Breakdown of Chlorophyll in Higher Plants--Phyllobilins as Abundant, Yet Hardly Visible Signs of Ripening, Senescence, and Cell Death

Novel Types of Phyllobilins in a Fern - Molecular Reporters of the Evolution of Chlorophyll Breakdown in the Paleozoic Era

Breakdown of chlorophyll (Chl), as studied in angiosperms, follows the pheophorbide a oxygenase/phyllobilin (PaO/PB) pathway, furnishing linear tetrapyrroles, named phyllobilins (PBs). In an investigation with fern leaves we have discovered iso-phyllobilanones (iPBs) with an intriguingly rearranged and oxidized carbon skeleton. We report here a key second group of iPBs from the fern and on their structure analysis. Previously, these additional Chl-catabolites escaped their characterization, since they exist in aqueous media as mixtures of equilibrating isomers. However, their chemical dehydration furnished stable iPB-derivatives that allowed the delineation of the enigmatic structures and chemistry of the original natural catabolites. The structures of all fern-iPBs reflect the early core steps of a PaO/PB-type pathway and the PB-to-iPB carbon skeleton rearrangement. A striking further degradative chemical ring-cleavage was observed, proposed to consume singlet molecular oxygen (1O2). Hence, Chl-catabolites may play a novel active role in detoxifying cellular 1O2. The critical deviations from the PaO/PB pathway, found in the fern, reflect evolutionary developments of Chl-breakdown in the green plants in the Paleozoic era.

Impact of Two Brown Seaweed (Ascophyllum nodosum L.) Biostimulants on the Quantity and Quality of Yield in Cucumber (Cucumis sativus L.)

Algal biostimulants are increasingly integral to vegetable cultivation due to their capacity to boost yield, alleviate abiotic and biotic stress, and enhance overall crop quality. This study evaluated the impact of two commercially available algal-based biostimulants on cucumber (Cucumis sativus L.), examining their effects on yield, number of fruits, dry weight, color, flesh thickness, skin thickness, plastid pigments, and tocopherol content. Both biostimulant treatments resulted in a roughly 13% decrease in yield and fruit number compared to the control treatment. Notably, the biostimulants positively influenced the fruit brightness parameter (L*), leading to darker fruits. Fitostim® algal biostimulant exhibited a positive effect on dry weight during the initial harvest. The predominant pigments were chlorophyll a and chlorophyll b (constituting 80% of all analyzed pigments), and the most abundant tocopherol was α-tocopherol, comprising 80% to 90% of tocopherols. Skin tissues contained significantly higher levels of pigments and tocopherols compared to flesh. Both biostimulants caused a notable decrease in total tocopherol content in the skin at the first harvest, with reductions of 19.91 mg/kg DW for Phylgreen® and 9.43 mg/kg DW for Fitostim® algae. The study underscores the variable efficacy of biostimulants, emphasizing their dependence on the specific biostimulant type and fruit part. The application of biostimulants has the potential to substantially enhance the internal quality of cucumbers, particularly in terms of plastid pigments and tocopherols, offering potential health benefits for consumers.

Phyllobilins - Bioactive Natural Products Derived from Chlorophyll - Plant Origins, Structures, Absorption Spectra, and Biomedical Properties

Phyllobilins are open-chain products of the biological degradation of chlorophyll a in higher plants. Recent studies reveal that phyllobilins exert anti-oxidative and anti-inflammatory properties, as well as activities against cancer cells, that contribute to the human health benefits of numerous plants. In general, phyllobilins have been overlooked in phytochemical analyses, and - more importantly - in the analyses of medicinal plant extracts. Nevertheless, over the past three decades, > 70 phyllobilins have been identified upon examination of more than 30 plant species. Eight distinct chromophoric classes of phyllobilins are known: phyllolumibilins (PluBs), phylloleucobilins (PleBs), phylloxanthobilins (PxBs), and phylloroseobilins (PrBs)-each in type-I or type-II groups. Here, we present a database of absorption and fluorescence spectra that has been compiled of 73 phyllobilins to facilitate identification in phytochemical analyses. The spectra are provided in digital form and can be viewed and downloaded at www.photochemcad.com. The present review describes the plant origin, molecular structure, and absorption and fluorescence features of the 73 phyllobilins, along with an overview of key medicinal properties. The review should provide an enabling tool for the community for the straightforward identification of phyllobilins in plant extracts, and the foundation for deeper understanding of these ubiquitous but underexamined plant-derived micronutrients for human health.

The advent of phyllobilins as bioactive phytochemicals – natural compounds derived from chlorophyll in medicinal plants and food with immunomodulatory activities

The degradation of the green plant pigment chlorophyll has fascinated chemists and biologists alike over the last few decades. Bioactivities of the compounds formed in this biochemical degradation pathway, however, have only come to light recently. These natural compounds that are formed from chlorophyll during plant senescence are now called phyllobilins. In this review, we shortly discuss chlorophyll degradation and outline the so-far known bioactivities of selected phyllobilins (phylloleucobilin, dioxobilin-type phylloleucobilin, and phylloxanthobilin), and we also highlight the recently discovered immunomodulatory effects of a yellow phylloxanthobilin.

Poplar leaf abscission through induced chlorophyll breakdown by Mg-dechelatase

Chlorophyll breakdown is observed during senescence. The first step in chlorophyll breakdown is the removal of central Mg by Mg-dechelatase. This reaction is the rate-limiting step in the chlorophyll breakdown pathway. We evaluated the effect of induced chlorophyll breakdown on abscission through the removal of Mg by Mg-dechelatase. Poplar transformants carrying the dexamethasone-inducible Mg-dechelatase gene were prepared using the Arabidopsis Stay-Green1 cDNA. When leaves were treated with dexamethasone, chlorophyll was degraded, photosynthetic capacity was reduced, and an abscission zone was formed, resulting in leaf abscission. In addition, ethylene, which plays an important role during senescence, was produced in this process. Thus, chlorophyll breakdown induces the phenotype in the same way as commonly observed during leaf senescence. This study suggests a physiological role of chlorophyll breakdown in the leaf abscission of deciduous trees. Furthermore, this study shows that the dexamethasone-inducible gene expression system is an available option for deciduous tree studies.

