Roberto Bassi

Land plant chloroplasts are characterized by a prominent morphological feature: the stacking of thylakoid membranes into grana, ie defining a continuous membranous structure surrounded by the stroma and folded into appressed grana and non-appressed domains, enclosing the thylakoid lumen. Photosystem II and its antenna system are concentrated, whereas Photosystem I and ATPase are located in the grana end membranes and, in the stroma, membranes connecting grana. Thylakoid stacking promotes the physical separation of Photosystems I and II, thereby ensuring a balanced flow of excitation energy between them. Through a genetic dissection of the light-harvesting complex subfamily Lhcb, we showed that removal of the antenna system serving Photosystem II completely abolished grana formation, whereas deletion of specific Lhcb subgroups reduced stacking by up to 40%. Lhcb5 alone was sufficient to partially restore stacking, whereas exclusive expression of Lhcb2 resulted in grana with an unusually large size, which disassembled upon illumination. Time-resolved fluorescence measurements revealed that the extent of excitation energy spillover from PSII to Photosystem I correlated with the level of Lhcb depletion, rather than with granal architecture. These findings provide genetic evidence for a functional link between Photosystem II antenna composition, lateral heterogeneity, and excitation energy partition between photosystems.

Seagrasses are marine angiosperms re-adapted to underwater life, forming productive ecosystems and long-term carbon sinks. Posidonia oceanica thrives up to 50 m depth, where light is scarce and spectrally shifted; yet, the molecular basis of its photosynthetic adaptation remains unclear. Here, we report that P. oceanica genetically adapts for highly efficient photon use under dim light by enhancing photosystem antenna size and reducing exciton trapping time. We determine the structures of P. oceanica PSI supercomplexes by cryo-electron microscopy, revealing an expanded antenna system composed of PSI-LHCI, a trimeric phospho-LHCII, and an additional LHCI heterodimer. Low-energy chlorophyll forms associated with LHCI are lost. Ultrafast spectroscopy shows that this loss correlates with faster exciton trapping, which compensates for antenna expansion and enhances light-use efficiency under dim light. We identify key residues responsible for the loss of low-energy forms. Reversion to land-plant ortholog sequences restores red-shifted emission, providing strategies to enhance light-use efficiency in crops.

Ranalli: Le alghe sono un gruppo estremamente eterogeneo di organismi viventi, che spaziano da forme microscopiche unicellulari (microalghe) a organismi giganti pluricellulari lunghi decine di metri (macroalghe). Accomunate dalla capacità di essere autotrofe, vivono principalmente in acqua (dolce o salata), producono circa la metà dell'ossigeno atmosferico e rappresentano la base della catena alimentare acquatica: semplici nella struttura ma vitali per il pianeta. Grazie alla loro velocità di crescita e alla ricchezza di nutrienti, sono considerate una delle risorse chiave per l'economia verde del futuro. Infatti, alcune specie di macroalghe possono crescere fino a 50 cm al giorno, pertanto costituiscono una delle biomasse più rinnovabili ed efficienti del pianeta. La loro composizione chimica varia enormemente tra le migliaia di specie esistenti e questa versatilità di nutrienti e molecole sintetizzate determina i loro molteplici impieghi. La ricerca scientifica oggi non si limita a usare l'alga "intera", ma lavora come in una bioraffineria: estrae selettivamente ogni singolo nutriente (proteine, oli, zuccheri) per destinarlo all'industria più appropriata, riducendo al minimo gli sprechi.Roberto, come esperto mondiale di fotosintesi, biologia vegetale e bioenergetica, con riguardo specificamente alle alghe, la tua attività si è concentrata principalmente su come questi organismi catturano la luce e la trasformano in energia chimica. È così?Bassi: Sì. Questa è stata la prima domanda che mi sono posto da ricercatore all’orto botanico di Padova a cavallo tra gli anni 70 e 80 del secolo scorso. Cercando risposte, mi sono addentrato in una complessità strutturale e funzionale inaspettata. Ricordo bene come, dopo aver pubblicato i primi risultati sulla identità biochimica del fotosistema II, che era ancora sconosciuta, i colleghi mi spingessero ad approfondire quell’argomento, considerato fondamentale. La comunità degli studiosi della fotosintesi era allora costituita soprattutto da bravissimi biofisici, come Pierre Joliot-Curie e Francis-André Wollman a Parigi, Jim Barber a Londra e Daniel Arnon a Berkeley. Wollman, mi ripeteva che la mia passione per la ricerca non poteva essere sprecata nello studio di quelle “scatole piene di clorofilla” che assorbivano fotoni ma erano prive di qualsiasi regolazione. Tuttavia, pur amando la fisica, la mia cultura era fondamentalmente biologica e mi portava a guardare ai genomi per capire cosa fosse importante nella vita degli organismi. Negli anni ’80 si accumulavano i dati sulle sequenze geniche delle piante ed era chiaro che, mentre i componenti dei centri di reazione erano tutti codificati da pochi geni, quasi identici tra loro in specie diverse, le proteine LHC (Light Harvesting Complexes, che codificavano per le proteine antenna), erano molti e diversificati. La grande capacità delle piante di adattarsi a climi contrastanti poteva solo essere basata su una grande diversità di strutture, funzioni e livelli di espressione come quelle che si vedevano nella famiglia multigenica lhc. Se lo scopo era quello di ottimizzare la crescita delle piante attraverso la genetica, pensavo, era meglio puntare su queste ultime proteine, allora ancora sconosciute nonostante fossero le proteine di membrana più abbondanti sulla Terra.

