Plant Protochlorophyllide Oxidoreductases A and B: CATALYTIC EFFICIENCY AND INITIAL REACTION STEPS

Molecular insights into the differences between cyanobacterial and plant LPORs and prolamellar body formation: In vitro studies

Light-dependent protochlorophyllide oxidoreductase (LPOR) has captivated the interest of the research community for decades. One reason is the photocatalytic nature of the reaction catalyzed by the enzyme, and the other is the involvement of LPOR in the formation of a paracrystalline lattice called a prolamellar body (PLB) that disintegrates upon illumination, initiating a process of photosynthetic membrane formation. In this paper, we have integrated three traditional methods previously employed to study the properties of the enzyme: molecular biology, spectroscopy, and electron microscopy. We found that for cyanobacterial LPOR, substrates binding appears to be independent of lipids, with membrane interaction primarily affecting the enzyme post-reaction, with MGDG and PG having opposite effects on SynPOR. In contrast, plant isoforms exhibit sequence alterations, rendering the enzyme effective in substrate binding mainly in the presence of anionic lipids, depending on residues at positions 122, 312, and 318. Moreover, we demonstrated that the interaction with MGDG could initially serve as enhancement of the substrate specificity towards monovinyl-protochlorophyllide (Pchlide). We have shown that the second LPOR isoforms of eudicots and monocots accumulated mutations that made these variants less and more dependent on anionic lipids, respectively. Finally, we have shown that in the presence of Pchlide, NADP+, and the lipids, plant but not cyanobacterial LPOR homolog remodel membranes into the cubic phase. The cubic phase is preserved if samples supplemented with NADP + are enriched with NADPH. The results are discussed in the evolutionary context, and the model of PLB formation is presented.

Light dependent protochlorophyllide oxidoreductase: a succinct look

Reducing protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is a major regulatory step in the chlorophyll biosynthesis pathway. This reaction is catalyzed by light-dependent protochlorophyllide oxidoreductase (LPOR) in oxygenic phototrophs, particularly angiosperms. LPOR-NADPH and Pchlide form a ternary complex to be efficiently photo-transformed to synthesize Chlide and, subsequently, chlorophyll during the transition from skotomorphogenesis to photomorphogenesis. Besides lipids, carotenoids and poly-cis xanthophylls influence the formation of the photoactive LPOR complexes and the PLBs. The crystal structure of LPOR reveals evolutionarily conserved cysteine residues implicated in the Pchlide binding and catalysis around the active site. Different isoforms of LPOR viz PORA, PORB, and PORC expressed at different stages of chloroplast development play a photoprotective role by quickly transforming the photosensitive Pchlide to Chlide. Non-photo-transformed Pchlide acts as a photosensitizer to generate singlet oxygen that causes oxidative stress and cell death. Therefore, different isoforms of LPOR have evolved and differentially expressed during plant development to protect plants from photodamage and thus play a pivotal role during photomorphogenesis. This review brings out the salient features of LPOR structure, structure-function relationships, and ultra-fast photo transformation of Pchlide to Chlide by oligomeric and polymeric forms of LPOR.

Genome-wide association of dry (Tamar) date palm fruit color

Date palm (Phoenix dactylifera) fruit (dates) are an economically and culturally significant crop in the Middle East and North Africa. There are hundreds of different commercial cultivars producing dates with distinctive shapes, colors, and sizes. Genetic studies of some date palm traits have been performed, including sex determination, sugar content, and fresh fruit color. In this study, we used genome sequences and image data of 199 dry dates (Tamar) collected from 14 countries to identify genetic loci associated with the color of this fruit stage. Here, we find loci across multiple linkage groups (LG) associated with dry fruit color phenotype. We recover both the previously identified VIRESCENS (VIR) genotype associated with fresh fruit yellow or red color and new associations with the lightness and darkness of dry fruit. This study will add resolution to our understanding of date color phenotype, especially at the most commercially important Tamar stage.

