Metabolic control of the tetrapyrrole biosynthetic pathway for porphyrin distribution in the barley mutant albostrians

Dual plastid targeting of protoporphyrinogen oxidase 2 in Amaranthaceae promotes herbicide tolerance

Plant tetrapyrrole biosynthesis (TPB) takes place in plastids and provides the chlorophyll and heme required for photosynthesis and many redox processes throughout plant development. TPB is strictly regulated, since accumulation of several intermediates causes photodynamic damage and cell death. Protoporphyrinogen oxidase (PPO) catalyzes the last common step before TPB diverges into chlorophyll and heme branches. Land plants possess two PPO isoforms. PPO1 is encoded as a precursor protein with a transit peptide, but in most dicotyledonous plants PPO2 does not possess a cleavable N-terminal extension. Arabidopsis (Arabidopsis thaliana) PPO1 and PPO2 localize in chloroplast thylakoids and envelope membranes, respectively. Interestingly, PPO2 proteins in Amaranthaceae contain an N-terminal extension that mediates their import into chloroplasts. Here, we present multiple lines of evidence for dual targeting of PPO2 to thylakoid and envelope membranes in this clade and demonstrate that PPO2 is not found in mitochondria. Transcript analyses revealed that dual targeting in chloroplasts involves the use of two transcription start sites and initiation of translation at different AUG codons. Among eudicots, the parallel accumulation of PPO1 and PPO2 in thylakoid membranes is specific for the Amaranthaceae and underlies PPO2-based herbicide resistance in Amaranthus species.

Inhibitory effect and mechanism of algicidal bacteria on Chaetomorpha valida

Chaetomorpha valida, filamentous green tide algae, poses a significant threat to both aquatic ecosystems and aquaculture. Vibrio alginolyticus Y20 is a new algicidal bacterium with an algicidal effect on C. valida. This study aimed to investigate the physiological and molecular responses of C. valida to exposure to V. alginolyticus Y20. The inhibitory effect of V. alginolyticus Y20 on C. valida was content dependent, with the lowest inhibitory content being 3 × 105 CFU mL-1. The microscopic results revealed that C. valida experienced severe morphological damage under the influence of V. alginolyticus Y20, with a dispersion of intracellular pigments. V. alginolyticus Y20 caused the decrease in chlorophyll-a content and Fv/Fm in C. valida. At the molecular level, V. alginolyticus Y20 downregulated the expression of genes related to photosynthetic pigment synthesis, light capture, and electron transport. Furthermore, V. alginolyticus Y20 induced oxidative damage to algal cells. The production of reactive oxygen species significantly increased after 11 days of exposure. Malondialdehyde content significantly increased, and the cell membranes were severely damaged by lipid peroxidation. The content of superoxide dismutase and peroxidase also markedly increased, whereas catalase content decreased significantly. Additionally, peroxisomes were inhibited due to the downregulation of PEX expression, leading to irreversible oxidative damage to algal cells. Our findings provided a new theoretical basis for exploring the interaction between algicidal bacteria and green tide algae at the molecular level.

Integrated Cytological, Physiological, and Transcriptome Analyses Provide Insight into the Albino Phenotype of Chinese Plum (Prunus salicina)

Albino seedlings that arise during seed reproduction can have a significant impact on plant growth and breeding. In this research, we present the first report of albino occurrences in the seed reproduction process of Prunus salicina and describe the cytological, physiological, and transcriptomic changes observed in albino seedlings. The albino seedlings which were observed in several plum cultivars exhibited abnormal chloroplast ultrastructure and perturbed stomatal structure. Compared to normal seedlings, the photosynthetic pigment contents in albino seedlings decreased by more than 90%, accompanied by significant reductions in several chlorophyll fluorescence parameters. Furthermore, substantially changed photosynthetic parameters indicated that the photosynthetic capacity and stomatal function were impaired in albino seedlings. Additionally, the activities of the antioxidant enzyme were drastically altered against the background of higher proline and lower ascorbic acid in leaves of albino seedlings. A total of 4048 differentially expressed genes (DEGs) were identified through transcriptomic sequencing, and the downregulated DEGs in albino seedlings were greatly enriched in the pathways for photosynthetic antenna proteins and flavonoid biosynthesis. GLK1 and Ftsz were identified as candidate genes responsible for the impaired chloroplast development and division in albino seedlings. Additionally, the substantial decline in the expression levels of examined photosystem-related chloroplast genes was validated in albino seedlings. Our findings shed light on the intricate physiological and molecular mechanisms driving albino plum seedling manifestation, which will contribute to improving the reproductive and breeding efforts of plums.

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.

Genetic mapping and physiological analysis of chlorophyll-deficient mutant in Brassica napus L

BACKGROUND

Leaf color mutants have reduced photosynthetic efficiency, which has severely negative impacts on crop growth and economic product yield. There are different chlorophyll mutants in Arabidopsis and crops that can be used for genetic control and molecular mechanism studies of chlorophyll biosynthesis, chloroplast development and photoefficiency. Chlorophyll mutants in Brassica napus are mostly used for mapping and location research but are rarely used for physiological research. The chlorophyll-deficient mutant in this experiment were both genetically mapped and physiologically analyzed.

RESULTS

In this study, yellow leaf mutant of Brassica napus L. mutated by ethyl methyl sulfone (EMS) had significantly lower chlorophyll a, b and carotenoid contents than the wild type, and the net photosynthetic efficiency, stomatal conductance and transpiration rate were all significantly reduced. The mutant had sparse chloroplast distribution and weak autofluorescence. The granule stacks were reduced, and the shape was extremely irregular, with more broken stromal lamella. Transcriptome data analysis enriched the differentially expressed genes mainly in phenylpropane and sugar metabolism. The mutant was mapped to a 2.72 Mb region on A01 by using BSA-Seq, and the region was validated by SSR markers.

CONCLUSIONS

The mutant chlorophyll content and photosynthetic efficiency were significantly reduced compared with those of the wild type. Abnormal chloroplasts and thylakoids less connected to the stroma lamella appeared in the mutant. This work on the mutant will facilitate the process of cloning the BnaA01.cd gene and provide more genetic and physiological information concerning chloroplast development in Brassica napus.

Identification and fine genetic mapping of the golden pod gene (pv-ye) from the snap bean (Phaseolus vulgaris L.)

