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LHCII can substitute for LHCI as an antenna for photosystem I but with reduced light-harvesting capacity

Nature Plants volume 2, Article number: 16131 (2016) Cite this article

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Abstract

Light-harvesting complexes (LHCs) are major constituents of the antenna systems in higher plant photosystems. Four Lhca subunits are tightly bound to the photosystem I (PSI) core complex, forming its outer antenna moiety called LHCI. The Arabidopsis thaliana mutant ΔLhca lacks all Lhca1–4 subunits and compensates for its decreased antenna size by binding LHCII trimers, the main constituent of the photosystem II antenna system, to PSI. In this work we have investigated the effect of LHCI/LHCII substitution by comparing the light harvesting and excitation energy transfer efficiency properties of PSI complexes isolated from ΔLhca mutants and from the wild type, as well as the consequences for plant growth. We show that the excitation energy transfer efficiency was not compromised by the substitution of LHCI with LHCII but a significant reduction in the absorption cross-section was observed. The absence of LHCI subunits in PSI thus significantly limits light harvesting, even on LHCII binding, inducing, as a consequence, a strong reduction in growth.

The conversion of light into chemical energy occurs in pigment–protein complexes, the photosystems. In eukaryotic photosynthetic organisms, two photosystems, namely PSI) and PSII, undergo light-driven charge separation. PSI and PSII bind chlorophyll (Chl) and carotenoid (Car) chromophores, whose spectroscopic properties are tuned by the protein environment. The initial reactions within PSI and PSII are catalysed by Chl a dimers, which absorb at 700 and 680 nm respectively. These dimers are served by a closely interacting array of Chl a and β-carotene pigments which are bound to plastid-encoded proteins in the so-called core complexes. More specifically, the PSI core (PSIc) complex comprises 95–98 Chls1,2, but the PSII core complex binds fewer Chls (36). The optical absorption cross-section of both core complexes is enhanced by an outer antenna system composed of nuclear-encoded LHC subunits3–6. Higher plants evolved four LHC subunits (Lhca1–4) arranged in a half-moon structure adjacent to the PsaF/PsaJ subunits of the PSI core complex1,2,7–9. The PSIc/LHCI stoichiometry has been shown to be maintained at 1:4 irrespective of light growth conditions10. In contrast, the dimeric PSII core is encircled by a larger LHC antenna system composed of Lhcb1–6 subunits. These are organized into monomers (Lhcb4–6) and trimers (Lhcb1–3, also known as LHCII complexes), with the monomers located in between the core complex and the more peripheral trimers. Lhcb proteins can, however, migrate from PSII to PSI along the thylakoid membranes, depending on photosystem excitation11–17. When PSI is preferentially excited, plastoquinole (PQH2) is oxidized and LHCII is almost exclusively associated to PSII (state 1). Upon preferential PSII excitation, however, PQH2 undergoes over-reduction and triggers a threonine kinase activity which targets LHCII, freeing it from PSII grana and allowing diffusion to PSI (state 2)16,18–21. This transition balances excitation energy pressure between the photosystems, restoring the plastoquinone/PQH2 ratio. It is reversed by TAP38/PPH1 phosphatase on PQH2 re-oxidation22–24. The interaction of LHCII with PSI was observed under several growth conditions as a consequence of acclimation to low or moderate light levels10,20,25, and was observed even in the absence of LHCII phosphorylation15. Upon binding to PSI, LHCII trimers have been shown to efficiently transfer excitation energy to the PSI reaction centre25–27. PSI supercomplexes binding either LHCI or LHCII, or both, differ as to their spectra and the strength of the PSIc–LHC interactions. In this work we investigated whether LHCI and LHCII are functionally equivalent when bound to PSI. To this extent, we isolated a ΔLhca mutant of A. thaliana which lacks the Lhca1–4 subunits and accumulates PSI core complexes with LHCII bound, analysing plant growth, light-harvesting and excitation energy transfer properties of its PSI complexes compared to wild type.

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Figure 1: Genetic and biochemical characterization of the ΔLhca mutant.
Figure 2: Biochemical characterization of pigment–protein complexes.
Figure 3: Absorption spectra of PSI complexes.
Figure 4: Role of red-shifted absorption forms under canopy conditions.
Figure 5: Time-resolved fluorescence maps of different supercomplexes.
Figure 6: Decay-associated spectra of PSI supercomplexes.
Figure 7: Trapping time and quantum yield of PSI supercomplexes.

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Acknowledgements

The work was financed by the Italian Ministry of Agriculture, Food and Forestry (MIPAAF) project HYDROBIO and by the Marie Curie Actions Initial Training Networks ACCLIPHOT (PITN-GA-2012-316427) and S2B (675006–SE2B) to R.B. G.C. acknowledges support from the European Research Council Advanced Grant STRATUS (ERC-2011-AdG No. 291198).

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Authors and Affiliations

  1. Dipartimento di Biotecnologie, Università di Verona, Strada Le Grazie 15, I-37134 Verona, Italy

    Mauro Bressan, Luca Dall'Osto, Roberto Bassi & Matteo Ballottari

  2. Centre for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, via Pascoli 70/3, 20133 Milan, Italy

    Ilaria Bargigia, Marcelo J. P. Alcocer & Cosimo D'Andrea

  3. Department of Physics, IFN-CNR, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy

    Marcelo J. P. Alcocer, Daniele Viola, Giulio Cerullo & Cosimo D'Andrea

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  1. Mauro Bressan
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Contributions

M.Ba., L.D. and R.B. conceived the work and designed the experiments. M.Br. and L.D. performed all the experiments for the isolation of the ΔLhca mutant, its physiological and biochemical characterization and purification of PSI complexes. M.Ba. performed all the experiments for the spectroscopic characterization of PSI complexes. C.D. and G.C. coordinated the time-resolved fluorescence analysis experiments. I.B., M.J.P.A. and C.D. contributed to the time-resolved fluorescence analysis experiments. I.B., M.J.P.A., D.V., G.C., M.Ba. and C.D. analysed the fluorescence decay results by global analysis. All of the authors contributed to writing the manuscript. All of the authors discussed the results and commented on the manuscript.

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Correspondence to Roberto Bassi.

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Supplementary Tables 1–5, Supplementary Figs 1–11 and Supplementary References. (PDF 1849 kb)

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Bressan, M., Dall'Osto, L., Bargigia, I. et al. LHCII can substitute for LHCI as an antenna for photosystem I but with reduced light-harvesting capacity. Nature Plants 2, 16131 (2016). https://doi.org/10.1038/nplants.2016.131

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