Redox signalling via thioredoxins plays central roles in the light-mediated regulation of metabolic pathways of the chloroplast. Recent observations indicate strongly that thiotransduction pathways not only take place in the chloroplast stroma but also regulate functions of the chloroplast lumen. Thioredoxin signalling is probably an intrinsic characteristic of the entire chloroplast. Central questions are: (a) if there are thioredoxins or related proteins that can reduce luminal proteins, what are their sources of regeneration and their target proteins? (b) Are there links to other luminal pathways and how is redoxregulated luminal signal transduction coupled to the function of photosynthesis and signalling in the chloroplast stroma? This study aims to identify luminal thioredoxin targets and their biochemical functions. The initial experimental set-up using the E. coli thioredoxin/thioredoxin reductase system and fluorescence electrophoresis was able to confirm the known prevalent luminal thioredoxin targets that include PsbO1, PsbO2, TL17 and FKBP13. In addition, a novel thioredoxin interaction was observed for the enzyme violaxanthin deeopoxidase implying a role of luminal thioredoxin signals for regulation of the xanthophyll cycle.