Phosphorus Acquisition Efficiency and Transcriptomic Changes in Maize Plants Treated with Two Lignohumates

Lignohumates are increasing in popularity in agriculture, but their chemistry and effects on plants vary based on the source and processing. The present study evaluated the ability of two humates (H1 and H2) to boost maize plant performance under different phosphorus (P) availability (25 and 250 μM) conditions in hydroponics, while understanding the underlying mechanisms. Humates differed in chemical composition, as revealed via elemental analysis, phenol and phytohormone content, and thermal and spectroscopic analyses. H1 outperformed H2 in triggering plant responses to low phosphorus by enhancing phosphatase and phytase enzymes, P acquisition efficiency, and biomass production. It contained higher levels of endogenous auxins, cytokinins, and abscisic acid, likely acting together to stimulate plant growth. H1 also improved the plant antioxidant capacity, thus potentially increasing plant resilience to external stresses. Both humates increased the nitrogen (N) content and acted as biostimulants for P and N acquisition. Consistent with the physiological and biochemical data, H1 upregulated genes involved in growth, hormone signaling and defense in all plants, and in P recycling particularly under low-P conditions. In conclusion, H1 showed promising potential for effective plant growth and nutrient utilization, especially in low-P plants, involving hormonal modulation, antioxidant enhancement, the stimulation of P uptake and P-recycling mechanisms.

Mutation in a chlorophyll-binding motif of Brassica ferrochelatase enhances both heme and chlorophyll biosynthesis

The heme branch of tetrapyrrole biosynthesis contributes to the regulation of chlorophyll levels. However, the mechanism underlying the balance between chlorophyll and heme synthesis remains elusive. Here, we identify a dark green leaf mutant, dg, from an ethyl methanesulfonate (EMS)-induced mutant library of Chinese cabbage. The dg phenotype is caused by an amino acid substitution in the conserved chlorophyll a/b-binding motif (CAB) of ferrochelatase 2 (BrFC2). This mutation increases the formation of BrFC2 homodimer to promote heme production. Moreover, wild-type BrFC2 and dBrFC2 interact with protochlorophyllide (Pchlide) oxidoreductase B1 and B2 (BrPORB1 and BrPORB2), and dBrFC2 exhibits higher binding ability to substrate Pchlide, thereby promoting BrPORBs-catalyzed production of chlorophyllide (Chlide), which can be directly converted into chlorophyll. Our results show that dBrFC2 is a gain-of-function mutation contributing to balancing heme and chlorophyll synthesis via a regulatory mechanism in which dBrFC2 promotes BrPORB enzymatic reaction to enhance chlorophyll synthesis.

Mechanism Underlying the Shading-Induced Chlorophyll Accumulation in Tea Leaves

Besides aroma and taste, the color of dry tea leaves, tea infusion, and infused tea leaves is also an important index for tea quality. Shading can significantly increase the chlorophyll content of tea leaves, leading to enhanced tea leaf coloration. However, the underlying regulatory mechanism remains unclear. In this study, we revealed that the expressions of chlorophyll synthesis genes were significantly induced by shading, specially, the gene encoding protochlorophyllide oxidoreductase (CsPOR). Indoor control experiment showed that decreased light intensity could significantly induce the expression of CsPOR, and thus cause the increase of chlorophyll content. Subsequently, we explored the light signaling pathway transcription factors regulating chlorophyll synthesis, including CsPIFs and CsHY5. Through expression level and subcellular localization analysis, we found that CsPIF3-2, CsPIF7-1, and CsHY5 may be candidate transcriptional regulators. Transcriptional activation experiments proved that CsHY5 inhibits CsPORL-2 transcription. In summary, we concluded that shading might promote the expression of CsPORL-2 by inhibiting the expression of CsHY5, leading to high accumulation of chlorophyll in tea leaves. The results of this study provide insights into the mechanism regulating the improvements to tea plant quality caused by shading.

