A COMPLETE FERREDOXIN/THIOREDOXIN SYSTEM REGULATES FUNDAMENTAL PROCESSES IN AMYLOPLASTS

Authors: BALMER, YVES - UC BEREKELY; Vensel, William; CAI, NICK - UC BERKELEY; MANIERI, WANDA - UNIV. OF SWITZERLAND; SCHURMANN, PETER - UNIV. OF SWITZERLAND; Hurkman Ii, William; BUCHANAN, BOB - UC BERKELEY

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2006
Publication Date: 2/21/2006
Citation: Balmer, Y., Vensel, W.H., Cai, N., Manieri, W., Schurmann, P., Hurkman, W.J., Buchanan, B. 2006. A complete ferredoxin/thioredoxin system regulates fundamental processes in amyloplasts. Proceedings of the National Academy of Sciences. 103(8): 2988-2993

Interpretive Summary: Proteins constitute a large percentage of the plant cell and are fundamental to all biological processes. The functions of many proteins are often facilitated by other proteins. Thioredoxin is a small regulatory disulfide protein that interacts with many other proteins to regulate a range of cellular activities. The current study reports thioredoxin target proteins in amyloplasts, organelles that synthesize and store starch in plants. Fluorescence gel electrophoresis and affinity column procedures were used to identify thioredoxin targets in amyloplasts isolated from developing wheat seeds. This analysis led to the identification of 42 potential thioredoxin target proteins, 13 of which were newly recognized. The proteins function in a range of processes including starch metabolism and the biosynthesis of lipids, amino acids and nucleotides. Among the proteins identified, all of the components of the ferredoxin/thioredoxin system (ferredoxin, ferredoxin/thioredoxin reductase and thioredoxin), originally described for chloroplasts, were found in amyloplasts. This finding suggests a mechanism whereby thioredoxin could coordinate biochemical activities between the two organelles. In effect, light illuminating chloroplasts in the leaves would inform amyloplasts via thioredoxin that biochemical processes in the grain should be adjusted accordingly. Knowledge of the coordinate regulation of cellular processes is essential for improving complex crop traits such as productivity and quality.

Technical Abstract: A growing number of processes throughout biology are regulated by redox via thiol-disulfide exchange. This mechanism is particularly widespread in plants where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protein. The current study extends regulation by Trx to amyloplasts—organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxin/thioredoxin system (ferredoxin, ferredoxin/thioredoxin reductase and Trx), originally described for chloroplasts, in amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin-NADP reductase (FNR). However, once reduced, ferredoxin appears to act as established for chloroplasts—i.e., via ferredoxin-thioredoxin reductase (FTR) and a Trx. A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism, whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves.