Chlorophyll a is a favorable substrate for Chlamydomonas Mg-dechelatase encoded by STAY-GREEN

Activity examination of plant Mg-dechelatase and its bacterial homolog in plants and in vitro

Chlorophyll a serves as a photosynthetic pigment in plants. Its degradation is initiated by the extraction of the central Mg by the Mg-dechelatase enzyme, which is encoded by Stay-Green (SGR). Plant SGR is believed to be derived from bacterial SGR homolog obtained through horizontal gene transfer into photosynthetic eukaryotes. However, it is not known how the bacterial SGR homolog was modified to function in plants. To assess its adaptation mechanism in plants, a bacterial SGR homolog derived from the Anaerolineae bacterium SM23_63 was introduced into plants. It was found that the bacterial SGR homolog metabolized chlorophyll in plants. However, its chlorophyll catabolic activity was lower than that of plant SGR. Recombinant proteins of the bacterial SGR homolog exhibited higher activity than those of the plant SGR. The reduced chlorophyll catabolic activity of bacterial SGR homologs in plants may be associated with low hydrophobicity of the entrance to the catalytic site compared to that of plant SGR. This hinders the enzyme access to chlorophyll, which is localized in hydrophobic environments. This study offers insights into the molecular changes underlying the optimization of enzyme function.

RsOBP2a, a member of OBF BINDING PROTEIN transcription factors, inhibits two chlorophyll degradation genes in green radish

The green radish (Raphanus sativus L.) contains abundant chlorophyll (Chl). DOF-type transcription factor OBF BINDING PROTEIN (OBP) plays crucial functions in plant growth, development, maturation and responses to various abiotic stresses. However, the metabolism by which OBP transcription factors regulate light-induced Chl metabolism in green radish is not well understood. In this study, six OBP genes were identified from the radish genome, distributed unevenly across five chromosomes. Among these genes, RsOBP2a showed significantly higher expression in the green flesh compared to the white flesh of green radish. Analysis of promoter elements suggested that RsOBPs might be involved in stress responses, particularly in light-related processes. Overexpression of RsOBP2a led to increase Chl levels in cotyledons and adventitious roots of radish, while silencing RsOBP2a expression through TYMV-induced gene silencing accelerated leaf senescence. Further investigations revealed that RsOBP2a was localized in the nucleus and served as a transcriptional repressor. RsOBP2a was induced by light and directly suppressed the expression of STAYGREEN (SGR) and RED CHLOROPHYLL CATABOLITE REDUCTASE (RCCR), thereby delaying senescence in radish. Overall, a novel regulatory model involving RsOBP2a, RsSGR, and RsRCCR was proposed to govern Chl metabolism in response to light, offering insights for the enhancement of green radish germplasm.

Transcriptome and Biochemical Analyses of a Chlorophyll-Deficient Bud Mutant of Tea Plant (Camellia sinensis)

Tea leaf-color mutants have attracted increasing attention due to their accumulation of quality-related biochemical components. However, there is limited understanding of the molecular mechanisms behind leaf-color bud mutation in tea plants. In this study, a chlorina tea shoot (HY) and a green tea shoot (LY) from the same tea plant were investigated using transcriptome and biochemical analyses. The results showed that the chlorophyll a, chlorophyll b, and total chlorophyll contents in the HY were significantly lower than the LY's, which might have been caused by the activation of several genes related to chlorophyll degradation, such as SGR and CLH. The down-regulation of the CHS, DFR, and ANS involved in flavonoid biosynthesis might result in the reduction in catechins, and the up-regulated GDHA and GS2 might bring about the accumulation of glutamate in HY. RT-qPCR assays of nine DEGs confirmed the RNA-seq results. Collectively, these findings provide insights into the molecular mechanism of the chlorophyll deficient-induced metabolic change in tea plants.