A Label-Free Proteomic and Complementary Metabolomic Analysis of Leaves of the Resurrection Plant Xerophytaschlechteri during Dehydration

Vegetative desiccation tolerance, or the ability to survive the loss of ~95% relative water content (RWC), is rare in angiosperms, with these being commonly called resurrection plants. It is a complex multigenic and multi-factorial trait, with its understanding requiring a comprehensive systems biology approach. The aim of the current study was to conduct a label-free proteomic analysis of leaves of the resurrection plant Xerophyta schlechteri in response to desiccation. A targeted metabolomics approach was validated and correlated to the proteomics, contributing the missing link in studies on this species. Three physiological stages were identified: an early response to drying, during which the leaf tissues declined from full turgor to a RWC of ~80–70%, a mid-response in which the RWC declined to 40% and a late response where the tissues declined to 10% RWC. We identified 517 distinct proteins that were differentially expressed, of which 253 proteins were upregulated and 264 were downregulated in response to the three drying stages. Metabolomics analyses, which included monitoring the levels of a selection of phytohormones, amino acids, sugars, sugar alcohols, fatty acids and organic acids in response to dehydration, correlated with some of the proteomic differences, giving insight into the biological processes apparently involved in desiccation tolerance in this species.

Chlorophyllides: Preparation, Purification, and Application

Chlorophyllides can be found in photosynthetic organisms. Generally, chlorophyllides have a-, b-, c-, d-, and f-type derivatives, and all chlorophyllides have a tetrapyrrole structure with a Mg ion at the center and a fifth isocyclic pentanone. Chlorophyllide a can be synthesized from protochlorophyllide a, divinyl chlorophyllide a, or chlorophyll. In addition, chlorophyllide a can be transformed into chlorophyllide b, chlorophyllide d, or chlorophyllide f. Chlorophyllide c can be synthesized from protochlorophyllide a or divinyl protochlorophyllide a. Chlorophyllides have been extensively used in food, medicine, and pharmaceutical applications. Furthermore, chlorophyllides exhibit many biological activities, such as anti-growth, antimicrobial, antiviral, antipathogenic, and antiproliferative activity. The photosensitivity of chlorophyllides that is applied in mercury electrodes and sensors were discussed. This article is the first detailed review dedicated specifically to chlorophyllides. Thus, this review aims to describe the definition of chlorophyllides, biosynthetic routes of chlorophyllides, purification of chlorophyllides, and applications of chlorophyllides.

The Role of Membranes and Lipid-Protein Interactions in the Mg-Branch of Tetrapyrrole Biosynthesis

Chlorophyll (Chl) is essential for photosynthesis and needs to be produced throughout the whole plant life, especially under changing light intensity and stress conditions which may result in the destruction and elimination of these pigments. All steps of the Mg-branch of tetrapyrrole biosynthesis leading to Chl formation are carried out by enzymes associated with plastid membranes. Still the significance of these protein-membrane and protein-lipid interactions in Chl synthesis and chloroplast differentiation are not very well-understood. In this review, we provide an overview on Chl biosynthesis in angiosperms with emphasis on its association with membranes and lipids. Moreover, the last steps of the pathway including the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the biosynthesis of the isoprenoid phytyl moiety and the esterification of Chlide are also summarized. The unique biochemical and photophysical properties of the light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) enzyme catalyzing Pchlide photoreduction and located to peculiar tubuloreticular prolamellar body (PLB) membranes of light-deprived tissues of angiosperms and to envelope membranes, as well as to thylakoids (especially grana margins) are also reviewed. Data about the factors influencing tubuloreticular membrane formation within cells, the spectroscopic properties and the in vitro reconstitution of the native LPOR enzyme complexes are also critically discussed.

Photocatalysis as the 'master switch' of photomorphogenesis in early plant development

Enzymatic photocatalysis is seldom used in biology. Photocatalysis by light-dependent protochlorophyllide oxidoreductase (LPOR)-one of only a few natural light-dependent enzymes-is an exception, and is responsible for the conversion of protochlorophyllide to chlorophyllide in chlorophyll biosynthesis. Photocatalysis by LPOR not only regulates the biosynthesis of the most abundant pigment on Earth but it is also a 'master switch' in photomorphogenesis in early plant development. Following illumination, LPOR promotes chlorophyll production, plastid membranes are transformed and the photosynthetic apparatus is established. Given these remarkable, light-induced pigment and morphological changes, the LPOR-catalysed reaction has been extensively studied from catalytic, physiological and plant development perspectives, highlighting vital, and multiple, cellular roles of this intriguing enzyme. Here, we offer a perspective in which the link between LPOR photocatalysis and plant photomorphogenesis is explored. Notable breakthroughs in LPOR structural biology have uncovered the structural-mechanistic basis of photocatalysis. These studies have clarified how photon absorption by the pigment protochlorophyllide-bound in a ternary LPOR-protochlorophyllide-NADPH complex-triggers photocatalysis and a cascade of complex molecular and cellular events that lead to plant morphological changes. Photocatalysis is therefore the master switch responsible for early-stage plant development and ultimately life on Earth.

