Antenna Complexes Research Articles

Multiexciton spectra of molecular aggregates: application to photosynthetic antenna complexes.

We perform theoretical studies of nonlinear spectral responses of molecular aggregates upon multiple electronic excitations. It is shown that the transient absorption (TA) spectra exhibit gradual shifting to short wavelengths upon an increase in excitation energy accompanied by population of higher-order exciton manifolds. This transformation of the TA profile reflects a character of the exciton splitting and, therefore, is strongly dependent on the aggregate shape and size as well as on the exciton couplings and disorder of the site energies. The theory is applied for modeling of the intensity-dependent TA spectra of a light-harvesting LH1 antenna from a photosynthetic purple bacterium. Fitting of the data allowed verification of the exciton model of the complex (enabling us to differentiate between the correlated (elliptical) and uncorrelated energetic disorder). We found that the difference between the TA spectra corresponding to the absorption of one and two quanta suggests the presence of strong uncorrelated disorder (in agreement with earlier models of bacterial LH1/LH2 antennas). Nonlinear spectroscopy with multiple excitations may also be useful for exploring the exciton structure of other photosynthetic antennas and similar molecular systems.

Biomimetic light-harvesting antennas via the self-assembly of chemically programmed chlorophylls.

The photosynthetic pigment "chlorophyll" possesses attractive photophysical properties, including efficient sunlight absorption, photoexcited energy transfer, and charge separation, which are advantageous for applications for photo- and electro-functional materials such as artificial photosynthesis and solar cells. However, these functions cannot be realized by individual chlorophyll molecules alone; rather, they are achieved by the formation of sophisticated supramolecules through the self-assembly of the pigments. Here, we present strategies for constructing and developing artificial light-harvesting systems by mimicking photosynthetic antenna complexes through the highly ordered supramolecular self-assembly of synthetic dyes, particularly chlorophyll derivatives.

An integrated approach towards extracting structural characteristics of chlorosomes from a bchQ mutant of Chlorobaculum tepidum.

Chlorosomes, the photosynthetic antenna complexes of green sulfur bacteria, are paradigms for light-harvesting elements in artificial designs, owing to their efficient energy transfer without protein participation. We combined magic angle spinning (MAS) NMR, optical spectroscopy and cryogenic electron microscopy (cryo-EM) to characterize the structure of chlorosomes from a bchQ mutant of Chlorobaculum tepidum. The chlorosomes of this mutant have a more uniform composition of bacteriochlorophyll (BChl) with a predominant homolog, [8Ethyl, 12Ethyl] BChl c, compared to the wild type (WT). Nearly complete 13C chemical shift assignments were obtained from well-resolved homonuclear 13C-13C RFDR data. For proton assignments heteronuclear 13C-1H (hCH) data sets were collected at 1.2 GHz spinning at 60 kHz. The CHHC experiments revealed intermolecular correlations between 132/31, 132/32, and 121/31, with distance constraints of less than 5 Å. These constraints indicate the syn-anti parallel stacking motif for the aggregates. Fourier transform cryo-EM data reveal an axial repeat of 1.49 nm for the helical tubular aggregates, perpendicular to the inter-tube separation of 2.1 nm. This axial repeat is different from WT and is in line with BChl syn-anti stacks running essentially parallel to the tube axis. Such a packing mode is in agreement with the signature of the Qy band in circular dichroism (CD). Combining the experimental data with computational insight suggests that the packing for the light-harvesting function is similar between WT and bchQ, while the chirality within the chlorosomes is modestly but detectably affected by the reduced compositional heterogeneity in bchQ.

Excitation landscape of the CP43 photosynthetic antenna complex from multiscale simulations

QM/MM simulations and the perturbed matrix method are used to investigate a crucial photosynthetic antenna complex, mapping its global excitonic structure and revealing the presence of a low-lying charge transfer state.

Histological and transcriptomic insights into the interaction between grapevine and Colletotrichum viniferum.

