Expression of multiple forms of ferredoxin NADP+ oxidoreductase in wheat leaves

In higher plants there are two forms of ferredoxin NADP(+) oxidoreductase (FNR), a photosynthetic pFNR primarily required for the photoreduction of NADP(+), and a heterotrophic hFNR which generates reduced ferredoxin by utilizing electrons from NADPH produced during carbohydrate oxidation. The aim of this study was to investigate the presence of multiple forms of FNR in wheat leaves and the capacity of FNR isoforms to respond to changes in reductant demand through varied expression and N-terminal processing. Two forms of pFNR mRNA (pFNRI and pFNRII) were expressed in a similar pattern along the 12 cm developing primary wheat leaf, with the highest levels observed in plants grown continuously in the dark in the presence (pFNRI) or absence (pFNRII) of nitrate respectively. pFNR protein increased from the leaf base to tip. hFNR mRNA and protein was in the basal part of the leaf in plants grown in the presence of nitrate. FNR activity in plants grown in a light/dark cycle without nitrate was mainly due to pFNR, whilst hFNR contributed significantly in nitrate-fed plants. The potential role of distinct forms of FNR in meeting the changing metabolic capacity and reductant demands along the linear gradient of developing cells of the leaf are discussed. Furthermore, evidence for alternative N-terminal cleavage sites of pFNR acting as a means of discriminating between ferredoxins and the implications of this in providing a more effective flow of electrons through a particular pathway in vivo is considered.