Feedback inhibition of chlorophyll synthesis in the phytochrome chromophore-deficient aurea and yellow-green-2 mutants of tomato

Molecular Mechanisms of Chlorophyll Deficiency in Ilex × attenuata 'Sunny Foster' Mutant

Ilex × attenuata 'Sunny Foster' represents a yellow leaf mutant originating from I. × attenuata 'Foster#2', a popular ornamental woody cultivar. However, the molecular mechanisms underlying this leaf color mutation remain unclear. Using a comprehensive approach encompassing cytological, physiological, and transcriptomic methodologies, notable distinctions were discerned between the mutant specimen and its wild type. The mutant phenotype displayed aberrant chloroplast morphology, diminished chlorophyll content, heightened carotenoid/chlorophyll ratios, and a decelerated rate of plant development. Transcriptome analysis identified differentially expressed genes (DEGs) related to chlorophyll metabolism, carotenoid biosynthesis and photosynthesis. The up-regulation of CHLD and CHLI subunits leads to decreased magnesium chelatase activity, while the up-regulation of COX10 increases heme biosynthesis-both impair chlorophyll synthesis. Conversely, the down-regulation of HEMD hindered chlorophyll synthesis, and the up-regulation of SGR enhanced chlorophyll degradation, resulting in reduced chlorophyll content. Additionally, genes linked to carotenoid biosynthesis, flavonoid metabolism, and photosynthesis were significantly down-regulated. We also identified 311 putative differentially expressed transcription factors, including bHLHs and GLKs. These findings shed light on the molecular mechanisms underlying leaf color mutation in I. × attenuata 'Sunny Foster' and provide a substantial gene reservoir for enhancing leaf color through breeding techniques.

Biochemical and Transcriptome Analysis Reveals Pigment Biosynthesis Influenced Chlorina Leaf Formation in Anoectochilus roxburghii (Wall.) Lindl

Anoectochilus roxburghii (Wall.) Lindl is a perennial herb of the Orchidaceae family; a yellow-green mutant and a yellow mutant were obtained from the wild type, thereby providing good material for the study of leaf color variation. Pigment content analysis revealed that chlorophyll, carotenoids, and anthocyanin were lower in the yellow-green and yellow mutants than in the wild type. Transcriptome analysis of the yellow mutant and wild type revealed that 78,712 unigenes were obtained, and 599 differentially expressed genes (120 upregulated and 479 downregulated) were identified. Using the Kyoto Encyclopedia of Genes and Genomes pathway analysis, candidate genes involved in the anthocyanin biosynthetic pathway (five unigenes) and the chlorophyll metabolic pathway (two unigenes) were identified. Meanwhile, the low expression of the chlorophyll and anthocyanin biosynthetic genes resulted in the absence of chlorophylls and anthocyanins in the yellow mutant. This study provides a basis for similar research in other closely related species.

Mutation to a cytochrome P450 -like gene alters the leaf color by affecting the heme and chlorophyll biosynthesis pathways in Brassica napus

The regulated biosynthesis of chlorophyll is important because of its effects on plant photosynthesis and dry biomass production. In this study, a map-based cloning approach was used to isolate the cytochrome P450 -like gene BnaC08g34840D (BnCDE1) from a chlorophyll-deficient mutant (cde1) of Brassica napus obtained by ethyl methanesulfonate (EMS) mutagenization. Sequence analyses revealed that BnaC08g34840D in the cde1 mutant (BnCDE1I320T ) encodes a substitution at amino acid 320 (Ile320Thr) in the conserved region. The over-expression of BnCDE1I320T in ZS11 (i.e., gene-mapping parent with green leaves) recapitulated a yellow-green leaf phenotype. The CRISPR/Cas9 genome-editing system was used to design two single-guide RNAs (sgRNAs) targeting BnCDE1I320T in the cde1 mutant. The knockout of BnCDE1I320T in the cde1 mutant via a gene-editing method restored normal leaf coloration (i.e., green leaves). These results indicate that the substitution in BnaC08g34840D alters the leaf color. Physiological analyses showed that the over-expression of BnCDE1I320T leads to decreases in the number of chloroplasts per mesophyll cell and in the contents of the intermediates of the chlorophyll biosynthesis pathway in leaves, while it increases heme biosynthesis, thereby lowering the photosynthetic efficiency of the cde1 mutant. The Ile320Thr mutation in the highly conserved region of BnaC08g34840D inhibited chlorophyll biosynthesis and disrupted the balance between heme and chlorophyll biosynthesis. Our findings may further reveal how the proper balance between the chlorophyll and heme biosynthesis pathways is maintained.

Mechanisms for leaf color changes in Osmanthus fragrans 'Ziyan Gongzhu' using physiology, transcriptomics and metabolomics

BACKGROUND

Color-leaved O. fragrans is a variety of Osmanthus fragrans, which has both the fragrance of Osmanthus and the color of color-leaved plants. However, the molecular mechanism of color change of color-leaved O. fragrans is not clear. In this study, we analyzed the regulatory mechanism of four different color leaves of 'Ziyan Gongzhu' through physiological, transcriptome and metabolome levels.

RESULTS

Firstly, we measured the leaf pigments content and leaf chromatic parameters for correlation analysis, indicating a significant correlation between them. Overall, the content of chlorophyll a + b is low and the content of anthocyanin is high in T1 and T2 leaves, along with low expression of chlorophyll synthesis genes (HEMA, CHLG, and CAO, etc.) and high expression of anthocyanin synthesis genes (F3H, F3'H, DFR and ANS, etc.), resulting purple red and light purple in T1 and T2 leaves, respectively. It was also found that the pigment closely related to the color leaves of 'Ziyan Gongzhu' was cyanidin. The content anthocyanins, may be regulated by two putative MYB activators (OfMYB3 and OfMYB4) and two putative MYB repressors (OfMYB1 and OfMYB2). In contrast, the content of chlorophyll a + b is high and the content of anthocyanin is low in T3 and T4 leaves, along with high expression of chlorophyll synthesis genes and low expression of anthocyanin synthesis genes, resulting yellow green and dark green in T3 and T4 leaves, respectively. And abnormal chloroplast development affects chlorophyll content in T1, T2, and T3 leaves. Although the content of carotenoids first dropped in T2 leaves, it then rapidly accumulated in T4 leaves, in sync with the increase in the expression of genes related to carotenoid biosynthesis (ZDS, LHYB, and ZEP, for example). Analysis of photosynthetic, carbohydrate and hormone-related differentially abundant metabolites (DAMs) and DEGs found that they may participate in the regulation of leaf color change of 'Ziyan Gongzhu' by affecting pigment synthesis.

CONCLUSION

Our results pave the way for a comprehensive knowledge of the regulatory processes governing leaf color in 'Ziyan Gongzhu' and identify possible genes for application regarding molecular colored-leaf cultivar breeding.

Melon yellow-green plant (Cmygp) encodes a Golden2-like transcription factor regulating chlorophyll synthesis and chloroplast development

A SNP mutation in CmYGP gene encoding Golden2-like transcription factor is responsible for melon yellow-green plant trait. Chlorophylls are essential and beneficial substances for both plant and human health. Identifying the regulatory network of chlorophyll is necessary to improve the nutritional quality of fruits. At least six etiolation genes have been identified in different melon varieties, but none of them have been cloned, and the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in melon remain unclear. Here, the NSL73046, a yellow-green plant (Cmygp) mutant, enabled the map-based cloning of the first etiolation gene in melon. CmYGP encodes a Golden2-like transcription factor. Spatiotemporal expression analyses confirmed the high CmYGP expression in all green tissues, particularly in young leaves and fruit peels. Virus-induced gene silencing and the development of near-isogenic line by marker-assisted selection further confirmed that downregulation of CmYGP can reduce chloroplast number and chlorophyll content, thereby resulting in yellow-green leaves and fruits in melon, and overexpression of CmYGP in tomatoes also led to dark-green leaves and fruits. RNA-seq analysis revealed that CmYGP greatly affected the expression of key genes associated with chloroplast development. Taken together, these findings demonstrated that CmYGP regulate chlorophyll synthesis and chloroplast development thus affect fruit development in melon. This study also offers a new strategy to enhance fruit quality in melon.

Phytol recycling: essential, yet not limiting for tomato fruit tocopherol accumulation under normal growing conditions

Tocopherols are potent membrane-bound antioxidant molecules that are paramount for plant physiology and also important for human health. In the past years, chlorophyll catabolism was identified as the primary source of phytyl diphosphate for tocopherol synthesis by the action of two enzymes, PHYTOL KINASE (VTE5) and PHYTHYL PHOSPHATE KINASE (VTE6) that are able to recycle the chlorophyll-derived phytol. While VTE5 and VTE6 were proven essential for tocopherol metabolism in tomato fruits, it remains unknown whether they are rate-limiting steps in this pathway. To address this question, transgenic tomato plants expressing AtVTE5 and AtVTE6 in a fruit-specific manner were generated. Although ripe transgenic fruits exhibited higher amounts of tocopherol, phytol recycling revealed a more intimate association with chlorophyll than with tocopherol content. Interestingly, protein-protein interactions assays showed that VTE5 and VTE6 are complexed, channeling free phytol and phytyl-P, thus mitigating their cytotoxic nature. Moreover, the analysis of tocopherol accumulation dynamics in roots, a chlorophyll-devoid organ, revealed VTE5-dependent tocopherol accumulation, hinting at the occurrence of shoot-to-root phytol trafficking. Collectively, these results demonstrate that phytol recycling is essential for tocopherol biosynthesis, even in chlorophyll-devoid organs, yet it is not the rate-limiting step for this pathway under normal growth conditions.