New insights into the response of maize to fluctuations in the light environment

Light is the most important environmental cue signaling the transition from skotomorphogenesis to photomorphogenesis, thus affecting plant development and metabolic activity. How the light response mechanisms of maize seedlings respond to fluctuations in the light environment has not been well characterized to date. In this study, we built a gene coexpression network from a dynamic transcriptomic map of maize seedlings exposed to different light environments. Coexpression analysis identified ten modules and multiple genes that closely correlate with photosynthesis and characterized hub genes associated with regulatory networks, duplication events, domestication and improvement. In addition, we identified that 38% of hub genes underwent duplication events, 74% of which are related to photosynthesis. Moreover, we captured the dynamic expression atlas of gene sets involved in the chloroplast photosynthetic apparatus and photosynthetic carbon assimilation in different light environments, which should help to elucidate the key mechanisms and regulatory networks that underlie photosynthesis in maize. Insights from this study provide a valuable resource to better understand the genetic mechanisms of the response to fluctuations in the light environment in maize.

Zucchini Plants Alter Gene Expression and Emission of (E)-β-Caryophyllene Following Aphis gossypii Infestation

Zucchini (Cucurbita pepo L.) is widely cultivated in temperate regions. One of the major production challenges is the damage caused by Aphis gossypii (Homoptera: Aphididae), a polyphagous aphid, which can negatively affect its host plant, both directly by feeding and indirectly by vectoring viruses. To gain insights into the transcriptome events that occur during the zucchini-aphid interaction and to understand the early-to-late defense response through gene expression profiles, we performed RNA-sequencing (RNA-Seq) on zucchini leaves challenged by A. gossypii (24, 48, and 96 h post-infestation; hpi). Data analysis indicated a complex and dynamic pattern of gene expression and a transient transcriptional reconfiguration that involved more than 700 differentially expressed genes (DEGs), including a large number of defense-related genes. The down-regulation of key genes of plant immunity, such as leucine-rich repeat (LRR) protein kinases, transcription factors, and genes associated with direct (i.e., protease inhibitors, cysteine peptidases, etc.) and indirect (i.e., terpene synthase) defense responses, suggests the aphid ability to manipulate plant immune responses. We also investigated the emission of volatile organic compounds (VOCs) from infested plants and observed a reduced emission of (E)-β-caryophyllene at 48 hpi, likely the result of aphid effectors, which reflects the down-regulation of two genes involved in the biosynthesis of terpenoids. We showed that (E)-β-caryophyllene emission was modified by the duration of plant infestation and by aphid density and that this molecule highly attracts Aphidius colemani, a parasitic wasp of A. gossypii. With our results we contributed to the identification of genes involved in cucurbit plant interactions with phloem feeders. Our findings may also help pave the way toward developing tolerant zucchini varieties and to identify molecules for sustainable management of harmful insect populations.

The origin, evolution and diversification of multiple isoforms of light-dependent protochlorophyllide oxidoreductase (LPOR): focus on angiosperms