Grape is of high economic value. Colletotrichum viniferum, a pathogen causing grape ripe rot and leaf spot, threatens grape production and quality. This study investigates the interplay between C. viniferum by Cytological study and transcriptome sequencing. Different grapevine germplasms, V. vinifera cv. Thompson Seedless (TS), V. labrusca accession Beaumont (B) and V. piasezkii Liuba-8 (LB-8) were classified as highly sensitive, moderate resistant and resistant to C. viniferum, respectively. Cytological study analysis reveals distinct differences between susceptible and resistant grapes post-inoculation, including faster pathogen development, longer germination tubes, normal appressoria of C. viniferum and absence of white secretions in the susceptible host grapevine. To understand the pathogenic mechanisms of C. viniferum, transcriptome sequencing was performed on the susceptible grapevine "TS" identifying 236 differentially expressed C. viniferum genes. These included 56 effectors, 36 carbohydrate genes, 5 P450 genes, and 10 genes involved in secondary metabolism. Fungal effectors are known as pivotal pathogenic factors that modulate plant immunity and affect disease development. Agrobacterium-mediated transient transformation in Nicotiana benthamiana screened 10 effectors (CvA13877, CvA01508, CvA05621, CvA00229, CvA07043, CvA05569, CvA12648, CvA02698, CvA14071 and CvA10999) that inhibited INF1 (infestans 1, P. infestans PAMP elicitor) induced cell death and 2 effectors (CvA02641 and CvA11478) that induced cell death. Additionally, transcriptome analysis of "TS" in response to C. viniferum identified differentially expressed grape genes related to plant hormone signaling (TGA, PR1, ETR, and ERF1/2), resveratrol biosynthesis genes (STS), phenylpropanoid biosynthesis genes (PAL and COMT), photosynthetic antenna proteins (Lhca and Lhcb), transcription factors (WRKY, NAC, MYB, ERF, GATA, bHLH and SBP), ROS (reactive oxygen species) clearance genes (CAT, GSH, POD and SOD), and disease-related genes (LRR, RPS2 and GST). This study highlights the potential functional diversity of C. viniferum effectors. Our findings lay a foundation for further research of infection mechanisms in Colletotrichum and identification of disease response targets in grape.

Morphology, photosynthetic and molecular mechanisms associated with powdery mildew resistance in Kentucky bluegrass.

Kentucky bluegrass (Poa pratensis L.), one of the most widely used cool-season turfgrasses around the world, is sensitive to powdery mildew (PM; Blumeria graminis). The PM strain identification and regulation mechanisms of Kentucky bluegrass in response to pathogens still remain unclear. Through morphological and molecular analyses, we identified that the pathogen in Kentucky bluegrass was B. graminis f. sp. poae. The infection of B. graminis led to a reduction of the sclerenchyma area, expansion of vesicular cells and movement of chloroplasts. The infected leaves had significantly lower values in net photosynthesis, stomatal conductance and transpiration rate, maximal quantum yield of PSII photochemistry, photochemical quenching and non-regulated energy dissipation compared to mock-inoculated leaves. Expressions of light-harvesting antenna protein genes LHCA and LHCB and photosynthetic electron transport genes petE and petH decreased significantly in infected leaves. Furthermore, upregulations of genes involved in plant-pathogen interaction, such as HSP90, RBOH, and RPM and downregulations of EDS, RPS and WRKY were observed in infected leaves. The findings may help design a feasible approach to effectively control the PM disease in Kentucky bluegrass and other related perennial grass species.

The Psb27 Assembly Factor and Histidine‐252 of the D1 Protein Modify Energy Transfer to Photosystem II in Synechocystis sp. PCC 6803