Seagrasses are marine angiosperms re-adapted to underwater life, forming productive ecosystems and long-term carbon sinks. Posidonia oceanica thrives up to 50 m depth, where light is scarce and spectrally shifted; yet, the molecular basis of its photosynthetic adaptation remains unclear. We showed that P. oceanica is genetically adapted to perform highly efficient photon use under dim light by enhancing photosystems antenna size and reducing exciton trapping time. We solved the structures of P. oceanica PSI supercomplexes by cryo-electron microscopy, revealing an expanded antenna system composed of PSI-LHCI, a trimeric phospho-LHCII, and an additional LHCI heterodimer. Low-energy chlorophyll spectral forms associated to LHCI, which modulate antenna-to-reaction center excitation transfer, are lost. Ultrafast spectroscopy revealed that this loss results in faster exciton trapping, thus compensating for antenna expansion and loss of quantum efficiency under dim light. Key residues responsible for the loss of low-energy forms were identified; replacing them with land-plant orthologs restored red-shifted emission, providing strategies to enhance light-use efficiency in crops.

Optimal photosynthetic performance of plants requires a balance between light energy capture and its use in downstream reactions. The process of light harvesting and its regulation are mediated by a complex array of antenna proteins, whose conservation throughout evolution suggests each complex serves a specific function in the diverse growth conditions in the natural subaerial environment. However, the specific roles of individual gene products in various antenna functions remain poorly understood. In this study, we investigated the Photosystem II antenna system by employing genome editing techniques targeted at subsets of LHCB genes and characterized Arabidopsis thaliana mutants missing specific components of the Photosystem II antenna: namely, the trimeric LHCII, the monomeric LHC, or both. The focus was on light-harvesting capabilities and photoprotective functions, which included exciton trapping cooperativity, non-photochemical quenching (NPQ) of excess excitation energy, and overall resistance to photoinhibition under excess irradiation, aiming to pinpoint the site(s) of the photoprotective responses. NPQ activity was present in all genotypes, indicating that each pigment-binding protein contributes to the overall quenching response. Within each antenna subgroup, NPQ activity did not rely on lutein, whereas zeaxanthin proved essential. Although trimeric LHCII provided the largest contribution to NPQ, the presence of monomeric Lhcbs was associated with enhanced Photosystem II phototolerance under excess light exposure. We conclude that the assembly of Photosystem II supercomplexes, including monomeric Lhcbs, is vital for maintaining PSII stability and functional integrity, playing a key role in preventing photoinhibition.

In plants and algae, photosynthesis is driven by the absorption of sunlight energy by networks of pigments housed within light‐harvesting proteins. Special photosynthetic complexes can intercept the low‐energy photons corresponding to the far‐red spectrum of the photosynthetically active radiation. These so‐called red chlorophyll forms are found in multiple lineages of the Viridiplantae clade, are formed upon a change in spatial organization of chromophores within specific subunits of the photosystem I supercomplex, and can be detected by their unique red‐shifted fluorescence emission signatures. Red forms enabled phototrophs to colonize light‐limited ecological niches, especially where far‐red radiation is enriched by leaf shading. The protein environment plays a key role in determining the occurrence of red forms, promoting strong excitonic interactions among chlorophyll a molecules and facilitating their excitation by low‐energy photons. In this review, we present a comprehensive account of the evolutionary diversity of long‐wavelength‐driven photosynthesis in eukaryotes, and detail the biophysical and structural determinants of this phenomenon. Finally, we discuss how this knowledge can be applied in biotechnology to engineer crop canopies with broadened light absorption and higher yield potential.