Identification and haplotype analysis of SiCHLI: a gene for yellow-green seedling as morphological marker to accelerate foxtail millet (Setaria italica) hybrid breeding

We cloned and developed functional markers for the SiCHLI gene, which is responsible for the yellow-green color of leaves in foxtail millet, a frequently used marker trait in the hybrid breeding of foxtail millet by using bulked segregant analysis sequencing and haplotype analysis on the F₂ and core-collected nature populations. The color of leaves has been widely used as a marker for the hybrid breeding of foxtail millet; however, few related gene have been cloned to date. Here, we used two F₂ populations generated from crosses between the highly male-sterile material 125A with yellow-green leaves, and CG58 and S410, which have green leaves, to identify the genes underlying the yellow-green color of the leaves of foxtail millet. The leaves of 125A seedlings were yellow-green, but they became green at the heading stage. The content of chlorophyll a and chlorophyll b was lower, the number of thylakoid lamellae and grana was reduced, and the chloroplasts was more rounded in 125A than in S410 at the yellow-green leaf stage; however, no differences were observed between 125A and S410 in these traits and photosynthetic at the heading stage. Bulked segregant analysis and map-based cloning revealed that the SiCHLI gene is responsible for the leaf colors of 125A. A nonsynonymous mutation (C/T) in exon 3 causes yellow-green leaves in 125A at the seedling stage. Haplotype analysis of the SiCHLI gene in 596 core collected accessions revealed a new haplotype associated with high photosynthetic metabolic potential at the heading and mature stages, which could be used to enhance sterile lines with yellow-green leaves. We developed a functional marker that will facilitate the identification of foxtail millet accessions with the different types of yellow-green leaves. Generally, our study provides new genetic resources to guide the future marker-assisted or target-base editing in foxtail millet hybrid breeding.

A mutation in SlCHLH encoding a magnesium chelatase H subunit is involved in the formation of yellow stigma in tomato (Solanum lycopersicum L.)

Chlorophylls are ubiquitous pigments responsible for the green color in plants. Changes in the chlorophyll content have a significant impact on photosynthesis, plant growth and development. In this study, we used a yellow stigma mutant (ys) generated from a green stigma tomato WT by using ethylmethylsulfone (EMS)-induced mutagenesis. Compared with WT, the stigma of ys shows low chlorophyll content and impaired chloroplast ultrastructure. Through map-based cloning, the ys gene is localized to a 100 kb region on chromosome 4 between dCAPS596 and dCAPS606. Gene expression analysis and nonsynonymous SNP determination identified the Solyc04g015750, as the potential candidate gene, which encodes a magnesium chelatase H subunit (CHLH). In ys mutant, a single base C to T substitution in the SlCHLH gene results in the conversion of Serine into Leucine (Ser92Leu) at the N-terminal region. The functional complementation test shows that the SlCHLH from WT can rescue the green stigma phenotype of ys. In contrast, knockdown of SlCHLH in green stigma tomato AC, observed the yellow stigma phenotype at the stigma development stage. Overexpression of the mutant gene Slys in green stigma tomato AC results in the light green stigma. These results indicate that the mutation of the N-terminal S92 to Leu in SlCHLH is the main reason for the formation of the yellow stigma phenotype. Characterization of the ys mutant enriches the current knowledge of the tomato chlorophyll mutant library and provides a novel and effective tool for understanding the function of CHLH in tomato.

Metabolic Pathways Involved in the Drought Stress Response of Nitraria tangutorum as Revealed by Transcriptome Analysis

Drought resistance in plants is controlled by multiple genes. To identify the genes that mediate drought stress responses and to assess the associated metabolic pathways in the desert shrub Nitraria tangutorum, we conducted a transcriptome analysis of plants under control (maximum field capacity) and drought (20% of the maximum field capacity) conditions. We analyzed differentially expressed genes (DEGs) of N. tangutorum and their enrichment in the KEGG metabolic pathways database, and explored the molecular biological mechanisms underlying the answer to its drought tolerance. Between the control and drought groups, 119 classified metabolic pathways annotated 3047 DEGs in the KEGG database. For drought tolerance, nitrate reductase (NR) gene expression was downregulated, indicating that NR activity was decreased to improve drought tolerance. In ammonium assimilation, drought stress inhibited glutamine formation. Protochlorophyllide reductase (1.3.1.33) expression was upregulated to promote chlorophyll a synthesis, whereas divinyl reductase (1.3.1.75) expression was downregulated to inhibit chlorophyll-ester a synthesis. The expression of the chlorophyll synthase (2.5.1.62) gene was downregulated, which affected the synthesis of chlorophyll a and b. Overall, drought stress appeared to improve the ability to convert chlorophyll b into chlorophyll a. Our data serve as a theoretical foundation for further elucidating the growth regulatory mechanism of desert xerophytes, thereby facilitating the development and cultivation of new, drought-resistant genotypes for the purpose of improving desert ecosystems.

Impact of Porphyrin Binding to GENOMES UNCOUPLED 4 on Tetrapyrrole Biosynthesis in planta

Plant tetrapyrrole biosynthesis (TPS) provides the indispensable chlorophyll (Chl) and heme molecules in photosynthetic organisms. Post-translational mechanisms control the enzymes to ensure a balanced flow of intermediates in the pathway and synthesis of appropriate amounts of both endproducts. One of the critical regulators of TPS is GENOMES UNCOUPLED 4 (GUN4). GUN4 interacts with magnesium chelatase (MgCh), and its binding of the catalytic substrate and product of the MgCh reaction stimulates the insertion of Mg²⁺ into protoporphyrin IX. Despite numerous in vitro studies, knowledge about the in vivo function of the GUN4:porphyrin interaction for the whole TPS pathway, particularly in plants, is still limited. To address this, we focused on two highly conserved amino acids crucial for porphyrin-binding to GUN4 and analyzed GUN4-F191A, R211A, and R211E substitution mutants in vitro and in vivo. Our analysis confirmed the importance of these amino acids for porphyrin-binding and the stimulation of plant MgCh by GUN4 in vitro. Expression of porphyrin-binding deficient F191A, R211A, and R211E in the Arabidopsis gun4-2 knockout mutant background revealed that, unlike in cyanobacteria and green algae, GUN4:porphyrin interactions did not affect the stability of GUN4 or other Arabidopsis TPS pathway enzymes in vivo. In addition, although they shared diminished porphyrin-binding and MgCh activation in vitro, expression of the different GUN4 mutants in gun4-2 had divergent effects on the TPS and the accumulation of Chl and Chl-binding proteins. For instance, expression of R211E, but not R211A, induced a substantial decrease of ALA synthesis rate, lower TPS intermediate and Chl level, and strongly impaired accumulation of photosynthetic complexes compared to wild-type plants. Furthermore, the presence of R211E led to significant growth retardation and paler leaves compared to GUN4 knockdown mutants, indicating that the exchange of R211 to glutamate compromised TPS and Chl accumulation more substantially than the almost complete lack of GUN4. Extensive in vivo analysis of GUN4 point mutants suggested that F191 and R211 might also play a role beyond porphyrin-binding.

AUREA maintains the balance between chlorophyll synthesis and adventitious root formation in tomato

Flooding tolerance is an important trait for tomato breeding. In this study, we obtained a recessive mutant exhibiting highly enhanced submergence resistance. Phenotypical analyses showed that this resistant to flooding (rf) mutant displays slightly chlorotic leaves and spontaneous initiation of adventitious roots (ARs) on stems. The mutation was mapped to the phytochromobilin synthase gene AUREA (AU), in which a single amino acid substitution from asparagine to tyrosine occurred. In addition to the classic function of AU in phytochrome and chlorophyll biogenesis in leaves, we uncovered its novel role in mediating AR formation on stems. We further observed temporal coincidence of the two phenotypes in the rf mutant: chlorosis and spontaneous AR formation and revealed that AU functions by maintaining heme homeostasis. Interestingly, our grafting results suggest that heme might play roles in AR initiation via long-distance transport from leaves to stems. Our results present genetic evidence for the involvement of the AU-heme oxygenase-1-heme pathway in AR initiation in tomato. As fruit production and yield in the rf mutant are minimally impacted, the mutation identified in this study may provide a target for biotechnological renovation of tomato germplasm in future breeding.

Mutation Mechanism of Leaf Color in Plants: A Review

Color mutation is a common, easily identifiable phenomenon in higher plants. Color mutations usually affect the photosynthetic efficiency of plants, resulting in poor growth and economic losses. Therefore, leaf color mutants have been unwittingly eliminated in recent years. Recently, however, with the development of society, the application of leaf color mutants has become increasingly widespread. Leaf color mutants are ideal materials for studying pigment metabolism, chloroplast development and differentiation, photosynthesis and other pathways that could also provide important information for improving varietal selection. In this review, we summarize the research on leaf color mutants, such as the functions and mechanisms of leaf color mutant-related genes, which affect chlorophyll synthesis, chlorophyll degradation, chloroplast development and anthocyanin metabolism. We also summarize two common methods for mapping and cloning related leaf color mutation genes using Map-based cloning and RNA-seq, and we discuss the existing problems and propose future research directions for leaf color mutants, which provide a reference for the study and application of leaf color mutants in the future.