AbstractPhotosystem II (PS II) is responsible for light‐driven water splitting in oxygenic photosynthesis. The Psb27 protein, an assembly factor required for biogenesis of PS II, is found associated with hydrophilic regions of the CP43 core antenna protein in the thylakoid lumen. CP43 and the D1 reaction center protein provide ligands for the Mn4CaO5 oxygen‐evolving complex (OEC). Release of Psb27 coincides with conformational changes that enable successful light‐driven assembly of the OEC. This stage in biogenesis also requires changes to allow electron transfer between plastoquinone electron acceptors on the opposite side of the membrane. We have introduced charge‐swap mutations to target the binding of Psb27 to CP43 during assembly. Here, we show that perturbation of the Psb27‐CP43 interaction results in elevated fluorescence, indicating enhanced energy transfer to PS II in fully assembled complexes. In a Psb27:Arg78 to Glu mutant, D1:His252 spontaneously mutated to Gln. D1:His252 is in the DE loop that contributes to quinone binding and protonation. Mutations targeting D1:His252 produced mutants with elevated PS II‐specific fluorescence that exceeded that observed in our Psb27 mutants and this was attenuated when the Psb27 charge‐swap mutations were introduced into H252Q cells. Perturbation of Psb27 binding to CP43 therefore modified structural changes on the opposite side of the membrane resulting from mutation of D1:His252. The peripheral phycobilisome antenna is lost during thylakoid isolation and thylakoids from our mutants did not display the increased PS II‐specific fluorescence. Hence Psb27 binding to CP43 during photoassembly of the OEC can modify phycobilisome‐dependent energy transfer into PS II.

Integrated transcriptome and metabolome analysis of salinity tolerance in response to foliar application of choline chloride in rice (Oryza sativa L.).

Salt stress is a major abiotic stress that affects crop growth and productivity. Choline Chloride (CC) has been shown to enhance salt tolerance in various crops, but the underlying molecular mechanisms in rice remain unclear. To investigate the regulatory mechanism of CC-mediated salt tolerance in rice, we conducted morpho-physiological, metabolomic, and transcriptomic analyses on two rice varieties (WSY, salt-tolerant, and HHZ, salt-sensitive) treated with 500 mg·L-1 CC under 0.3% NaCl stress. Our results showed that foliar application of CC improved morpho-physiological parameters such as root traits, seedling height, seedling strength index, seedling fullness, leaf area, photosynthetic parameters, photosynthetic pigments, starch, and fructose content under salt stress, while decreasing soluble sugar, sucrose, and sucrose phosphate synthase levels. Transcriptomic analysis revealed that CC regulation combined with salt treatment induced changes in the expression of genes related to starch and sucrose metabolism, the citric acid cycle, carbon sequestration in photosynthetic organs, carbon metabolism, and photosynthetic antenna proteins in both rice varieties. Metabolomic analysis further supported these findings, indicating that photosynthesis, carbon metabolism, and carbon fixation pathways were crucial in CC-mediated salt tolerance. The combined transcriptomic and metabolomic data suggest that CC treatment enhances rice salt tolerance by activating distinct transcriptional cascades and phytohormone signaling, along with multiple antioxidants and unique metabolic pathways. These findings provide a basis for further understanding the mechanisms of metabolite synthesis and gene regulation induced by CC in rice in response to salt stress, and may inform strategies for improving crop resilience to salt stress.

An Efficient Goal-Oriented Adaptive Finite Element Method for Accurate Simulation of Complex Electromagnetic Radiation Problems

A goal-oriented adaptive frequency domain finite element method for solving electromagnetic radiation problems including complex structures is presented in this paper. Compared with the traditional adaptive finite element method, the goal-oriented method can flexibly control the refined regions according to the parameters of interest; therefore it has better convergence and has made significant progress in scattering problems and eigenvalue problems. To simulate complex antennas, this paper proposes an error indicator with high accuracy and low computational cost, and it uses the adjoint problem to weight element residuals without additional degrees of freedom. Moreover, high-quality mesh refinement algorithms adapted to this indicator are developed using a suitable point insertion strategy for multiscale structures. By simulating two practical antennas, comparisons with the traditional goal-oriented FEM and the well-developed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</i> -adaptive finite element method in commercial software demonstrate the accuracy and efficiency of the proposed method.

Analyzing the Performance of TAS/MRC with Decode-and-Forward Relaying for Multihop Transmission over Fisher-Snedecor F Fading Channels