A Chlorophyll-Derived Phylloxanthobilin Is a Potent Antioxidant That Modulates Immunometabolism in Human PBMC

Phyllobilins are natural products derived from the degradation of chlorophyll, which proceeds via a common and strictly controlled pathway in higher plants. The resulting tetrapyrrolic catabolites—the phyllobilins—are ubiquitous in nature; despite their high abundance, there is still a lack of knowledge about their physiological properties. Phyllobilins are part of human nutrition and were shown to be potent antioxidants accounting with interesting physiological properties. Three different naturally occurring types of phyllobilins—a phylloleucobilin, a dioxobilin-type phylloleucobilin and a phylloxanthobilin (PxB)—were compared regarding potential antioxidative properties in a cell-free and in a cell-based antioxidant activity test system, demonstrating the strongest effect for the PxB. Moreover, the PxB was investigated for its capacity to interfere with immunoregulatory metabolic pathways of tryptophan breakdown in human blood peripheral mononuclear cells. A dose-dependent inhibition of tryptophan catabolism to kynurenine was observed, suggesting a suppressive effect on pathways of cellular immune activation. Although the exact mechanisms of immunomodulatory effects are yet unknown, these prominent bioactivities point towards health-relevant effects, which warrant further mechanistic investigations and the assessment of the in vivo extrapolatability of results. Thus, phyllobilins are a still surprisingly unexplored family of natural products that merit further investigation.

Differential Expression of Genes between a Tolerant and a Susceptible Maize Line in Response to a Sugarcane Mosaic Virus Infection

To uncover novel genes associated with the Sugarcane mosaic virus (SCMV) response, we used RNA-Seq data to analyze differentially expressed genes (DEGs) and transcript expression pattern clusters between a tolerant/resistant (CI-RL1) and a susceptible (B73) line, in addition to the F1 progeny (CI-RL1xB73). A Gene Ontology (GO) enrichment of DEGs led us to propose three genes possibly associated with the CI-RL1 response: a heat shock 90-2 protein and two ABC transporters. Through a clustering analysis of the transcript expression patterns (CTEPs), we identified two genes putatively involved in viral systemic spread: the maize homologs to the PIEZO channel (ZmPiezo) and to the Potyvirus VPg Interacting Protein 1 (ZmPVIP1). We also observed the complex behavior of the maize eukaryotic factors ZmeIF4E and Zm-elfa (involved in translation), homologs to eIF4E and eEF1α in A. thaliana. Together, the DEG and CTEPs results lead us to suggest that the tolerant/resistant CI-RL1 response to the SCMV encompasses the action of diverse genes and, for the first time, that maize translation factors are associated with viral interaction.

Side-chain modifications of phyllobilins may not be essential for chlorophyll degradation in Arabidopsis

Disposing efficiently and safely chlorophyll derivatives during senescence requires a coordinated pathway that is well conserved throughout green plants. The PAO/phyllobilin pathway catalyzes the degradation of the chlorophyll during senescence and allows detoxification of the pigment and its subsequent export from the chloroplast. Although most of the chloroplastic reactions involved in chlorophyll degradation are well understood, the diversity of enzymes responsible for downstream modifications of non-phototoxic phyllobilins remains to be explored. More than 40 phyllobilins have been described to date, but only three enzymes catalyzing side-chain reactions have been identified in Arabidopsis thaliana, namely, TIC55, CYP89A9, and MES16. Here, by generating a triple mutant, we evaluate the extent to which these enzymes are influencing the rate and amplitude of chlorophyll degradation at the metabolite as well as its regulation at the transcriptome level. Our data show that major side-chain modifications of phyllobilins do not influence significantly chlorophyll degradation or leaf senescence, letting the physiological relevance of their striking diversity an open question.

Overexpression of SlPRE5, an atypical bHLH transcription factor, affects plant morphology and chlorophyll accumulation in tomato

The basic helix-loop-helix (bHLH) transcription factors play vital regulatory roles in a series of metabolic, physiological, and developmental processes of plants. Here, SlPRE5, an atypical bHLH gene, was isolated from tomato. SlPRE5 was noticeably expressed in young leaves, sepals, and flowers. SlPRE5-overexpressing plants exhibited rolling leaves with reduced chlorophyll content, increased stem internode length, leaf angle, and compound leaf length. The water loss rate of mature leaves and the content of starch were significantly reduced, while the content of gibberellin was significantly increased in transgenic plants. Yeast two-hybrid and bimolecular fluorescence complementation (BiFC) showed that SlPRE5 could interact with SlAIF1, SlAIF2, and SlPAR1. qRT-PCR and RNA-seq results revealed that the expression levels of genes related to chloroplast development, chlorophyll metabolism, gibberellin metabolism and signal transduction, starch, photosynthesis, and cell expansion were significantly altered in SlPRE5-overexpression plants. Collectively, our results suggest that SlPRE5 is a crucial transcription factor involved in plant morphology and chlorophyll accumulation in tomato leaves.

Evolution of chlorophyll degradation is associated with plant transition to land

Chlorophyll, the central pigment of photosynthesis, is highly photo‐active and degraded enzymatically during leaf senescence. Merging comparative genomics and metabolomics, we evaluate the extent to which the chlorophyll detoxification pathway has evolved in Viridiplantae. We argue that cytosolic detoxification of phyllobilins in particular was a critical process to the green lineage’s transition to land.

Facile retro-Dieckmann cleavage of a pink phyllobilin: new type of potential downstream steps of natural chlorophyll breakdown

In senescent leaves of higher plants, colourless chlorophyll (Chl) catabolites typically accumulate temporarily, and undergo natural oxidation, in part, to yellow- and pink-coloured phyllobilins (PBs). The latter, also classified as phylloroseobilins (PrBs), represent the final currently established products of Chl-breakdown, possibly playing important roles in metabolism. However, PrBs, themselves, do not accumulate in the leaves. Indeed, the original PrB identified, then classified as a pink Chl-catabolite (PiCC), is remarkably instable in methanolic solution. As reported here, PiCC readily converts at room temperature into yellow tetrapyrroles. The deduced main process, a retro-Dieckmann reaction, cleaves open its ring E moiety, the α-methoxycarbonyl-cyclopentanone unit characteristic of the Chls and of the natural Chl-derived PBs. This readily occurring reaction of the PiCC represents an unprecedented skeletal transformation of a PB, furnishing a cross-conjugated biladiene with a basic structure more similar to the heme-derived bilins.

Graphical abstract

Chlorophylls: A Personal Snapshot

Chlorophylls provide the basis for photosynthesis and thereby most life on Earth. Besides their involvement in primary charge separation in the reaction center, they serve as light-harvesting and light-sensing pigments, they also have additional functions, e.g., in inter-system electron transfer. Chlorophylls also have a wealth of applications in basic science, medicine, as colorants and, possibly, in optoelectronics. Considering that there has been more than 200 years of chlorophyll research, one would think that all has been said on these pigments. However, the opposite is true: ongoing research evidenced in this Special Issue brings together current work on chlorophylls and on their carotenoid counterparts. These introductory notes give a very brief and in part personal account of the history of chlorophyll research and applications, before concluding with a snapshot of this year’s publications.