Chloroplast development at low temperature requires the pseudouridine synthase gene TCD3 in rice

Low temperature affects a broad spectrum of cellular components in plants, such as chloroplasts, as well as plant metabolism. On the other hand, pseudouridine (Ψ) synthases are required for the most abundant post-transcriptional modification of RNA in Escherichia coli. However, the role of rice Ψ synthases in regulating chloroplast development at low temperature remains elusive. In this study, we identified the rice thermo-sensitive chlorophyll-deficient (tcd3) mutant, which displays an albino phenotype before the 4-leaf stage and ultimately dies when grown at 20 °C, but can grow normally at 32 °C. Genetic analysis showed that the mutant trait is controlled by a single recessive nuclear gene (tcd3). Map-based cloning, complementation and knockout tests revealed that TCD3 encodes a chloroplast-localized Ψ synthase. TCD3 is a cold-induced gene that is mainly expressed in leaves. The disruption of TCD3 severely affected the transcript levels of various chloroplast-associated genes, as well as ribosomal genes involved in chloroplast rRNA assembly at low temperature (20 °C), whereas the transcript levels of these genes were normal at high temperature (32 °C). These results provide a first glimpse into the importance of rice Ψ synthase gene in chloroplast development at low temperatures.

Overexpression of chloroplast-targeted ferrochelatase 1 results in a genomes uncoupled chloroplast-to-nucleus retrograde signalling phenotype

Chloroplast development requires communication between the progenitor plastids and the nucleus, where most of the genes encoding chloroplast proteins reside. Retrograde signals from the chloroplast to the nucleus control the expression of many of these genes, but the signalling pathway is poorly understood. Tetrapyrroles have been strongly implicated as mediators of this signal with the current hypothesis being that haem produced by the activity of ferrochelatase 1 (FC1) is required to promote nuclear gene expression. We have tested this hypothesis by overexpressing FC1 and specifically targeting it to either chloroplasts or mitochondria, two possible locations for this enzyme. Our results show that targeting of FC1 to chloroplasts results in increased expression of the nuclear-encoded chloroplast genes GUN4, CA1, HEMA1, LHCB2.1, CHLH after treatment with Norflurazon (NF) and that this increase correlates to FC1 gene expression and haem production measured by feedback inhibition of protochlorophyllide synthesis. Targeting FC1 to mitochondria did not enhance the expression of nuclear-encoded chloroplast genes after NF treatment. The overexpression of FC1 also increased nuclear gene expression in the absence of NF treatment, demonstrating that this pathway is operational in the absence of a stress treatment. Our results therefore support the hypothesis that haem synthesis is a promotive chloroplast-to-nucleus retrograde signal. However, not all FC1 overexpression lines enhanced nuclear gene expression, suggesting there is still a lot we do not understand about the role of FC1 in this signalling pathway. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

A Simple Method for Quantification of Protochlorophyllide in Etiolated Arabidopsis Seedlings

Etiolated seedlings accumulate the chlorophyll biosynthesis intermediate protochlorophyllide (Pchlide) and measuring Pchlide can be important for characterizing photomorphogenic mutants that may be affected in chloroplast development. In this chapter we outline a simple and sensitive method for quantifying Pchlide in extracts of Arabidopsis seedlings using fluorescence spectroscopy. This method can be easily adapted to study chloroplast development in a wide range of plant species.

Identification of two recessive etiolation genes (py1, py2) in pakchoi (Brassica rapa L. ssp. chinensis)

BACKGROUND

Leaf color is a major agronomic trait, which has a strong influence on crop yields. Isolating leaf color mutants can represent valuable materials for research in chlorophyll (Chl) biosynthesis and metabolism regulation.

RESULTS

In this study, we identified a stably inherited yellow leaf mutant derived from 'Huaguan' pakchoi variety via isolated microspore culture and designated as pylm. This mutant displayed yellow leaves after germination. Its etiolated phenotype was nonlethal and stable during the whole growth period. Its growth was weak and its hypocotyls were markedly elongated. Genetic analysis revealed that two recessive nuclear genes, named py1 and py2, are responsible for the etiolation phenotype. Bulked segregant RNA sequencing (BSR-Seq) showed that py1 and py2 were mapped on chromosomes A09 and A07, respectively. The genes were single Mendelian factors in F_3:4 populations based on a 3:1 phenotypic segregation ratio. The py1 was localized to a 258.3-kb interval on a 34-gene genome. The differentially expressed gene BraA09004189 was detected in the py1 mapping region and regulated heme catabolism. One single-nucleotide polymorphism (SNP) of BraA09004189 occurred in pylm. A candidate gene-specific SNP marker in 1520 F_3:4 yellow-colored individuals co-segregated with py1. For py2, 1860 recessive homozygous F_3:4 individuals were investigated and localized py2 to a 4.4-kb interval. Of the five genes in this region, BraA07001774 was predicted as a candidate for py2. It encoded an embryo defective 1187 and a phosphotransferase related to chlorophyll deficiency and hypocotyl elongation. One SNP of BraA07001774 occurred in pylm. It caused a single amino acid mutation from Asp to Asn. According to quantitative real-time polymerase chain reaction (qRT-PCR), BraA07001774 was downregulated in pylm.

CONCLUSIONS

Our study identified a Chl deficiency mutant pylm in pakchoi. Two recessive nuclear genes named py1 and py2 had a significant effect on etiolation. Candidate genes regulating etiolation were identified as BraA09004189 and BraA07001774, respectively. These findings will elucidate chlorophyll metabolism and the molecular mechanisms of the gene interactions controlling pakchoi etiolation.

The Reason for Growth Inhibition of Ulmus pumila 'Jinye': Lower Resistance and Abnormal Development of Chloroplasts Slow Down the Accumulation of Energy

Ulmus pumila 'Jinye', the colorful leaf mutant of Ulmus pumila L., is widely used in landscaping. In common with most leaf color mutants, U. pumila 'Jinye' exhibits growth inhibition. In this study, U. pumila L. and U. pumila 'Jinye' were used to elucidate the reasons for growth inhibition at the physiological, cellular microstructural, and transcriptional levels. The results showed that the pigment (chlorophyll a, chlorophyll b, and carotenoids) content of U. pumila L. was higher than that of U. pumila 'Jinye', whereas U. pumila 'Jinye' had a higher proportion of carotenoids, which may be the cause of the yellow leaves. Examination of the cell microstructure and RNA sequencing analysis showed that the leaf color and growth inhibition were mainly due to the following reasons: first, there were differences in the structure of the thylakoid grana layer. U. pumila L. has a normal chloroplast structure and clear thylakoid grana slice layer structure, with ordered and compact thylakoids. However, U. pumila 'Jinye' exhibited the grana lamella stacking failures and fewer thylakoid grana slice layers. As the pigment carrier and the key location for photosynthesis, the close stacking of thylakoid grana could combine more chlorophyll and promote efficient electron transfer promoting the photosynthesis reaction. In addition, U. pumila 'Jinye' had a lower capacity for light energy absorption, transformation, and transportation, carbon dioxide (CO₂) fixation, lipopolysaccharide biosynthesis, auxin synthesis, and protein transport. The genes related to respiration and starch consumption were higher than those of U. pumila L., which indicated less energy accumulation caused the growth inhibition of U. pumila 'Jinye'. Finally, compared with U. pumila 'Jinye', the transcription of genes related to stress resistance all showed an upward trend in U. pumila L. That is to say, U. pumila L. had a greater ability to resist adversity, which could maintain the stability of the intracellular environment and maintain normal progress of physiological metabolism. However, U. pumila 'Jinye' was more susceptible to changes in the external environment, which affected normal physiological metabolism. This study provides evidence for the main cause of growth inhibition in U. pumila 'Jinye', information for future cultivation, and information on the mutation mechanism for the breeding of colored leaf trees.

The GluTR-binding protein is the heme-binding factor for feedback control of glutamyl-tRNA reductase

Synthesis of 5-aminolevulinic acid (ALA) is the rate-limiting step in tetrapyrrole biosynthesis in land plants. In photosynthetic eukaryotes and many bacteria, glutamyl-tRNA reductase (GluTR) is the most tightly controlled enzyme upstream of ALA. Higher plants possess two GluTR isoforms: GluTR1 is predominantly expressed in green tissue, and GluTR2 is constitutively expressed in all organs. Although proposed long time ago, the molecular mechanism of heme-dependent inhibition of GluTR in planta has remained elusive. Here, we report that accumulation of heme, induced by feeding with ALA, stimulates Clp-protease-dependent degradation of Arabidopsis GluTR1. We demonstrate that binding of heme to the GluTR-binding protein (GBP) inhibits interaction of GBP with the N-terminal regulatory domain of GluTR1, thus making it accessible to the Clp protease. The results presented uncover a functional link between heme content and the post-translational control of GluTR stability, which helps to ensure adequate availability of chlorophyll and heme.

Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxin-dependent manner

Plastids are organelles responsible for essential aspects of plant development, including carbon fixation and synthesis of several secondary metabolites. Chloroplast differentiation and activity are highly regulated by light, and several proteins involved in these processes have been characterised. Such is the case of the GOLDEN 2-LIKE (GLK) transcription factors, which induces the expression of genes related to chloroplast differentiation and photosynthesis. The tomato (Solanum lycopersicum) genome harbours two copies of this gene, SlGLK1 and SlGLK2, each with distinct expression patterns. While the former predominates in leaves, the latter is mainly expressed in fruits, precisely at the pedicel region. During tomato domestication, the selection of fruits with uniform ripening fixed the mutation Slglk2, nowadays present in most cultivated varieties, what penalised fruit metabolic composition. In this study, we investigated how SlGLK2 is regulated by light, auxin and cytokinin and determined the effect of SlGLK2 on tocopherol (vitamin E) and sugar metabolism, which are components of the fruit nutritional and industrial quality. To achieve this, transcriptional profiling and biochemical analysis were performed throughout fruit development and ripening from SlGLK2, Slglk2, SlGLK2-overexpressing genotypes, as well as from phytochrome and hormonal deficient mutants. The results revealed that SlGLK2 expression is regulated by phytochrome-mediated light perception, yet this gene can induce chloroplast differentiation even in a phytochrome-independent manner. Moreover, auxin was found to be a negative regulator of SlGLK2 expression, while SlGLK2 enhances cytokinin responsiveness. Additionally, SlGLK2 enhanced chlorophyll content in immature green fruits, leading to an increment in tocopherol level in ripe fruits. Finally, SlGLK2 overexpression resulted in higher total soluble solid content, possibly by the regulation of sugar metabolism enzyme-encoding genes. The results obtained here shed light on the regulatory network that interconnects SlGLK2, phytohormones and light signal, promoting the plastidial activity and consequently, influencing the quality of tomato fruit.