This study comprehensively assesses a decode-and-forward relaying system that employs Transmit Antenna Selection/Maximal Ratio Combining (TAS/MRC) for multi-hop communication across Fisher-Snedecor F fading channels. The study primarily focuses on the average symbol error rate (ASER) and outage probability (OP) while using cross QAM (XQAM) and rectangular quadrature amplitude modulation (RQAM). The proposed MIMO system effectively decreases the hardware complexity and cost of large antennas by implementing Transmit Antenna Selection (TAS) at the source and relay nodes and Maximum Ratio Combining (MRC) diversity at the destination and relays. Analytical formulations for OP (outage probability) and ASER (average symbol error rate) are derived using PDF (Probability Density Function)-based methods. Monte-Carlo simulations further validate these formulas. The investigation indicates that system performance is improved under conditions of lighter shadowing compared to moderate and extreme shadowing settings. Additionally, increasing the hop count amplifies the observed performance. Moreover, when the fading parameter increases, both the OP and ASER decrease. This study comprehensively examines the impact of many parameters, including m, p, L, R, and o, on the behaviour of the system under Fisher-Snedecor F fading. Based on the findings, the combination of shadowing and multipath fading significantly affects the system's performance, with both m and p significantly influencing this. When examining the Signal-to-Noise Ratio (SNR), a comparison between RQAM and XQAM demonstrates that XQAM is superior. The research uncovers crucial information for planning and executing the TAS/MRC-based MIMO multi-hop communication system. This technology possesses the capability to enhance the effectiveness of similar communication systems, as seen by the encouraging advancements in performance, especially in scenarios including signal attenuation.

Expanding the toolbox for phycobiliprotein assembly: phycoerythrobilin biosynthesis in Synechocystis

AbstractPhycobiliproteins (PBPs) play a vital role in light harvesting by cyanobacteria, which enables efficient utilization of photon energy for oxygenic photosynthesis. The PBPs carry phycobilins, open‐chain tetrapyrrole chromophores derived from heme. The structure and chromophore composition of PBPs is dependent on the organism's ecological niche. In cyanobacteria, these holo‐proteins typically form large, macromolecular antenna complexes called phycobilisomes (PBSs). The PBS of Synechocystis sp. PCC 6803 (hereafter Synechocystis) consists of allophycocyanin (APC) and phycocyanin (PC), which exclusively harbor phycocyanobilin (PCB) as a chromophore. Investigations into heterologous PBP biosynthesis in E. coli have proven limiting with respect to PBP assembly and their functional characterization. Consequently, we wanted to engineer a platform for the investigation of heterologously produced PBPs, focusing on unusual, phycoerythrobilin (PEB)‐containing light‐harvesting proteins called phycoerythrins (PEs) in Synechocystis. As a first step, a gene encoding for the synthesis of the natural cyanobacterial chromophore, PEB, was introduced into Synechocystis. We provide spectroscopic evidence for heterologous PEB formation and show covalent attachment of PEB to the α‐subunit of PC, CpcA, by HPLC and LC–MS/MS analyses. Fluorescence microscopy and PBS isolation demonstrate a cellular dispersal of PBPs with modified phycobilin content. However, these modifications have minor effects on physiological responses, as demonstrated by growth rates, oxygen evolution, nutrient accumulation, and PBP content analyses. As a result, Synechocystis demonstrates the capacity to efficiently manage PEB biosynthesis and therefore reflects a promising platform for both biochemical and physiological investigations of foreign and unusual PEs.

CiAP2/ERF65 and CiAP2/ERF106, a pair of homologous genes in pecan (Carya illinoensis), regulate plant responses during submergence in transgenic Arabidopsis thaliana

When plants are entirely submerged, photosynthesis and respiration are severely restricted, affecting plant growth and potentially even causing plant death. The AP2/ERF superfamily has been widely reported to play a vital role in plant growth, development and resistance to biotic and abiotic stresses. However, no relevant studies exist on flooding stress in pecan. In this investigation, we observed that CiAP2/ERF65 positively modulated the hypoxia response during submergence, whereas CiAP2/ERF106 was sensitive to submergence. The levels of physiological and biochemical indicators, such as POD, CAT and among others, in CiAP2/ERF65-OE lines were significantly higher than those in wild-type Arabidopsis thaliana, indicating that the antioxidant capacity of CiAP2/ERF65-OE lines was enhanced under submergence. The RNA-seq results revealed that the maintenance of the expression levels of the antenna protein gene, different signaling pathways for regulation, as well as the storage and consumption of ATP, might account for the opposite phenotypes of CiAP2/ERF65 and CiAP2/ERF106. Furthermore, the expression of some stress-related genes was altered during submergence and reoxygenation. Overall, these findings enhance our understanding of submergence stress in pecan, providing important candidate genes for the molecular design and breeding of hypoxia resistant in plants.