Light-dependent protochlorophyllide oxidoreductase (LPOR) catalyzes the reduction of protochlorophyllide to chlorophyllide, which is a key reaction for angiosperm development. Dark operative light-independent protochlorophyllide oxidoreductase (DPOR) is the other enzyme able to catalyze this reaction, however, it is not present in angiosperms. LPOR, which evolved later than DPOR, requires light to trigger the reaction. The ancestors of angiosperms lost DPOR genes and duplicated the LPORs, however, the LPOR evolution in angiosperms has not been yet investigated. In the present study, we built a phylogenetic tree using 557 nucleotide sequences of LPORs from both bacteria and plants to uncover the evolution of LPOR. The tree revealed that all modern sequences of LPOR diverged from a single sequence ∼1.36 billion years ago. The LPOR gene was then duplicated at least 10 times in angiosperms, leading to the formation of two or even more LPOR isoforms in multiple species. In the case of Arabidopsis thaliana, AtPORA and AtPORB originated in one duplication event, in contrary to the isoform AtPORC, which diverged first. We performed biochemical characterization of these isoforms in vitro, revealing differences in the lipid-driven properties. The results prone us to hypothesize that duplication events of LPOR gave rise to the isoforms having different lipid-driven activity, which may predispose them for functioning in different locations in plastids. Moreover, we showed that LPOR from Synechocystis operated in the lipid-independent manner, revealing differences between bacterial and plant LPORs. Based on the presented results, we propose a novel classification of LPOR enzymes based on their biochemical properties and phylogenetic relationships.

SCARECROW gene function is required for photosynthetic development in maize

C4 photosynthesis in grasses relies on a specialized leaf anatomy. In maize, this "Kranz" leaf anatomy is patterned in part by the duplicated SCARECROW (SCR) genes ZmSCR1 and ZmSCR1h. Here we show that in addition to patterning defects, chlorophyll content and levels of transcripts encoding Golden2-like regulators of chloroplast development are significantly lower in Zmscr1; Zmscr1h mutants than in wild-type. These perturbations are not associated with changes in chloroplast number, size, or ultrastructure. However, the maximum rates of carboxylation by ribulose bisphosphate carboxylase/oxygenase (RuBisCO, V cmax) and phosphoenolpyruvate carboxylase (PEPC, V pmax) are both reduced, leading to perturbed plant growth. The CO2 compensation point and 13C‰ of Zmscr1;Zmscr1h plants are both normal, indicating that a canonical C4 cycle is operating, albeit at reduced overall capacity. Taken together, our results reveal that the maize SCR genes, either directly or indirectly, play a role in photosynthetic development.

SIGNIFICANCE STATEMENT

SCARECROW (SCR) is one of the best studied plant developmental regulators, however, its role in downstream plant physiology is less well-understood. Here, we have demonstrated that SCR is required to establish and/or maintain photosynthetic capacity in maize leaves.

Crystal structures of cyanobacterial light-dependent protochlorophyllide oxidoreductase

The reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is the penultimate step of chlorophyll biosynthesis. In oxygenic photosynthetic bacteria, algae, and plants, this reaction can be catalyzed by the light-dependent Pchlide oxidoreductase (LPOR), a member of the short-chain dehydrogenase superfamily sharing a conserved Rossmann fold for NAD(P)H binding and the catalytic activity. Whereas modeling and simulation approaches have been used to study the catalytic mechanism of this light-driven reaction, key details of the LPOR structure remain unclear. We determined the crystal structures of LPOR from two cyanobacteria, Synechocystis sp. PCC 6803 and Thermosynechococcus elongatus Structural analysis defines the LPOR core fold, outlines the LPOR-NADPH interaction network, identifies the residues forming the substrate cavity and the proton-relay path, and reveals the role of the LPOR-specific loop. These findings provide a basis for understanding the structure-function relationships of the light-driven Pchlide reduction.

Comparative Proteomic Analysis Reveals That Chlorophyll Metabolism Contributes to Leaf Color Changes in Wucai ( Brassica campestris L.) Responding to Cold Acclimation

