoxin (Trx) appears to play a role in this transition in cereals. It is not known, however, whether Trx-linked redox changes are restricted to cereals or whether they take place more broadly in germinating seeds. To gain information on this point, we have investigated a model legume, Medicago truncatula. Two complementary gel-based proteomic approaches were followed to identify Trx targets in seeds: proteins were (i) labeled with a thiol-specific probe, monobromobimane (mBBr), following in vitro reduction by an NADP/Trx system (NTS), or (ii) isolated on a mutant Trx affinity column. Altogether, 111 Trx-linked proteins were identified with few differences between axes and cotyledons. Fifty-nine were new, 33 found previously in cereal or peanut seeds and 19 in other plants or photosynthetic organisms. In parallel, the redox state of proteins assessed in germinating seeds using mBBr revealed that a substantial number of proteins that are oxidized or partly reduced in dry seeds became more reduced upon germination. The patterns were similar for proteins reduced in vivo during germination or in vitro by Trx. In contrast, glutathione and glutaredoxin were less effective as reductants in vitro. Overall, more than half of the potential targets identified with the mBBr labelling procedure were reduced during germination. The results provide evidence that Trx functions in the germination of seeds of dicotyledons as well as monocotyledons.