Transcription factor LcNAC002 coregulates chlorophyll degradation and anthocyanin biosynthesis in litchi

Chlorophyll degradation and anthocyanin biosynthesis, which often occur almost synchronously during fruit ripening, are crucial for vibrant coloration of fruits. However, the interlink point between their regulatory pathways remains largely unknown. Here, 2 litchi (Litchi chinensis Sonn.) cultivars with distinctively different coloration patterns during ripening, i.e. slow-reddening/stay-green "Feizixiao" (FZX) vs rapid-reddening/degreening "Nuomici" (NMC), were selected as the materials to study the key factors determining coloration. Litchi chinensis STAY-GREEN (LcSGR) was confirmed as the critical gene in pericarp chlorophyll loss and chloroplast breakdown during fruit ripening, as LcSGR directly interacted with pheophorbide a oxygenase (PAO), a key enzyme in chlorophyll degradation via the PAO pathway. Litchi chinensis no apical meristem (NAM), Arabidopsis transcription activation factor 1/2, and cup-shaped cotyledon 2 (LcNAC002) was identified as a positive regulator in the coloration of litchi pericarp. The expression of LcNAC002 was significantly higher in NMC than in FZX. Virus-induced gene silencing of LcNAC002 significantly decreased the expression of LcSGR as well as L. chinensis MYELOBLASTOSIS1 (LcMYB1), and inhibited chlorophyll loss and anthocyanin accumulation. A dual-luciferase reporter assay revealed that LcNAC002 significantly activates the expression of both LcSGR and LcMYB1. Furthermore, yeast-one-hybrid and electrophoretic mobility shift assay results showed that LcNAC002 directly binds to the promoters of LcSGR and LcMYB1. These findings suggest that LcNAC002 is an important ripening-related transcription factor that interlinks chlorophyll degradation and anthocyanin biosynthesis by coactivating the expression of both LcSGR and LcMYB1.

Receptor for Activated C Kinase1B (RACK1B) Delays Salinity-Induced Senescence in Rice Leaves by Regulating Chlorophyll Degradation

The widely conserved Receptor for Activated C Kinase1 (RACK1) protein is a WD-40 type scaffold protein that regulates diverse environmental stress signal transduction pathways. Arabidopsis RACK1A has been reported to interact with various proteins in salt stress and Light-Harvesting Complex (LHC) pathways. However, the mechanism of how RACK1 contributes to the photosystem and chlorophyll metabolism in stress conditions remains elusive. In this study, using T-DNA-mediated activation tagging transgenic rice (Oryza sativa L.) lines, we show that leaves from rice RACK1B gene (OsRACK1B) gain-of-function (RACK1B-OX) plants exhibit the stay-green phenotype under salinity stress. In contrast, leaves from down-regulated OsRACK1B (RACK1B-UX) plants display an accelerated yellowing. qRT-PCR analysis revealed that several genes which encode chlorophyll catabolic enzymes (CCEs) are differentially expressed in both RACK1B-OX and RACK1B-UX rice plants. In addition to CCEs, stay-green (SGR) is a key component that forms the SGR-CCE complex in senescing chloroplasts, and which causes LHCII complex instability. Transcript and protein profiling revealed a significant upregulation of OsSGR in RACK1B-UX plants compared to that in RACK1B-OX rice plants during salt treatment. The results imply that senescence-associated transcription factors (TFs) are altered following altered OsRACK1B expression, indicating a transcriptional reprogramming by OsRACK1B and a novel regulatory mechanism involving the OsRACK1B-OsSGR-TFs complex. Our findings suggest that the ectopic expression of OsRACK1B negatively regulates chlorophyll degradation, leads to a steady level of LHC-II isoform Lhcb1, an essential prerequisite for the state transition of photosynthesis for adaptation, and delays salinity-induced senescence. Taken together, these results provide important insights into the molecular mechanisms of salinity-induced senescence, which can be useful in circumventing the effect of salt on photosynthesis and in reducing the yield penalty of important cereal crops, such as rice, in global climate change conditions.