Using bulked segregant analysis combined with next-generation sequencing, we delimited the pv-ye gene responsible for the golden pod trait of snap bean cultivar A18-1. Sequence analysis identified Phvul.002G006200 as the candidate gene. The pod is the main edible part of snap beans (Phaseolus vulgaris L.). The commercial use of the pods is mainly affected by their color. Consumers seem to prefer golden pods. The aim of the present study was to identify the gene responsible for the golden pod trait in the snap bean. 'A18-1' (a golden bean cultivar) and 'Renaya' (a green bean cultivar) were chosen as the experimental materials. Genetic analysis indicated that a single recessive gene, pv-ye, controls the golden pod trait. A candidate region of 4.24 Mb was mapped to chromosome Pv 02 using bulked-segregant analysis coupled with whole-genome sequencing. In this region, linkage analysis in an F₂ population localized the pv-ye gene to an interval of 182.9 kb between the simple sequence repeat markers SSR77 and SSR93. This region comprised 16 genes (12 annotated genes from the P. vulgaris database and 4 functionally unknown genes). Combined with transcriptome sequencing results, we identified Phvul.002G006200 as the potential candidate gene for pv-ye. Sequencing of Phvul.002G006200 identified a single-nucleotide polymorphism (SNP) in pv-ye. A pair of primers covering the SNP were designed, and the fragment was sequenced to screen 1086 F₂ plants with the 'A18-1' phenotype. Our findings showed that among the 1086 mapped individuals, the SNP cosegregated with the 'A18-1' phenotype. The findings presented here could form the basis to reveal the molecular mechanism of the golden pod trait in the snap bean.

Blocked chlorophyll synthesis leads to the production of golden snap bean pods

The main edible organ of snap bean (Phaseolus vulgaris L.) is the pod, whose color is a main characteristic affecting its commercial use. Golden pods are popular with consumers; however, color instability affects their commercial exploitation and causes economic losses to the planters. In this study, we focused on the different pod color of two varieties of snap bean. The golden yellow color of snap bean pods is controlled by a single recessive nuclear gene located at 1-4.24 Mb of chromosome 2. To explore the physiological and molecular mechanism of the golden pod color, the golden bean line 'A18-1' and the green bean line 'Renaya' were selected as experimental materials. We analyzed the pigment contents, detected the intermediate products of chlorophyll biosynthesis, and identified differentially expressed genes using RNA-seq. The formation of golden bean pods reflects a chlorophyll deficiency, which was speculated to be caused by impairment of the Mg-protoporphyrin IX to chlorophyllide step. In 'A18-1' and 'Renaya' pods on 10, 14, and 18 days, five genes related to this step were differentially expressed, all of which were protochlorophyllide oxidoreductase (POR) genes. Among them, the expression changes of the Phvul. 004G112700, Phvul.007G157500, and Phvul. 004G112400 genes were consistent with the color change and physiological data during pod development in 'A18-1' and 'Renaya'. We speculated that the altered expression of these three POR genes might be related to changes in the chlorophyllide content. The results might provide insight into the understanding of chlorophyll biosynthesis and crop breeding for snap bean.

Barley's Second Spring as A Model Organism for Chloroplast Research

Barley (Hordeum vulgare) has been widely used as a model crop for studying molecular and physiological processes such as chloroplast development and photosynthesis. During the second half of the 20th century, mutants such as albostrians led to the discovery of the nuclear-encoded, plastid-localized RNA polymerase and the retrograde (chloroplast-to-nucleus) signalling communication pathway, while chlorina-f2 and xantha mutants helped to shed light on the chlorophyll biosynthetic pathway, on the light-harvesting proteins and on the organization of the photosynthetic apparatus. However, during the last 30 years, a large fraction of chloroplast research has switched to the more “user-friendly” model species Arabidopsis thaliana, the first plant species whose genome was sequenced and published at the end of 2000. Despite its many advantages, Arabidopsis has some important limitations compared to barley, including the lack of a real canopy and the absence of the proplastid-to-chloroplast developmental gradient across the leaf blade. These features, together with the availability of large collections of natural genetic diversity and mutant populations for barley, a complete genome assembly and protocols for genetic transformation and gene editing, have relaunched barley as an ideal model species for chloroplast research. In this review, we provide an update on the genomics tools now available for barley, and review the biotechnological strategies reported to increase photosynthesis efficiency in model species, which deserve to be validated in barley.

Effect of Low Temperature on Chlorophyll Biosynthesis and Chloroplast Biogenesis of Rice Seedlings during Greening

Rice (Oryza sativa L.) frequently suffers in late spring from severe damage due to cold spells, which causes the block of chlorophyll biosynthesis during early rice seedling greening. However, the inhibitory mechanism by which this occurs is still unclear. To explore the responsive mechanism of rice seedlings to low temperatures during greening, the effects of chilling stress on chlorophyll biosynthesis and plastid development were studied in rice seedlings. Chlorophyll biosynthesis was obviously inhibited and chlorophyll accumulation declined under low temperatures during greening. The decrease in chlorophyll synthesis was due to the inhibited synthesis of δ-aminolevulinic acid (ALA) and the suppression of conversion from protochlorophyllide (Pchlide) into chlorophylls (Chls). Meanwhile, the activities of glutamate-1-semialdehyde transaminase (GSA-AT), Mg-chelatase, and protochlorophyllide oxidoreductase (POR) were downregulated under low temperatures. Further investigations showed that chloroplasts at 18 °C had loose granum lamellae, while the thylakoid and lamellar structures of grana could hardly develop at 12 °C after 48 h of greening. Additionally, photosystem II (PSII) and photosystem I (PSI) proteins obviously declined in the stressed seedlings, to the point that the PSII and PSI proteins could hardly be detected after 48 h of greening at 12 °C. Furthermore, the accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA) and cell death were all induced by low temperature. Chilling stress had no effect on the development of epidermis cells, but the stomata were smaller under chilling stress than those at 28 °C. Taken together, our study promotes more comprehensive understanding in that chilling could inhibit chlorophyll biosynthesis and cause oxidative damages during greening.

Differential accumulation of specialized metabolite l-theanine in green and albino-induced yellow tea (Camellia sinensis) leaves

l-Theanine is a specialized metabolite in tea (Camellia sinensis) leaves that contributes to tea function and quality. Yellow tea leaves (albino) generally have higher l-theanine contents than green tea leaves (normal), but the reason is unknown. The objective of this study was to investigate why l-theanine is accumulated in yellow tea leaves. We compared original normal leaves (green) and light-sensitive albino leaves (yellow) of cv. Yinghong No. 9. The l-theanine content was significantly higher in yellow leaves than in green leaves (p ≤ 0.01). After supplementation with [²H₅]-l-theanine, yellow leaves catabolized less [²H₅]-l-theanine than green leaves (p ≤ 0.05). Furthermore, most plants contained the enzyme catalyzing l-theanine conversion to ethylamine and l-glutamic acid. In conclusion, l-theanine accumulation in albino-induced yellow tea leaves was due to weak l-theanine catabolism. The differential accumulation mechanism differed from the l-theanine accumulation mechanism in tea and other plants.

