Functional relationship of atypical thioredoxins with NADPH-thioredoxin reductase C and 2-Cys peroxiredoxins in Arabidopsis chloroplasts.

In addition to typical thioredoxins (TRXs) of the types m, f, x, y, and z, Arabidopsis (Arabidopsis thaliana) chloroplasts contain atypical TRXs such as cysteine- and histidine-rich TRXs (ACHTs) and TRXs-like 2 (TRXs L2), but their contribution to the redox regulation of the organelle and their functional relationship with the NADPH-TRX reductase C (NTRC)-2-Cys peroxiredoxin (PRX) redox system are poorly understood. To address these issues, we have focused on atypical TRXs ACHT3, ACHT4, and TRX L2.2. Either single or multiple Arabidopsis mutants combining deficiencies in these atypical TRXs showed wild-type-like phenotypes, but impaired light- and dark-dependent redox regulation of chloroplast enzymes. Their functional relationship with the NTRC-2-Cys PRXs redox system was tested in mutants lacking both the atypical TRXs analyzed here and NTRC or 2-Cys PRXs. Our results show the contribution of these atypical TRXs to chloroplast reductive and oxidative pathways, a function exerted via 2-Cys PRXs, which allows modulation of the redox state of chloroplast enzymes in response to light and darkness. The effects of different light regimes on the growth of the single and multiple mutants suggest that the contribution of atypical TRXs to chloroplast redox regulation might be relevant for plant acclimation to changes in light intensity.