Differential expression of two hemA mRNAs encoding glutamyl-tRNA reductase proteins in greening cucumber seedlings

Downregulation of Brassica napus MYB69 (BnMYB69) increases biomass growth and disease susceptibility via remodeling phytohormone, chlorophyll, shikimate and lignin levels

MYB transcription factors are major actors regulating plant development and adaptability. Brassica napus is a staple oil crop and is hampered by lodging and diseases. Here, four B. napus MYB69 (BnMYB69s) genes were cloned and functionally characterized. They were dominantly expressed in stems during lignification. BnMYB69 RNA interference (BnMYB69i) plants showed considerable changes in morphology, anatomy, metabolism and gene expression. Stem diameter, leaves, roots and total biomass were distinctly larger, but plant height was significantly reduced. Contents of lignin, cellulose and protopectin in stems were significantly reduced, accompanied with decrease in bending resistance and Sclerotinia sclerotiorum resistance. Anatomical detection observed perturbation in vascular and fiber differentiation in stems, but promotion in parenchyma growth, accompanied with changes in cell size and cell number. In shoots, contents of IAA, shikimates and proanthocyanidin were reduced, while contents of ABA, BL and leaf chlorophyll were increased. qRT-PCR revealed changes in multiple pathways of primary and secondary metabolisms. IAA treatment could recover many phenotypes and metabolisms of BnMYB69i plants. However, roots showed trends opposite to shoots in most cases, and BnMYB69i phenotypes were light-sensitive. Conclusively, BnMYB69s might be light-regulated positive regulators of shikimates-related metabolisms, and exert profound influences on various internal and external plant traits.

Genetic manipulation of bermudagrass photosynthetic biosynthesis using Agrobacterium-mediated transformation

Bermudagrass is one of the most extensively used warm-season grasses. It is widely used in landscaping, stadium construction and soil remediation due to its excellent regeneration, trampling and stress tolerances. However, studies on its regulatory mechanism and variety improvement by genetic engineering are still at a standstill, owing to its genetic variability and intrinsic limits linked with some resistance to Agrobacterium infection. In this study, we established a higher efficient Agrobacterium-mediated transformation via screening for vital embryogenic callus and improving infection efficiency. The superior callus was light yellow, hard granular and compact, determined with a differentiation rate of more than 95%. The optimized infestation courses by gentle shaking, vacuuming and sonicating were used. The infested calluses were co-cultured for 3 days, followed by desiccation treatments for 1 day to get higher infection efficiency. Then the CdHEMA1 gene, essential for chlorophyll biosynthesis, was cloned and transferred into bermudagrass to validate the aforementioned optimization procedures integrally. Molecular-level analyses indicated that the CdHEMA1 gene had successfully integrated and was greatly increased in transgenic seedlings. Results of the photosynthetic capacity assessment showed that CdHEMA1 overexpression may considerably enhance the contents of photosynthetic pigments, OJIP curve and reaction center density (RC/CSo) to absorb (ABS/CSo, ABS/CSM) and capture (TRo/CSo) more light energy, hence improve the performance indices PI_ABS and PI_CS compared to the wild type. The successful completion of this project would provide a solid platform for further gene function study and molecular breeding of bermudagrass.

Effect of chlorophyll biosynthesis-related genes on the leaf color in Hosta (Hosta plantaginea Aschers) and tobacco (Nicotiana tabacum L.)

BACKGROUND

'Regal Splendour' (Hosta variety) is famous for its multi-color leaves, which are useful resources for exploring chloroplast development and color changes. The expressions of chlorophyll biosynthesis-related genes (HrHEMA, HrPOR and HrCAO) in Hosta have been demonstrated to be associated with leaf color. Herein, we isolated, sequenced, and analyzed HrHEMA, HrPOR and HrCAO genes. Subcellular localization was also performed to determine the location of the corresponding enzymes. After plasmid construction, virus-induced gene silencing (VIGS) was carried out to reduce the expressions of those genes. In addition, HrHEMA-, HrPOR- and HrCAO-overexpressing tobacco plants were made to verify the genes function. Changes of transgenic tobacco were recorded under 2000 lx, 6000 lx and 10,000 lx light intensity. Additionally, the contents of enzyme 5-aminolevulinic acid (5-ALA), porphobilinogen (PBG), chlorophyll a and b (Chla and Chlb), carotenoid (Cxc), superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA), proline (Pro) and catalase (CAT) under different light intensities were evaluated.

