Chloroplast remodeling during state transitions in Chlamydomonas reinhardtii as revealed by noninvasive techniques in vivo

International audiencePlants respond to changes in light quality by regulating the absorption capacity of their photosystems. These short-term adaptations use redox-controlled, reversible phosphorylation of the light-harvesting complexes (LHCIIs) to regulate the relative absorption cross-section of the two photosystems (PSs), commonly referred to as state transitions. It is acknowledged that state transitions induce substantial reorganizations of the PSs. However, their consequences on the chloroplast structure are more controversial. Here, we investigate how state transitions affect the chloroplast structure and function using complementary approaches for the living cells of Chlamydomonas reinhardtii. Using small-angle neutron scattering, we found a strong periodicity of the thylakoids in state 1, with characteristic repeat distances of ∼200 Å, which was almost completely lost in state 2. As revealed by circular dichroism, changes in the thylakoid periodicity were paralleled by modifications in the long-range order arrangement of the photosynthetic complexes, which was reduced by ∼20% in state 2 compared with state 1, but was not abolished. Furthermore, absorption spectroscopy reveals that the enhancement of PSI antenna size during state 1 to state 2 transition (∼20%) is not commensurate to the decrease in PSII antenna size (∼70%), leading to the possibility that a large part of the phosphorylated LHCIIs do not bind to PSI, but instead form energetically quenched complexes, which were shown to be either associated with PSII supercomplexes or in a free form. Altogether these noninvasive in vivo approaches allow us to present a more likely scenario for state transitions that explains their molecular mechanism and physiological consequences

Modulation of the multilamellar membrane organization and of the chiral macrodomains in the diatom Phaeodactylum tricornutum revealed by small-angle neutron scattering and circular dichroism spectroscopy

Diatoms possess effective photoprotection mechanisms, which may involve reorganizations in the photosynthetic machinery. We have shown earlier, by using circular dichroism (CD) spectroscopy, that in Phaeodactylum tricornutum the pigment-protein complexes are arranged into chiral macrodomains, which have been proposed to be associated with the multilamellar organization of the thylakoid membranes and shown to be capable of undergoing light-induced reversible reorganizations (Szabó et al. Photosynth Res 95:237, 2008). Recently, by using small-angle neutron scattering (SANS) on the same algal cells we have determined the repeat distances and revealed reversible light-induced reorganizations in the lamellar order of thylakoids (Nagy et al. Biochem J 436:225, 2011). In this study, we show that in moderately heat-treated samples, the weakening of the lamellar order is accompanied by the diminishment of the psi-type CD signal associated with the long-range chiral order of the chromophores (psi, polymer or salt-induced). Further, we show that the light-induced reversible increase in the psi-type CD is associated with swelling in the membrane system, with magnitudes larger in high light than in low light. In contrast, shrinkage of the membrane system, induced by sorbitol, brings about a decrease in the psi-type CD signal; this shrinkage also diminishes the non-photochemical quenching capability of the cells. These data shed light on the origin of the psi-type CD signal, and confirm that both CD spectroscopy and SANS provide valuable information on the macro-organization of the thylakoid membranes and their dynamic properties; these parameters are evidently of interest with regard to the photoprotection in whole algal cells. © 2011 Springer Science+Business Media B.V

Thylakoid membrane reorganizations revealed by small-angle neutron scattering of Monstera deliciosa leaves associated with non-photochemical quenching

Non-photochemical quenching (NPQ) is an important photoprotective mechanism in plants and algae. Although the process is extensively studied, little is known about its relationship with ultrastructural changes of the thylakoid membranes. In order to better understand this relationship, we studied the effects of illumination on the organization of thylakoid membranes in Monstera deliciosa leaves. This evergreen species is known to exhibit very large NPQ and to possess giant grana with dozens of stacked thylakoids. It is thus ideally suited for small-angle neutron scattering measurements (SANS)—a non-invasive technique, which is capable of providing spatially and statistically averaged information on the periodicity of the thylakoid membranes and their rapid reorganizations in vivo. We show that NPQ-inducing illumination causes a strong decrease in the periodic order of granum thylakoid membranes. Development of NPQ and light-induced ultrastructural changes, as well as the relaxation processes, follow similar kinetic patterns. Surprisingly, whereas NPQ is suppressed by diuron, it impedes only the relaxation of the structural changes and not its formation, suggesting that structural changes do not cause but enable NPQ. We also demonstrate that the diminishment of SANS peak does not originate from light-induced redistribution and reorientation of chloroplasts inside the cells

A voltage-dependent chloride channel fine-tunes photosynthesis in plants.

In natural habitats, plants frequently experience rapid changes in the intensity of sunlight. To cope with these changes and maximize growth, plants adjust photosynthetic light utilization in electron transport and photoprotective mechanisms. This involves a proton motive force (PMF) across the thylakoid membrane, postulated to be affected by unknown anion (Cl(-)) channels. Here we report that a bestrophin-like protein from Arabidopsis thaliana functions as a voltage-dependent Cl(-) channel in electrophysiological experiments. AtVCCN1 localizes to the thylakoid membrane, and fine-tunes PMF by anion influx into the lumen during illumination, adjusting electron transport and the photoprotective mechanisms. The activity of AtVCCN1 accelerates the activation of photoprotective mechanisms on sudden shifts to high light. Our results reveal that AtVCCN1, a member of a conserved anion channel family, acts as an early component in the rapid adjustment of photosynthesis in variable light environments

Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria

In this paper we propose an energy dissipation mechanism that iscompletely reliant on changes in the aggregation state of thephycobilisome light-harvesting antenna components. All photosyntheticorganisms regulate the efficiency of excitation energy transfer(EET) to fit light energy supply to biochemical demands. Not many dothis to the extent required of desert crust cyanobacteria. Followingpredawn dew deposition, they harvest light energy with maximumefficiency until desiccating in the early morning hours. In thedesiccated state, absorbed energy is completely quenched. Timeand spectrally resolved fluorescence emission measurements of thedesiccated desert crust Leptolyngbya ohadii strain identified (i) reducedEET between phycobilisome components, (ii) shorter fluorescencelifetimes, and (iii) red shift in the emission spectra, comparedwith the hydrated state. These changes coincide with a loss of theordered phycobilisome structure, evident from small-angle neutronand X-ray scattering and cryo-transmission electron microscopy data.Based on these observations we propose a model where in the hydratedstate the organized rod structure of the phycobilisome supportsdirectional EET to reaction centers with minimal losses due tothermal dissipation. In the desiccated state this structure is lost, givingway to more random aggregates. The resulting EET path will exhibitincreased coupling to the environment and enhanced quenching