Structure of the antenna complex expressed during far-red light photoacclimation in Synechococcus sp. PCC 7335

Far-red light photoacclimation, or FaRLiP, is a facultative response exhibited by some cyanobacteria that allows them to absorb and utilize lower energy light (700 to 800 nm) than the wavelengths typically used for oxygenic photosynthesis (400 to 700 nm). During this process, three essential components of the photosynthetic apparatus are altered: photosystem I, photosystem II, and the phycobilisome. In all three cases, at least some of the chromophores found in these pigment-protein complexes are replaced by chromophores that have red-shifted absorbance relative to the analogous complexes produced in visible light. Recent structural and spectroscopic studies have elucidated important features of the two photosystems when altered to absorb and utilize far-red light, but much less is understood about the modified phycobiliproteins made during FaRLiP. We used single-particle, cryo-electron microscopy to determine the molecular structure of a phycobiliprotein core complex comprising allophycocyanin variants that absorb far-red light during FaRLiP in the marine cyanobacterium Synechococcus sp. PCC 7335. The structure reveals the arrangement of the numerous red-shifted allophycocyanin variants and the probable locations of the chromophores that serve as the terminal emitters in this complex. It also suggests how energy is transferred to the photosystem II complexes produced during FaRLiP. The structure additionally allows comparisons with other previously studied allophycocyanins to gain insights into how phycocyanobilin chromophores can be tuned to absorb far-red light. These studies provide new insights into how far-red light is harvested and utilized during FaRLiP, a widespread cyanobacterial photoacclimation mechanism.

Computing the Relative Affinity of Chlorophylls a and b to Light-Harvesting Complex II.

In plants and algae, the primary antenna protein bound to photosystem II is light-harvesting complex II (LHCII), a pigment-protein complex that binds eight chlorophyll (Chl) a molecules and six Chl b molecules. Chl a and Chl b differ only in that Chl a has a methyl group (-CH3) on one of its pyrrole rings, while Chl b has a formyl group (-CHO) at that position. This blue-shifts the Chl b absorbance relative to Chl a. It is not known how the protein selectively binds the right Chl type at each site. Knowing the selection criteria would allow the design of light-harvesting complexes that bind different Chl types, modifying an organism to utilize the light of different wavelengths. The difference in the binding affinity of Chl a and Chl b in pea and spinach LHCII was calculated using multiconformation continuum electrostatics and free energy perturbation. Both methods have identified some Chl sites where the bound Chl type (a or b) has a significantly higher affinity, especially when the protein provides a hydrogen bond for the Chl b formyl group. However, the Chl a sites often have little calculated preference for one Chl type, so they are predicted to bind a mixture of Chl a and b. The electron density of the spinach LHCII was reanalyzed, which, however, confirmed that there is negligible Chl b in the Chl a-binding sites. It is suggested that the protein chooses the correct Chl type during folding, segregating the preferred Chl to the correct binding site.

Nitraria sibirica adapts to long-term soil water deficit by reducing photosynthesis, stimulating antioxidant systems, and accumulating osmoregulators

Amid climate change and shifts in precipitation patterns, drought conditions are expanding worldwide. Drought stress severely threatens plant growth in arid and semi-arid regions, wherein shrubs play a crucial role in maintaining ecological stability. Despite its ecological significance, studies are lacking on how Nitraria sibirica adapts to long-term drought stress. Therefore, in this study, to elucidate the mechanism of drought stress adaptation in N. sibirica, we analysed morphological, physiological, and transcriptional characteristics of plants in two soil habitats: riparian (moist) and desert (arid). The results showed that in desert soils, as soil water content decreased, leaf thickness increased, while plant height and leaf area decreased. Physiologically, photosynthesis decreased; soluble sugar, starch, proline, and hydrogen peroxide content increased significantly; while soluble proteins decreased significantly. Additionally, membrane lipid peroxidation products and antioxidant enzyme activities significantly increased under drought stress. Then, Kyoto Encyclopaedia of Genes and Genomes (KEGG) enrichment analysis identified 313 key genes, which were considered the most significantly enriched in the photosynthesis and photosynthetic antenna protein pathways. Further, we found that the proteins encoding photosystem II (PsbP, PsbQ, PsbR, PsbY, and Psb27), photosystem I (PsaD, PsaF, PsaG, PsaH, PsaK, and PsaO), photosynthetic electron transport (PetF), and light-trapping antenna proteins were significantly downregulated under drought stress. Taken together, these results suggest that N. sibirica adapts to long-term drought conditions by suppressing photosynthesis, activating antioxidant systems, and recruiting osmoregulators. This study provides a basis for elucidating the growth mechanisms of N. sibirica under long-term drought stress conditions.