The NAC transcription factor ZmNAC132 regulates leaf senescence and male fertility in maize

Leaf senescence is an integral step in the final stages of plant development, as nutrient remobilization from leaves to sink organs is accomplished during this process. NACs compose a large superfamily of plant-specific TFs involved in multiple plant development processes. Here, we identified a maize NAC TF, ZmNAC132, involved in leaf senescence and male fertility. ZmNAC132 expression was tightly linked to leaf senescence in an age-dependent manner. Knockout of ZmNAC132 led to delays in chlorophyll degradation and leaf senescence, whereas overexpression of ZmNAC132 had the opposite effects. ZmNAC132 could bind to and transactivate the promoter of ZmNYE1, a major chlorophyll catabolic gene, to accelerate chlorophyll degradation during leaf senescence. Moreover, ZmNAC132 affected male fertility through the upregulation of ZmEXPB1, an expansin-encoding gene associated with sexual reproduction and other related genes. Together, the results show that ZmNAC132 participates in the regulation of leaf senescence and male fertility through the targeting of different downstream genes in maize.

FsHemF is involved in the formation of yellow Forsythia leaves by regulating chlorophyll synthesis in response to light intensity

The leaves of Forsythia koreana 'Suwon Gold' are yellow under natural light condition and can revert to green when the light intensity is reduced. To understand the molecular mechanism of leaf color changes in response to light intensity, we compared the chlorophyll content and precursor content between yellow- and green-leaf Forsythia under shade and light-recovery conditions. We identified the conversion of coproporphyrin III (Coprogen III) to protoporphyrin IX (Proto IX) as the primary rate-limiting step of chlorophyll biosynthesis in yellow-leaf Forsythia. Further analysis of the activity of the enzymes that catalyze this step and the expression pattern of the chlorophyll biosynthesis-related genes under different light intensities revealed that the negatively regulated expression of FsHemF by light intensity was the major cause affecting the leaf color change in response to light intensity in yellow-leaf Forsythia. To further understand the cause of differential expression pattern of FsHemF in yellow- and green-leaf lines, we compared the coding sequence and promoter sequence of FsHemF between yellow- and green-leaf Forsythia. We found that one G-box light-responsive cis-element was absent in the promoter region of green-leaf lines. To investigate the functional role of FsHemF, we performed virus-induced gene silencing (VIGS) of FsHemF in green-leaf Forsythia, which leads to yellowing leaf veins, decreased chlorophyll b content, and inhibition of chlorophyll biosynthesis. The results will assist in elucidating the mechanism of yellow-leaf Forsythia in response to light intensity.

Phosphorylation of MdERF17 by MdMPK4 promotes apple fruit peel degreening during light/dark transitions

As apple fruits (Malus domestica) mature, they accumulate anthocyanins concomitantly with losing chlorophyll (Chl); however, the molecular pathways and events that coordinate Chl degradation and fruit coloration have not been elucidated. We showed previously that the transcription factor ETHYLENE RESPONSE FACTOR17 (MdERF17) modulates Chl degradation in apple fruit peels and that variation in the pattern of MdERF17 serine (Ser) residues is responsible for differences in its transcriptional regulatory activity. Here, we report that MdERF17 interacts with and is phosphorylated by MAP KINASE4 (MdMPK4-14G). Phosphorylation of MdERF17 at residue Thr67 by MdMPK4-14G is necessary for its transcriptional regulatory activity and its regulation of Chl degradation. We also show that MdERF17 mutants with different numbers of Ser repeat insertions exhibit altered phosphorylation profiles, with more repeats increasing its interaction with MdMPK4. MdMPK4-14G can be activated by exposure to darkness and is involved in the dark-induced degreening of fruit peels. We also demonstrate that greater phosphorylation of MdERF17 by MdMPK4-14G is responsible for the regulation of Chl degradation during light/dark transitions. Overall, our findings reveal the mechanism by which MdMPK4 controls fruit peel coloration.

