Localisation of Ferredoxin-NADP+ reductase

Abstract

The principle research goal in this thesis is to investigate the effects of ferredoxin-NADP+ reductase (FNR) sub-chloroplast location on its function and proposed role in regulation of electron transport. It was found before that FNR in Arabidopsis thaliana can either be soluble in the stroma or tethered to the thylakoid membrane by two identi fied tethers. Its primary function is to deliver electrons from ferredoxin to NADP+. Its primary location therefore should be at the thylakoid membrane and the function of stroma soluble FNR is debatable. Hence, to facilitate localization studies and functional investigations, previously generated A. thaliana plants expressing isoforms of FNR that mainly bind to one specifi c tether were used as models. I explored several imaging approaches in the scope of this thesis and found that only electron microscopy delivers a sufficient resolution for conclusive assumptions about the absolute location of FNR. Hence, I developed a method to quantify the FNR labelling density on immunogold labelled chloroplasts for the de fined regions of grana, margins of grana, lamellae and stroma in each genotype. The striking result is that FNR is not found soluble in the stroma in the wild-type but mainly tightly associated to the lamellae and margin regions of the grana. My work supports the hypothesis that linear electron transport takes place at the margin regions, whereas cyclic electron transport happens at the lamellae. A functional analysis of photosynthetic electron transport in these plants reveals that FNR location/differential tether binding impacts the electron transport mainly during adaptation to light or a change in light intensity and not so much in steady-state conditions. These fi ndings support the idea that FNR plays a regulatory role in electron transport.