Functional Coupling of Biohybrid Photosynthetic Antennae and Reaction Center Complexes: Quantitative Comparison with Native Antennae.

Light-harvesting (LH) complexes in photosynthetic organisms absorb photons within limited wavelength ranges over a broad solar spectrum. Extension of the LH wavelength has been realized by attaching artificial fluorophores to LH complexes (biohybrid LH complexes) for complementing the limited-wavelength regions. However, how efficiently such fluorophores in biohybrid LH complexes function to drive the photocatalytic reaction center (RC) has not been quantitatively evaluated, specifically in comparison with native LH antenna complexes. In this study, we prepared various biohybrid LH1-RC complexes (from Rhodopseudomonas palustris), to quantitatively evaluate the LH activity of the attached external chromophores through a photocurrent generation reaction by LH1-RC on an electrode. For a direct comparison of the LH activity among the LH chromophores that were examined, we introduced the k1 term, which represents the extent of the functional coupling of LH and the photochemical reactions in the RC. We determined that the hydrophobic fluorophore ATTO647N attached to LH1 possesses the highest LH activity among the examined hydrophilic fluorophores such as Alexa647, and its activity is comparable to that of native LH1(-RC). The LH activity of LH2 (from Rhodoblastus acidophilus strain 10050) and its biohybrid LH2s were examined for the comprehensive assessment of their LH activity.

Integrative metabolomic and transcriptomic reveals potential mechanism for promotion of ginsenoside synthesis in Panax ginseng leaves under different light intensities

Panax ginseng C.A. Meyer is a shade plant, and its leaves are an important medicinal part of P. ginseng. Light intensity plays a crucial role in physiological activities and metabolite accumulation in P. ginseng. Currently, little is known about the molecular mechanisms underlying physiological changes and quality under different light intensities in P. ginseng leaves. Therefore, we investigated the changes in photosynthetic physiology, secondary metabolism, transcriptomics, and metabolomics of P. ginseng leaves under different light intensities [T20 (20 µmol m-2·s-1), T50 (50 µmol m-2·s−1), T100 (100 μmol m−2·s−1)]]. Higher light intensity positively influenced the yield, photosynthesis, and accumulation of polysaccharides, soluble sugars, terpenoids, and ginsenosides in P. ginseng leaves. The T100 treatment notably promoted the accumulation of ginsenosides in the leaves, resulting in a 68.32% and 45.55% increase in total ginsenosides compared to the T20 and T50 treatments, respectively. Ginsenosides Rg1, Re, Rb1, Rc, Rg2, Rb2, Rb3, and Rd were 1.28-, 1.47-, 2.32-, 1.64-, 1.28-, 2.59-, 1.66-, and 2.28-times higher than in the T20 treatment. Furthermore, 285 differentially accumulated metabolites (DAMs) and 4218 differentially expressed genes (DEGs) in the metabolome and transcriptome of P. ginseng leaves, respectively, were identified. 13 triterpenoid saponins were significantly upregulated, and three were downregulated. The expression of genes encoding photosystem II reaction center proteins was upregulated under the T100 treatment, thereby increasing photosynthetic activity. The T100 treatment enhanced the expression of genes involved in photosynthetic carbon and energy metabolism in P. ginseng. The expression of antenna protein synthesis genes was upregulated under the T20, which increased the ability to capture light in P. ginseng leaves. T100 upregulated the expression of HMGR, SS, CYP716A53v2, UGT74AE, PgUGT1, and UGTPg45, thereby promoting terpene and ginsenoside synthesis. In summary, 100 µmol m−2·s−1 was conducive to quality formation of P. ginseng leaves. This study elucidates molecular mechanisms underlying the photosynthetic physiology and ginsenoside synthesis in P. ginseng under varying light intensities and provides a theoretical basis for the P. ginseng cultivation and its industrial production of secondary metabolites.

