The light-harvesting antenna of higher plant photosystem II (LHCII) has the intrinsic capacity to dissipate excess light energy as heat in a process termed nonphotochemical quenching (NPQ). Recent studies suggest that zeaxanthin and lutein both contribute to the rapidly relaxing component of NPQ, qE, possibly acting in the minor monomeric antenna complexes and the major trimeric LHCII, respectively. To distinguish whether zeaxanthin and lutein act independently as quenchers at separate sites, or alternatively whether zeaxanthin fulfills an allosteric role regulating lutein-mediated quenching, the kinetics of qE and the qE-related conformational changes (DA 535 ) were compared in Arabidopsis (Arabidopsis thaliana) mutant/antisense plants with altered contents of minor antenna (kolhcb6, aslhcb4), trimeric LHCII (aslhcb2), lutein (lut2, lut2npq1, lut2npq2), and zeaxanthin (npq1, npq2). The kinetics of the two components of NPQ induction arising from zeaxanthin-independent and zeaxanthin-dependent qE were both sensitive to changes in the protein composition of the photosystem II antenna. The replacement of lutein by zeaxanthin or violaxanthin in the internal Lhcb protein-binding sites affected the kinetics and relative amplitude of each component as well as the absolute chlorophyll fluorescence lifetime. Both components of qE were characterized by a conformational change leading to nearly identical absorption changes in the Soret region that indicated the involvement of the LHCII lutein 1 domain. Based on these observations, we suggest that both components of qE arise from a common quenching mechanism based upon a conformational change within the photosystem II antenna, optimized by Lhcb subunit-subunit interactions and tuned by the synergistic effects of external and internally bound xanthophylls.

During desiccation, homoiochlorophyllous resurrection plants retain most of their photosynthetic apparatus, allowing them to resume photosynthetic activity quickly upon water availability. These plants rely on various mechanisms to prevent the formation of reactive oxygen species and/or protect their tissues from the damage they inflict. In this work, we addressed the issue of how homoiochlorophyllous resurrection plants deal with the problem of excessive excitation/electron pressures during dehydration using Craterostigma pumilum as a model plant. To investigate the alterations in the supramolecular organization of photosynthetic protein complexes, we examined cryoimmobilized, freeze-fractured leaf tissues using (cryo)scanning electron microscopy. These examinations revealed rearrangements of photosystem II (PSII) complexes, including a lowered density during moderate dehydration, consistent with a lower level of PSII proteins, as shown by biochemical analyses. The latter also showed a considerable decrease in the level of cytochrome f early during dehydration, suggesting that initial regulation of the inhibition of electron transport is achieved via the cytochrome b 6 f complex. Upon further dehydration, PSII complexes are observed to arrange into rows and semicrystalline arrays, which correlates with the significant accumulation of sucrose and the appearance of inverted hexagonal lipid phases within the membranes. As opposed to PSII and cytochrome f, the light-harvesting antenna complexes of PSII remain stable throughout the course of dehydration. Altogether, these results, along with photosynthetic activity measurements, suggest that the protection of retained photosynthetic components is achieved, at least in part, via the structural rearrangements of PSII and (likely) light-harvesting antenna complexes into a photochemically quenched state.

The xanthophylls of the light-harvesting complexes of photosystem II (LHCII), zeaxanthin, and lutein are thought to be essential for non-photochemical quenching (NPQ). NPQ is a process of photoprotective energy dissipation in photosystem II (PSII). The major rapidly reversible component of NPQ, qE, is activated by the transmembrane proton gradient, and involves the quenching of antenna chlorophyll excited states by the xanthophylls lutein and zeaxanthin. Using diaminodurene (DAD), a mediator of cyclic electron flow around photosystem I, to enhance ΔpH we demonstrate that qE can still be formed in the absence of lutein and light-induced formation of zeaxanthin in chloroplasts derived from the normally qE-deficient lut2npq1 mutant of Arabidopsis. The qE induced by high ΔpH in lut2npq1 chloroplasts quenched the level of fluorescence when all PSII reaction centers were in the open state (F (o) state), protected PSII reaction centers from photoinhibition, was sensitive to the uncoupler nigericin, and was accompanied by absorption changes in the 410-565 nm region. Titrations show the ΔpH threshold for activation of qE in lut2npq1 chloroplasts lies outside the normal physiological range and is highly cooperative. Comparison of quenching in isolated trimeric (LHCII) and monomeric (CP26) light-harvesting complexes from lut2npq1 plants revealed a similarly shifted pH dependency compared with wild-type LHCII. The implications for the roles of lutein and zeaxanthin as direct quenchers of excitation energy are discussed. Furthermore, we argue that the control over the proton-antenna association constant, pK, occurs via influence of xanthophyll structure on the interconnected phenomena of light-harvesting antenna reorganization/aggregation and hydrophobicity.

Nonphotochemical quenching (NPQ) is the photoprotective dissipation of energy in photosynthetic membranes. The hypothesis that the DpH-dependent component of NPQ (qE) component of non-photochemical quenching is controlled allosterically by the xanthophyll cycle has been tested using Arabidopsis mutants with different xanthophyll content and composition of Lhcb proteins. The titration curves of qE against DpH were different in chloroplasts containing zeaxanthin or violaxanthin, proving their roles as allosteric activator and inhibitor, respectively. The curves differed in mutants deficient in lutein and specific Lhcb proteins. The results show that qE is determined by xanthophyll occupancy and the structural interactions within the antenna that govern allostericity.