Stromal Loop of Lhca6 is Responsible for the Linker Function Required for the NDH-PSI Supercomplex Formation

The transcription factor MfbHLH104 from Myrothamnus flabellifolia promotes drought tolerance of Arabidopsis thaliana by enhancing stability of the photosynthesis system

The resurrection plant Myrothamnus flabellifolia can survive extreme drought and desiccation conditions, and quickly recover after rewatering. However, little is known about the mechanism underlying the drought tolerance of M. flabellifolia. In this study, MfbHLH104 was cloned and introduced into Arabidopsis thaliana due to the lack of a transgenic system for M. flabellifolia. MfbHLH104 is localized in the nucleus. Its N-terminal region has transactivation ability in yeast, and the C-terminal region may inhibit the transactivation ability. Overexpressing MfbHLH104 significantly increased drought and salt tolerance of A. thaliana at both seedling and adult stages. It enhanced leaf water retention capacity by decreasing water loss rate and increasing drought- and abscisic acid (ABA) -induced stomatal closure. Additionally, it boosted osmolyte accumulation and ROS scavenging ability by up-regulating genes associated with osmolyte biosynthesis and antioxidant enzymes, and enhancing antioxidant enzyme activities. The expression of ABA-responsive genes were also promoted by MfbHLH104. Remarkably, RNA-seq analysis indicated that MfbHLH104 significantly up-regulated 32 genes (FDR < 0.05 and fold change ≥1.5) involved in photosynthesis related pathways (KEGG pathway No: ko00195, ko00196) under drought, which account for 18.7 % of the total up-regulated genes and the most enriched KEGG pathways. This result suggested that it may help to maintain the stability of the photosynthesis system under drought conditions.

Molecular Genetic Dissection of the Regulatory Network of Proton Motive Force in Chloroplasts

The proton motive force (pmf) generated across the thylakoid membrane rotates the Fo-ring of ATP synthase in chloroplasts. The pmf comprises two components: membrane potential (∆Ψ) and proton concentration gradient (∆pH). Acidification of the thylakoid lumen resulting from ∆pH downregulates electron transport in the cytochrome b6f complex. This process, known as photosynthetic control, is crucial for protecting photosystem I (PSI) from photodamage in response to fluctuating light. To optimize the balance between efficient photosynthesis and photoprotection, it is necessary to regulate pmf. Cyclic electron transport around PSI and pseudo-cyclic electron transport involving flavodiiron proteins contribute to the modulation of pmf magnitude. By manipulating the ratio between the two components of pmf, it is possible to modify the extent of photosynthetic control without affecting the pmf size. This adjustment can be achieved by regulating the movement of ions (such as K+ and Cl-) across the thylakoid membrane. Since ATP synthase is the primary consumer of pmf in chloroplasts, its activity must be precisely regulated to accommodate other mechanisms involved in pmf optimization. Although fragments of information about each regulatory process have been accumulated, a comprehensive understanding of their interactions is lacking. Here, I summarize current knowledge of the network for pmf regulation, mainly based on genetic studies.

The structure of the Physcomitrium patens photosystem I reveals a unique Lhca2 paralogue replacing Lhca4

The moss Physcomitrium patens diverged from green algae shortly after the colonization of land by ancient plants. This colonization posed new environmental challenges, which drove evolutionary processes. The photosynthetic machinery of modern flowering plants is adapted to the high light conditions on land. Red-shifted Lhca4 antennae are present in the photosystem I light-harvesting complex of many green-lineage plants but absent in P. patens. The cryo-EM structure of the P. patens photosystem I light-harvesting complex I supercomplex (PSI-LHCI) at 2.8 Å reveals that Lhca4 is replaced by a unique Lhca2 paralogue in moss. This PSI-LHCI supercomplex also retains the PsaM subunit, present in Cyanobacteria and several algal species but lost in vascular plants, and the PsaO subunit responsible for binding light-harvesting complex II. The blue-shifted Lhca2 paralogue and chlorophyll b enrichment relative to flowering plants make the P. patens PSI-LHCI spectroscopically unique among other green-lineage supercomplexes. Overall, the structure represents an evolutionary intermediate PSI with the crescent-shaped LHCI common in vascular plants, and contains a unique Lhca2 paralogue that facilitates the moss's adaptation to low-light niches.