Research Progress in the Interconversion, Turnover and Degradation of Chlorophyll

Chlorophylls (Chls, Chl a and Chl b) are tetrapyrrole molecules essential for photosynthetic light harvesting and energy transduction in plants. Once formed, Chls are noncovalently bound to photosynthetic proteins on the thylakoid membrane. In contrast, they are dismantled from photosystems in response to environmental changes or developmental processes; thus, they undergo interconversion, turnover, and degradation. In the last twenty years, fruitful research progress has been achieved on these Chl metabolic processes. The discovery of new metabolic pathways has been accompanied by the identification of enzymes associated with biochemical steps. This article reviews recent progress in the analysis of the Chl cycle, turnover and degradation pathways and the involved enzymes. In addition, open questions regarding these pathways that require further investigation are also suggested.

Tetrapyrrolic Pigments from Heme- and Chlorophyll Breakdown are Actin-Targeting Compounds

Chlorophyll and heme are among the "pigments of life", tetrapyrrolic structures, without which life on Earth would not be possible. Their catabolites, the phyllobilins and the bilins, respectively, share not only structural features, but also a similar story: Long considered waste products of detoxification processes, important bioactivities for both classes have now been demonstrated. For phyllobilins, however, research on physiological roles is sparse. Here, we introduce actin, the major component of the cytoskeleton, as the first discovered target of phyllobilins and as a novel target of bilins. We demonstrate the inhibition of actin dynamics in vitro and effects on actin and related processes in cancer cells. A direct interaction with G-actin is shown by in silico studies and confirmed by affinity chromatography. Our findings open a new chapter in bioactivities of tetrapyrroles-especially phyllobilins-for which they form the basis for broad implications in plant science, ecology, and physiology.

Comparative transcriptome analysis reveals that chlorophyll metabolism contributes to leaf color changes in wucai (Brassica campestris L.) in response to cold

BACKGROUND

Chlorophyll (Chl) is a vital photosynthetic pigment involved in capturing light energy and energy conversion. In this study, the color conversion of inner-leaves from green to yellow in the new wucai (Brassica campestris L.) cultivar W7-2 was detected under low temperature. The W7-2 displayed a normal green leaf phenotype at the seedling stage, but the inner leaves gradually turned yellow when the temperature was decreased to 10 °C/2 °C (day/night), This study facilitates us to understand the physiological and molecular mechanisms underlying leaf color changes in response to low temperature.

RESULTS

A comparative leaf transcriptome analysis of W7-2 under low temperature treatment was performed on three stages (before, during and after leaf color change) with leaves that did not change color under normal temperature at the same period as a control. A total of 67,826 differentially expressed genes (DEGs) were identified. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Ontology (GO) analysis revealed that the DEGs were mainly enriched in porphyrin and Chl metabolism, carotenoids metabolism, photosynthesis, and circadian rhythm. In the porphyrin and chlorophyll metabolic pathways, the expression of several genes was reduced [i.e. magnesium chelatase subunit H (CHLH)] under low temperature. Almost all genes [i.e. phytoene synthase (PSY)] in the carotenoids (Car) biosynthesis pathway were downregulated under low temperature. The genes associated with photosynthesis [i.e. photosystem II oxygen-evolving enhancer protein 1 (PsbO)] were also downregulated under LT. Our study also showed that elongated hypocotyl5 (HY5), which participates in circadian rhythm, and the metabolism of Chl and Car, is responsible for the regulation of leaf color change and cold tolerance in W7-2.

CONCLUSIONS

The color of inner-leaves was changed from green to yellow under low temperature in temperature-sensitive mutant W7-2. Physiological, biochemical and transcriptomic studies showed that HY5 transcription factor and the downstream genes such as CHLH and PSY, which regulate the accumulation of different pigments, are required for the modulation of leaf color change in wucai under low temperature.

Rising levels of antioxidative phyllobilins in stored agricultural produce and their impact on consumer acceptance

Consumers often throw away faded greens, because taste and appearance are less appealing compared to fresh ones. We report here a family of antioxidants, the phyllobilins, which increase during storage in iceberg lettuce and cucumber. We show that informing consumers about rising levels of phyllobilins leads to a longer willingness to consume faded lettuce and to an improved health and safety perception.

An evergreen mind and a heart for the colors of fall

With the finest biochemical and molecular approaches, convincing explorative strategies, and long-term vision, Stefan Hörtensteiner succeeded in elucidating the biochemical pathway responsible for chlorophyll degradation. After having contributed to the identification of key chlorophyll degradation products in the course of the past 25 years, he gradually identified and characterized most of the crucial players in the PAO/phyllobilin degradation pathway of chlorophyll. He was one of the brightest plant biochemists of his generation, and his work opened doors to a better understanding of plant senescence, tetrapyrrole homeostasis, and their complex regulation. He sadly passed away on 5 December 2020, aged 57.

Phyllobilins as a challenging diverse natural product class: Exploration of pharmacological activities

Phyllobilins are a group of chlorophyll-derived bilin-type linear tetrapyrroles, generated in the process of chlorophyll breakdown. Since the first phyllobilin was isolated and characterized in 1991, more and more structures of these chlorophyll catabolites were identified alongside the biochemical players involved in chlorophyll breakdown. In the meantime, phyllobilins are known to occur in a large natural structural variety, and new modifications are still being discovered. Phyllobilins have been regarded as products of chlorophyll detoxification for a very long time, hence they have been completely overlooked as a natural product class in terms of their biological role or pharmacological activity. A change of this paradigm, however, is long overdue. Here, we review the current knowledge of the pharmacological activities of phyllobilins and give an overview of the diverse structural modifications, laying the groundwork for analyzing their role(s) as active components in medicinal plants.

Degradation of the photosystem II core complex is independent of chlorophyll degradation mediated by Stay-Green Mg2+ dechelatase in Arabidopsis

During leaf senescence, the degradation of photosystems and photosynthetic pigments proceeds in a coordinated manner, which would minimize the potential photodamage to cells. Both photosystem I and II are composed of core complexes and peripheral antenna complexes, with the former binding chlorophyll a and the latter binding chlorophyll a and b. Although the degradation of peripheral antenna complexes is initiated by chlorophyll degradation, it remains unclear whether the degradation of core complexes and chlorophyll is coordinated. In this study, we examined the degradation of peripheral antenna and core complexes in the Arabidopsis sgr1/sgr2/sgrl triple mutant, lacking all the isoforms of chlorophyll a:Mg²⁺ dechelatase. In this mutant, the degradation of peripheral antenna complexes and photosystem I core complexes was substantially retarded, but the core complexes of photosystem II were rapidly degraded during leaf senescence. On the contrary, the photosynthetic activity declined at a similar rate as in the wild type plants. These results suggest that the degradation of photosystem II core complexes is regulated independently of the major chlorophyll degradation pathway mediated by the dechelatase. The study should contribute to the understanding of the complex molecular mechanisms underlying the degradation of photosystems, which is an essential step during leaf senescence.