In-Depth Characterization of greenflesh Tomato Mutants Obtained by CRISPR/Cas9 Editing: A Case Study With Implications for Breeding and Regulation

Gene editing has already proved itself as an invaluable tool for the generation of mutants for crop breeding, yet its ultimate impact on agriculture will depend on how crops generated by gene editing technologies are regulated, and on our ability to characterize the impact of mutations on plant phenotype. A starting operational strategy for evaluating gene editing-based approaches to plant breeding might consist of assessing the effect of the induced mutations in a crop- and locus-specific manner: this involves the analysis of editing efficiency in different cultivars of a crop, the assessment of potential off-target mutations, and a phenotypic evaluation of edited lines carrying different mutated alleles. Here, we targeted the GREENFLESH (GF) locus in two tomato cultivars ('MoneyMaker' and 'San Marzano') and evaluated the efficiency, specificity and mutation patterns associated with CRISPR/Cas9 activity for this gene. The GF locus encodes a Mg-dechelatase responsible for initiating chlorophyll degradation; in gf mutants, ripe fruits accumulate both carotenoids and chlorophylls. Phenotypic evaluations were conducted on two transgene-free T₂ 'MoneyMaker' gf lines with different mutant alleles (a small insertion of 1 nucleotide and a larger deletion of 123 bp). Both lines, in addition to reduced chlorophyll degradation, showed a notable increase in carotenoid and tocopherol levels during fruit ripening. Infection of gf leaves and fruits with Botrytis cinerea resulted in a significant reduction of infected area and pathogen proliferation compared to the wild type (WT). Our data indicates that the CRISPR/Cas9-mediated mutation of the GF locus in tomato is efficient, specific and reproducible and that the resulting phenotype is robust and consistent with previously characterized greenflesh mutants obtained with different breeding techniques, while also shedding light on novel traits such as vitamin E overaccumulation and pathogen resistance. This makes GF an appealing target for breeding tomato cultivars with improved features for cultivation, as well as consumer appreciation and health.

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.

Insights into the structure and function of the rate-limiting enzyme of chlorophyll degradation through analysis of a bacterial Mg-dechelatase homolog

The Mg-dechelatase enzyme encoded by the Stay-Green (SGR) gene catalyzes Mg²⁺ dechelation from chlorophyll a. This reaction is the first committed step of chlorophyll degradation pathway in plants and is thus indispensable for the process of leaf senescence. There is no structural information available for this or its related enzymes. This study aims to provide insights into the structure and reaction mechanism of the enzyme through biochemical and computational analysis of an SGR homolog from the Chloroflexi Anaerolineae (AbSGR-h). Recombinant AbSGR-h with its intact sequence and those with mutations were overexpressed in Escherichia coli and their Mg-dechelatase activity were compared. Two aspartates - D34 and D62 were found to be essential for catalysis, while R26, Y28, T29 and D114 were responsible for structural maintenance. Gel filtration analysis of the recombinant AbSGR-h indicates that it forms a homo-oligomer. The three-dimensional structure of AbSGR-h was predicted by a deep learning-based method, which was evaluated by protein structure quality evaluation programs while structural stability of wild-type and mutant forms were investigated through molecular dynamics simulations. Furthermore, in concordance with the results of enzyme assay, molecular docking concluded the significance of D34 in ligand interaction. By combining biochemical analysis and computational prediction, this study unveils the detailed structural characteristics of the enzyme, including the probable pocket of interaction and the residues of structural and functional importance. It also serves as a basis for further studies on Mg-dechelatase such as elucidation of its reaction mechanism or inhibitor screening.