Perturbations in the Photosynthetic Pigment Status Result in Photooxidation-Induced Crosstalk between Carotenoid and Porphyrin Biosynthetic Pathways

Possible crosstalk between the carotenoid and porphyrin biosynthetic pathways under photooxidative conditions was investigated by using their biosynthetic inhibitors, norflurazon (NF) and oxyfluorfen (OF). High levels of protoporphyrin IX (Proto IX) accumulated in rice plants treated with OF, whereas Proto IX decreased in plants treated with NF. Both NF and OF treatments resulted in greater decreases in MgProto IX, MgProto IX methyl ester, and protochlorophyllide. Activities and transcript levels of most porphyrin biosynthetic enzymes, particularly in the Mg-porphyrin branch, were greatly down-regulated in NF and OF plants. In contrast, the transcript levels of GSA, PPO1, and CHLD as well as FC2 and HO2 were up-regulated in NF-treated plants, while only moderate increases in FC2 and HO2 were observed in the early stage of OF treatment. Phytoene, antheraxanthin, and zeaxanthin showed high accumulation in NF-treated plants, whereas other carotenoid intermediates greatly decreased. Transcript levels of carotenoid biosynthetic genes, PSY1 and PDS, decreased in response to NF and OF, whereas plants in the later stage of NF treatment exhibited up-regulation of BCH and VDE as well as recovery of PDS. However, perturbed porphyrin biosynthesis by OF did not noticeably influence levels of carotenoid metabolites, regardless of the strong down-regulation of carotenoid biosynthetic genes. Both NF and OF plants appeared to provide enhanced protection against photooxidative damage, not only by scavenging of Mg-porphyrins, but also by up-regulating FC2, HO2, and Fe-chelatase, particularly with increased levels of zeaxanthin via up-regulation of BCH and VDE in NF plants. On the other hand, the up-regulation of GSA, PPO1, and CHLD under inhibition of carotenogenic flux may be derived from the necessity to recover impaired chloroplast biogenesis during photooxidative stress. Our study demonstrates that perturbations in carotenoid and porphyrin biosynthesis coordinate the expression of their biosynthetic genes to sustain plastid function at optimal levels by regulating their metabolic flux in plants under adverse stress conditions.

The discovery of plastid-to-nucleus retrograde signaling-a personal perspective

DNA and machinery for gene expression have been discovered in chloroplasts during the 1960s. It was soon evident that the chloroplast genome is relatively small, that most genes for chloroplast-localized proteins reside in the nucleus and that chloroplast membranes, ribosomes, and protein complexes are composed of proteins encoded in both the chloroplast and the nuclear genome. This situation has made the existence of mechanisms highly probable that coordinate the gene expression in plastids and nucleus. In the 1970s, the first evidence for plastid signals controlling nuclear gene expression was provided by studies on plastid ribosome deficient mutants with reduced amounts and/or activities of nuclear-encoded chloroplast proteins including the small subunit of Rubisco, ferredoxin NADP+ reductase, and enzymes of the Calvin cycle. This review describes first models of plastid-to-nucleus signaling and their discovery. Today, many plastid signals are known. They do not only balance gene expression in chloroplasts and nucleus during developmental processes but are also generated in response to environmental changes sensed by the organelles.

Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide-resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide-resistant respiration was also accompanied by the increase of plastid NADPH/NADP(+) ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin-Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX-derived plastidial NADPH/NADP(+) ratio change. Further, our results thus revealed a molecular mechanism of chloroplast-mitochondria interactions.

De novo Sequencing of the Leaf Transcriptome Reveals Complex Light-Responsive Regulatory Networks in Camellia sinensis cv. Baijiguan

Tea plants (Camellia sinensis L.) possess high genetic diversity that is important for breeding. One cultivar, Baijiguan, exhibits a yellow leaf phenotype, reduced chlorophyll (Chl) content, and aberrant chloroplast structures under high light intensity. In contrast, under low light intensity, the flush shoot from Baijiguan becomes green, the Chl content increases significantly, and the chloroplasts exhibit normal structures. To understand the underlying molecular mechanisms for these observations, we performed de novo transcriptome sequencing and digital gene expression (DGE) profiling using Illumina sequencing technology. De novo transcriptome assembly identified 88,788 unigenes, including 1652 transcription factors from 25 families. In total, 1993 and 2576 differentially expressed genes (DEGs) were identified in Baijiguan plants treated with 3 and 6 days of shade, respectively. Gene Ontology (GO) and pathway enrichment analyses indicated that the DEGs are predominantly involved in the ROS scavenging system, chloroplast development, photosynthetic pigment synthesis, secondary metabolism, and circadian systems. The light-responsive gene POR (protochlorophyllide oxidoreductase) and transcription factor HY5 were identified. Quantitative real-time PCR (qRT-PCR) analysis of 20 selected DEGs confirmed the RNA-sequencing (RNA-Seq) results. Overall, these findings suggest that high light intensity inhibits the expression of photosystem II 10-kDa protein (PsbR) in Baijiguan, thus affecting PSII stability, chloroplast development and chlorophyll biosynthesis.

Effect of low temperature on chlorophyll biosynthesis in albinism line of wheat (Triticum aestivum) FA85

The 'stage albinism line of winter wheat' FA85 exhibits a severe block in chlorophyll (Chl) biosynthesis with prolonged low-temperature treatment. The correlations between leaf color and low temperature provide more comprehensive understanding of low temperature as an environmental signal that regulate the metabolic changes in the entire Chl-synthesizing pathway. In this study, we investigated differences in Chl biosynthesis between leaves of Aibian1 and FA85 by measuring their Chl precursors and heme content, transcripts for key genes of Chl biosynthesis and key enzyme activities. With prolonged low-temperature treatment, the Chl content gradually decreased, but Chl precursors, including protoporphyrin IX, Mg-protoporphyrin IX and protochlorophyllide (Pchlide), simultaneously accumulated. Parallel to the decline in Chl content, the protoporphyrin IX distribution toward Chl synthesis was less than that in heme synthesis in the leaves of FA85. Corresponding to the change of protoporphyrin IX distribution, the relative changes in magnesium chelatase (EC 6.6.1.1) and ferrochelatase (EC 4.99.1.1) activities in the leaves of FA85 also indirectly reflected channeling of the metabolic flow into heme rather than Chl. A drastic loss in the transcripts for Pchlide oxidoreductase (EC 1.3.1.33) and Chl synthase (EC 2.5.1.62) accounted for a decrease in the metabolic flux and the re-direction of metabolites. The high-level accumulations of Chl precursors and traces of Chl in the leaves of FA85 suggest that a severe block between the steps from Pchlide to Chl formation during Chl biosynthesis is partially derived from the transcriptional downregulation of Pchlide oxidoreductase and Chl synthase.