Genome-Wide Analysis of the G2-Like Transcription Factor Genes and Their Expression in Different Senescence Stages of Tobacco (Nicotiana tabacum L.)

The Golden2-like (GLK) transcription factors play important roles in regulating chloroplast growth, development, and senescence in plants. In this study, a total of 89 NtGLK genes (NtGLK1-NtGLK89) were identified in the tobacco genome and were classified into 10 subfamilies with variable numbers of exons and similar structural organizations based on the gene structure and protein motif analyses. Twelve segmental duplication pairs of NtGLK genes were identified in the genome. These NtGLK genes contain two conserved helix regions related to the HLH structure, and the sequences of the first helix region are less conserved than that of the second helix motif. Cis-regulatory elements of the NtGLK promoters were widely involved in light responsiveness, hormone treatment, and physiological stress. Moreover, a total of 206 GLK genes from tomato, tobacco, maize, rice, and Arabidopsis were retrieved and clustered into eight subgroups. Our gene expression analysis indicated that NtGLK genes showed differential expression patterns in tobacco leaves at five senescence stages. The expression levels of six NtGLK genes in group C were reduced, coinciding precisely with the increment of the degree of senescence, which might be associated with the function of leaf senescence of tobacco. Our results have revealed valuable information for further functional characterization of the GLK gene family in tobacco.

Detection of 132-carboxy-chlorin produced by the in vitro BciC enzymatic hydrolysis of zinc chlorophyllide

Green photosynthetic bacteria with an efficient light-harvesting system contain special chlorophyll molecules, called bacteriochlorophylls c, d, e, in their main antennae. In the biosynthetic pathway, a BciC enzyme is proposed to catalyze the hydrolysis of the C13²-methoxycarbonyl group of chlorophyllide a, but the resulting C13²-carboxy group has not been detected yet because it is spontaneously removed due to the instability of the β-keto-carboxylic acid. In this study, the in vitro BciC enzymatic reactions of zinc methyl (13¹R/S)-hydroxy-mesochlorophyllides a were examined and a carboxylic acid possessing the C13²S-OH was first observed as the hydrolyzed product of the C13²-COOCH₃.

A yellow chlorophyll catabolite in leaves of Urtica dioica L.: An overlooked phytochemical that contributes to health benefits of stinging nettle

Stinging nettle is appreciated for its antioxidant and anti-inflammatory properties, which renders the plant a popular ingredient in a healthy diet in form of salads or smoothies. The most common use, presumably, is of dried leaves as ingredient in tea mixtures. The plant's health benefits are attributed primarily to phenolic phytochemicals. Here we describe the characterization and quantification of a phylloxanthobilin (PxB), a yellow chlorophyll catabolite, in nettle tea. Despite their abundance in the plant kingdom, chlorophyll catabolites have been overlooked as phytochemicals and as part of human nutrition. Our investigations of tea reveal that one cup of nettle tea contains about 50 µg of PxB with large variations depending on the supplier. When investigating the bioactivities of PxB, our observations show that PxB has antioxidative and anti-inflammatory activities comparable to known bioactive small molecules found in nettle, indicating the phylloxanthobilin to be an overlooked ingredient of nettle tea.

Tea as a Source of Biologically Active Compounds in the Human Diet

Due to the different levels of bioactive compounds in tea reported in the literature, the aim of this study was to determine whether commercially available leaf teas could be an important source of phenolics and selected minerals (copper, manganese, iron, zinc, magnesium, calcium, sodium, potassium) and if the differences in the content of these components between various types of tea are significant. It was found that both the amount of these compounds in tea and the antioxidant activity of tea infusions were largely determined by the origin of tea leaves as well as the processing method, which can modify the content of the studied components up to several hundred-fold. The group of green teas was the best source of phenolic compounds (110.73 mg/100 mL) and magnesium (1885 µg/100 mL) and was also characterised by the highest antioxidant activity (59.02%). This type of tea is a great contributor to the daily intake of the studied components. The average consumption of green tea infusions, assumed to be 3–4 cups (1 L) a day, provides the body with health-promoting polyphenol levels significantly exceeding the recommended daily dose. Moreover, drinking one litre of an unfermented tea infusion provides more than three times the recommended daily intake of manganese. Tea infusions can be a fairly adequate, but only a supplementary, source of potassium, zinc, magnesium, and copper in the diet. Moreover, it could be concluded that the antioxidant activity of all the analysed types of tea infusions results not only from the high content of phenolic compounds and manganese but is also related to the presence of magnesium and potassium.

The fate of chlorophyll in phytophagous insects goes beyond nutrition

Chlorophyll (Chl) is a natural compound that is found in all autotrophic plants. Since phytophagous insects ingest the photosynthetically active material with the plant leaves, the question arises if and how herbivores deal with Chl and its degradation products. Here we review findings on Chl degradation in phytophagous insects and highlight the role of these ubiquitous plant metabolites for plant-feeding insects. Due to the anaerobic gut of many insects, the degradation is limited to the removal of the peripheral substituents, while the tetrapyrrole core remains intact. Proteins, such as red fluorescent protein, P252 (a novel 252-kDa protein), and chlorophyllide binding protein have been reported to occur in the insect gut and might be indirectly connected to Chl degradation. Besides of an nutritional value, e.g., by taking up Mg²⁺ ions or by sequestration of carbon from the phytol side chain, the Chl degradation products may serve the insect, after binding to certain proteins, as antimicrobial, antifungal, and antiviral factors. The protein complexes may also confer protection against reactive oxygen species. The antibiotic potential of proteins and degradation products does not only benefit phytophagous insects but also human being in medical application of cancer treatment for instance. This review highlights these aspects from a molecular, biochemical, and ecological point of view.

