Photoprotective Energy Dissipation in Higher Plants Involves Alteration of the Excited State Energy of the Emitting Chlorophyll(s) in the Light Harvesting Antenna II (LHCII)

Abstract

The photosynthetic apparatus of plants and algae has evolved a process of feedback control of energy input into the photosynthetic reaction centers, revealed in observations of the nonphotochemical quenching of chlorophyll fluorescence (NPQ) 2 (1-3). Exposure to high light intensity leads to closure of a large fraction of photosystem II (PSII) reaction centers and the buildup of the proton gradient across the photosynthetic membrane (⌬pH). The latter prompts the transition of the PSII light harvesting antenna (LHCII) into the photoprotective state (4 -6), where a large proportion of the absorbed energy is rapidly converted into heat, preventing photoinhibitory damage of reaction centers. The molecular mechanism of NPQ is currently under scrupulous multidisciplinary investigation (7-14).Many of these studies have centered on the search for pigments associated with light harvesting complexes that may act as energy traps/quenchers. In recent years certain evidence has accumulated that xanthophylls may play a central role in photoprotective energy dissipation. A carotenoid radical cation, suggested to be zeaxanthin, was found to correlate with the level of NPQ and was proposed to act as a direct quencher of chlorophyll excitation (7, 10). Other studies indicated the involvement of energy transfer to the S 1 state of LHCII-bound lutein 1 (9, 15). Evidence of these putative quenchers occurring in vivo was provided (7, 9), yet little is known about how ⌬pH activates them. ⌬pH is believed to protonate luminal residues on the PSII subunit PsbS and LHC complexes causing certain conformational changes to occur, which are necessary for NPQ (5, 6). These conformational changes are not well understood but can be traced by changes in LHCII-bound pigments detected by absorption and resonance Raman spectroscopy (9,(15)(16)(17)(18)(19)(20).Both the proposed zeaxanthin-and lutein-quenching mechanisms assume very close (van der Waals) contact between the quenching xanthophyll and chlorophyll. Such an interaction would, most likely, affect the energy levels of the pigments involved, which may be required to convert the xanthophyll into an energy quencher. Indeed, a subpopulation of zeaxanthin molecules in PSII was reported to undergo an absorption redshift in the NPQ state (18 -20). Recently, lutein absorption was also found to be decreased in the dissipative state (20), interestingly this loss of molar extinction was enhanced by the presence of zeaxanthin.Whereas many studies have probed the role of xanthophylls, relatively little is known about the state of chlorophylls upon formation of NPQ. Some research suggests that chlorophyll associates may be formed in the NPQ state, which potentially could act as quenchers (21-23). It has been proposed that such associates have red-shifted absorption and fluorescence spectra. Low temperature (77K) fluorescence spectra of quenched LHCII aggregates possess a 20-nm red-shifted band, F700 (22), which was also detected in leaves and thylakoids frozen in the NPQ state (21, 24). The F700 emitting...

Economic photoprotection in photosystem II that retains a complete light-harvesting system with slow energy traps

“…Therefore, in light of this model PSII cross-section is expected to decrease during NPQ. However, it must be mentioned that a later work 8 on diluted chloroplasts did not reveal any long-wavelength components, which could be attributed to detached LHCII, although the resolution there was almost an order of magnitude higher than in the work of Holzwarth and colleagues 6,7 . Moreover, in the same work, low-temperature excitation fluorescence spectroscopy did not reveal any alterations in the PSII core antenna cross-section resulting from the detachment of chlorophyll b-enriched LHCII 8 .…”

“…In the F/S regime, the transfer of energy from the antenna to the NPQ quenching site is fast (comparable to trapping by the reaction centres) but the dissipation of the trapped exciton is much slower (B100 ps). The chlorophyll-chlorophyll charge transfer state quenching model, for example, can be described as a F/S process [6][7][8] . In the S/F regime, energy delivery to the quencher is slow (B100 ps or even slower), while the dissipation of trapped energy is fast (B10 ps).…”

“…On the other hand, Holzwarth and colleagues 6,7 argued that the excitation fluorescence spectroscopy applied in ref. 8 was not a fit test of the RCII cross-section because of the strong overlap between PSII core and LHCII fluorescence, the measurements were performed at 77 K and the spectra were normalized at the chlorophyll a Soret band maximum. Taking into account all these controversial interpretations of the experimental observations, it seemed crucial to further investigate the important mechanistic point-whether the functional PSII antenna size changes during NPQ or not-with an independent approach.…”

Nonphotochemical Chlorophyll Fluorescence Quenching: Mechanism and Effectiveness in Protecting Plants from Photodamage

“…Results from early fluorescence lifetime analysis were consistent with quenching taking place in the PSII antenna (Genty et al, 1992). Later, this type of spectroscopy revealed similarities between decay-associated spectral changes upon the transition into the quenching state in both isolated LHCII complexes and intact chloroplasts (Johnson and Ruban, 2009). Plants lacking a majority of LHCII antenna complexes display strongly reduced NPQ (Jahns and Krause, 1994;Havaux et al, 2007).…”

“…The remaining quenching in the chlorina mutants or plants grown under intermittent light was attributed to the presence of some minor LHCII antenna complexes (Jahns and Krause, 1994;Havaux et al, 2007), as was previously proposed (Andrews et al, 1995). NPQ is modulated by cross-linkers, tertiary amines, antimycin A, DCCD, and magnesium in the same way as quenching in isolated LHCII antenna complexes Johnson and Ruban, 2009). The latter is induced at detergent concentrations below critical micelle concentration and leads to the aggregation of the complex.…”