Tomato fruit photosynthesis is seemingly unimportant in primary metabolism and ripening but plays a considerable role in seed development

Fruit of tomato (Solanum lycopersicum), like those from many species, have been characterized to undergo a shift from partially photosynthetic to truly heterotrophic metabolism. While there is plentiful evidence for functional photosynthesis in young tomato fruit, the rates of carbon assimilation rarely exceed those of carbon dioxide release, raising the question of its role in this tissue. Here, we describe the generation and characterization of lines exhibiting a fruit-specific reduction in the expression of glutamate 1-semialdehyde aminotransferase (GSA). Despite the fact that these plants contained less GSA protein and lowered chlorophyll levels and photosynthetic activity, they were characterized by few other differences. Indeed, they displayed almost no differences in fruit size, weight, or ripening capacity and furthermore displayed few alterations in other primary or intermediary metabolites. Although GSA antisense lines were characterized by significant alterations in the expression of genes associated with photosynthesis, as well as with cell wall and amino acid metabolism, these changes were not manifested at the phenotypic level. One striking feature of the antisense plants was their seed phenotype: the transformants displayed a reduced seed set and altered morphology and metabolism at early stages of fruit development, although these differences did not affect the final seed number or fecundity. Taken together, these results suggest that fruit photosynthesis is, at least under ambient conditions, not necessary for fruit energy metabolism or development but is essential for properly timed seed development and therefore may confer an advantage under conditions of stress.

Characterization and genetic analysis of a light- and temperature-sensitive spotted-leaf mutant in rice

A rice spotted-leaf mutant was isolated from an ethane methyl sulfonate (EMS) -induced IR64 mutant bank. The mutant, designated as spl30 (spotted-leaf30), displayed normal green leaf color under shade but exhibited red-brown lesions under natural summer field conditions. Initiation of the lesions was induced by light and the symptom was enhanced at 33 (°) C relative to 26 (°) C. Histochemical staining did not show cell death around the red-brown lesions. Chlorophyll contents in the mutant were significantly lower than those of the wild type while the ratio of chlorophyll a/b remained the same, indicating that spl30 was impaired in biosynthesis or degradation of chlorophyll. Disease reaction patterns of the mutant to Xanthomonas oryzae pv. oryzae were largely unchanged to most races tested except for a few strains. Genetic analysis showed that the mutation was controlled by a single recessive gene, tentatively named spl30(t), which co-segregated with RM15380 on chromosome 3, and was delimited to a 94 kb region between RM15380 and RM15383. Spl30(t) is likely a novel rice spotted-leaf gene since no other similar genes have been identified near the chromosomal region. The genetic data and recombination populations provided in this study will enable further fine-mapping and cloning of the gene.

An intermediate membrane subfraction in cyanobacteria is involved in an assembly network for Photosystem II biogenesis

Early steps in the biogenesis of Photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803 are thought to occur in a specialized membrane fraction that is characterized by the specific accumulation of the PSII assembly factor PratA and its interaction partner pD1, the precursor of the D1 protein of PSII. Here, we report the molecular characterization of this membrane fraction, called the PratA-defined membrane (PDM), with regard to its lipid and pigment composition and its association with PSII assembly factors, including YCF48, Slr1471, Sll0933, and Pitt. We demonstrate that YCF48 and Slr1471 are present and that the chlorophyll precursor chlorophyllide a accumulates in the PDM. Analysis of PDMs from various mutant lines suggests a central role for PratA in the spatial organization of PSII biogenesis. Moreover, quantitative immunoblot analyses revealed a network of interdependences between several PSII assembly factors and chlorophyll synthesis. In addition, formation of complexes containing both YCF48 and Sll0933 was substantiated by co-immunoprecipitation experiments. The findings are integrated into a refined model for PSII biogenesis in Synechocystis 6803.

Rapid dark repression of 5-aminolevulinic acid synthesis in green barley leaves

In photosynthetic organisms chlorophyll and heme biosynthesis is tightly regulated at various levels in response to environmental adaptation and plant development. The formation of 5-aminolevulinic acid (ALA) is the key regulatory step and provides adequate amounts of the common precursor molecule for the Mg and Fe branches of tetrapyrrole biosynthesis. Pathway control prevents accumulation of metabolic intermediates and avoids photo-oxidative damage. In angiosperms reduction of protochlorophyllide (Pchlide) to chlorophyllide is catalyzed by the light-dependent NADPH:Pchlide oxidoreductase (POR). Although a correlation between down-regulated ALA synthesis and accumulation of Pchlide in the dark was proposed a long time ago, the time-resolved mutual dependency has never been analyzed. Taking advantage of the high metabolic activity of young barley (Hordeum vulgare L.) seedlings, in planta ALA synthesis could be determined with high time-resolution. ALA formation declined immediately after transition from light to dark and correlated with an immediate accumulation of POR-bound Pchlide within the first 60 min in darkness. The flu homologous barley mutant tigrina d(12) uncouples ALA synthesis from dark-suppression and continued to form ALA in darkness without a significant change in synthesis rate in this time interval. Similarly, inhibition of protoporphyrinogen IX oxidase by acifluorfen resulted in a delayed accumulation of Pchlide during the entire dark period and a weak repression of ALA synthesis in darkness. Moreover, it is demonstrated that dark repression of ALA formation relies rather on rapid post-translational regulation in response to accumulating Pchlide than on changes in nuclear gene expression.

Comparative transcriptome analysis of green/white variegated sectors in Arabidopsis yellow variegated2: responses to oxidative and other stresses in white sectors

The yellow variegated2 (var2) mutant in Arabidopsis thaliana has been studied as a typical leaf-variegated mutant whose defect results from the lack of FtsH2 metalloprotease in chloroplasts. The var2 green sectors suffer from photo-oxidative stress and accumulate high levels of reactive oxygen species (ROS) because of compromised Photosystem II repair. This study investigated and compared microarray-based expression profiles of green and white sectors of var2 leaves. Results suggest that ROS that accumulate in chloroplasts of var2 green sectors do not cause much significant change in the transcriptional profile related to ROS signalling and scavenging. By contrast, transcriptome in the white sectors apparently differs from those in the green sectors and wild type. Numerous genes related to photosynthesis and chloroplast functions were repressed in the white sectors. Furthermore, many genes related to oxidative stress were up-regulated. Among them, ROS scavenging genes were specifically examined, such as Cu/Zn superoxide dismutase 2 (CSD2), that were apparently up-regulated in white but not in the green sectors. Up-regulation of CSD2 appears to be partly attributable to the lack of a microRNA (miR398) in the white sectors. It was concluded that the white sectors exhibit a response to oxidative and other stresses, including CSD2 up-regulation, which might be commonly found in tissues with abnormal chloroplast differentiation.