Suppression of a hexokinase gene, SlHXK1, leads to accelerated leaf senescence and stunted plant growth in tomato

Sugars are the key regulatory molecules that impact diverse biological processes in plants. Hexokinase, the key rate-limiting enzyme in hexose metabolism, takes part in the first step of glycolytic pathway. Acting as a sensor that mediates sugar regulation, hexokinase has been proved to play significant roles in regulating plant growth and development. Here, we isolated a hexokinase gene SlHXK1 from tomato. Its transcript levels were higher in flowers and leaves than in other organs and decreased during leaf and petiole development. SlHXK1-RNAi lines displayed advanced leaf senescence and stunted plant growth. Physiological features including plant height, leaf length, thickness and size, the contents of chlorophyll, starch and MDA, and hexokinase activity were dramatically altered in SlHXK1-RNAi plants. Dark-induced leaf senescence were advanced and the transcripts of senescence-related genes after darkness treatment were markedly increased in SlHXK1-RNAi plants. RNA-seq and qRT-PCR analyses showed that the transcripts of genes related to plant hormones, photosynthesis, chloroplast development, chlorophyll synthesis and metabolism, cellular process, starch and sucrose metabolism, and senescence were significantly altered in SlHXK1-RNAi plants. Taken together, our data demonstrate that SlHXK1 is a significant gene involved in leaf senescence and plant growth and development in tomato through affecting starch turnover.

Phyllobilins from Senescence-Associated Chlorophyll Breakdown in the Leaves of Basil (Ocimum basilicum) Show Increased Abundance upon Herbivore Attack

In view of the common use of the herb basil (Ocimum basilicum) in nutrition and in phytomedicine, the contents of its leaves are of obvious interest. In extracts of fresh yellowish-green basil leaves, phyllobilins (PBs), which are bilin-type catabolites of chlorophyll (Chl), were detected using high-performance liquid chromatography (HPLC). Two such PBs, provisionally named Ob-nonfluorescent chlorophyll catabolite (NCC)-40 and Ob-YCC-45, exhibited previously unknown structures that were delineated by a thorough spectroscopic characterization. When basil leaves were infested with aphids or thrips or underwent fungal infections, areas with chlorosis were observed. HPLC analyses of the infested parts of leaves compared to those of the healthy parts showed a significant accumulation of PBs in the infested areas, demonstrating that the senescence-associated pheophorbide a oxygenase/phyllobilin (PAO/PB) pathway is activated by herbivore feeding and fungal infection.

The role of lyases, NblA and NblB proteins and bilin chromophore transfer in restructuring the cyanobacterial light-harvesting complex‡

Phycobilisomes are large light-harvesting complexes attached to the stromal side of thylakoids in cyanobacteria and red algae. They can be remodeled or degraded in response to changing light and nutritional status. Both the core and the peripheral rods of phycobilisomes contain biliproteins. During biliprotein biosynthesis, open-chain tetrapyrrole chromophores are attached covalently to the apoproteins by dedicated lyases. Another set of non-bleaching (Nb) proteins has been implicated in phycobilisome degradation, among them NblA and NblB. We report in vitro experiments with lyases, biliproteins and NblA/B which imply that the situation is more complex than currently discussed: lyases can also detach the chromophores and NblA and NblB can modulate lyase-catalyzed binding and detachment of chromophores in a complex fashion. We show: (i) NblA and NblB can interfere with chromophorylation as well as chromophore detachment of phycobiliprotein, they are generally inhibitors but in some cases enhance the reaction; (ii) NblA and NblB promote dissociation of whole phycobilisomes, cores and, in particular, allophycocyanin trimers; (iii) while NblA and NblB do not interact with each other, both interact with lyases, apo- and holo-biliproteins; (iv) they promote synergistically the lyase-catalyzed chromophorylation of the β-subunit of the major rod component, CPC; and (v) they modulate lyase-catalyzed and lyase-independent chromophore transfers among biliproteins, with the core protein, ApcF, the rod protein, CpcA, and sensory biliproteins (phytochromes, cyanobacteriochromes) acting as potential traps. The results indicate that NblA/B can cooperate with lyases in remodeling the phycobilisomes to balance the metabolic requirements of acclimating their light-harvesting capacity without straining the overall metabolic economy of the cell.

The red chlorophyll catabolite (RCC) is an inefficient sensitizer of singlet oxygen - photochemical studies of the methyl ester of RCC

The red chlorophyll catabolite (RCC) is a proposed cryptic intermediate of chlorophyll (Chl) breakdown in higher plants. Its accumulation in higher plants is believed to be metabolically suppressed, as RCC is commonly suspected to efficiently sensitize for the formation of the cell poison singlet oxygen (¹O₂). We report here a study on luminescence of the methyl ester of RCC (Me-RCC) and of its capacity to generate ¹O₂ in ethanolic solution. A solution of Me-RCC fluoresces at room temperature with a maximum near 670 nm and features a fluorescence spectrum with pronounced vibrational spacing at 77 K. As shown here, sensitization of the generation of ¹O₂ by Me-RCC in an oxygen-saturated solution in hexadeutero-ethanol occurs with a maximal quantum yield of only about 0.015. This low quantum yield suggests that the specific catabolic suppression of the accumulation of RCC during Chl breakdown is not primarily a countermeasure against the formation of ¹O₂ by RCC in the plant, but has other crucial reasons mainly.

Comparative Proteomic Analysis by iTRAQ Reveals that Plastid Pigment Metabolism Contributes to Leaf Color Changes in Tobacco (Nicotiana tabacum) during Curing

Tobacco (Nicotiana tabacum), is a world's major non-food agricultural crop widely cultivated for its economic value. Among several color change associated biological processes, plastid pigment metabolism is of trivial importance in postharvest plant organs during curing and storage. However, the molecular mechanisms involved in carotenoid and chlorophyll metabolism, as well as color change in tobacco leaves during curing, need further elaboration. Here, proteomic analysis at different curing stages (0 h, 48 h, 72 h) was performed in tobacco cv. Bi'na1 with an aim to investigate the molecular mechanisms of pigment metabolism in tobacco leaves as revealed by the iTRAQ proteomic approach. Our results displayed significant differences in leaf color parameters and ultrastructural fingerprints that indicate an acceleration of chloroplast disintegration and promotion of pigment degradation in tobacco leaves due to curing. In total, 5931 proteins were identified, of which 923 (450 up-regulated, 452 down-regulated, and 21 common) differentially expressed proteins (DEPs) were obtained from tobacco leaves. To elucidate the molecular mechanisms of pigment metabolism and color change, 19 DEPs involved in carotenoid metabolism and 12 DEPs related to chlorophyll metabolism were screened. The results exhibited the complex regulation of DEPs in carotenoid metabolism, a negative regulation in chlorophyll biosynthesis, and a positive regulation in chlorophyll breakdown, which delayed the degradation of xanthophylls and accelerated the breakdown of chlorophylls, promoting the formation of yellow color during curing. Particularly, the up-regulation of the chlorophyllase-1-like isoform X2 was the key protein regulatory mechanism responsible for chlorophyll metabolism and color change. The expression pattern of 8 genes was consistent with the iTRAQ data. These results not only provide new insights into pigment metabolism and color change underlying the postharvest physiological regulatory networks in plants, but also a broader perspective, which prompts us to pay attention to further screen key proteins in tobacco leaves during curing.