Genome-Wide Investigation of the PtrCHLP Family Reveals That PtrCHLP3 Actively Mediates Poplar Growth and Development by Regulating Photosynthesis

Chlorophyll (Chl) plays a crucial role in plant photosynthesis. The geranylgeraniol reductase gene (CHLP) participates in the terminal hydrogenation of chlorophyll biosynthesis. Although there are many studies related to the genome-wide analysis of Populus trichocarpa, little research has been conducted on CHLP family genes, especially those concerning growth and photosynthesis. In this study, three CHLP genes were identified in Populus. The evolutionary tree indicated that the CHLP family genes were divided into six groups. Moreover, one pair of genes was derived from segmental duplications in Populus. Many elements related to growth were detected by cis-acting element analysis of the promoters of diverse PtrCHLPs. Furthermore, PtrCHLPs exhibit different tissue expression patterns. In addition, PtrCHLP3 is preferentially expressed in the leaves and plays an important role in regulating chlorophyll biosynthesis. Silencing of PtrCHLP3 in poplar resulted in a decrease in chlorophyll synthesis in plants, thus blocking electron transport during photosynthesis. Furthermore, inhibition of PtrCHLP3 expression in poplar can inhibit plant growth through the downregulation of photosynthesis. Ultimately, PtrCHLP3 formed a co-expression network with photosynthesis and chlorophyll biosynthesis-related genes, which synergistically affected the growth and photosynthesis of poplars. Thus, this study provides genetic resources for the improved breeding of fast-growing tree traits.

Architecture of the chloroplast PSI-NDH supercomplex in Hordeum vulgare

The chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and has an important role in mediating photosystem I (PSI) cyclic electron transport (CET)^1-3. The NDH complex associates with PSI to form the PSI-NDH supercomplex and fulfil its function. Here, we report cryo-electron microscopy structures of a PSI-NDH supercomplex from barley (Hordeum vulgare). The structures reveal that PSI-NDH is composed of two copies of the PSI-light-harvesting complex I (LHCI) subcomplex and one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast-specific NDH subunits are presented and their exact positions as well as their interactions with other subunits in NDH are elucidated. In all, this study provides a structural basis for further investigations on the functions and regulation of PSI-NDH-dependent CET.

The acidic domain of the chloroplast RNA-binding protein CP31A supports cold tolerance in Arabidopsis thaliana

The processing of chloroplast RNA requires a large number of nuclear-encoded RNA-binding proteins (RBPs) that are imported post-translationally into the organelle. The chloroplast ribonucleoprotein 31A (CP31A) supports RNA editing at 13 sites and also supports the accumulation of multiple chloroplast mRNAs. In cp31a mutants it is the ndhF mRNA (coding for a subunit of the NDH complex) that is most strongly affected. CP31A becomes particularly important at low temperatures, where it is essential for chloroplast development in young tissue. Next to two RNA-recognition motifs (RRMs), CP31A has an N-terminal acidic domain that is phosphorylated at several sites. We investigated the function of the acidic domain in the role of CP31A in RNA metabolism and cold resistance. Using point mutagenesis, we demonstrate that the known phosphorylation sites within the acidic domain are irrelevant for any of the known functions of CP31A, both at normal and at low temperatures. Even when the entire acidic domain is removed, no effects on RNA editing were observed. By contrast, loss of the acidic domain reduced the ability of CP31A to stabilize the ndhF mRNA, which was associated with reduced NDH complex activity. Most importantly, acidic domain-less CP31A lines displayed bleached young tissue in the cold. Together, these data show that the different functions of CP31A can be assigned to different regions of the protein: the RRMs are sufficient to maintain RNA editing and to allow the accumulation of basal amounts of ndhF mRNA, while chloroplast development under cold conditions critically depends on the acidic domain.

Evolution of an assembly factor-based subunit contributed to a novel NDH-PSI supercomplex formation in chloroplasts

Chloroplast NADH dehydrogenase-like (NDH) complex is structurally related to mitochondrial Complex I and forms a supercomplex with two copies of Photosystem I (the NDH-PSI supercomplex) via linker proteins Lhca5 and Lhca6. The latter was acquired relatively recently in a common ancestor of angiosperms. Here we show that NDH-dependent Cyclic Electron Flow 5 (NDF5) is an NDH assembly factor in Arabidopsis. NDF5 initiates the assembly of NDH subunits (PnsB2 and PnsB3) and Lhca6, suggesting that they form a contact site with Lhca6. Our analysis of the NDF5 ortholog in Physcomitrella and angiosperm genomes reveals the subunit PnsB2 to be newly acquired via tandem gene duplication of NDF5 at some point in the evolution of angiosperms. Another Lhca6 contact subunit, PnsB3, has evolved from a protein unrelated to NDH. The structure of the largest photosynthetic electron transport chain complex has become more complicated by acquiring novel subunits and supercomplex formation with PSI.