Regulation of carotenoid and chlorophyll pools in hesperidia, anatomically unique fruits found only in Citrus

Domesticated citrus varieties are woody perennials and interspecific hybrid crops of global economic and nutritional importance. The citrus fruit "hesperidium" is a unique morphological innovation not found in any other plant lineage. Efforts to improve the nutritional quality of the fruit are predicated on understanding the underlying regulatory mechanisms responsible for fruit development, including temporal control of chlorophyll degradation and carotenoid biosynthesis. Here, we investigated the molecular basis of the navel orange (Citrus sinensis) brown flavedo mutation, which conditions flavedo that is brown instead of orange. To overcome the limitations of using traditional genetic approaches in citrus and other woody perennials, we developed a strategy to elucidate the underlying genetic lesion. We used a multi-omics approach to collect data from several genetic sources and plant chimeras to successfully decipher this mutation. The multi-omics strategy applied here will be valuable in driving future gene discovery efforts in citrus as well as in other woody perennial plants. The comparison of transcriptomic and genomic data from multiple genotypes and plant sectors revealed an underlying lesion in the gene encoding STAY-GREEN (SGR) protein, which simultaneously regulates carotenoid biosynthesis and chlorophyll degradation. However, unlike SGR of other plant species, we found that the carotenoid and chlorophyll regulatory activities could be uncoupled in the case of certain SGR alleles in citrus and thus we propose a model for the molecular mechanism underlying the brown flavedo phenotype. The economic and nutritional value of citrus makes these findings of wide interest. The strategy implemented, and the results obtained, constitute an advance for agro-industry by driving opportunities for citrus crop improvement.

Chlorophyll dephytylation in chlorophyll metabolism: a simple reaction catalyzed by various enzymes

Chlorophyll (Chl) is composed of a tetrapyrrole ring and a phytol tail, which facilitate light energy absorbance and assembly with photosynthetic protein complexes, respectively. Chl dephytylation, the hydrolytic removal of the phytol tail, is considered a pivotal step in diverse physiological processes, such as Chl salvage during repair of the photosystem, the Chl cycle in the adjustment of antenna size, and Chl breakdown in leaf senescence and fruit maturation. Moreover, phytol is a component of the tocopherols, a major form of vitamin E that is essential in the human diet. This phytol mostly comes from Chl hydrolysis. However, the authentic enzyme responsible for Chl dephytylation has proved elusive. CHLOROPHYLLASE (CLH) which was discovered over a century ago, was the first enzyme found to have dephytylation activity in vitro, but its role in Chl metabolism has been questioned and remains under debate. Recently, novel dephytylases, i.e., PHEOPHYTINASE (PPH) and CHLOROPHYLL DEPHYTYLASE1 (CLD1) have emerged from genetic studies, indicating that dephytylation in Chl catabolism involves different players and is more complicated than previously thought. Based on sequence homology, substrate specificity, and subcellular localization, CLH, PPH, and CLD1 belong to different types of dephytylase, which prompted us to re-examine the dilemmas and missing links that still exist in Chl metabolism. This review thus focuses on the hitherto unanswered questions involving the Chl dephytylation reaction by highlighting relevant literature, updating recent progress, and synthesizing ideas.

SlSGRL, a tomato SGR-like protein, promotes chlorophyll degradation downstream of the ABA signaling pathway

Chlorophyll (chl) degradation plays a vital role during green plant growth and development, including nutrient metabolism, fruit and seed maturation, and phototoxic detoxification. STAY-GREEN (SGR) is a plant-specific regulator involved in chl degradation. Previous studies showed that SlSGR1 functioned in chl degradation and lycopene accumulation during fruit ripening of tomato (Solanum lycopersicum). However, little is known about SlSGR-LIKE (SlSGRL) gene, which is a homolog of SlSGR1. We cloned the SlSGRL gene and created transgenic tomato plants overexpressing (OE) SlSGRL. Expression analysis showed that SlSGRL was up-regulated by abscisic acid (ABA). Our data showed that SlSGRL-OE lines exhibited earlier leaf yellowing than wild-type (WT) lines under ABA treatment. Yeast two-hybrid (Y2H) assay revealed that SlSGRL interacted with pheophytin pheophorbide hydrolase (SlPPH) and light-harvesting complex a2 (SlLHCa2) to promote the chl degradation. Further analysis demonstrated that ABA-INSENSITIVE5 (SlABI5) and SlABI5-LIKE regulated SlSGRL expression by directly binding to the sequence (-611 to -582) of the SlSGRL promoter that included an ABRE cis-element. We proposed that SlSGRL, which was regulated by SlABI5/SlABI5-LIKE, mainly acted in ABA-induced chl degradation via interacting with SlPPH and SlLHCa2.