Chlorophyll is a vital photosynthetic pigment that plays a key role in plant development, participating in light energy capture and energy conversion. In this study, a novel wucai ( Brassica campestris L.) germplasm with green outer leaves and yellow inner leaves at the adult stage (W7-2) was used to examine chlorophyll metabolism response to cold acclimation. A green leaf wucai genotype without leaf color changes named W7-1 was selected as the control to evaluate the chlorophyll metabolism changes of W7-2. Compared to W7-1, the contents of chlorophyll a (Chl a) and chlorophyll b (Chl b) in W7-2 were significantly reduced at five developmental stages (13, 21, 29, 37, and 45 days after planting (DAP)). An iTRAQ-based quantitative proteomic analysis was carried out at 21 and 29 DAP according to the leaf color changes in both of genotypes. 1409 proteins were identified, while 218 of them displayed differential accumulations between W7-2 and W7-1 during the two developmental stages. The differentially expressed proteins (DEPs) mainly assigned to chlorophyll biosynthesis, photosynthesis, carbohydrate metabolism, ribosome metabolism and posttranslational modification. Among these DEPs, NADPH-protochlorophyllide oxidoreductase (PORB) and Mg-protoporphyrin IX chelatase 1 (CHLI1) were the key enzymes participating in chlorophyll (Chl) biosynthesis, which was down-regulated at 21 DAP and up-regulated at 29 DAP in W7-2 compared with W7-1, respectively. The expression analysis of genes of three subunits of Mg-chelatase ( CHLI1, CHLD, and CHLH), Genomes Uncoupled 4 ( GUN4), and Thioredoxin ( TRX3) associated with chlorophyll metabolism also displayed significant down-regulation in W7-2. In particular, PORB showed significant up-regulation in W7-2, significantly affecting chlorophyll biosynthesis. Additionally, differences in chlorophyll metabolism between W7-2 and W7-1 were in terms of altered photosynthesis, carbohydrate, and energy metabolism. We found that the transcription levels of most photosynthesis proteins showed significantly lower levels, and the genes expression level, associated with carbohydrate and energy metabolism, were lower in W7-2 than in W7-1. Therefore, the present study results help understand the physiological and molecular mechanisms underlying leaf coloring responding to cold acclimation.

Theoretical Model of the Protochlorophyllide Oxidoreductase from a Hierarchy of Protocols

The enzyme protochlorophyllide oxidoreductase (LPOR) catalyzes the light-driven reduction of protochlorophyllide (Pchlide), a crucial step in chlorophyll biosynthesis. Molecular understanding of the photocatalytic mechanism of LPOR is essential for harnessing light energy to mediate enzymatic reactions. The absence of X-ray crystal structure has promoted the development of LPOR homology models that lack a catalytically competent active site and could not explain the variously reported spectroscopic evidence, including time-resolved optical spectroscopy data. We have refined previous structural models to account for the catalytic active site and the characteristic experimental spectral features of Pchlide binding, including the 26 cm^-1 red shift of the C₁₃₍₁₎ carbonyl stretch vibration in the mid-infrared (IR) and the 12 nm red shift of the Q _x electronic band. A hierarchy of theoretical methods, including homology modeling, molecular dynamics simulations, hybrid quantum mechanics [(TD-)DFT]/molecular mechanics [AMBER] calculations, and computational vibrational and electronic spectroscopies, have been combined in an iterative protocol to reproduce experimental evidence and to predict ultrafast transient IR spectroscopic fingerprints associated with the catalytic process. The successful application to the LPOR enzyme indicates that the presented hierarchical protocol provides a general workflow to protein structure refinement.

NADPH:protochlorophyllide oxidoreductase B (PORB) action in Arabidopsis thaliana revisited through transgenic expression of engineered barley PORB mutant proteins

NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme for the light-induced greening of etiolated angiosperm plants. It belongs to the 'RED' family of reductases, epimerases and dehydrogenases. All POR proteins characterized so far contain evolutionarily conserved cysteine residues implicated in protochlorophyllide (Pchlide)-binding and catalysis. cDNAs were constructed by site-directed mutagenesis that encode PORB mutant proteins with defined Cys→Ala exchanges. These cDNAs were expressed in transgenic plants of a PORB-deficient knock-out mutant (porB) of Arabidopsis thaliana. Results show that porB plants expressing PORB mutant proteins with Ala substitutions of Cys276 or Cys303 are hypersensitive to high-light conditions during greening. Hereby, failure to assemble higher molecular weight complexes of PORB with its twin isoenzyme, PORA, as encountered with (Cys303→Ala)-PORB plants, caused more severe effects than replacing Cys276 by an Ala residue in the active site of the enzyme, as encountered in (Cys276→Ala)-PORB plants. Our results are consistent with the presence of two distinct pigment binding sites in PORB, with Cys276 establishing the active site of the enzyme and Cys303 providing a second, low affinity pigment binding site that is essential for the assembly of higher molecular mass light-harvesting PORB::PORA complexes and photoprotection of etiolated seedlings. Failure to assemble such complexes provoked photodynamic damage through the generation of singlet oxygen. Together, our data highlight the importance of PORB for Pchlide homoeostasis and greening in Arabidopsis.

