The plastid-encoded PsaI subunit stabilizes photosystem I during leaf senescence in tobacco.

Mark Aurel Schöttler,Sandra Stegemann,Karolina Belkius,Wolfram Thiele,Stephanie Ruf,Ralph Bock,Sonja Verena Bergner,Shreya Agrawal,Claudia Flügel,Gal Wittenberg

doi:10.1093/jxb/erx009

Mark Aurel Schöttler, Sandra Stegemann + Show 8 more

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https://doi.org/10.1093/jxb/erx009

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Abstract

PsaI is the only subunit of PSI whose precise physiological function has not yet been elucidated in higher plants. While PsaI is involved in PSI trimerization in cyanobacteria, trimerization was lost during the evolution of the eukaryotic PSI, and the entire PsaI side of PSI underwent major structural remodelling to allow for binding of light harvesting complex II antenna proteins during state transitions. Here, we have generated a tobacco (Nicotiana tabacum) knockout mutant of the plastid-encoded psaI gene. We show that PsaI is not required for the redox reactions of PSI. Neither plastocyanin oxidation nor the processes at the PSI acceptor side are impaired in the mutant, and both linear and cyclic electron flux rates are unaltered. The PSI antenna cross section is unaffected, state transitions function normally, and binding of other PSI subunits to the reaction centre is not compromised. Under a wide range of growth conditions, the mutants are phenotypically and physiologically indistinguishable from wild-type tobacco. However, in response to high-light and chilling stress, and especially during leaf senescence, PSI content is reduced in the mutants, indicating that the I-subunit plays a role in stabilizing PSI complexes.

Highlights

photosystem I (PSI) catalyses the final step of linear photosynthetic electron transport, oxidizing plastocyanin and reducing ferredoxin and, via the ferredoxin NADP+ oxidoreductase, NADP+ in a light-driven reaction sequence
While PsaI is involved in PSI trimerization in cyanobacteria, trimerization was lost during the evolution of the eukaryotic PSI, and the entire PsaI side of PSI underwent major structural remodelling to allow for binding of light harvesting complex II antenna proteins during state transitions
The small plastome-encoded PsaI protein is the only subunit of PSI whose precise physiological function has not been elucidated in higher plants to date

Summary

Introduction

PSI catalyses the final step of linear photosynthetic electron transport, oxidizing plastocyanin and reducing ferredoxin and, via the ferredoxin NADP+ oxidoreductase, NADP+ in a light-driven reaction sequence. In addition to NADPH production, electrons on the PSI acceptor side can be partitioned into other reactions, most notably cyclic electron transport around PSI, which generates an extra pmf to produce. PSI is composed of 14 subunits (PsaA to PSAL, PSAN, and PSAO), with an additional complement of four different light harvesting complex A (LHCA) antenna proteins, which are stably bound to one side of PSI and form a half-moon-shaped belt (reviewed by Jensen et al, 2007; Schöttler et al, 2011). PsaA and B bind the redox-active chlorophyll-a dimer P700, where the light-induced charge separation occurs, as well as the subsequent components of the intra-complex electron transport chain, such as the first non-chlorophyll electron acceptor A1, a phylloquinone, and FX, the first iron-sulfur (FeS) cluster on the PSI acceptor side. Knocking out PSAD or PSAE severely affects PSI function; in Arabidopsis thaliana, the D-subunit is essential for PSI stability and photoautotrophic growth (Haldrup et al, 2003, Ihnatowicz et al, 2004)

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