Propentdyopents: Brief history of a family of dipyrrolic pigments

Propentdyopents are naturally occurring dipyrroles deriving from the metabolism of heme and characterized by a dipyrrin-1,9-dione motif. The unusual name propentdyopent is due to the first colorimetric method (the Stokvis reaction) for the detection of these compounds, which were initially isolated from urine samples. Upon reduction in alkaline solutions, they produced red species that were termed pentdyopents to describe with Greek numerals their absorption maximum (525 nm) in the visible range. The precursors to the red pentdyopents were thus indicated as propentdyopents.Over the course of several decades, these dipyrrolic compounds have appeared in several studies of human physiology, typically associated to conditions of abnormal heme metabolism and/or oxidative stress. Concurrently, synthetic investigations have confirmed their chemical structure, reactivity, and ability to coordinate metals as bidentate monoanionic ligands. Notably, the planar dipyrrindione platform can undergo reversible one-electron redox processes and thereby act as an electron reservoir in metal complexes. In combination with the documented ability of the carbonyl groups to act as hydrogen-bonding acceptors, the coordination chemistry of propentdyopents could lead to new applications for this old class of pigments. Furthermore, the observation of these pigments in several clinical contexts could potentially delineate a role of propentdyopents as diagnostic biomarkers. This mini-review summarizes both the chemistry and biology of propentdyopents while highlighting the ample space for new discoveries.

Subcellular localization of chlorophyllase2 reveals it is not involved in chlorophyll degradation during senescence in Arabidopsis thaliana

Chlorophyllase (CLH), which catalyzes the release of the phytol chain from chlorophyll (Chl), has been long considered to catalyze the first step of Chl degradation. Arabidopsis contains two isoforms of CLH (CLH1 and CLH2), and CLH1 was previously demonstrated to be localized in tonoplast and endoplasmic reticulum, and not be involved in Chl degradation. In contrast, CLH2 possesses a predicted signal-peptide for chloroplast localization, and phylogenetic analysis of CLHs in Arabidopsis and other species also indicate that CLH2 forms a different clade than CLH1. Therefore, the possibility remains that CLH2 is involved in the breakdown of Chl. In the current study, clh mutants lacking CLH2 or both CLH isoforms were analyzed after the induction of senescence. Results indicated that the clh knockout lines were still able to degrade Chl at the same rate as wild-type plants. Transgenic Arabidopsis plants were generated that constitutively expressed either CLH2 or CLH2 fused to a yellow fluorescent protein (YFP). Observations made using confocal microscopy indicated that CLH2-YFP was located external to chloroplasts. Additionally, in overexpression plants, CLH2 was enriched in tonoplast and endoplasmic reticulum fractions following membrane fractionation. Based on the collective data, we conclude that CLH2 is not involved in Chl breakdown during senescence in Arabidopsis.

Synthesis of chlorophyll-a homologs by C132-substitutions and their physico- and biochemical properties

A mixture of pheophytins-a/a', metal-free forms of photosynthetically active chlorophyll(Chl)s-a/a' bearing the 13²-methoxycarbonyl group, was substituted at the C13²-position by bimolecular nucleophilic substitution with methyl bromoacetate or Michael addition with methyl acrylate, followed by C13²-demethoxycarbonylation and magnesium insertion at the central position, to afford Chl-a/a' homologs possessing a methoxycarbonylmethyl or 2-methoxycarbonylethyl group at the C13²-position, respectively. These C13²-methylene- and ethylene-inserted homologs were characterized by 1D/2D ¹H NMR spectroscopy, and the optical properties of their C13²-epimerically pure samples are investigated using visible absorption, fluorescence emission, and circular dichroism spectroscopies. The stereochemistry at the C13²-chiral center of these Chl-a/a' homologs was not inverted in a basic solution, and the Chl-a homologs were effective for the substrates for the chlorophyllase reaction, hydrolysis of the phytyl ester.

Cryptic chlorophyll breakdown in non-senescent green Arabidopsis thaliana leaves

Chlorophyll (Chl) breakdown is a diagnostic visual process of leaf senescence, which furnishes phyllobilins (PBs) by the PAO/phyllobilin pathway. As Chl breakdown disables photosynthesis, it appears to have no role in photoactive green leaves. Here, colorless PBs were detected in green, non-senescent leaves of Arabidopsis thaliana. The PBs from the green leaves had structures entirely consistent with the PAO/phyllobilin pathway and the mutation of a single Chl catabolic enzyme completely abolished PBs with the particular modification. Hence, the PAO/phyllobilin pathway was active in the absence of visible senescence and expression of genes encoding Chl catabolic enzymes was observed in green Arabidopsis leaves. PBs accumulated to only sub-% amounts compared to the Chls present in the green leaves, excluding a substantial contribution of Chl breakdown from rapid Chl turnover associated with photosystem II repair. Indeed, Chl turnover was shown to involve a Chl a dephytylation and Chl a reconstitution cycle. However, non-recyclable pheophytin a is also liberated in the course of photosystem II repair, and is proposed here to be scavenged and degraded to the observed PBs. Hence, a cryptic form of the established pathway of Chl breakdown is indicated to play a constitutive role in photoactive leaves.

Isolation, characterization, and antioxidative activity of a dioxobilin-type phylloxanthobilin from savoy cabbage

The degradation of the green pigment chlorophyll in plants is known to yield phyllobilins as highly abundant linear tetrapyrroles. Recently, a split path of the degradation pathway has been discovered, leading to so-called dioxobilin-type (or type-II) phyllobilins. The first characterized type-II phyllobilin was colorless featuring four deconjugated pyrrole units. Similar to the type-I branch, for which yellow oxidation products of the colorless phyllobilins – the type-I phylloxanthobilins – are known, a type-II phylloxanthobilin has recently been characterized from senescent leaves of grapevine. Type-I phylloxanthobilins appear to be actively produced in the plant, are known to possess interesting chemical properties, and were shown to act as potent antioxidants that can protect cells from oxidative stress. Here we report the isolation and structural characterization of a type-II phylloxanthobilin from de-greened leaves of savoy cabbage, which turned out to be structurally closely related to bilirubin. Bilirubin is known to possess high antioxidative activity; in addition, savoy cabbage is considered to promote health benefits due to its high content in antioxidants. We therefore investigated the in vitro antioxidative potential of the newly identified type-II phylloxanthobilin using two different approaches, both of which revealed an even higher antioxidative activity for the type-II phylloxanthobilin from savoy cabbage compared to bilirubin.