Roles of stay-green (SGR) homologs during chlorophyll degradation in green plants

Chlorophyll (Chl) degradation is one of the most obvious signs of leaf senescence and fruit ripening. Stay-green (SGR) homologs that can remove magnesium from Chl a are the most important components in Chl degradation pathway in green plants. SGR homologs are not only universally involved in Chl breakdown during the senescence of green organs, but also play crucial roles in other organs during plant growth and development, such as fruit mature and nodule development. In this review, we focus on the diverse functions of SGR homologs in plant growth and development. A better understanding of SGR would be helpful for providing a theoretical basis for further illustrating the regulatory mechanism of SGR homologs.

Horizontal Transfer of Promiscuous Activity from Nonphotosynthetic Bacteria Contributed to Evolution of Chlorophyll Degradation Pathway

The relationship between enzymes and substrates does not perfectly match the "lock and key" model, because enzymes act on molecules other than their true substrate in different catalytic reactions. Such biologically nonfunctional reactions are called "promiscuous activities." Promiscuous activities are apparently useless, but they can be an important starting point for enzyme evolution. It has been hypothesized that enzymes with low promiscuous activity will show enhanced promiscuous activity under selection pressure and become new specialists through gene duplication. Although this is the prevailing scenario, there are two major problems: 1) it would not apply to prokaryotes because horizontal gene transfer is more significant than gene duplication and 2) there is no direct evidence that promiscuous activity is low without selection pressure. We propose a new scenario including various levels of promiscuous activity throughout a clade and horizontal gene transfer. STAY-GREEN (SGR), a chlorophyll a-Mg dechelating enzyme, has homologous genes in bacteria lacking chlorophyll. We found that some bacterial SGR homologs have much higher Mg-dechelating activities than those of green plant SGRs, while others have no activity, indicating that the level of promiscuous activity varies. A phylogenetic analysis suggests that a bacterial SGR homolog with high dechelating activity was horizontally transferred to a photosynthetic eukaryote. Some SGR homologs acted on various chlorophyll molecules that are not used as substrates by green plant SGRs, indicating that SGR acquired substrate specificity after transfer to eukaryotes. We propose that horizontal transfer of high promiscuous activity is one process of new enzyme acquisition.

Defect in Brnym1, a magnesium-dechelatase protein, causes a stay-green phenotype in an EMS-mutagenized Chinese cabbage (Brassica campestris L. ssp. pekinensis) line

Leaf color is an important target trait in Chinese cabbage breeding programs. Leaf yellowing may reduce crop commercial and nutritional values. Some plants with the "stay-green" trait maintain leaf greenness during senescence and even after death. Stay-green Chinese cabbage may be a focal point of future breeding projects because it could improve crop quality and yield and prolong shelf life. A new stay-green mutant, non-yellowing mutant 1 (nym1), was identified in Chinese cabbage derived from an ethyl methane sulfonate (EMS)-mutagenized population. The mutant had stay-green characteristics and a higher chlorophyll content than the wild-type during leaf senescence. The stay-green trait in the mutant Chinese cabbage was controlled by the recessive gene Brnym1. MutMap and KASP analyses showed that Brnym1 (BraA03g050600.3C) encodes an mg-dechelatase (SGR protein), which might be the causal gene of the mutation in Chinese cabbage. A nonsynonymous single nucleotide base substitution (G to A) in the third exon of Brnym1 caused an amino acid substitution from L to F in the highly conserved domain of the magnesium-dechelatase. Ectopic overexpression showed that the BrNYM1 gene of wild-type Chinese cabbage complemented the SGR-defective stay-green mutant nye1-1 of Arabidopsis. The magnesium-dechelatase activity in the nym1 mutant was significantly downregulated compared to that in the wild type. Brnym1 was relatively upregulated in the mutant during late senescence, and BrNYM1 was localized to the chloroplasts. These results indicate that Brnym1 (BraA03g050600.3C) is the causal gene of the stay-green mutation and could be of particular significance in the genetic improvement of Chinese cabbage.