The chloroplast NDH complex interacts with Photosystem I to form the NDH-PSI supercomplex. Here the authors show that Arabidopsis NDF5 shares a common ancestor with the NDH subunit PnsB2 and acts as an NDH assembly factor initiating the assembly of PnsB2 and the evolutionarily distinct PnsB3.

Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.

Stepwise evolution of supercomplex formation with photosystem I is required for stabilization of chloroplast NADH dehydrogenase-like complex: Lhca5-dependent supercomplex formation in Physcomitrella patens

In angiosperms, such as Arabidopsis and barley, the chloroplast NADH dehydrogenase-like (NDH) complex associates with two copies of photosystem I (PSI) supercomplex to form an NDH-PSI supercomplex for the stabilization of the NDH complex. Two linker proteins, Lhca5 and Lhca6, are members of the light-harvesting complex I (LHCI) family and mediate this supercomplex formation. The liverwort Marchantia polymorpha has branched from the basal land plant lineage and has neither Lhca5 nor Lhca6. Consequently, the NDH complex does not form a supercomplex with PSI in this plant. The Lhca6 gene does not seem to exist also in the moss Physcomitrella patens (Physcomitrella). Conversely, the Lhca5 gene has been found in Physcomitrella, although experimental evidence is still lacking for its contribution to NDH-PSI supercomplex formation as a linker. Here, we biochemically characterized the Lhca5 knock-out mutant (lhca5) in Physcomitrella. The NDH-PSI supercomplex observed in wild-type Physcomitrella was absent in the lhca5 mutant. Lhca5 protein was detected in this NDH-PSI supercomplex. Some PSI and NDH subunits were co-immunoprecipitated with Lhca5-HA. These results indicate that the Physcomitrella gene is the functional ortholog of Lhca5 reported in Arabidopsis. Between Physcomitrella and Arabidopsis, the stromal loop region is highly conserved in Lhca5 proteins but not in other LHCI members. We found that Lhca5 contributed to the stable accumulation of the NDH complex, but part of the NDH complex was still sensitive to high light intensity, even in the wild-type. We considered that angiosperms acquired another linker protein, Lhca6, to further stabilize the NDH complex.

Specific substitutions of light-harvesting complex I proteins associated with photosystem I are required for supercomplex formation with chloroplast NADH dehydrogenase-like complex

In Arabidopsis, the chloroplast NADH-dehydrogenase-like (NDH) complex is sandwiched between two copies of photosystem I (PSI) supercomplex, consisting of a PSI core and four light-harvesting complex I (LHCI) proteins (PSI-LHCI) to form the NDH-PSI supercomplex. Two minor LHCI proteins, Lhca5 and Lhca6, contribute to the interaction of each PSI-LHCI copy with the NDH complex. Here, large-pore blue-native gel electrophoresis revealed that, in addition to this complex, there were at least two types of higher-order association of more LHCI copies with the NDH complex. In single-particle images, this higher-order association of PSI-LHCI preferentially occurs at the left side of the NDH complex when viewed from the stromal side, placing subcomplex A at the top (Yadav et al., Biochim. Biophys. Acta - Bioenerg., 1858, 2017, 12). The association was impaired in the lhca6 mutant but not in the lhca5 mutant, suggesting that the left copy of PSI-LHCI was linked to the NDH complex via Lhca6. From an analysis of subunit compositions of the NDH-PSI supercomplex in lhca5 and lhca6 mutants, we propose that Lhca6 substitutes for Lhca2 in the left copy of PSI-LHCI, whereas Lhca5 substitutes for Lhca4 in the right copy. In the lhca2 mutant, Lhca3 was specifically stabilized in the NDH-PSI supercomplex through heterodimer formation with Lhca6. In the left copy of PSI-LHCI, subcomplex B, Lhca6 and NdhD likely formed the core of the supercomplex interaction. In contrast, a larger protein complex, including at least subcomplexes B and L and NdhB, was needed to form the contact site with Lhca5 in the right copy of PSI-LHCI.