RESULTS

The silencing of HrHEMA, HrPOR and HrCAO genes can induce leaf yellowing and chloroplast structure changes in Hosta. Specifically, leaves of Hosta with HrCAO silencing were the most affected, while those with HrPOR silencing were the least affected. Moreover, all three genes in tobacco were highly expressed, whereas no expression was detected in wild-type (WT). However, the sensitivities of the three genes to different light intensities were different. The highest expression level of HrHEMA and HrPOR was detected under 10,000 lx of illumination, while HrCAO showed the highest expression level under 6000 lx. Lastly, the 5-ALA, Chla, Cxc, SOD, POD, MDA, Pro and CAT contents in different transgenic tobaccos changed significantly under different light intensities.

CONCLUSION

The overexpression of these three genes in tobacco enhanced photosynthesis by accumulating chlorophyll content, but the influential level varied under different light intensities. Furthermore, HrHEMA-, HrPOR- and HrCAO- overexpressing in tobacco can enhance the antioxidant capacity of plants to cope with stress under higher light intensity. However, under lower light intensity, the antioxidant capacity was declined in HrHEMA-, HrPOR- and HrCAO- overexpressing tobaccos.

Loss of fumarylacetoacetate hydrolase causes light-dependent increases in protochlorophyllide and cell death in Arabidopsis

Fumarylacetoacetate hydrolase (FAH) catalyses the final step of the tyrosine degradation pathway, which is essential to animals but was of unknown importance in plants until we found that mutation of Short-day Sensitive Cell Death1 (SSCD1), encoding Arabidopsis FAH, results in cell death under short-day conditions. The sscd1 mutant accumulates succinylacetone (SUAC), an abnormal metabolite caused by loss of FAH. Succinylacetone is an inhibitor of δ-aminolevulinic acid (ALA) dehydratase (ALAD), which is involved in chlorophyll (Chl) biosynthesis. In this study, we investigated whether sscd1 cell death is mediated by Chl biosynthesis and found that ALAD activity is repressed in sscd1 and that protochlorophyllide (Pchlide), an intermediate of Chl biosynthesis, accumulates at lower levels in etiolated sscd1 seedlings. However, it was interesting that Pchlide in sscd1 might increase after transfer from light to dark and that HEMA1 and CHLH are upregulated in the light-dark transition before Pchlide levels increased. Upon re-illumination after Pchlide levels had increased, reactive oxygen species marker genes, including singlet oxygen-induced genes, are upregulated, and the sscd1 cell death phenotype appears. In addition, Arabidopsis WT seedlings treated with SUAC mimic sscd1 in decline of ALAD activity and accumulation of Pchlide as well as cell death. These results demonstrate that increase in Pchlide causes cell death in sscd1 upon re-illumination and suggest that a decline in the Pchlide pool due to inhibition of ALAD activity by SUAC impairs the repression of ALA synthesis from the light-dark transition by feedback control, resulting in activation of the Chl biosynthesis pathway and accumulation of Pchlide in the dark.

Field-grown soybean transcriptome shows diurnal patterns in photosynthesis-related processes

Many plant physiological processes have diurnal patterns regulated by diurnal environmental changes and circadian rhythms, but the transcriptional underpinnings of many of these cycles have not been studied in major crop species under field conditions. Here, we monitored the transcriptome of field-grown soybean (Glycine max) during daylight hours in the middle of the growing season with RNA-seq. The analysis revealed 21% of soybean genes were differentially expressed over the course of the day. Expression of some circadian-related genes in field-grown soybean differed from previously reported expression patterns measured in controlled environments. Many genes in functional groups contributing to and/or depending on photosynthesis showed differential expression, with patterns particularly evident in the chlorophyll synthesis pathway. Gene regulatory network inference also revealed seven diurnally sensitive gene nodes involved with circadian rhythm, transcription regulation, cellular processes, and water transport. This study provides a diurnal overview of the transcriptome for an economically important field-grown crop and a basis for identifying pathways that could eventually be tailored to optimize diurnal regulation of carbon gain.