First-principles simulation of excitation energy transfer and transient absorption spectroscopy in the CP29 light-harvesting complex.

The dynamics of delocalized excitons in light-harvesting complexes (LHCs) can be investigated using different experimental techniques, and transient absorption (TA) spectroscopy is one of the most valuable methods for this purpose. A careful interpretation of TA spectra is essential for the clarification of excitation energy transfer (EET) processes occurring during light-harvesting. However, even in the simplest LHCs, a physical model is needed to interpret transient spectra as the number of EET processes occurring at the same time is very large to be disentangled from measurements alone. Physical EET models are commonly built by fittings of the microscopic exciton Hamiltonians and exciton-vibrational parameters, an approach that can lead to biases. Here, we present a first-principles strategy to simulate EET and transient absorption spectra in LHCs, combining molecular dynamics and accurate multiscale quantum chemical calculations to obtain an independent estimate of the excitonic structure of the complex. The microscopic parameters thus obtained are then used in EET simulations to obtain the population dynamics and the related spectroscopic signature. We apply this approach to the CP29 minor antenna complex of plants for which we follow the EET dynamics and transient spectra after excitation in the chlorophyll b region. Our calculations reproduce all the main features observed in the transient absorption spectra and provide independent insight on the excited-state dynamics of CP29. The approach presented here lays the groundwork for the accurate simulation of EET and unbiased interpretation of transient spectra in multichromophoric systems.

Molecular dissection of the soluble photosynthetic antenna from the cryptophyte alga Hemiselmis andersenii

Cryptophyte algae have a unique phycobiliprotein light-harvesting antenna that fills a spectral gap in chlorophyll absorption from photosystems. However, it is unclear how the antenna transfers energy efficiently to these photosystems. We show that the cryptophyte Hemiselmis andersenii expresses an energetically complex antenna comprising three distinct spectrotypes of phycobiliprotein, each composed of two αβ protomers but with different quaternary structures arising from a diverse α subunit family. We report crystal structures of the major phycobiliprotein from each spectrotype. Two-thirds of the antenna consists of open quaternary form phycobiliproteins acting as primary photon acceptors. These are supplemented by a newly discovered open-braced form (~15%), where an insertion in the α subunit produces ~10 nm absorbance red-shift. The final components (~15%) are closed forms with a long wavelength spectral feature due to substitution of a single chromophore. This chromophore is present on only one β subunit where asymmetry is dictated by the corresponding α subunit. This chromophore creates spectral overlap with chlorophyll, thus bridging the energetic gap between the phycobiliprotein antenna and the photosystems. We propose that the macromolecular organization of the cryptophyte antenna consists of bulk open and open-braced forms that transfer excitations to photosystems via this bridging closed form phycobiliprotein.

Possible Contribution of Corticular Photosynthesis to Grapevine Winter Hardiness

Numerous studies show that photosynthesis in non-foliar tissues contributes to plant productivity. Here, we demonstrate that in chlorenchyma tissues of lignified branches of grape vines, photosynthetic activity is maintained during winter and provide evidence that corticular photosynthesis could contribute to the plant’s freeze tolerance. In a collection of grape varieties that varied noticeably in freeze tolerance, a positive correlation between the maximum quantum yield of photosystem II in the wintering vines and the ability to survive harsh winter temperatures was observed. A more detailed comparison of two grapevine varieties differing in freeze tolerance showed that the vines of the more tolerant variety have more abundant corticular chlorenchyma with chloroplasts containing a better developed network of photosynthetic membranes, characterized by a higher photosynthetic pigments content, higher efficiency of both photosystems, and higher mobility of antennae complexes under the changing light intensity. In addition, we found that freezing temperatures induced more damage in vine samples when they were preliminarily treated with a specific inhibitor of photosynthetic electron transfer. The data obtained could be useful in the generation of freeze-tolerant grape varieties.