Identification of a Mg-protoporphyrin IX monomethyl ester cyclase homologue, EaZIP, differentially expressed in variegated Epipremnum aureum 'Golden Pothos' is achieved through a unique method of comparative study using tissue regenerated plants

Variegated plants provide a valuable tool for studying chloroplast biogenesis by allowing direct comparison between green and white/yellow sectors within the same leaf. While variegated plants are abundant in nature, the mechanism of leaf variegation remains largely unknown. Current studies are limited to a few mutants in model plant species, and are complicated by the potential for cross-contamination during dissection of leaf tissue into contrasting sectors. To overcome these obstacles, an alternative approach was explored using tissue-culture techniques to regenerate plantlets from unique sectors. Stable green and pale yellow plants were developed from a naturally variegated Epipremnum aureum 'Golden Pothos'. By comparing the gene expression between green and pale yellow plants using suppression subtractive hybridization in conjunction with homologous sequence search, nine down-regulated and 18 up-regulated genes were identified in pale yellow plants. Transcript abundance for EaZIP (Epipremnum aureum leucine zipper), a nuclear gene homologue of tobacco NTZIP and Arabidopsis CHL27, was reduced more than 4000-fold in qRT-PCR analysis. EaZIP encodes the Mg-protoporphyrin IX monomethyl ester cyclase, one of the key enzymes in the chlorophyll biosynthesis pathway. Examination of EaZIP expression in naturally variegated 'Golden Pothos' confirmed that EaZIP transcript levels were correlated with leaf chlorophyll contents, suggesting that this gene plays a major role in the loss of chlorophyll in the pale yellow sectors of E. aureum 'Golden Pothos'. This study further suggests that tissue-culture regeneration of plantlets from different coloured sectors of variegated leaves can be used to investigate the underlying mechanisms of variegation.

The variegated mutants lacking chloroplastic FtsHs are defective in D1 degradation and accumulate reactive oxygen species

In the photosynthetic apparatus, a major target of photodamage is the D1 reaction center protein of photosystem II (PSII). Photosynthetic organisms have developed a PSII repair cycle in which photodamaged D1 is selectively degraded. A thylakoid membrane-bound metalloprotease, FtsH, was shown to play a critical role in this process. Here, the effect of FtsHs in D1 degradation was investigated in Arabidopsis (Arabidopsis thaliana) mutants lacking FtsH2 (yellow variegated2 [var2]) or FtsH5 (var1). Because these mutants are characterized by variegated leaves that sometimes complicate biochemical studies, we employed another mutation, fu-gaeri1 (fug1), that suppresses leaf variegation in var1 and var2 to examine D1 degradation. Two-dimensional blue native PAGE showed that var2 has less PSII supercomplex and more PSII intermediate lacking CP43, termed RC47, than the wild type under normal growth light. Moreover, our histochemical and quantitative analyses revealed that chloroplasts in var2 accumulate significant levels of reactive oxygen species, such as superoxide radical and hydrogen peroxide. These results indicate that the lack of FtsH2 leads to impaired D1 degradation at the step of RC47 formation in PSII repair and to photooxidative stress even under nonphotoinhibitory conditions. Our in vivo D1 degradation assays, carried out by nonvariegated var2 fug1 and var1 fug1 leaves, demonstrated that D1 degradation was impaired in different light conditions. Taken together, our results suggest the important role of chloroplastic FtsHs, which was not precisely examined in vivo. Attenuated D1 degradation in the nonvariegated mutants also suggests that leaf variegation seems to be independent of the PSII repair.

Expression of chlorophyll synthase is also involved in feedback-control of chlorophyll biosynthesis

At the last step of the chlorophyll biosynthetic pathway chlorophyll synthase (CHLG) esterifies chlorophyllide a and b with phytyl or geranyl-geranyl pyrophosphate in chloroplasts. Transgenic tobacco plants expressing CHLG RNA in sense and antisense orientation were examined for the effects of excessive and reduced ectopic CHLG expression, respectively, on the chlorophyll biosynthetic pathway and the expression of chlorophyll-binding proteins. Reduced chlorophyll synthase activity does not result in accumulation of chlorophyllide and caused reduced ALA formation and Mg and ferrochelatase activity, while CHLG overexpression correlated with enhanced ALA synthesizing capacity and more chelatase activities. The transcript levels of genes expressing proteins of chlorophyll biosynthesis and chlorophyll-binding proteins were down-regulated in response to reduced CHLG expression. Thus, reduced expression and activity of chlorophyll synthase caused a feedback-controlled inactivation of the initial and rate limiting step of the pathway leading to down regulation of the metabolic flow, while overexpression can mediate a stimulation of the pathway. Chlorophyll synthase is proposed to be important for the co-regulation of the entire pathway and the coordination of synthesis of chlorophyll and the chlorophyll-binding proteins.

Parallel pigment and transcriptomic analysis of four barley albina and xantha mutants reveals the complex network of the chloroplast-dependent metabolism

We investigated the pigment composition and the transcriptome of albina (alb-e ( 16 ) and alb-f ( 17 )) and xantha (xan-s ( 46 ) and xan-b ( 12 )) barley mutants to provide an overall transcriptional picture of genes whose expression is interconnected with chloroplast activities and to search for candidate genes associated with the mutations. Beside those encoding plastid-localized proteins, more than 3,000 genes involved in non-chloroplast localized metabolism were up-/down-regulated in the mutants revealing the network of chloroplast-dependent metabolic pathways. The alb-e ( 16 ) mutant was characterized by overaccumulation of protoporphyrin IX upon ALA (5-amino levulinic acid) feeding and down-regulation of the gene encoding one subunit of Mg-chelatase, suggesting a block of the chlorophyll biosynthetic pathway before Mg-protoporphyrin IX biosynthesis, while alb-f ( 17 ) overaccumulated Mg-protoporphyrin IX and repressed PorA expression, without alterations in Mg-chelatase mRNA level. The alb-f ( 17 )mutant also showed overexpression of several genes involved in phytochrome and in phytochrome-dependent pathways. The results indicate that the down-regulation of Lhcb genes in alb-e ( 16 ) cannot be mediated by the accumulation of Mg-protoporphyrin IX. After ALA treatment, xan-s ( 46 ) showed overaccumulation of Mg-protoporphyrin IX, while the relative porphyrin composition of xan-b ( 12 ) was similar to wild type. The transcripts encoding the components of several mitochondrial metabolic pathways were up-regulated in albina/xantha leaves to compensate for the absence of active chloroplasts. The mRNAs encoding gun3, gun4, and gun5 barley homologous genes showed significant expression variations and were used to search for co-expressed genes across all samples. These analyses provide additional evidences on a chloroplast-dependent covariation of large sets of nuclear genes.

Tetrapyrrole profiling in Arabidopsis seedlings reveals that retrograde plastid nuclear signaling is not due to Mg-protoporphyrin IX accumulation

Chloroplast biogenesis involves careful coordination of both plastid and nuclear gene expression, which is achieved in part by retrograde signaling from the chloroplast to the nucleus. This can be demonstrated by the fact that the herbicide, Norflurazon (NF), which causes bleaching of chloroplasts, prevents the light induction of photosynthesis-related genes in the nucleus. It has been proposed that the tetrapyrrole pathway intermediate Mg-protoporphyrin IX acts as the signaling molecule in this pathway and accumulates in the chloroplasts and cytosol of the cell after NF treatment. Here we present data that demonstrate that this model is too simplistic. We have developed a sensitive liquid chromatography-mass spectrometry (LC/MS) method to measure tetrapyrrole intermediates and have shown that no Mg-protoporphyrin IX, nor indeed any other chlorophyll-biosynthesis intermediate, can be detected in NF-treated plants under conditions in which nuclear gene expression is repressed. Conversely when endogenous Mg-protoporphyrin IX levels are artificially increased by supplementation with the tetrapyrrole precursor, 5-aminolevulinic acid, the expression of nuclear-encoded photosynthetic genes is induced, not repressed. We also demonstrate that NF-treatment leads to a strong down-regulation of tetrapyrrole biosynthesis genes, consistent with the absence of an accumulation of tetrapyrrole intermediates. Finally, there is no correlation between nuclear-gene expression and any of the chlorophyll biosynthetic intermediates over a range of growth conditions and treatments. Instead, it is possible that a perturbation of tetrapyrrole synthesis may lead to localized ROS production or an altered redox state of the plastid, which could mediate retrograde signaling.

