Light-harvesting in photosystem I

This review focuses on the light-harvesting properties of photosystem I (PSI) and its LHCI outer antenna. LHCI consists of different chlorophyll a/b binding proteins called Lhca’s, surrounding the core of PSI. In total, the PSI-LHCI complex of higher plants contains 173 chlorophyll molecules, most of which are there to harvest sunlight energy and to transfer the created excitation energy to the reaction center (RC) where it is used for charge separation. The efficiency of the complex is based on the capacity to deliver this energy to the RC as fast as possible, to minimize energy losses. The performance of PSI in this respect is remarkable: on average it takes around 50 ps for the excitation to reach the RC in plants, without being quenched in the meantime. This means that the internal quantum efficiency is close to 100 % which makes PSI the most efficient energy converter in nature. In this review, we describe the light-harvesting properties of the complex in relation to protein and pigment organization/composition, and we discuss the important parameters that assure its very high quantum efficiency. Excitation energy transfer and trapping in the core and/or Lhcas, as well as in the supercomplexes PSI-LHCI and PSI-LHCI-LHCII are described in detail with the aim of giving an overview of the functional behavior of these complexes

Instantaneous switching between different modes of non-photochemical quenching in plants. Consequences for increasing biomass production

Photosynthetic productivity usually saturates far below the maximum solar light intensity, meaning that in those conditions many absorbed photons and the resulting electronic excitations of the pigment molecules can no longer be utilized for photosynthesis. To avoid photodamage, various protection mechanisms are induced that dissipate excess excitations, which otherwise could lead to the formation of harmful molecular species like singlet oxygen. This Non-Photochemical Quenching (NPQ) of excitations can be monitored via a decrease of the chlorophyll fluorescence. There is consensus that in plants 1) there are at least two major NPQ (sub)processes and 2) NPQ (de)activation occurs on various time scales, ranging from (tens of) seconds to minutes. This relatively slow switching has a negative effect on photosynthetic efficiency, and Kromdijk et al. demonstrated in 2016 (Science 354, 857) that faster switching rates can lead to increased crop productivity. Very recently, we were involved in the discovery of a new NPQ process that switches off well within a millisecond (Farooq et al. (2018) Nat. Plants 4, 225). Here we describe the current level of knowledge regarding this process and discuss its implications.</p

Understanding the Changes in the Circular Dichroism of Light Harvesting Complex II upon Varying Its Pigment Composition and Organization

In this work we modeled the circular dichroism (CD) spectrum of LHCII, the main light harvesting antenna of photosystem II of higher plants. Excitonic calculations are performed for a monomeric subunit, taken from the crystal structure of trimeric LHCII from spinach. All of the major features of the CD spectrum above 450 nm are satisfactorily reproduced, and possible orientations of the Chl and carotenoid transition dipole moments are identified. The obtained modeling parameters are used to simulate the CD spectra of two complexes with altered pigment composition: a mutant lacking Chls a 611-612 and a complex lacking the carotenoid neoxanthin. By removing the relevant pigment(s) from the structure, we are able to reproduce their spectra, which implies that the alteration does not disturb the overall structure. The CD spectrum of trimeric LHCII shows a reversed relative intensity of the two negative bands around 470 and 490 nm as compared to monomeric LHCII. The simulations reproduce this reversal, indicating that it is mainly due to interactions between chromophores in different monomeric subunits, and the trimerization does not induce observable changes in the monomeric structure. Our simulated spectrum resembles one of two different trimeric CD spectra reported in literature. We argue that the differences in the experimental trimeric CD spectra are caused by changes in the strength of the monomer-monomer interactions due to the differences in detergents used for the purification of the complexes.

Disturbed excitation energy transfer in Arabidopsis thaliana mutants lacking minor antenna complexes of photosystem II.

Abstract Minor light-harvesting complexes (Lhcs) CP24, CP26 and CP29 occupy a position in photosystem II (PSII) of plants between the major light-harvesting complexes LHCII and the PSII core subunits. Lack of minor Lhcs in vivo causes impairment of PSII organization, and negatively affects electron transport rates and photoprotection capacity. Here we used picosecond-fluorescence spectroscopy to study excitation-energy transfer (EET) in thylakoid membranes isolated from Arabidopsis thaliana wild-type plants and knockout lines depleted of either two (koCP26/24 and koCP29/24) or all minor Lhcs (NoM). In the absence of all minor Lhcs, the functional connection of LHCII to the PSII cores appears to be seriously impaired whereas the "disconnected" LHCII is substantially quenched. For both double knock-out mutants, excitation trapping in PSII is faster than in NoM thylakoids but slower than in WT thylakoids. In NoM thylakoids, the loss of all minor Lhcs is accompanied by an over-accumulation of LHCII, suggesting a compensating response to the reduced trapping efficiency in limiting light, which leads to a photosynthetic phenotype resembling that of low-light-acclimated plants. Finally, fluorescence kinetics and biochemical results show that the missing minor complexes are not replaced by other Lhcs, implying that they are unique among the antenna subunits and crucial for the functioning and macro-organization of PSII

Pathways for Energy Transfer in the Core Light-Harvesting Complexes CP43 and CP47 of Photosystem II

AbstractThe pigment-protein complexes CP43 and CP47 transfer excitation energy from the peripheral antenna of photosystem II toward the photochemical reaction center. We measured the excitation dynamics of the chlorophylls in isolated CP43 and CP47 complexes at 77K by time-resolved absorbance-difference and fluorescence spectroscopy. The spectral relaxation appeared to occur with rates of 0.2–0.4ps and 2–3ps in both complexes, whereas an additional relaxation of 17ps was observed only in CP47. Using the 3.8-Å crystal structure of the photosystem II core complex from Synechococcus elongatus (A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, and P. Orth, 2001, Nature, 409:739–743), excitation energy transfer kinetics were calculated and a Monte Carlo simulation of the absorption spectra was performed. In both complexes, the rate of 0.2–0.4ps can be ascribed to excitation energy transfer within a layer of chlorophylls near the stromal side of the membrane, and the slower 2–3-ps process to excitation energy transfer to the calculated lowest excitonic state. We conclude that excitation energy transfer within CP43 and CP47 is fast and does not contribute significantly to the well-known slow trapping of excitation energy in photosystem II