Map-Based Cloning and Functional Analysis of YE1 in Rice, Which Is Involved in Light-Dependent Chlorophyll Biogenesis and Photoperiodic Flowering Pathway

Light is one of the most important environmental factors that affect many aspects of plant growth, including chlorophyll (Chl) synthesis and flowering time. Here, we identified a rice mutant, yellow leaf and early flowering (ye1), and characterized the gene YE1 by using a map-based cloning method. YE1 encodes a heme oxygenase, which is localized to the chloroplasts. YE1 is expressed in various green tissues, especially in leaves, with a diurnal-rhythmic expression pattern, and its transcripts is also induced by light during leaf-greening. The mutant displays decreased Chl contents with less and disorderly thylakoid lamellar layers in chloroplasts, which reduced the photosynthesis rate. The early flowering phenotype of ye1 was not photoperiod-sensitive. Furthermore, the expression levels of Chl biosynthetic genes were downregulated in ye1 seedlings during de-etiolation responses to light. We also found that rhythmic expression patterns of genes involved in photoperiodic flowering were altered in the mutant. Based on these results, we infer that YE1 plays an important role in light-dependent Chl biogenesis as well as photoperiodic flowering pathway in rice.

Fluorescence in blue light (FLU) is involved in inactivation and localization of glutamyl-tRNA reductase during light exposure

Fluorescent in blue light (FLU) is a negative regulator involved in dark repression of 5-aminolevulinic acid (ALA) synthesis and interacts with glutamyl-tRNA reductase (GluTR), the rate-limiting enzyme of tetrapyrrole biosynthesis. In this study, we investigated FLU's regulatory function in light-exposed FLU-overexpressing (FLUOE) Arabidopsis lines and under fluctuating light intensities in wild-type (WT) and flu seedlings. FLUOE lines suppress ALA synthesis in the light, resulting in reduced chlorophyll content, but more strongly in low and high light than in medium growth light. This situation indicates that FLU's impact on chlorophyll biosynthesis depends on light intensity. FLU overexpressors contain strongly increased amounts of mainly membrane-associated GluTR. These findings correlate with FLU-dependent localization of GluTR to plastidic membranes and concomitant inhibition, such that only the soluble GluTR fraction is active. The overaccumulation of membrane-associated GluTR indicates that FLU binding enhances GluTR stability. Interestingly, under fluctuating light, the leaves of flu mutants contain less chlorophyll compared with WT and become necrotic. We propose that FLU is basically required for fine-tuned ALA synthesis. FLU not only mediates dark repression of ALA synthesis, but functions also to control balanced ALA synthesis under variable light intensities to ensure the adequate supply of chlorophyll.

Allopolyploidization in Cucumis contributes to delayed leaf maturation with repression of redundant homoeologous genes

The important role of polyploidy in plant evolution is widely recognized. However, many questions remain to be explored to address how polyploidy affects the phenotype of the plant. To shed light on the phenotypic and molecular impacts of allopolyploidy, we investigated the leaf development of a synthesized allotetraploid (Cucumis × hytivus), with an emphasis on chlorophyll development. Delayed leaf maturation was identified in C. × hytivus, based on delayed leaf expansion, initial chlorophyll deficiency in the leaves and disordered sink-source transition. Anatomical observations also revealed disturbed chloroplast development in C. ×hytivus. The determination of chlorophyll biosynthesis intermediates suggested that the chlorophyll biosynthesis pathway of C. × hytivus is blocked at the site at which uroporphyrinogen III is catalysed to coproporphyrinogen III. Three chlorophyll biosynthesis-related genes, HEMA1, HEME2 and POR, were significantly repressed in C. × hytivus. Sequence alignment showed both synonymous and non-synonymous substitutions in the HEMA1, HEME2 and POR genes of the parents. Cloning of the chlorophyll biosynthetic genes suggested the retention of homoeologs. In addition, a chimeric clone of the HEMA1 gene that consisted of homologous genes from the parents was identified in C. × hytivus. Overall, our results showed that allopolyploidization in Cucumis has resulted in disturbed chloroplast development and reduced chlorophyll biosynthesis caused by the repressed expression of duplicated homologous genes, which further led to delayed leaf maturation in the allotetraploid, C. × hytivus. The preferential retention/loss of certain types of genes and non-reciprocal homoeologous recombination were also supported in the present study, which provides new insights into the impact of allopolyploidy.

Haem oxygenase: A functionally diverse enzyme of photosynthetic organisms and its role in phytochrome chromophore biosynthesis, cellular signalling and defence mechanisms

Haem oxygenase (HO) is a universal enzyme that catalyses stereospecific cleavage of haem to BV IX α and liberates Fe⁺² ion and CO as by-product. Beside haem degradation, it has important functions in plants that include cellular defence, stomatal regulation, iron mobilization, phytochrome chromophore synthesis, and lateral root formation. Phytochromes are an extended family of photoreceptors with a molecular mass of 250 kDa and occur as a dimer made up of 2 equivalent subunits of 125 kDa each. Each subunit is made of two components: the chromophore, a light-capturing pigment molecule and the apoprotein. Biosynthesis of phytochrome (phy) chromophore includes the oxidative splitting of haem to biliverdin IX by an enzyme HO, which is the decisive step in the biosynthesis. In photosynthetic organisms, BVα is reduced to 3Z PΦB by a ferredoxin-dependent PΦB synthase that finally isomerised to PΦB. The synthesized PΦB assembles with the phytochrome apoprotein in the cytoplasm to generate holophytochrome. Thus, necessary for photomorphogenesis in plants, which has confirmed from the genetic studies, conducted on Arabidopsis thaliana and pea. Besides the phytochrome chromophore synthesis, the review also emphasises on the current advances conducted in plant HO implying its developmental and defensive role.

LLM-Domain B-GATA Transcription Factors Play Multifaceted Roles in Controlling Greening in Arabidopsis

Chlorophyll accumulation and chloroplast development are regulated at multiple levels during plant development. The paralogous LLM-domain B-GATA transcription factors GNC and GNL contribute to chlorophyll biosynthesis and chloroplast formation in light-grown Arabidopsis thaliana seedlings. Whereas there is already ample knowledge about the transcriptional regulation of GNC and GNL, the identity of their downstream targets is largely unclear. Here, we identified genes controlling greening directly downstream of the GATAs by integrating data from RNA-sequencing and microarray data sets. We found that genes encoding subunits of the Mg-chelatase complex and 3,8-divinyl protochlorophyllide a 8-vinyl reductase (DVR) likely function directly downstream of the GATAs and that DVR expression is limiting in the pale-green gnc gnl mutants. The GATAs also regulate the nucleus-encoded SIGMA (SIG) factor genes, which control transcription in the chloroplast and suppress the greening defects of sig mutants. Furthermore, GNC and GNL act, at the gene expression level, in an additive manner with the GOLDEN2-LIKE1 (GLK1) and GLK2 transcription factor genes, which are also important for proper chlorophyll accumulation. We thus reveal that chlorophyll biosynthesis genes are directly controlled by LLM-domain B-GATAs and demonstrate that these transcription factors play an indirect role in the control of greening through regulating SIGMA factor genes.

Evaluation of heme oxygenase 1 (HO 1) in Cd and Ni induced cytotoxicity and crosstalk with ROS quenching enzymes in two to four leaf stage seedlings of Vigna radiata

Research on heme oxygenase in plants has received consideration in recent years due to its several roles in development, defense, and metabolism during various environmental stresses. In the current investigation, the role of heme oxygenase (HO) 1 was evaluated in reducing heavy metal (Cd and Ni) uptake and alleviating Cd and Ni toxicity effects in the hydroponically grown seedlings of Vigna radiata var. PDM 54. Seedlings were subjected to Cd- and Ni-induced oxidative stress independently at different concentrations ranging from 10 to 100 μM. After 96 h (fourth day) of treatment, the stressed plants were harvested to study the cellular homeostasis and detoxification mechanism by examining the growth, stress parameters (LPX, H₂O₂ content), and non-enzymatic and enzymatic parameters (ascorbate peroxidase (APX), guaicol peroxidase (GPX), and catalase (CAT)) including HO 1. At 50 μM CdCl₂ and 60 μM NiSO₄, HO 1 activity was found to be highest in leaves which were 1.39 and 1.16-fold, respectively. The greatest HO 1 activity was reflected from the reduction of H₂O₂ content at these metal concentrations (50 μM CdCl₂ and 60 μM NiSO₄) which is correlated with the increasing activity of other antioxidant enzymes (CAT, APX). Thus, HO 1 works within a group that generates the defense machinery for the plant's survival by scavenging ROS which is confirmed by a time-dependent study. Hence, it is concluded that seedlings of V. radiata were more tolerant towards metal-induced oxidative stress in which HO 1 is localized in its residential area (plastids).