The C-terminal extension of ferrochelatase is critical for enzyme activity and for functioning of the tetrapyrrole pathway in Synechocystis strain PCC 6803

Heme and chlorophyll (Chl) share a common biosynthetic pathway up to the branch point where magnesium chelatase and ferrochelatase (FeCH) insert either magnesium for Chl biosynthesis or ferrous iron for heme biosynthesis. A distinctive feature of FeCHs in cyanobacteria is their C-terminal extension, which forms a putative transmembrane segment containing a Chl-binding motif. We analyzed the deltaH324 strain of Synechocystis sp. strain PCC 6803, which contains a truncated FeCH enzyme lacking this C-terminal domain. Truncated FeCH was localized to the membrane fraction, suggesting that the C-terminal domain is not necessary for membrane association of the enzyme. Measurements of enzyme activity and complementation experiments revealed that the deltaH324 mutation dramatically reduced activity of the FeCH, which resulted in highly upregulated 5-aminolevulinic acid synthesis in the deltaH324 mutant, implying a direct role for heme in the regulation of flux through the pathway. Moreover, the deltaH324 mutant accumulated a large amount of protoporphyrin IX, and levels of Chl precursors were also significantly increased, suggesting that some, but not all, of the "extra" flux can be diverted down the Chl branch. Analysis of the recombinant full-length and truncated FeCHs demonstrated that the C-terminal extension is critical for activity of the FeCH and that it is strictly required for oligomerization of this enzyme. The observed changes in tetrapyrrole trafficking and the role of the C terminus in the functioning of FeCH are discussed.

The light stress-induced protein ELIP2 is a regulator of chlorophyll synthesis in Arabidopsis thaliana

The early light-induced proteins (ELIPs) belong to the multigenic family of pigment-binding light-harvesting complexes. ELIPs accumulate transiently and are believed to play a protective role in plants exposed to high levels of light. Constitutive expression of the ELIP2 gene in Arabidopsis resulted in a marked reduction of the pigment content of the chloroplasts, both in mature leaves and during greening of etiolated seedlings. The chlorophyll loss was associated with a decrease in the number of photosystems in the thylakoid membranes, but the photosystems present were fully assembled and functional. A detailed analysis of the chlorophyll-synthesizing pathway indicated that ELIP2 accumulation downregulated the level and activity of two important regulatory steps: 5-aminolevulinate synthesis and Mg-protoporphyrin IX (Mg-Proto IX) chelatase activity. The contents of glutamyl tRNA reductase and Mg chelatase subunits CHLH and CHLI were lowered in response to ELIP2 accumulation. In contrast, ferrochelatase activity was not affected and the inhibition of Heme synthesis was null or very moderate. As a result of reduced metabolic flow from 5-aminolevulinic acid, the steady state levels of various chlorophyll precursors (from protoporphyrin IX to protochlorophyllide) were strongly reduced in the ELIP2 overexpressors. Taken together, our results indicate that the physiological function of ELIPs could be related to the regulation of chlorophyll concentration in thylakoids. This seems to occur through an inhibition of the entire chlorophyll biosynthesis pathway from the initial precursor of tetrapyrroles, 5-aminolevulinic acid. We suggest that ELIPs work as chlorophyll sensors that modulate chlorophyll synthesis to prevent accumulation of free chlorophyll, and hence prevent photooxidative stress.

The balance between protein synthesis and degradation in chloroplasts determines leaf variegation in Arabidopsis yellow variegated mutants

An Arabidopsis thaliana leaf-variegated mutant yellow variegated2 (var2) results from loss of FtsH2, a major component of the chloroplast FtsH complex. FtsH is an ATP-dependent metalloprotease in thylakoid membranes and degrades several chloroplastic proteins. To understand the role of proteolysis by FtsH and mechanisms leading to leaf variegation, we characterized the second-site recessive mutation fu-gaeri1 (fug1) that suppressed leaf variegation of var2. Map-based cloning and subsequent characterization of the FUG1 locus demonstrated that it encodes a protein homologous to prokaryotic translation initiation factor 2 (cpIF2) located in chloroplasts. We show evidence that cpIF2 indeed functions in chloroplast protein synthesis in vivo. Suppression of leaf variegation by fug1 is observed not only in var2 but also in var1 (lacking FtsH5) and var1 var2. Thus, suppression of leaf variegation caused by loss of FtsHs is most likely attributed to reduced protein synthesis in chloroplasts. This hypothesis was further supported by the observation that another viable mutation in chloroplast translation elongation factor G also suppresses leaf variegation in var2. We propose that the balance between protein synthesis and degradation is one of the determining factors leading to the variegated phenotype in Arabidopsis leaves.

Variegation mutants and mechanisms of chloroplast biogenesis

Variegated plants typically have green- and white-sectored leaves. Cells in the green sectors contain normal-appearing chloroplasts, whereas cells in the white sectors lack pigments and appear to be blocked at various stages of chloroplast biogenesis. Variegations can be caused by mutations in nuclear, chloroplast or mitochondrial genes. In some plants, the green and white sectors have different genotypes, but in others they have the same (mutant) genotype. One advantage of variegations is that they provide a means of studying genes for proteins that are important for chloroplast development, but for which mutant analysis is difficult, either because mutations in a gene of interest are lethal or because they do not show a readily distinguishable phenotype. This paper focuses on Arabidopsis variegations, for which the most information is available at the molecular level. Perhaps the most interesting of these are variegations caused by defective nuclear gene products in which the cells of the mutant have a uniform genotype. Two questions are of paramount interest: (1) What is the gene product and how does it function in chloroplast biogenesis? (2) What is the mechanism of variegation and why do green sectors arise in plants with a uniform (mutant) genotype? Two paradigms of variegation mechanism are described: immutans (im) and variegated2 (var2). Both mechanisms emphasize compensating activities and the notion of plastid autonomy, but redundant gene products are proposed to play a role in var2, but not in im. It is hypothesized that threshold levels of certain activities are necessary for normal chloroplast development.

