Electron transfer activity. A range of concentrations of AtFd1 (black circles), AtFd2 (white circles), AtFd3 (black squares), AtFd4 (white squares) were used to mediate electron flow: A, from photoreduced PSI to leaf FNR (L-FNR); B, from NADPH reduced L-FNR to cytochrome c; C, from NADPH reduced root FNR (R-FNR) to cytochrome c; D, from NADPH reduced R-FNR to SiR. Results are single data sets representative of three to four independent measurements.