Inducible Repression of Nuclear-Encoded Subunits of the Cytochrome b6f Complex in Tobacco Reveals an Extraordinarily Long Lifetime of the Complex.

Marta Hojka,Szilvia Z Tóth,Wolfgang Lein,Mark Aurel Schöttler,Ralph Bock,Wolfram Thiele

doi:10.1104/pp.114.243741

Marta Hojka, Szilvia Z Tóth + Show 4 more

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https://doi.org/10.1104/pp.114.243741

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Abstract

The biogenesis of the cytochrome b6f complex in tobacco (Nicotiana tabacum) seems to be restricted to young leaves, suggesting a high lifetime of the complex. To directly determine its lifetime, we employed an ethanol-inducible RNA interference (RNAi) approach targeted against the essential nuclear-encoded Rieske protein (PetC) and the small M subunit (PetM), whose function in higher plants is unknown. Young expanding leaves of both PetM and PetC RNAi transformants bleached rapidly and developed necroses, while mature leaves, whose photosynthetic apparatus was fully assembled before RNAi induction, stayed green. In line with these phenotypes, cytochrome b6f complex accumulation and linear electron transport capacity were strongly repressed in young leaves of both RNAi transformants, showing that the M subunit is as essential for cytochrome b6f complex accumulation as the Rieske protein. In mature leaves, all photosynthetic parameters were indistinguishable from the wild type even after 14 d of induction. As RNAi repression of PetM and PetC was highly efficient in both young and mature leaves, these data indicate a lifetime of the cytochrome b6f complex of at least 1 week. The switch-off of cytochrome b6f complex biogenesis in mature leaves may represent part of the first dedicated step of the leaf senescence program.

Highlights

The biogenesis of the cytochrome b6f complex in tobacco (Nicotiana tabacum) seems to be restricted to young leaves, suggesting a high lifetime of the complex
Notable species-dependent variations seem to exist for cytochrome f: In Lemna perpusilla (Bruce and Malkin, 1991) and tobacco (Nicotiana tabacum; Hager et al, 1999; Schwenkert et al, 2007), no cytochrome f accumulates in the absence of one of the other essential cyt-bf subunits, but both in C. reinhardtii and Arabidopsis (Arabidopsis thaliana), unassembled PetA can still be detected in the absence of the other
As two highly homologous PetC isoforms exist in tobacco, which differ only in four amino acids and 15 nucleotides within the coding region, respectively (Madueño et al, 1992), we targeted both isoforms with our RNA interference (RNAi) construct

Summary

Introduction

The biogenesis of the cytochrome b6f complex in tobacco (Nicotiana tabacum) seems to be restricted to young leaves, suggesting a high lifetime of the complex. Young expanding leaves of both PetM and PetC RNAi transformants bleached rapidly and developed necroses, while mature leaves, whose photosynthetic apparatus was fully assembled before RNAi induction, stayed green In line with these phenotypes, cytochrome b6f complex accumulation and linear electron transport capacity were strongly repressed in young leaves of both RNAi transformants, showing that the M subunit is as essential for cytochrome b6f complex accumulation as the Rieske protein. In knockout mutants of the nuclear-encoded Rieske-2Fe2S-protein, the other subunits of the cyt-bf are still synthesized, but they cannot assemble into a complex (Bruce and Malkin, 1991; Maiwald et al, 2003) They are instable and rapidly degraded so that no or only very low levels of most cyt-bf subunits are detectable (Bruce and Malkin, 1991; Maiwald et al, 2003). A PetM knockout mutant (DpetM) of Synechocystis PCC 6803 accumulates wild-type levels of cyt-bf, while the accumulation of PSI and of phycobilisomes is reduced, suggesting a signaling function of PetM

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