Total synthesis campaigns toward chlorophylls and related natural hydroporphyrins - diverse macrocycles, unrealized opportunities

Covering: up to 2018 Chlorophylls, bacteriochlorophylls and related hydroporphyrins constitute invaluable natural products but have largely remained outside the scope of viable syntheses. The campaign toward chlorophyll a by Woodward and coworkers is a deservedly celebrated landmark in organic synthesis yet the route entailed 49 steps, relied on semisynthetic replenishment of advanced intermediates, and then pointed to (but did not implement) uncertain literature procedures for the final transformations. Indeed, the full synthesis at any scale of any (bacterio)chlorophylls - conversion of small-molecule starting materials to the product - has never been accomplished. Herein, the reported syntheses of (±)-bonellin dimethyl ester (0.93 mg) and tolyporphin A O,O-diacetate (0.38 mg), as well as the never-fully traversed route to chlorophyll a, have been evaluated in a quantitative manner. Bonellin and tolyporphin A are naturally occurring chlorin and bacteriochlorin macrocycles, respectively, that lack the characteristic fifth ring of (bacterio)chlorophylls. A practical assessment is provided by the cumulative reaction mass efficiency (cRME) of the entire synthetic process. The cRME for the route to chlorophyll a would be 4.3 × 10-9 (230 kg of all reactants and reagents in total would yield 1.0 mg of chlorophyll a), whereas that for (±)-bonellin dimethyl ester or tolyporphin A O,O-diacetate is approximately 6.4 × 10-4 or 3.6 × 10-5, respectively. Comparison of the three syntheses reveals insights for designing hydroporphyrin syntheses. Development of syntheses with cRME > 10-5 (if not 10-4), as required to obtain 10 mg quantities of hydroporphyrin for diverse physicochemical, biochemical and medicinal chemistry studies, necessitates significant further advances in tetrapyrrole chemistry.

Cytotoxic Effects of Chlorophyllides in Ethanol Crude Extracts from Plant Leaves

Chlorophyllide (chlide) is a natural catabolic product of chlorophyll (Chl), produced through the activity of chlorophyllase (chlase). The growth inhibitory and antioxidant effects of chlide from different plant leaf extracts have not been reported. The aim of this study is to demonstrate that chlide in crude extracts from leaves has the potential to exert cytotoxic effects on cancer cell lines. The potential inhibitory and antioxidant effects of chlide in crude extracts from 10 plant leaves on breast cancer cells (MCF7 and MDA-MB-231), hepatocellular carcinoma cells (Hep G2), colorectal adenocarcinoma cells (Caco2), and glioblastoma cells (U-118 MG) were studied using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) and DPPH (1,1-diphenyl-2-picrylhydrazyl) assays. The results of the MTT assay showed that chlide in crude extracts from sweet potato were the most effective against all cancer cell lines tested. U-118 MG cells were the most sensitive, while Caco2 cells were the most resistant to the tested crude extracts. The cytotoxic effects of chlide and Chl in crude extracts from sweet potato and of commercial chlorophyllin (Cu-chlin), in descending order, were as follows: chlide > Chl > Cu-chlin. Notably, the IC₅₀ of sweet potato in U-118 MG cells was 45.65 μg/mL while those of Chl and Cu-chlin exceeded 200 μg/mL. In the DPPH assay, low concentrations (100 μg/mL) of chlide and Cu-chlin from crude extracts of sweet potato presented very similar radical scavenging activity to vitamin B2. The concentration of chlide was negatively correlated with DPPH activity. The current study was the first to demonstrate that chlide in crude extracts from leaves have potential cytotoxicity in cancer cell lines. Synergism between chlide and other compounds from leaf crude extracts may contribute to its cytotoxicity.

Jasmonate production through chlorophyll a degradation by Stay-Green in Arabidopsis thaliana

Leaf color change through chlorophyll degradation is a characteristic symptom of senescence. Magnesium removal from chlorophyll a is the initial step in chlorophyll a degradation, in a reaction catalyzed by Stay-Green (SGR). Arabidopsis thaliana has three SGR homologs, SGR1, SGR2, and SGR-like. When SGR1 is overexpressed, both chlorophyll a and b are degraded and leaves turn yellow. This process is visually identical to senescence, suggesting that SGR1 overexpression affects various physiological processes in plants. To examine this possibility, gene expression associated with chlorophyll metabolism and senescence was analyzed following dexamethasone-inducible SGR1 introduction into Arabidopsis. When SGR1 was overexpressed following 18 h of dexamethasone treatment, genes involved in chlorophyll degradation were upregulated, as were senescence-associated genes. These observations suggested that chlorophyll a degradation promotes senescence. As jasmonate is the plant hormone responsible for senescence and was expected to be involved in the regulation of gene expression after dexamethasone treatment, the level of jasmonoyl-isoleucine, the active form of jasmonate, was measured. The jasmonoyl-isoleucine level increased slightly after 10 h of SGR1 overexpression, and this increase became significant after 18 h. These observations suggest that jasmonate is produced through chlorophyll a degradation and affects the promotion of senescence.

Re-opening the stage for Echinacea research - Characterization of phylloxanthobilins as a novel anti-oxidative compound class in Echinacea purpurea

BACKGROUND

Phylloxanthobilins are tetrapyrrolic natural products that arise from the degradation of chlorophyll. Phylloxanthobilins have been discovered roughly 10 years ago in the leaves of deciduous trees, and are now considered a compound class with high and still unexplored potential of bioactivities. To date, however, there are no reports on the occurrence of phylloxanthobilins in parts of a medicinal plant used for pharmaceutical preparations.

PURPOSE

The relevance of Echinacea purpurea as medicinal plant is undoubtedly high, and a large variety of pharmaceutical preparations is available on the market, mostly for the treatment of the common cold. Nevertheless, its phytochemical profiling has been limited to analysis for previously characterized substances, and this has not explained all its pharmacological efficacies. We therefore set out to investigate the occurrence of phylloxanthobilins in Echinacea purpurea.

METHODS

Phylloxanthobilins in leaf extracts of Echinacea purpurea were detected using analytical HPLC. Identified phyllobilins were purified from plant material and characterized by UV/Vis, mass spectrometry, MS/MS, and confirmed by co-injections with previously published phyllobilins from different sources. The anti-oxidant activity of selected isolated phylloxanthobilins was assessed by an in vitro ferric reducing antioxidant power (FRAP) assay; in addition, the ability to scavenge ROS in cells caused by hydrogen peroxide stimulation was determined by measuring H₂DCF-DA fluorescence and by assessing cellular GSH levels.