Reduced activity of plastid protoporphyrinogen oxidase causes attenuated photodynamic damage during high-light compared to low-light exposure

Protoporphyrinogen oxidase (EC 1.3.3.4, PPOX) is the last enzyme in the branched tetrapyrrole biosynthetic pathway, before its substrate protoporphyrin is directed to the Mg and Fe branches for chlorophyll and haem biosynthesis, respectively. The enzyme exists in many plants in two similar isoforms, which are either exclusively located in plastids (PPOX I) or in mitochondria and plastids (PPOX II). Antisense RNA expression inhibited the formation of PPOX I in transgenic tobacco plants, which showed reduced growth rate and necrotic leaf damage. The cytotoxic effect is attributed to accumulation of photodynamically acting protoporphyrin. The expression levels of PPOX I mRNA and protein and the cellular enzyme activities were reduced to similar extents in transgenic plants grown under low- or high-light conditions (70 and 530 mumol photons m(-2) sec(-1)). More necrotic leaf lesions were surprisingly generated under low- than under high-light exposure. Several reasons were explored to explain this paradox and the intriguing necrotic phenotype of PPOX-deficient plants under both light intensity growth conditions. The same reduction of PPOX expression and activity under both light conditions led to similar initial protoporphyrin, but to faster decrease in protoporphyrin content during high light. It is likely that a light intensity-dependent degradation of reduced and oxidized porphyrins prevents severe photodynamic leaf damage. Moreover, under high-light conditions, elevated contents of reduced and total low-molecular-weight antioxidants contribute to the protection against photosensitizing porphyrins. These reducing conditions stabilize protoporphyrinogen in plastids and allow their redirection into the metabolic pathway.

Cytokinin effects on tetrapyrrole biosynthesis and photosynthetic activity in barley seedlings

Cytokinin promotes morphological and physiological processes including the tetrapyrrole biosynthetic pathway during plant development. Only a few steps of chlorophyll (Chl) biosynthesis, exerting the phytohormonal influence, have been individually examined. We performed a comprehensive survey of cytokinin action on the regulation of tetrapyrrole biosynthesis with etiolated and greening barley seedlings. Protein contents, enzyme activities and tetrapyrrole metabolites were analyzed for highly regulated metabolic steps including those of 5-aminolevulinic acid (ALA) biosynthesis and enzymes at the branch point for protoporphyrin IX distribution to Chl and heme. Although levels of the two enzymes of ALA synthesis, glutamyl-tRNA reductase and glutamate 1-semialdehyde aminotransferase, were elevated in dark grown kinetin-treated barley seedlings, the ALA synthesis rate was only significantly enhanced when plant were exposed to light. While cytokinin do not stimulatorily affect Fe-chelatase activity and heme content, it promotes activities of the first enzymes in the Mg branch, Mg protoporphyrin IX chelatase and Mg protoporphyrin IX methyltransferase, in etiolated seedlings up to the first 5 h of light exposure in comparison to control. This elevated activities result in stimulated Chl biosynthesis, which again parallels with enhanced photosynthetic activities indicated by the photosynthetic parameters F(V)/F(M), J (CO2max) and J (CO2) in the kinetin-treated greening seedlings during the first hours of illumination. Thus, cytokinin-driven acceleration of the tetrapyrrole metabolism supports functioning and assembly of the photosynthetic complexes in developing chloroplasts.

Regulation of tetrapyrrole biosynthesis in higher plants

Plant tetrapyrroles are the most abundant biomolecules on the earth and are cofactors of many apoproteins essential for plant function. The four end-products sirohaem, chlorophyll, haem and phytochromobilin are synthesized by a common branched pathway, which is tightly regulated to ensure a continuous supply to the cognate apoproteins. This may induce strong competition between different branches of the pathway for common substrates. In addition, the intermediates, which are phototoxic, must not be allowed to accumulate in the cell. The major control points are during the synthesis of the initial precursor, ALA (5-aminolaevulinic acid), and at the branch points for the insertion of metal ions into the porphyrin macrocycle. A recent study has also suggested that tetrapyrroles are involved in the communication between the chloroplast and the nucleus, strengthening the necessity for tight regulation. However, intermediates remain difficult to quantify mainly due to their low content and the different properties of the intermediates. In this paper, we summarize the regulation of this pathway and we detail why it is important to have an accurate method for the determination of tetrapyrroles in plants.

Genomics-based dissection of the cross-talk of chloroplasts with the nucleus and mitochondria in Arabidopsis

Post-endosymbiotic evolution of chloroplasts was characterized by a massive transfer of cyanobacterial genes to the nucleus, followed by re-routing of many of their encoded proteins. In consequence, most plastid proteins are nucleus-encoded, enabling an anterograde (nucleus-to-plastid) control of the organelle. The regulation of chloroplast functions includes also cross-talk between chloroplasts and mitochondria, as well as retrograde (plastid-to-nucleus) signalling. Genetic analyses reveal that redox state, flux through the chlorophyll biosynthetic pathway, sugar sensing and reactive oxygen species contribute to retrograde signalling. However, the identity of the messenger molecule(s) remains largely unknown. Novel facets of the chloroplast-mitochondrion cross-talk have been revealed by the characterization of mitochondrial mutants affected in chloroplast properties. Studies of the nuclear chloroplast transcriptome imply the existence of at least three distinct types of transcriptional regulation: a master switch, acting in a binary mode by either inducing or repressing the same large set of genes; a "mixed response" with about equal numbers of up- and down-regulated genes; and mechanisms supporting the specific co-regulation of nuclear genes for photosynthesis and for plastid gene expression. The recent discovery of the latter mode of control highlights a possibly ancient route to co-ordinate chloroplast and nuclear genome expression.

Photosystem II Assembly in CP47 Mutant of Synechocystis sp. PCC 6803 Is Dependent on the Level of Chlorophyll Precursors Regulated by Ferrochelatase

Accumulation of chlorophyll and expression of the chlorophyll (Chl)-binding CP47 protein that serves as the core antenna of photosystem II are indispensable for the assembly of a functional photosystem II. We have characterized the CP47 mutant with an impaired photosystem II assembly and its two spontaneous pseudorevertants with their much improved photoautotrophic growth. The complementing mutations in these pseudorevertants were previously mapped to the ferrochelatase gene (1). We demonstrated that complementing mutations dramatically decrease ferrochelatase activity in pseudorevertants and that this decrease is responsible for their improved photoautotrophic growth. Photoautotrophic growth of the CP47 mutant was also restored by in vivo inhibition of ferrochelatase by a specific inhibitor. The decrease in ferrochelatase activity in pseudorevertants was followed by increased steady-state levels of Chl precursors and Chl, leading to CP47 accumulation and photosystem II assembly. Similarly, supplementation of the CP47 mutant with the Chl precursor Mg-protoporphyrin IX increased the number of active photosystem-II centers, suggesting that synthesis of the mutated CP47 protein is enhanced by an increased Chl availability in the cell. The probable role of ferrochelatase in the regulation of Chl biosynthesis is discussed.

