Chloroplast NADPH Thioredoxin Reductase: A Novel Modulator of Plastidial Amino Acid and Hormone Metabolism

Abstract

A chloroplast thioredoxin reductase utilizing NADPH as reducing power (NTRC) is a recently found component of thioredoxin system in chloroplast. Characterization of the Arabidopsis SALK T-DNA insertion lines of the ntrc gene (ntrc) established an essential impact of ntrc on the regulation of chloroplast metabolism. Besides retarded growth, ntrc plants showed distinct developmental disorders when grown under short day conditions. Mesophyll cells had irregular shape and small size, and possessed lowered number of chloroplasts. The mutant phenotype was less severe in plants grown under long day conditions. Such photoperiod-dependent developmental disorders of ntrc suggest that ntrc regulates basic metabolism in chloroplasts. Indeed, amino acid analysis showed distinctly different profiles of plastid-synthesized amino acids between wild type and ntrc plants. In particular, the levels of aromatic amino acids, which serve as precursors for a growth hormone auxin and secondary metabolites, were significantly increased. Intriguingly, the content of auxin was remarkably reduced in ntrc plants grown under short day conditions. These metabolic changes correlated with the photoperiod-dependent phenotype of ntrc. Finally, growth of ntrc seedlings on a medium supplemented with auxin or aromatic amino acids restored the cell size and the number of chloroplasts in the ntrc leaves.