RESULTS

In extracts of Echinacea purpurea leaves, an unprecedented diversity of phylloxanthobilins was detected; surprisingly, not only in senescent yellow leaves, but also in green leaves with no visible chlorophyll degradation. Six phylloxanthobilins were identified and structurally characterized. The uptake of phylloxanthobilins by human endothelial kidney cells was demonstrated. When investigating the anti-oxidative activity of these natural products, a potent in vitro activity was demonstrated; in addition, phylloxanthobilins possess intracellular ROS scavenging ability and can prevent oxidative stress as assessed by total cellular GSH levels.

CONCLUSION

Phylloxanthobilins are important constituents of Echinacea purpurea extracts, and our first exploratory studies hint towards promising bioactivities of these natural products, which may be relevant for understanding Echinacea efficacies.

Transcriptome analysis based on a combination of sequencing platforms provides insights into leaf pigmentation in Acer rubrum

BACKGROUND

Red maple (Acer rubrum L.) is one of the most common and widespread trees with colorful leaves. We found a mutant with red, yellow, and green leaf phenotypes in different branches, which provided ideal materials with the same genetic relationship, and little interference from the environment, for the study of complex metabolic networks that underly variations in the coloration of leaves. We applied a combination of NGS and SMRT sequencing to various red maple tissues.

RESULTS

A total of 125,448 unigenes were obtained, of which 46 and 69 were thought to be related to the synthesis of anthocyanins and carotenoids, respectively. In addition, 88 unigenes were presumed to be involved in the chlorophyll metabolic pathway. Based on a comprehensive analysis of the pigment gene expression network, the mechanisms of leaf color were investigated. The massive accumulation of Cy led to its higher content and proportion than other pigments, which caused the redness of leaves. Yellow coloration was the result of the complete decomposition of chlorophyll pigments, the unmasking of carotenoid pigments, and a slight accumulation of Cy.

CONCLUSIONS

This study provides a systematic analysis of color variations in the red maple. Moreover, mass sequence data obtained by deep sequencing will provide references for the controlled breeding of red maple.

In Search of Bioactivity - Phyllobilins, an Unexplored Class of Abundant Heterocyclic Plant Metabolites from Breakdown of Chlorophyll

The fate of the green plant pigment chlorophyll (Chl) in de-greening leaves has long been a fascinating biological puzzle. In the course of the last three decades, various bilin-type products of Chl breakdown have been identified, named phyllobilins (PBs). Considered 'mere' leftovers of a controlled biological Chl detoxification originally, the quest for finding relevant bioactivities of the PBs has become a new paradigm. Indeed, the PBs are abundant in senescent leaves, in ripe fruit and in some vegetables, and they display an exciting array of diverse heterocyclic structures. This review outlines briefly which types of Chl breakdown products occur in higher plants, describes basics of their bio-relevant structural and chemical properties and gives suggestions as to 'why' the plants produce vast amounts of uniquely 'decorated' heterocyclic compounds. Clearly, it is worthwhile to consider crucial metabolic roles of PBs in plants, which may have practical consequences in agriculture and horticulture. However, PBs are also part of our plant-based nutrition and their physiological and pharmacological effects in humans are of interest, as well.

Yellow Dioxobilin-Type Tetrapyrroles from Chlorophyll Breakdown in Higher Plants-A New Class of Colored Phyllobilins

In senescent leaves chlorophyll (Chl) catabolites typically accumulate as colorless tetrapyrroles, classified as formyloxobilin-type (or type-I) or dioxobilin-type (type-II) phyllobilins (PBs). Yellow type-I Chl catabolites (YCCs) also occur in some senescent leaves, in which they are generated by oxidation of colorless type-I PBs. A yellow type-II PB was recently proposed to occur in extracts of fall leaves of grapevine (Vitis vinifera), tentatively identified by its mass and UV/Vis absorption characteristics. Here, the first synthesis of a yellow type-II Chl catabolite (DYCC) from its presumed natural colorless type-II precursor is reported. A homogenate of a Spatiphyllum wallisii leaf was used as "green" means of effective and selective oxidation. The synthetic DYCC was fully characterized and identified with the yellow grapevine leaf pigment. As related yellow type-I PBs do, the DYCC functions as a reversible photoswitch by undergoing selective photo-induced Z/E isomerization of its C15=C16 bond.

Chlorophyll catabolism precedes changes in chloroplast structure and proteome during leaf senescence

The earliest visual changes of leaf senescence occur in the chloroplast as chlorophyll is degraded and photosynthesis declines. Yet, a comprehensive understanding of the sequence of catabolic events occurring in chloroplasts during natural leaf senescence is still missing. Here, we combined confocal and electron microscopy together with proteomics and biochemistry to follow structural and molecular changes during Arabidopsis leaf senescence. We observed that initiation of chlorophyll catabolism precedes other breakdown processes. Chloroplast size, stacking of thylakoids, and efficiency of PSII remain stable until late stages of senescence, whereas the number and size of plastoglobules increase. Unlike catabolic enzymes, whose level increase, the level of most proteins decreases during senescence, and chloroplast proteins are overrepresented among these. However, the rate of their disappearance is variable, mostly uncoordinated and independent of their inherent stability during earlier developmental stages. Unexpectedly, degradation of chlorophyll-binding proteins lags behind chlorophyll catabolism. Autophagy and vacuole proteins are retained at relatively high levels, highlighting the role of extra-plastidic degradation processes especially in late stages of senescence. The observation that chlorophyll catabolism precedes all other catabolic events may suggest that this process enables or signals further catabolic processes in chloroplasts.

Bacterio- and Isobacteriodilactones by Stepwise or Direct Oxidations of meso-Tetrakis(pentafluorophenyl)porphyrin

Porpholactones are porphyrinoids in which one or more β,β'-bonds of the parent chromophore were replaced by lactone moieties. Accessible to varying degrees by direct and nonselective oxidations of porphyrins, the rational syntheses of all five dilactone isomers along stepwise, controlled, and high-yielding routes via porphyrin → tetrahydroxyisobacteriochlorin metal complexes → isobacteriochlorindilactone metal complexes or porphyrin → tetrahydroxybacteriochlorin → bacteriochlorindilactone (and related) pathways, respectively, are described. A major benefit of these complementary routes over established methods is the simplicity of the isolation of the dilactones because of the reduced number of side products formed. In an alternative approach we report the direct and selective conversion of free base meso-tetrakis(pentafluorophenyl)porphyrin to all isomers of free base isobacteriodilactones using the oxidant cetyltrimethylN⁺MnO₄^-. The solid-state structures of some of the isomers and their precursors are reported, providing data on the conformational modulation induced by the derivatizations. We also rationalize computationally their differing thermodynamic stability and electronic properties. In making new efficient routes toward these dilactone isomers available, we enable the further study of this diverse class of porphyrinoids.