Effects of different growth temperatures on growth, development, and plastid pigments metabolism of tobacco (Nicotiana tabacum L.) plants

BACKGROUND

Temperature remarkably affects the growth and metabolism of plants. Tobacco is an important cash crop, and the long-term effects of different growth temperatures (18.5, 23.5 and 28.5 °C, daily average) on growth, development and plastid pigments metabolism of tobacco plants were investigated in this study.

RESULTS

Compared with tobacco plants grown under 23.5 °C, treatments with 18.5 and 28.5 °C inhibited the expansion of leaves. The contents of superoxide anion (O 2·- ), hydrogen peroxide (H2O2) and malonaldehyde (MDA) in the leaves were significantly increased under 28.5 °C from 0 to 60 days, which in turn accelerated the flowering and senescence of tobacco plants. By contrast, the treatment with 18.5 °C remarkably decreased O 2.- , H2O2 and MDA, and delayed the flowering and senescence. Furthermore, treatment with 18.5 °C significantly up-regulated the expression of glutamyl-tRNA reductase (Glu-TR) and magnesium chelatase (MgCH), and down-regulated the ferri chelatase (FeCH), protochlorophyllide oxidoreductase, chlorophyllase (CHLase), phaeophorbide a monooxygenase (PaO) and phytoene synthase (PSY), which further promoted the accumulation of chlorophyll (Chls) and reduced the carotenoids (Cars) in leaves. On the contrary, exposing to 28.5 °C remarkably down-regulated the Glu-TR and MgCH, and up-regulated the FeCH, CHLase, PaO and PSY, which in turn decreased the Chls and increased the Cars in tobacco leaves.

CONCLUSION

As compared with the plants grown under 23.5 °C, lower (18.5 °C) and higher (28.5 °C) growth temperature inhibited the growth of tobacco plants. In general, treatment with 28.5 °C accelerated the flowering and senescence of tobacco plants by enhancing the accumulation of O 2.- and H2O2 in leaves, while exposing to 18.5 °C had the opposite effects. Treatment with 18.5 °C increased the content of Chls and reduced the Cars in leaves. In contrast, Treatment with 28.5 °C decreased the Chls and increased the Cars. Moreover, both O 2.- and H2O2 took part in the breakdown of Chls in tobacco leaves to some extent. The results suggest that growth temperature could regulate growth, development, and plastid pigments metabolism, and 23.5 °C could be an optimal temperature for growth, development and metabolism of plastid pigments of tobacco plants under the experimental conditions.

Proteome and Acetyl-Proteome Profiling of Camellia sinensis cv. 'Anjin Baicha' during Periodic Albinism Reveals Alterations in Photosynthetic and Secondary Metabolite Biosynthetic Pathways

Tea leaf color is not only important from an aesthetics standpoint but is also related to tea quality. To investigate the molecular mechanisms that determine tea leaf color, we examined Camellia sinensis cv. 'Anjin Baicha' (an albino tea cultivar) by tandem mass tag isobaric labeling to generate a high-resolution proteome and acetyl-proteome atlas of three leaf developmental stages. We identified a total of 7,637 proteins and quantified 6,256; of these, 3,232 were classified as differentially accumulated proteins (DAPs). We also identified 3,161 lysine acetylation sites in 1,752 proteins and quantified 2,869 in 1,612 proteins. The acetylation levels at 468 sites were significantly altered across the three developmental stages during periodic albinism; the corresponding proteins were associated with a variety of biological processes. Interestingly, a large number of DAPs and acetylated proteins with increased/decreased acetylation were related to photosynthesis and secondary metabolite biosynthetic pathways, suggesting that the accumulation or acetylation level of these proteins regulates periodic albinism in 'Anjin Baicha.' Additionally, overlap between succinylome and acetylome among three 'Anjin Baicha' developmental stages were found. These data provide important insight into the mechanisms of leaf coloration in the tea plant. The mass spectrometry data have been deposited to Proteome X change via the PRIDE partner repository with the data set identifier PXD008134.

Characterization of a Thermo-Inducible Chlorophyll-Deficient Mutant in Barley

Leaf color is an important trait for not only controlling crop yield but also monitoring plant status under temperature stress. In this study, a thermo-inducible chlorophyll-deficient mutant, named V-V-Y, was identified from a gamma-radiated population of the barley variety Vlamingh. The leaves of the mutant were green under normal growing temperature but turned yellowish under high temperature in the glasshouse experiment. The ratio of chlorophyll a and chlorophyll b in the mutant declined much faster in the first 7-9 days under heat treatment. The leaves of V-V-Y turned yellowish but took longer to senesce under heat stress in the field experiment. Genetic analysis indicated that a single nuclear gene controlled the mutant trait. The mutant gene (vvy) was mapped to the long arm of chromosome 4H between SNP markers 1_0269 and 1_1531 with a genetic distance of 2.2 cM and a physical interval of 9.85 Mb. A QTL for grain yield was mapped to the same interval and explained 10.4% of the yield variation with a LOD score of 4. This QTL is coincident with the vvy gene interval that is responsible for the thermo-inducible chlorophyll-deficient trait. Fine mapping, based on the barley reference genome sequence, further narrowed the vvy gene to a physical interval of 0.428 Mb with 11 annotated genes. This is the first report of fine mapping a thermo-inducible chlorophyll-deficient gene in barley.

Phytochromobilin deficiency impairs sugar metabolism through the regulation of cytokinin and auxin signaling in tomato fruits

Phytochomes and plant hormones have been emerging as important regulators of fleshy fruit biology and quality traits; however, the relevance of phytochrome-hormonal signaling crosstalk in controlling fruit development and metabolism remains elusive. Here, we show that the deficiency in phytochrome chromophore phytochromobilin (PΦB) biosynthesis inhibits sugar accumulation in tomato (Solanum lycopersicum) fruits by transcriptionally downregulating sink- and starch biosynthesis-related enzymes, such as cell-wall invertases, sucrose transporters and ADP-glucose pyrophosphorylases. PΦB deficiency was also shown to repress fruit chloroplast biogenesis, which implicates more limited production of photoassimilates via fruit photosynthesis. Genetic and physiological data revealed the involvement of auxins and cytokinins in mediating the negative impact of PΦB deficiency on fruit sink strength and chloroplast formation. PΦB deficiency was shown to transcriptionally repress type-A TOMATO RESPONSE REGULATORs and AUXIN RESPONSE FACTORs both in pericarp and columella, suggesting active phytochrome-hormonal signaling crosstalk in these tissues. Data also revealed that PΦB deficiency influences fruit ripening by delaying the climacteric rise in ethylene production and signaling. Altogether, the data uncover the impact of phytochromobilin deficiency in fine-tuning sugar metabolism, chloroplast formation and the timing of fruit ripening and also reveal a link between auxins, cytokinins and phytochromes in regulating sugar import and accumulation in fruits.

Monogalactosyldiacylglycerol Facilitates Synthesis of Photoactive Protochlorophyllide in Etioplasts

Cotyledon cells of dark-germinated angiosperms develop etioplasts that are plastids containing unique internal membranes called prolamellar bodies (PLBs). Protochlorophyllide (Pchlide), a precursor of chlorophyll, accumulates in PLBs and forms a ternary complex with NADPH and light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR), which allows for the rapid formation of chlorophyll after illumination while avoiding photodamage. PLBs are 3D lattice structures formed by the lipid bilayer rich in monogalactosyldiacylglycerol (MGDG). Although MGDG was found to be required for the formation and function of the thylakoid membrane in chloroplasts in various plants, the roles of MGDG in PLB formation and etioplast development are largely unknown. To analyze the roles of MGDG in etioplast development, we suppressed MGD1 encoding the major isoform of MGDG synthase by using a dexamethasone-inducible artificial microRNA in etiolated Arabidopsis (Arabidopsis thaliana) seedlings. Strong MGD1 suppression caused a 36% loss of MGDG in etiolated seedlings, together with a 41% decrease in total Pchlide content. The loss of MGDG perturbed etioplast membrane structures and impaired the formation of the photoactive Pchlide-LPOR-NADPH complex and its oligomerization, without affecting LPOR accumulation. The MGD1 suppression also impaired the formation of Pchlide from protoporphyrin IX via multiple enzymatic reactions in etioplast membranes, which suggests that MGDG is required for the membrane-associated processes in the Pchlide biosynthesis pathway. Suppressing MGD1 at several germination stages revealed that MGDG biosynthesis at an early germination stage is particularly important for Pchlide accumulation. MGDG biosynthesis may provide a lipid matrix for Pchlide biosynthesis and the formation of Pchlide-LPOR complexes as an initial step of etioplast development.

Light Controls Cytokinin Signaling via Transcriptional Regulation of Constitutively Active Sensor Histidine Kinase CKI1

In plants, the multistep phosphorelay (MSP) pathway mediates a range of regulatory processes, including those activated by cytokinins. The cross talk between cytokinin response and light has been known for a long time. However, the molecular mechanism underlying the interaction between light and cytokinin signaling remains elusive. In the screen for upstream regulators we identified a LONG PALE HYPOCOTYL (LPH) gene whose activity is indispensable for spatiotemporally correct expression of CYTOKININ INDEPENDENT1 (CKI1), encoding the constitutively active sensor His kinase that activates MSP signaling. lph is a new allele of HEME OXYGENASE1 (HY1) that encodes the key protein in the biosynthesis of phytochromobilin, a cofactor of photoconvertible phytochromes. Our analysis confirmed the light-dependent regulation of the CKI1 expression pattern. We show that CKI1 expression is under the control of phytochrome A (phyA), functioning as a dual (both positive and negative) regulator of CKI1 expression, presumably via the phyA-regulated transcription factors (TF) PHYTOCHROME INTERACTING FACTOR3 and CIRCADIAN CLOCK ASSOCIATED1. Changes in CKI1 expression observed in lph/hy1-7 and phy mutants correlate with misregulation of MSP signaling, changed cytokinin sensitivity, and developmental aberrations that were previously shown to be associated with cytokinin and/or CKI1 action. Besides that, we demonstrate a novel role of phyA-dependent CKI1 expression in the hypocotyl elongation and hook development during skotomorphogenesis. Based on these results, we propose that the light-dependent regulation of CKI1 provides a plausible mechanistic link underlying the well-known interaction between light- and cytokinin-controlled plant development.

