cpSecA, a thylakoid protein translocase subunit, is essential for photosynthetic development in Arabidopsis

The endosymbiont-derived Sec-dependent protein sorting pathway is essential for protein import into the thylakoid lumen and is important for the proper functioning of the chloroplast. Two loss-of-function mutants of cpSecA, the ATPase subunit of the chloroplast Sec translocation machinery, were analysed in Arabidopsis. The homozygous mutants were albino and seedling lethal under autotrophic conditions and remained dwarf and infertile with an exogenous carbon supply. They were subject to oxidative stress and accumulated superoxide under normal lighting conditions. Electron microscopy revealed that the chloroplast of the mutants had underdeveloped thylakoid structures. Histochemical GUS assay of the AtcpSecA::GUS transgenic plants confirmed that AtcpSecA was expressed in green organs in a light-inducible way. Real-time RT-PCR and microarray analysis revealed repressed transcription of nucleus- and chloroplast- encoded subunits of photosynthetic complexes, and induced transcription of chloroplast protein translocation machinery and mitochondrion-encoded respiratory complexes in the mutants. It is inferred that AtcpSecA plays an essential role in chloroplast biogenesis, the absence of which triggered a retrograde signal, eventually leading to a reprogramming of chloroplast and mitochondrial gene expression.