The redox state of the chloroplast is maintained by a delicate balance between energy production and consumption and is affected by the need to avoid increased production of reactive oxygen species (ROS). Redox power and ROS generated in the chloroplast are essential for maintaining physiological metabolic pathways and for optimizing chloroplast functions. The redox poise of photosynthetic electron transport components like plastoquinone is crucial to initiate signaling cascades and might also be involved in key biosynthetic pathways such as chlorophyll biosynthesis. We, therefore, explored this well characterized pathway focusing on cyclase reaction in which the fifth ring of the chlorophyll molecule is formed. So far only one catalytic subunit of the cyclase has been identified, but, in order to complete its catalytic cycle the enzyme requires also a reductase.

Since, at present, no reductase protein has been characterized we searched for alternative ways to reduce the enzyme. We concluded that, in this case, reduced plastoquinone might act as an electron donor for the cyclase reaction thereby fulfilling the role of a cyclase reductase and providing a functional connection between the redox status of the plastoquinone pool and chlorophyll biosynthesis. Furthermore, in the plant cell, the equilibrium between redox reactions and ROS signals is also maintained by various balancing mechanisms among which the thioredoxin reductase-thioredoxin system (TR-Trx) stands out as a mediator between environmental signals and enzyme activities.

A particular kind of TR-Trx system is NADPH-thioredoxin reductase C (NTRC) which comprises both a NADPH-reductase and a thioredoxin domain in a single enzyme. For this second project, the moss Physcomitrella patens was chosen as model organism because it lacks some of the key enzymes for ROS scavenging and it is the only, so far, known species to have two NTRC genes. Our aim was to elucidate the role of the two NTRC isoforms found in moss as an alternative system for protection against oxidative damage, providing the first partial attempt of a molecular and biochemical analysis as well as the first functional characterization of the two NTRC in Physcomitrella patens.