Heme oxygenase 1 defects lead to reduced chlorophyll in Brassica napus

We previously described a Brassica napus chlorophyll-deficient mutant (ygl) with yellow-green seedling leaves and mapped the related gene, BnaC.YGL, to a 0.35 cM region. However, the molecular mechanisms involved in this chlorophyll defect are still unknown. In this study, the BnaC07.HO1 gene (equivalent to BnaC.YGL) was isolated by the candidate gene approach, and its function was confirmed by genetic complementation. Comparative sequencing analysis suggested that BnaC07.HO1 was lost in the mutant, while a long noncoding-RNA was inserted into the promoter of the homologous gene BnaA07.HO1. This insert was widely present in B. napus cultivars and down-regulated BnaA07.HO1 expression. BnaC07.HO1 was highly expressed in the seedling leaves and encoded heme oxygenase 1, which was localized in the chloroplast. Biochemical analysis showed that BnaC07.HO1 can catalyze heme conversion to form biliverdin IXα. RNA-seq analysis revealed that the loss of BnaC07.HO1 impaired tetrapyrrole metabolism, especially chlorophyll biosynthesis. According, the levels of chlorophyll intermediates were reduced in the ygl mutant. In addition, gene expression in multiple pathways was affected in ygl. These findings provide molecular evidences for the basis of the yellow-green leaf phenotype and further insights into the crucial role of HO1 in B. napus.

Comparative Transcriptome Analysis Reveals Effects of Exogenous Hematin on Anthocyanin Biosynthesis during Strawberry Fruit Ripening

Anthocyanin in strawberries has a positive effect on fruit coloration. In this study, the role of exogenous hematin on anthocyanin biosynthesis was investigated. Our result showed that the white stage of strawberries treated with exogenous hematin had higher anthocyanin content, compared to the control group. Among all treatments, 5 μM of hematin was the optimal condition to promote color development. In order to explore the molecular mechanism of fruit coloring regulated by hematin, transcriptomes in the hematin- and non-hematin-treated fruit were analyzed. A large number of differentially expressed genes (DEGs) were identified in regulating anthocyanin synthesis, including the DEGs involved in anthocyanin biosynthesis, hormone signaling transduction, phytochrome signaling, starch and sucrose degradation, and transcriptional pathways. These regulatory networks may play an important role in regulating the color process of strawberries treated with hematin. In summary, exogenous hematin could promote fruit coloring by increasing anthocyanin content in the white stage of strawberries. Furthermore, transcriptome analysis suggests that hematin-promoted fruit coloring occurs through multiple related metabolic pathways, which provides valuable information for regulating fruit color via anthocyanin biosynthesis in strawberries.

Comparative Proteomic and Physiological Analysis Reveals the Variation Mechanisms of Leaf Coloration and Carbon Fixation in a Xantha Mutant of Ginkgo biloba L

Yellow-green leaf mutants are common in higher plants, and these non-lethal chlorophyll-deficient mutants are ideal materials for research on photosynthesis and plant development. A novel xantha mutant of Ginkgo biloba displaying yellow-colour leaves (YL) and green-colour leaves (GL) was identified in this study. The chlorophyll content of YL was remarkably lower than that in GL. The chloroplast ultrastructure revealed that YL had less dense thylakoid lamellae, a looser structure and fewer starch grains than GL. Analysis of the photosynthetic characteristics revealed that YL had decreased photosynthetic activity with significantly high nonphotochemical quenching. To explain these phenomena, we analysed the proteomic differences in leaves and chloroplasts between YL and GL of ginkgo using two-dimensional gel electrophoresis (2-DE) coupled with MALDI-TOF/TOF MS. In total, 89 differential proteins were successfully identified, 82 of which were assigned functions in nine metabolic pathways and cellular processes. Among them, proteins involved in photosynthesis, carbon fixation in photosynthetic organisms, carbohydrate/energy metabolism, amino acid metabolism, and protein metabolism were greatly enriched, indicating a good correlation between differentially accumulated proteins and physiological changes in leaves. The identifications of these differentially accumulated proteins indicates the presence of a specific different metabolic network in YL and suggests that YL possess slower chloroplast development, weaker photosynthesis, and a less abundant energy supply than GL. These studies provide insights into the mechanism of molecular regulation of leaf colour variation in YL mutants.

Fine Mapping of Virescent Leaf Gene v-1 in Cucumber (Cucumis sativus L.)

Leaf color mutants are common in higher plants that can be used as markers in crop breeding or as an important tool in understanding regulatory mechanisms in chlorophyll biosynthesis and chloroplast development. In virescent leaf mutants, young leaves are yellow in color, which gradually return to normal green when the seedlings grow large. In the present study, we conducted phenotypic characterization and genetic mapping of the cucumber virescent leaf mutant 9110Gt conferred by the v-1 locus. Total chlorophyll and carotenoid content in 9110Gt was reduced by 44% and 21%, respectively, as compared with its wild type parental line 9110G. Electron microscopic investigation revealed fewer chloroplasts per cell and thylakoids per chloroplast in 9110Gt than in 9110G. Fine genetic mapping allowed for the assignment of the v-1 locus to a 50.4 kb genomic DNA region in chromosome 6 with two flanking markers that were 0.14 and 0.16 cM away from v-1, respectively. Multiple lines of evidence supported CsaCNGCs as the only candidate gene for the v-1 locus, which encoded a cyclic-nucleotide-gated ion channel protein. A single nucleotide change in the promoter region of v-1 seemed to be associated with the virescent color change in 9110Gt. Real-time PCR revealed significantly lower expression of CsaCNGCs in the true leaves of 9110Gt than in 9110G. This was the first report that connected the CsaCNGCs gene to virescent leaf color change, which provided a useful tool to establish linkages among virescent leaf color change, chloroplast development, chlorophyll biosynthesis, and the functions of the CsaCNGCs gene.

Fine mapping of a dominant gene conferring chlorophyll-deficiency in Brassica napus

Leaf colour regulation is important in photosynthesis and dry material production. Most of the reported chlorophyll-deficient loci are recessive. The dominant locus is rarely reported, although it may be more important than the recessive locus in the regulation of photosynthesis efficiency. During the present study, we mapped a chlorophyll-deficient dominant locus (CDE1) from the ethyl methanesulfonate-mutagenized Brassica napus line NJ7982. Using an F₂ population derived from the chlorophyll-deficient mutant (cde1) and the canola variety ‘zhongshuang11’, a high-density linkage map was constructed, consisting of 19 linkage groups with 2,878 bins containing 13,347 SNP markers, with a total linkage map length of 1,968.6 cM. Next, the CDE1 locus was mapped in a 0.9-cM interval of chromosome C08 of B. napus, co-segregating with nine SNP markers. In the following fine-mapping of the gene using the inherited F_2:3 populations of 620 individuals, the locus was identified in an interval with a length of 311 kb. A bioinformatics analysis revealed that the mapping interval contained 22 genes. These results produced a good foundation for continued research on the dominant locus involved in chlorophyll content regulation.

Arabidopsis HY1-Modulated Stomatal Movement: An Integrative Hub Is Functionally Associated with ABI4 in Dehydration-Induced ABA Responsiveness

Heme oxygenase (HO; EC 1.14.99.3) has recently been proposed as a novel component in mediating wide ranges of the plant adaptive signaling processes. However, the physiological significance and molecular basis underlying Arabidopsis (Arabidopsis thaliana) HO1 (HY1) functioning in drought tolerance remained unclear. Here, we report that mutation of HY1 promoted, but overexpression of this gene impaired, Arabidopsis drought tolerance. This was attributed to the abscisic acid (ABA)-hypersensitive or -hyposensitive phenotypes, with the regulation of stomatal closure in particular. However, comparative transcriptomic profile analysis showed that the induction of numerous ABA/stress-dependent genes in dehydrated wild-type plants was differentially impaired in the hy1 mutant. In agreement, ABA-induced ABSCISIC ACID-INSENSITIVE4 (ABI4) transcript accumulation was strengthened in the hy1 mutant. Genetic analysis further identified that the hy1-associated ABA hypersensitivity and drought tolerance were arrested in the abi4 background. Moreover, the promotion of ABA-triggered up-regulation of RbohD abundance and reactive oxygen species (ROS) levels in the hy1 mutant was almost fully blocked by the mutation of ABI4, suggesting that the HY1-ABI4 signaling in the wild type involved in stomatal closure was dependent on the RbohD-derived ROS production. However, hy1-promoted stomatal closure was not affected by a nitric oxide scavenger. Correspondingly, ABA-insensitive behaviors in rbohD stomata were not affected by either the mutation of HY1 or its ectopic expression in the rbohD background, both of which responded significantly to exogenous ROS. These data indicate that HY1 functioned negatively and acted upstream of ABI4 in drought signaling, which was casually dependent on the RbohD-derived ROS in the regulation of stomatal closure.