An Arabidopsis mutant that is resistant to the protoporphyrinogen oxidase inhibitor acifluorfen shows regulatory changes in tetrapyrrole biosynthesis

Several Arabidopsis mutants of the ecotype Dijon were isolated that show resistance to the herbicide acifluorfen, which inactivates protoporphyrinogen oxidase (PPOX), an enzyme of tetrapyrrole biosynthesis. This enzyme provides protoporphyrin for both Mg chelatase and ferrochelatase at the branchpoint, which leads to chlorophyll and heme, respectively. One of the mutations, aci5-3, displays semidominant inheritance. Heterozygous progeny showed yellow-green leaves, while the homozygous seedlings were white and inviable, but could be rescued by supplementing the medium with sugar. Interestingly, the expression of neither of the two forms of PPOX was altered in the mutant, but the rate of synthesis of 5-aminolevulinate, the precursor of all tetrapyrroles, was drastically reduced. Genetic mapping revealed the mutant locus is closely linked to the ch42 marker, which is itself located in the CHLI-1 gene which codes for one of the three subunits of Mg chelatase. The cs mutant also shows a defect in this gene, and test for allelism with aci5-3 confirmed that the two mutations are allelic. Sequencing of the wild type and aci5-3 alleles of CHLI-1 revealed a single base change (G718A), which results in a D240N substitution in the CHLI-1 protein. In the homozygous aci5-3 mutant no CHLI-1 RNA or protein could be detected. Strikingly, CHLH and CHLI-2 transcripts were also absent. This indicates the existence of a feedback-regulatory mechanism that inactivates the genes encoding certain Mg chelatase subunits. The basis for the semidominant inheritance pattern and the relationship between herbicide resistance and modified gene expression is discussed.

The FLP proteins act as regulators of chlorophyll synthesis in response to light and plastid signals in Chlamydomonas

In photosynthetic organisms the accumulation of harmful photodynamic chlorophyll precursors is prevented because of the tight regulation of the tetrapyrrole pathway. FLU is one of the regulatory factors involved in this process in land plants. We have examined the function of a Flu-like gene (FLP) from Chlamydomonas that gives rise to two FLP transcripts through alternative splicing. These transcripts are translated into a short and a long protein that differ by only 12 amino acids but that interact differently with glutamyl-tRNA reductase, an enzyme involved in an early step of the chlorophyll biosynthetic pathway. Expression of FLPs is light-regulated at the level of RNA accumulation and splicing and is altered by mutations affecting the pathway. The relative levels of the long and short forms of FLP can be correlated with the accumulation of specific porphyrin intermediates, some of which have been implicated in a signaling chain from the chloroplast to the nucleus. Reciprocally, reduction of the FLP proteins by RNA interference leads to the accumulation of several porphyrin intermediates and to photobleaching when cells are transferred from the dark to the light. Thus the FLP proteins act as regulators of chlorophyll synthesis, and their expression is controlled by light and plastid signals.

Arabidopsis cue mutants with defective plastids are impaired primarily in the photocontrol of expression of photosynthesis-associated nuclear genes

Plant photoreceptors detect light cues and initiate responses ranging from chloroplast differentiation to the control of morphogenesis and flowering. The photocontrol of photosynthesis-related nuclear genes appears closely related to 'retrograde plastid signals' by which the status of the organelle controls the expression of nuclear genes. However, what specific role, if any, plastid-originated signals play in light responses is poorly understood: it has in the past been proposed that plastid signals play a role in all responses to 'high fluence' far-red light perceived by the light-labile phytochrome A, irrespective of whether they involve photosynthesis-related genes. To explore this further, we have re-examined the phenotype of three cue (cab-underexpressed) Arabidopsis mutants, defective in chloroplast development. The mutants have underdeveloped etioplasts, with increasing impairments in cue6, cue8 and cue3. The mutants show only small defects in photocontrol of hypocotyl elongation and cotyledon opening under prolonged far-red or red light, and normal photocontrol under blue. On the other hand, the expression of photosynthesis-associated nuclear genes is much more impaired in the mutants in the dark and following red or far-red light short treatments or continuous light, than that of those phytochrome-dependent genes tested which are not associated with photosynthesis. Furthermore, red/far-red photoreversible responses involving photosynthesis-related genes (induction of Lhcb1-cab promoter activity, and photoreversible extent of greening) mediated by phytochrome B and other photo-stable phytochromes, both show a reduction in the cue mutants, which correlates with the etioplast defect. Our evidence demonstrates that plastid-derived signals need to be operational in order for the phytochrome control of photosynthetic nuclear genes to occur.

Concurrent interactions of heme and FLU with Glu tRNA reductase (HEMA1), the target of metabolic feedback inhibition of tetrapyrrole biosynthesis, in dark- and light-grown Arabidopsis plants

The regulation of tetrapyrrole biosynthesis in higher plants has been attributed to metabolic feedback inhibition of Glu tRNA reductase by heme. Recently, another negative regulator of tetrapyrrole biosynthesis has been discovered, the FLU protein. During an extensive second site screen of mutagenized flu seedlings a suppressor of flu, ulf3, was identified that is allelic to hy1 and encodes a heme oxygenase. Increased levels of heme in the hy1 mutant have been implicated with inhibiting Glu tRNA reductase and suppressing the synthesis of delta-aminolevulinic acid (ALA) and Pchlide accumulation. When combined with hy1 or ulf3 upregulation of ALA synthesis and overaccumulation of protochlorophyllide in the flu mutants were severely suppressed supporting the notion that heme antagonizes the effect of the flu mutation by inhibiting Glu tRNA reductase independently of FLU. The coiled-coil domain at the C-terminal end of Glu tRNA reductase interacts with FLU, whereas the N-terminal site of Glu tRNA reductase that is necessary for the inhibition of the enzyme by heme is not required for this interaction. The interaction with FLU is specific for the Glu tRNA reductase encoded by HEMA1 that is expressed in photosynthetically active tissues. FLU seems to be part of a second regulatory circuit that controls chlorophyll biosynthesis by interacting directly with Glu tRNA reductase not only in etiolated seedlings but also in light-adapted green plants.

Recent advances in chlorophyll biosynthesis and breakdown in higher plants

Chlorophyll (Chl) has unique and essential roles in photosynthetic light-harvesting and energy transduction, but its biosynthesis, accumulation and degradation is also associated with chloroplast development, photomorphogenesis and chloroplast-nuclear signaling. Biochemical analyses of the enzymatic steps paved the way to the identification of their encoding genes. Thus, important progress has been made in the recent elucidation of almost all genes involved in Chl biosynthesis and breakdown. In addition, analysis of mutants mainly in Arabidopsis , genetically engineered plants and the application of photo-reactive herbicides contributed to the genetic and regulatory characterization of the formation and breakdown of Chl. This review highlights recent progress in Chl metabolism indicating highly regulated pathways from the synthesis of precursors to Chl and its degradation to intermediates, which are not longer photochemically active.