Transcriptional and post-translational control of chlorophyll biosynthesis by dark-operative protochlorophyllide oxidoreductase in Norway spruce

Unlike angiosperms, gymnosperms use two different enzymes for the reduction of protochlorophyllide to chlorophyllide: the light-dependent protochlorophyllide oxidoreductase (LPOR) and the dark-operative protochlorophyllide oxidoreductase (DPOR). In this study, we examined the specific role of both enzymes for chlorophyll synthesis in response to different light/dark and temperature conditions at different developmental stages (cotyledons and needles) of Norway spruce (Picea abies Karst.). The accumulation of chlorophyll and chlorophyll-binding proteins strongly decreased during dark growth in secondary needles at room temperature as well as in cotyledons at low temperature (7 °C) indicating suppression of DPOR activity. The levels of the three DPOR subunits ChlL, ChlN, and ChlB and the transcripts of their encoding genes were diminished in dark-grown secondary needles. The low temperature had minor effects on the transcription and translation of these genes in cotyledons, which is suggestive for post-translational control in chlorophyll biosynthesis. Taking into account the higher solubility of oxygen at low temperature and oxygen sensitivity of DPOR, we mimicked low-temperature condition by the exposure of seedlings to higher oxygen content (33%). The treatment resulted in an etiolated phenotype of dark-grown seedlings, confirming an oxygen-dependent control of DPOR activity in spruce cotyledons. Moreover, light-dependent suppression of mRNA and protein level of DPOR subunits indicates that more efficiently operating LPOR takes over the DPOR function under light conditions, especially in secondary needles.

Vertebrate Cryptochromes are Vestigial Flavoproteins

All cryptochromes are currently classified as flavoproteins. In animals their best-described role is as components of the circadian clock. This circadian function is variable, and can be either light-dependent or -independent; the molecular origin of this difference is unknown. Type I animal cryptochromes are photoreceptors that entrain an organism's clock to its environment, whereas Type II (including mammals) regulate circadian timing in a light-independent manner. Here, we reveal that, in contrast to Type I, Type II animal cryptochromes lack the structural features to securely bind the photoactive flavin cofactor. We provide a molecular basis for the distinct circadian roles of different animal cryptochromes, which also has significant implications for the putative role of Type II cryptochromes in animal photomagnetoreception.

Multiple active site residues are important for photochemical efficiency in the light-activated enzyme protochlorophyllide oxidoreductase (POR)

Protochlorophyllide oxidoreductase (POR) catalyzes the light-driven reduction of protochlorophyllide (Pchlide), an essential, regulatory step in chlorophyll biosynthesis. The unique requirement of the enzyme for light has provided the opportunity to investigate how light energy can be harnessed to power biological catalysis and enzyme dynamics. Excited state interactions between the Pchlide molecule and the protein are known to drive the subsequent reaction chemistry. However, the structural features of POR and active site residues that are important for photochemistry and catalysis are currently unknown, because there is no crystal structure for POR. Here, we have used static and time-resolved spectroscopic measurements of a number of active site variants to study the role of a number of residues, which are located in the proposed NADPH/Pchlide binding site based on previous homology models, in the reaction mechanism of POR. Our findings, which are interpreted in the context of a new improved structural model, have identified several residues that are predicted to interact with the coenzyme or substrate. Several of the POR variants have a profound effect on the photochemistry, suggesting that multiple residues are important in stabilizing the excited state required for catalysis. Our work offers insight into how the POR active site geometry is finely tuned by multiple active site residues to support enzyme-mediated photochemistry and reduction of Pchlide, both of which are crucial to the existence of life on Earth.

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Identified several active site residues that can interact with coenzyme/substrate

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Multiple residues are important in excited state POR–protochlorophyllide interactions.

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New structural model for T. elongatus POR to rationalize mutagenesis outcomes

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POR active site geometry is finely-tuned to support photochemistry.