Summary: Higher plants Photosystem I absorbs far‐red light, enriched under vegetation canopies, through long‐wavelength Chls to enhance photon capture. Far‐red absorption originates from Chl pairs within the Lhca3 and Lhca4 subunits of the LHCI antenna, known as the ‘red cluster’, including Chls a 603 and a 609. We used reverse genetics to produce an Arabidopsis mutant devoid of red‐shifted absorption, and we obtained high‐resolution cryogenic electron microscopy structures of PSI‐LHCI complexes from both wild‐type and mutant plants. Computed excitonic coupling values suggested contributions from additional nearby pigment molecules, namely Chl a 615 and violaxanthin in the L2 site, to far‐red absorption. We investigated the structural determinants of far‐red absorption by producing further Arabidopsis transgenic lines and analyzed the spectroscopic effects of mutations targeting these chromophores. The two structures solved were used for quantum mechanics calculations, revealing that excitonic interactions alone cannot explain far‐red absorption, while charge transfer states were needed for accurate spectral simulations. Our findings demonstrate that the molecular mechanisms of light‐harvesting under shaded conditions rely on very precise tuning of chromophore interactions, whose understanding is crucial for designing light‐harvesting complexes with engineered absorption spectra.

Photosystem II (PSII) is the pigment-protein complex catalysing light-induced water oxidation. In Arabidopsis thaliana, it includes three Lhcb4–6 proteins linking the core complex to peripheral trimeric antennae. While Lhcb5 and Lhcb6 are encoded by single genes, Lhcb4 is encoded by three isoforms: Lhcb4.1 and Lhcb4.2, constitutively expressed, and Lhcb4.3 (Lhcb8), which accumulates under prolonged abiotic stress. Lhcb8 substitutes for Lhcb4, preventing Lhcb6 accumulation and resulting in a smaller PSII with high quantum yield. Cryo-electron microscopy reveals that Lhcb8 has a shorter carboxy-terminal domain, lacks two chlorophylls, and interacts more tightly with the PSII core, inducing structural changes in the PSII antenna system, ultimately inhibiting the formation of PSII arrays and favouring plastoquinone diffusion. We suggest that dynamic Lhcb4 vs Lhcb8 expression allows for PSII acclimation to contrasting light conditions, offering the potential for engineering crops with improved light use efficiency.

SUMMARY: - Higher plants Photosystem I absorbs near-infrared light through long-wavelength chlorophylls, enriched under vegetation canopies, to enhance photon capture. Far-red absorption originates from chlorophylls pairs within the Lhca3 and Lhca4 subunits of the LHCI antenna, known as the “red cluster” composed of chlorophylls a603 and a609. - We used reverse genetics to produce an Arabidopsis mutant devoid of red-shifted absorption, and we obtained high-resolution cryo-EM structures from purified PSI-LHCI complexes in both wild-type and mutant plants. - Computed excitonic coupling values suggested a possible contribution of additional nearby pigment molecules, namely chlorophyll a615 and violaxanthin in L2 site, to far-red absorption. Therefore, we investigated the structural determinants of far-red absorption and analyzed the spectroscopic effects of these additional pigments by producing further Arabidopsis transgenic lines. The two experimental structures were used for quantum mechanics calculations, revealing that excitonic interactions alone cannot explain far-red absorption, while charge transfer states were needed for accurate spectral simulations. - Our findings demonstrate that the molecular mechanisms of light-harvesting under shaded conditions rely on very precise tuning of chromophore interactions, an understanding of which is crucial for designing light-harvesting complexes with engineered absorption spectra

Photosynthesis underpins life on Earth, serving as the primary energy source while regulating global carbon and water cycles, thereby shaping climate and vegetation. Advancing photosynthesis research is essential for improving crop productivity and refining photosynthesis models across scales, ultimately addressing critical global challenges such as food security and environmental sustainability. This minireview synthesizes a selection of recent advancements presented at the 2nd European Congress of Photosynthesis Research, focusing on improving photosynthesis efficiency and modelling across the scales. We explore strategies to optimize light harvesting and carbon fixation, leading to canopy level improvements. Alongside synthetic biology, we examine recent advances in harnessing natural variability in key photosynthetic traits, considering both methodological innovations and the vast reservoir of opportunities they present. Additionally, we highlight unique insights gained from plants adapted to extreme environments, offering pathways to improve photosynthetic efficiency and resilience simultaneously. We emphasize the importance of a holistic approach, integrating dynamic modeling of metabolic processes to bridge these advancements. Beyond photosynthesis improvements, we discuss the progress of improving photosynthesis simulations, particularly through improved parametrization of mesophyll conductance, crucial for enhancing leaf‐to‐global scale simulations. Recognizing the need for greater interdisciplinary collaboration to tackle the grand challenges put on photosynthesis research, we highlight two initiatives launched at the congress—an open science platform and a dedicated journal for plant ecophysiology. We conclude this minireview with a forward‐looking outline, highlighting key next steps toward achieving meaningful improvements in photosynthesis, yield, resilience and modeling.