Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have similar to 50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Automated summary

In this study, a viable fdC2 mutant line with near-depleted FdC2 protein levels was generated using RNAi silencing in Arabidopsis thaliana. The mutant leaves showed a decrease of approximately 50% in chlorophyll a and b, and the chloroplasts had poorly developed thylakoid membrane structure. Transcriptomics analysis revealed upregulation of stress response genes. Contrary to previous literature, the mutant plants exhibited the same recovery rate as the wild type at photosystem II (PSII) in the dark, indicating that FdC2 may not regulate translation of the D1 subunit of PSII by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates showed an accumulation of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. Furthermore, FdC2 was localized to the inner chloroplast envelope and the FdC2 RNAi line exhibited a disproportionately lower protein abundance of antennae proteins, which undergo refolding at the envelope after import.