Chlorophyll Oxidative Metabolism During the Phototrophic and Heterotrophic Growth of Scenedesmus obliquus

Different cultivation strategies have been developed with the aim of increasing the production rate of microalgal pigments. Specifically, biotechnological approaches are designed to increase antioxidant metabolites as chlorophyll and carotenoids. However, although significant advances have been built up, available information regarding both the chlorophyll metabolism and their oxidative reactions in photobioreactors is scarce. To unravel such processes, the detailed chlorophyll and carotenoid fraction of Scenedesmus obliquus has been studied by HPLC-ESI/APCI-hrTOF-MS from phototrophic and heterotrophic cultures. Scenedesmus is provided with a controlled strategy of interconversion between chlorophyll a and b to avoid the formation of reactive oxygen species (ROS) at high irradiances in addition to the photoacclimation of carotenoids. Indeed, precise kinetics of 132-hydroxy- and 151-hydroxy-lactone chlorophyll metabolites shows the existence of a chlorophyll oxidative metabolism as a tool to manage the excess of energy at high light conditions. Unexpectedly, the oxidation under phototrophy favored chlorophyll b metabolites over the chlorophyll a series, while the heterotrophic conditions exclusively induced the formation of 132-hydroxy-chlorophyll a. In parallel, during the first 48 h of growth in the dark, the chlorophyll fraction maintained a promising steady state. Although future studies are required to resolve the biochemical reactions implied in the chlorophyll oxidative metabolism, the present results agree with phytoplankton metabolism.

The C-terminal cysteine-rich motif of NYE1/SGR1 is indispensable for its function in chlorophyll degradation in Arabidopsis

The C-terminal cysteine-rich motif of NYE1/SGR1 affects chlorophyll degradation likely by mediating its self-interaction and conformational change, and somehow altering its Mg-dechelating activity in response to the changing redox potential. During green organ senescence in plants, the most prominent phenomenon is the degreening caused by net chlorophyll (Chl) loss. NON-YELLOWING1/STAY-GREEN1 (NYE1/SGR1) was recently reported to be able to dechelates magnesium (Mg) from Chl a to initiate its degradation, but little is known about the domain/motif basis of its functionality. In this study, we carried out a protein truncation assay and identified a conserved cysteine-rich motif (CRM, P-X3-C-X3-C-X-C2-F-P-X5-P) at its C terminus, which is essential for its function. Genetic analysis showed that all four cysteines in the CRM were irreplaceable, and enzymatic assays demonstrated that the mutation of each of the four cysteines affected its Mg-dechelating activity. The CRM plays a critical role in the conformational change and self-interaction of NYE1 via the formation of inter- and intra-molecular disulfide bonds. Our results may provide insight into how NYE1 responds to rapid redox changes during leaf senescence and in response to various environmental stresses.

Mg-dechelatase is involved in the formation of photosystem II but not in chlorophyll degradation in Chlamydomonas reinhardtii

The STAY-GREEN (SGR) gene encodes Mg-dechelatase which catalyzes the conversion of chlorophyll (Chl) a to pheophytin (Pheo) a. This reaction is the first and most important regulatory step in the Chl degradation pathway. Conversely, Pheo a is an indispensable molecule in photosystem (PS) II, suggesting the involvement of SGR in the formation of PSII. To investigate the physiological functions of SGR, we isolated Chlamydomonas sgr mutants by screening an insertion-mutant library. The sgr mutants had reduced maximum quantum efficiency of PSII (F_v /F_m ) and reduced Pheo a levels. These phenotypes were complemented by the introduction of the Chlamydomonas SGR gene. Blue Native polyacrylamide gel electrophoresis and immunoblotting analysis showed that although PSII levels were reduced in the sgr mutants, PSI and light-harvesting Chl a/b complex levels were unaffected. Under nitrogen starvation conditions, Chl degradation proceeded in the sgr mutants as in the wild type, indicating that ChlamydomonasSGR is not required for Chl degradation and primarily contributes to the formation of PSII. In contrast, in the Arabidopsis sgr triple mutant (sgr1 sgr2 sgrL), which completely lacks SGR activity, PSII was synthesized normally. These results suggest that the Arabidopsis SGR participates in Chl degradation while the ChlamydomonasSGR participates in PSII formation despite having the same catalytic property.