Phytochrome B Mediates the Regulation of Chlorophyll Biosynthesis through Transcriptional Regulation of ChlH and GUN4 in Rice Seedlings

Accurate regulation of chlorophyll synthesis is crucial for chloroplast formation during the greening process in angiosperms. In this study, we examined the role of phytochrome B (phyB) in the regulation of chlorophyll synthesis in rice seedlings (Oryza sativa L.) through the characterization of a pale-green phenotype observed in the phyB mutant grown under continuous red light (Rc) irradiation. Our results show that the Rc-induced chlorophyll accumulation can be divided into two components—a phyB-dependent and a phyB-independent component, and that the pale-green phenotype is caused by the absence of the phyB-dependent component. To elucidate the role of the missing component we established an Rc-induced greening experiment, the results of which revealed that several genes encoding proteins on the chlorophyll branch were repressed in the phyB mutant. Notable among them were ChlH and GUN4 genes, which encode subunit H and an activating factor of magnesium chelatase (Mg-chelatase), respectively, that were largely repressed in the mutant. Moreover, the kinetic profiles of chlorophyll precursors suggested that Mg-chelatase activity simultaneously decreased with the reduction in the transcript levels of ChlH and GUN4. These results suggest that phyB mediates the regulation of chlorophyll synthesis through transcriptional regulation of these two genes, whose products exert their action at the branching point of the chlorophyll biosynthesis pathway. Reduction of 5-aminolevulinic acid (5-ALA) synthesis could be detected in the mutant, but the kinetic profiles of chlorophyll precursors indicated that it was an event posterior to the reduction of the Mg-chelatase activity. It means that the repression of 5-ALA synthesis should not be a triggering event for the appearance of the pale-green phenotype. Instead, the repression of 5-ALA synthesis might be important for the subsequent stabilization of the pale-green phenotype for preventing excessive accumulation of hazardous chlorophyll precursors, which is an inevitable consequence of the reduction of Mg-chelatase activity.

InsP6-sensitive variants of the Gle1 mRNA export factor rescue growth and fertility defects of the ipk1 low-phytic-acid mutation in Arabidopsis

Myo-inositol-1,2,3,4,5,6-hexakisphosphate (InsP(6)), also known as phytic acid, accumulates in large quantities in plant seeds, serving as a phosphorus reservoir, but is an animal antinutrient and an important source of water pollution. Here, we report that Gle1 (GLFG lethal 1) in conjunction with InsP(6) functions as an activator of the ATPase/RNA helicase LOS4 (low expression of osmotically responsive genes 4), which is involved in mRNA export in plants, supporting the Gle1-InsP(6)-Dbp5 (LOS4 homolog) paradigm proposed in yeast. Interestingly, plant Gle1 proteins have modifications in several key residues of the InsP(6) binding pocket, which reduce the basicity of the surface charge. Arabidopsis thaliana Gle1 variants containing mutations that increase the basic charge of the InsP(6) binding surface show increased sensitivity to InsP(6) concentrations for the stimulation of LOS4 ATPase activity in vitro. Expression of the Gle1 variants with enhanced InsP(6) sensitivity rescues the mRNA export defect of the ipk1 (inositol 1,3,4,5,6-pentakisphosphate 2-kinase) InsP(6)-deficient mutant and, furthermore, significantly improves vegetative growth, seed yield, and seed performance of the mutant. These results suggest that Gle1 is an important factor responsible for mediating InsP(6) functions in plant growth and reproduction and that Gle1 variants with increased InsP(6) sensitivity may be useful for engineering high-yielding low-phytate crops.

Young Leaf Chlorosis 2 encodes the stroma-localized heme oxygenase 2 which is required for normal tetrapyrrole biosynthesis in rice

Rice heme oxygenase 2 (OsHO2) mutants are chlorophyll deficient with distinct tetrapyrrole metabolite and transcript profiles, suggesting a potential regulatory role of the stromal-localized OsHO2 in tetrapyrrole biosynthesis. In plants, heme oxygenases (HOs) are classified into the subfamilies HO1 and HO2. HO1 are highly conserved plastid enzymes required for synthesizing the chromophore in phytochromes which mediate a number of light-regulated responses. However, the physiological and biochemical functions of HO2, which are distantly related to HO1, are not well understood, especially in crop plants. From a population of (60)Coγ-irradiated rice mutants, we identified the ylc2 (young leaf chlorosis 2) mutant which displays a chlorosis phenotype in seedlings with substantially reduced chlorophyll content. Normal leaf pigmentation is gradually restored in older plants while newly emerged leaves remain yellow. Transmission electron microscopy further revealed defective chloroplast structures in the ylc2 seedlings. Map-based cloning located the OsYLC2 gene on chromosome 3 and it encodes the OsHO2 protein. The gene identification was confirmed by complementation and T-DNA mutant analyses. Subcellular localization and chloroplast fractionation experiments indicated that OsHO2 resides in the stroma. However, recombinant enzyme assay demonstrated that OsHO2 is not a functional HO enzyme. Analysis of tetrapyrrole metabolites revealed the reduced levels of most chlorophyll and phytochromobilin precursors in the ylc2 mutant. On the other hand, elevated accumulation of 5-aminolevulinic acid and Mg-protoporphyrin IX was observed. These unique metabolite changes are accompanied by consistent changes in the expression levels of the corresponding tetrapyrrole biosynthesis genes. Taken together, our work suggests that OsHO2 has a potential regulatory role for tetrapyrrole biosynthesis in rice.

Arabidopsis miR171-targeted scarecrow-like proteins bind to GT cis-elements and mediate gibberellin-regulated chlorophyll biosynthesis under light conditions

An extraordinarily precise regulation of chlorophyll biosynthesis is essential for plant growth and development. However, our knowledge on the complex regulatory mechanisms of chlorophyll biosynthesis is very limited. Previous studies have demonstrated that miR171-targeted scarecrow-like proteins (SCL6/22/27) negatively regulate chlorophyll biosynthesis via an unknown mechanism. Here we showed that SCLs inhibit the expression of the key gene encoding protochlorophyllide oxidoreductase (POR) in light-grown plants, but have no significant effect on protochlorophyllide biosynthesis in etiolated seedlings. Histochemical analysis of β-glucuronidase (GUS) activity in transgenic plants expressing pSCL27::rSCL27-GUS revealed that SCL27-GUS accumulates at high levels and suppresses chlorophyll biosynthesis at the leaf basal proliferation region during leaf development. Transient gene expression assays showed that the promoter activity of PORC is indeed regulated by SCL27. Consistently, chromatin immunoprecipitation and quantitative PCR assays showed that SCL27 binds to the promoter region of PORC in vivo. An electrophoretic mobility shift assay revealed that SCL27 is directly interacted with G(A/G)(A/T)AA(A/T)GT cis-elements of the PORC promoter. Furthermore, genetic analysis showed that gibberellin (GA)-regulated chlorophyll biosynthesis is mediated, at least in part, by SCLs. We demonstrated that SCL27 interacts with DELLA proteins in vitro and in vivo by yeast-two-hybrid and coimmunoprecipitation analysis and found that their interaction reduces the binding activity of SCL27 to the PORC promoter. Additionally, we showed that SCL27 activates MIR171 gene expression, forming a feedback regulatory loop. Taken together, our data suggest that the miR171-SCL module is critical for mediating GA-DELLA signaling in the coordinate regulation of chlorophyll biosynthesis and leaf growth in light.

Chlorophyll biosynthesis is essential for plant growth and development. To date, the regulatory mechanisms of chlorophyll biosynthesis have been well understood only in dark conditions. Previous reports showed that miR171-targeted SCL6/22/27 proteins were involved in chlorophyll biosynthesis. However, the molecular mechanism of SCL action remains unclear. In this study, we found that SCLs negatively regulated chlorophyll biosynthesis though suppressing the expression of the key gene PROTOCHLOROPHYLLIDE OXIDOREDUCTASE (POR). SCL27 is highly expressed at the basal cell proliferation region of young leaves, suggesting an important role of SCLs in inhibiting chloroplast development before cell expansion. In addition, GT-cis elements were required for SCL27 directly binding to the PORC promoter. Furthermore, we showed that SCLs mediated GA-regulated chlorophyll biosynthesis through direct interaction with DELLA proteins. The interaction between SCLs and DELLAs reduced the DNA binding activity of SCL27. Our uncovered GA-DELLA-SCL module and its DNA binding targets provide new insights into molecular mechanisms by which chlorophyll biosynthesis and cell proliferation are coordinately regulated during leaf development in response to developmental and environmental cues.

A knockdown mutation of YELLOW-GREEN LEAF2 blocks chlorophyll biosynthesis in rice

An insert mutation of YELLOW-GREEN LEAF2 , encoding Heme Oxygenase 1 , results in significant reduction of its transcript levels, and therefore impairs chlorophyll biosynthesis in rice. Heme oxygenase (HO) in higher plants catalyzes the degradation of heme to synthesize phytochrome precursor and its roles conferring the photoperiodic control of flowering in rice have been revealed. However, its involvement in regulating rice chlorophyll (Chl) synthesis is not fully explored. In this study, we isolated a rice mutant named yellow-green leaf 2 (ygl2) from a (60)Co-irradiated population. Normal grown ygl2 seedlings showed yellow-green leaves with reduced contents of Chl and tetrapyrrole intermediates whereas an increase of Chl a/b ratio. Ultrastructural analyses demonstrated grana were poorly stacked in ygl2 mutant, resulting in underdevelopment of chloroplasts. The ygl2 locus was mapped to chromosome 6 and isolated via map-based cloning. Sequence analysis indicated that it encodes the rice HO1 and its identity was verified by transgenic complementation test and RNA interference. A 7-Kb insertion was found in the first exon of YGL2/HO1, resulting in significant reduction of YGL2 expressions in the ygl2 mutant. YGL2 was constitutively expressed in a variety of rice tissues with the highest levels in leaves and regulated by temperature. In addition, we found expression levels of some genes associated with Chl biosynthesis and photosynthesis were concurrently altered in ygl2 mutant. These results provide direct evidence that YGL2 has a vital function in rice Chl biosynthesis.