Mg-dechelation of chlorophyll a by Stay-Green activates chlorophyll b degradation through expressing Non-Yellow Coloring 1 in Arabidopsis thaliana

The first step in chlorophyll a degradation is the extraction of the central Mg. This reaction is catalyzed by Mg-dechelatase encoded by Stay-Green (SGR) in land plants. SGR extracts Mg from chlorophyll a but not from chlorophyll b, and chlorophyll b must be converted to chlorophyll a before degradation. The first reaction of the chlorophyll b to chlorophyll a conversion is catalyzed by chlorophyll b reductase. Non-Yellow Coloring 1 (NYC1) and NYC1 like (NOL) are isozymes of chlorophyll b reductase. When SGR was transiently overexpressed in Arabidopsis, both chlorophyll a and b were degraded, suggesting that the chlorophyll b to chlorophyll a conversion is activated by SGR overexpression. To examine the involvement of chlorophyll b reductases in SGR-induced chlorophyll b degradation, SGR was transiently overexpressed in nyc1, nol, and nyc1 nol double mutants by dexamethasone treatment. It was found that in the wild type and nol mutant, chlorophyll a and b were degraded and all the chlorophyll-binding proteins decreased. Meanwhile, in nyc1 and nyc1 nol mutants, chlorophyll b degradation was suppressed and the light-harvesting complex of photosystem II remained. The mRNA and protein levels of NYC1 increased after SGR overexpression in wild type plants. These results suggest that Mg-dechelation of chlorophyll a by SGR activates chlorophyll b degradation by inducing the expression of NYC1. This is an effective regulation of a metabolic pathway.

The biochemistry and molecular biology of chlorophyll breakdown

Chlorophyll breakdown is one of the most obvious signs of leaf senescence and fruit ripening. The resulting yellowing of leaves can be observed every autumn, and the color change of fruits indicates their ripening state. During these processes, chlorophyll is broken down in a multistep pathway, now termed the 'PAO/phyllobilin' pathway, acknowledging the core enzymatic breakdown step catalysed by pheophorbide a oxygenase, which determines the basic linear tetrapyrrole structure of the products of breakdown that are now called 'phyllobilins'. This review provides an update on the PAO/phyllobilin pathway, and focuses on recent biochemical and molecular progress in understanding phyllobilin-modifying reactions as the basis for phyllobilin diversity, on the evolutionary diversity of the pathway, and on the transcriptional regulation of the pathway genes.

Pilot scale flat panel photobioreactor system for mass production of Ulva lactuca (Chlorophyta)

Conventional open-sea cultivation constrained by environmental factors is singly incompetent to sustain the rising seaweed demand. This necessitates a complementary strategy to reinforce the existing cultivation system and expand the global seaweed industry. Present study proposes cultivation of Ulva lactuca in temperature controlled flat panel photobioreactors under natural illumination. Adaptability of U. lactuca to the flat panel system is apparent through growth studies and photosynthetic performance (F_v/F_m) across individual panels. Evident effect of annual variation in irradiance on daily growth rates, biomass productivity and composition is portrayed. Significance of initial stocking density and harvesting frequency is highlighted. Poultry litter extract was used as an alternative N-source for sustainable cultivation. The maximum achievable productivity was 303gm^-2d^-1 (fresh weight) expanding to 910tonsha^-1yr^-1 including biomass composition consistent with the control media. The present pilot scale study delivers valuable information for commercial scale photobioreactors for seaweed cultivation.