Chlorophyll deficiency in the maize elongated mesocotyl2 mutant is caused by a defective heme oxygenase and delaying grana stacking

BACKGROUND

Etiolated seedlings initiate grana stacking and chlorophyll biosynthesis in parallel with the first exposure to light, during which phytochromes play an important role. Functional phytochromes are biosynthesized separately for two components. One phytochrome is biosynthesized for apoprotein and the other is biosynthesized for the chromophore that includes heme oxygenase (HO).

METHODOLOGY/PRINCIPAL FINDING

We isolated a ho1 homolog by map-based cloning of a maize elongated mesocotyl2 (elm2) mutant. cDNA sequencing of the ho1 homolog in elm2 revealed a 31 bp deletion. De-etiolation responses to red and far-red light were disrupted in elm2 seedlings, with a pronounced elongation of the mesocotyl. The endogenous HO activity in the elm2 mutant decreased remarkably. Transgenic complementation further confirmed the dysfunction in the maize ho1 gene. Moreover, non-appressed thylakoids were specifically stacked at the seedling stage in the elm2 mutant.

CONCLUSION

The 31 bp deletion in the ho1 gene resulted in a decrease in endogenous HO activity and disrupted the de-etiolation responses to red and far-red light. The specific stacking of non-appressed thylakoids suggested that the chlorophyll biosynthesis regulated by HO1 is achieved by coordinating the heme level with the regulation of grana stacking.

Cell growth defect factor1/chaperone-like protein of POR1 plays a role in stabilization of light-dependent protochlorophyllide oxidoreductase in Nicotiana benthamiana and Arabidopsis

Angiosperms require light for chlorophyll biosynthesis because one reaction in the pathway, the reduction of protochlorophyllide (Pchlide) to chlorophyllide, is catalyzed by the light-dependent protochlorophyllide oxidoreductase (POR). Here, we report that Cell growth defect factor1 (Cdf1), renamed here as chaperone-like protein of POR1 (CPP1), an essential protein for chloroplast development, plays a role in the regulation of POR stability and function. Cdf1/CPP1 contains a J-like domain and three transmembrane domains, is localized in the thylakoid and envelope membranes, and interacts with POR isoforms in chloroplasts. CPP1 can stabilize POR proteins with its holdase chaperone activity. CPP1 deficiency results in diminished POR protein accumulation and defective chlorophyll synthesis, leading to photobleaching and growth inhibition of plants under light conditions. CPP1 depletion also causes reduced POR accumulation in etioplasts of dark-grown plants and as a result impairs the formation of prolamellar bodies, which subsequently affects chloroplast biogenesis upon illumination. Furthermore, in cyanobacteria, the CPP1 homolog critically regulates POR accumulation and chlorophyll synthesis under high-light conditions, in which the dark-operative Pchlide oxidoreductase is repressed by its oxygen sensitivity. These findings and the ubiquitous presence of CPP1 in oxygenic photosynthetic organisms suggest the conserved nature of CPP1 function in the regulation of POR.

Intracellular signaling from plastid to nucleus

Intracellular signaling from plastids to the nucleus, called retrograde signaling, coordinates the expression of nuclear and plastid genes and is essential for plastid biogenesis and for maintaining plastid function at optimal levels. Recent identification of several components involved in plastid retrograde generation, transmission, and control of nuclear gene expression has provided significant insight into the regulatory network of plastid retrograde signaling. Here, we review the current knowledge of multiple plastid retrograde signaling pathways, which are derived from distinct sources, and of possible plastid signaling molecules. We describe the retrograde signaling-dependent regulation of nuclear gene expression, which involves multilayered transcriptional control, as well as the transcription factors involved. We also summarize recent advances in the identification of key components mediating signal transduction from plastids to the nucleus.

Mutation of Arabidopsis HY1 causes UV-C hypersensitivity by impairing carotenoid and flavonoid biosynthesis and the down-regulation of antioxidant defence

Previous pharmacological results confirmed that haem oxygenase-1 (HO-1) is involved in protection of cells against ultraviolet (UV)-induced oxidative damage in soybean [Glycine max (L.) Merr.] seedlings, but there remains a lack of genetic evidence. In this study, the link between Arabidopsis thaliana HO-1 (HY1) and UV-C tolerance was investigated at the genetic and molecular levels. The maximum inducible expression of HY1 in wild-type Arabidopsis was observed following UV-C irradiation. UV-C sensitivity was not observed in ho2, ho3, and ho4 single and double mutants. However, the HY1 mutant exhibited UV-C hypersensitivity, consistent with the observed decreases in chlorophyll content, and carotenoid and flavonoid metabolism, as well as the down-regulation of antioxidant defences, thereby resulting in severe oxidative damage. The addition of the carbon monoxide donor carbon monoxide-releasing molecule-2 (CORM-2), in particular, and bilirubin (BR), two catalytic by-products of HY1, partially rescued the UV-C hypersensitivity, and other responses appeared in the hy1 mutant. Transcription factors involved in the synthesis of flavonoid or UV responses were induced by UV-C, but reduced in the hy1 mutant. Overall, the findings showed that mutation of HY1 triggered UV-C hypersensitivity, by impairing carotenoid and flavonoid synthesis and antioxidant defences.

Distinct phytochrome actions in nonvascular plants revealed by targeted inactivation of phytobilin biosynthesis

The red/far-red light photoreceptor phytochrome mediates photomorphological responses in plants. For light sensing and signaling, phytochromes need to associate with open-chain tetrapyrrole molecules as the chromophore. Biosynthesis of tetrapyrrole chromophores requires members of ferredoxin-dependent bilin reductases (FDBRs). It was shown that LONG HYPOCOTYL 2 (HY2) is the only FDBR in flowering plants producing the phytochromobilin (PΦB) for phytochromes. However, in the moss Physcomitrella patens, we found a second FDBR that catalyzes the formation of phycourobilin (PUB), a tetrapyrrole pigment usually found as the protein-bound form in cyanobacteria and red algae. Thus, we named the enzyme PUB synthase (PUBS). Severe photomorphogenic phenotypes, including the defect of phytochrome-mediated phototropism, were observed in Physcomitrella patens when both HY2 and PUBS were disrupted by gene targeting. This indicates HY2 and PUBS function redundantly in phytochrome-mediated responses of nonvascular plants. Our studies also show that functional PUBS orthologs are found in selected lycopod and chlorophyte genomes. Using mRNA sequencing for transcriptome profiling, we demonstrate that expression of the majority of red-light-responsive genes are misregulated in the pubs hy2 double mutant. These studies showed that moss phytochromes rapidly repress expression of genes involved in cell wall organization, transcription, hormone responses, and protein phosphorylation but activate genes involved in photosynthesis and stress signaling during deetiolation. We propose that, in nonvascular plants, HY2 and PUBS produce structurally different but functionally similar chromophore precursors for phytochromes. Holophytochromes regulate biological processes through light signaling to efficiently reprogram gene expression for vegetative growth in the light.

Assembly of synthetic locked phycocyanobilin derivatives with phytochrome in vitro and in vivo in Ceratodon purpureus and Arabidopsis

Phytochromes are photoreceptors with a bilin chromophore in which light triggers the conversion between the red light-absorbing form, Pr, and the far-red-light-absorbing form, Pfr. Here we performed in vitro and in vivo studies using locked phycocyanobilin derivatives, termed 15 Z anti phycocyanobilin (15ZaPCB) and 15 E anti PCB (15EaPCB). Recombinant bacterial and plant phytochromes incorporated either chromophore in a noncovalent or covalent manner. All adducts were photoinactive. The absorption spectra of the 15ZaPCB and 15EaPCB adducts were comparable with those of the Pr and Pfr form, respectively. Feeding of 15EaPCB, but not 15ZaPCB, to protonemal filaments of the moss Ceratodon purpureus resulted in increased chlorophyll accumulation, modulation of gravitropism, and induction of side branches in darkness. The effect of locked chromophores on phytochrome responses, such as induction of seed germination, inhibition of hypocotyl elongation, induction of cotyledon opening, randomization of gravitropism, and gene regulation, were investigated in wild-type Arabidopsis thaliana and the phytochrome-chromophore-deficient long hypocotyl mutant hy1. All phytochrome responses were induced in darkness by 15EaPCB, not only in the mutant but also in the wild type. These studies show that the 15Ea stereochemistry of the chromophore results in the formation of active Pfr-like phytochrome in the cell. Locked chromophores might be used to investigate phytochrome responses in many other organisms without the need to isolate mutants. The induction of phytochrome responses in the hy1 mutant by 15EaPCB were however less efficient than by red light irradiation given to biliverdin-rescued seeds or seedlings.

Post-translational control of tetrapyrrole biosynthesis in plants, algae, and cyanobacteria

The tetrapyrrole biosynthetic pathway provides the vital cofactors and pigments for photoautotrophic growth (chlorophyll), several essential redox reactions in electron transport chains (haem), N- and S-assimilation (sirohaem), and photomorphogenic processes (phytochromobilin). While the biochemistry of the pathway is well understood and almost all genes encoding enzymes of tetrapyrrole biosynthesis have been identified in plants, the post-translational control and organization of the pathway remains to be clarified. Post-translational mechanisms controlling metabolic activities are of particular interest since tetrapyrrole biosynthesis needs adaptation to environmental challenges. This review surveys post-translational mechanisms that have been reported to modulate metabolic activities and organization of the tetrapyrrole biosynthesis pathway.