Controlled Partitioning of Glutamyl-tRNA Reductase in Stroma- and Membrane-Associated Fractions Affects the Synthesis of 5-Aminolevulinic Acid

The synthesis of 5-aminolevulinic acid (ALA) determines adequate amounts of metabolites for the tetrapyrrole biosynthetic pathway. Glutamyl-tRNA reductase (GluTR) catalyzes the rate-limiting step of ALA synthesis and was previously considered to be exclusively localized in the chloroplast stroma of light-exposed plants. To assess the intraplastidic localization of GluTR, we developed a fast separation protocol of soluble and membrane-bound proteins and reassessed the subplastidal allocation of GluTR in stroma and membrane fractions of Arabidopsis plants grown under different light regimes as well as during de-etiolation and dark incubations. Under the examined conditions, the amount of stroma-localized GluTR correlated with the ALA synthesis rate. The transfer to dark repression of ALA synthesis resulted in a loss of soluble GluTR. Arabidopsis mutants lacking one of the GluTR-interacting factors FLUORESCENT (FLU), the GluTR-binding protein (GBP) or ClpC, a chaperone of the Clp protease system, were applied to examine the amount of GluTR and its distribution to the stroma or membrane in darkness and light. Taking into consideration the different compartmental allocation of GluTR, its stability and ALA synthesis rates, the post-translational impact of these regulatory factors on GluTR activity and plastidic sublocalization is discussed.

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.

The rice TCD11 encoding plastid ribosomal protein S6 is essential for chloroplast development at low temperature

Plastid ribosome proteins (PRPs) are important components for chloroplast biogenesis and early chloroplast development. Although it has been known that chloroplast ribosomes are similar to bacterial ones, the precise molecular function of ribosomal proteins remains to be elucidated in rice. Here, we identified a novel rice mutant, designated tcd11 (thermo-sensitive chlorophyll-deficient mutant 11), characterized by the albino phenotype until it died at 20°C, while displaying normal phenotype at 32°C. The alteration of leaf color in tcd11 mutants was aligned with chlorophyll (Chl) content and chloroplast development. The map-based cloning and molecular complementation showed that TCD11 encodes the ribosomal small subunit protein S6 in chloroplasts (RPS6). TCD11 was abundantly expressed in leaves, suggesting its different expressions in tissues. In addition, the disruption of TCD11 greatly reduced the transcript levels of certain chloroplasts-associated genes and prevented the assembly of ribosome in chloroplasts at low temperature (20°C), whereas they recovered to nearly normal levels at high temperature (32°C). Thus, our data indicate that TCD11 plays an important role in chloroplast development at low temperature. Upon our knowledge, the observations from this study provide a first glimpse into the importance of RPS6 function in rice chloroplast development.

Modulation of chlorophyll biosynthesis by water stress in rice seedlings during chloroplast biogenesis

To understand the impact of water stress on the greening process, water stress was applied to 6-day-old etiolated seedlings of a drought-sensitive cultivar of rice (Oryza sativa), Pusa Basmati-1 by immersing their roots in 40 mm polyethylene glycol (PEG) 6000 (-0.69 MPa) or 50 mm PEG 6000 (-1.03 MPa) dissolved in half-strength Murashige and Skoog (MS)-nutrient-solution, 16 h prior to transfer to cool-white-fluorescent + incandescent light. Chlorophyll (Chl) accumulation substantially declined in developing water-stressed seedlings. Reduced Chl synthesis was due to decreased accumulation of chlorophyll biosynthetic intermediates, that is, glutamate-1-semialdehyde (GSA), 5-aminolevulinic acid, Mg-protoporphyrin IX monomethylester and protochlorophyllide. Although 5-aminolevulinic acid synthesis decreased, the gene expression and protein abundance of the enzyme responsible for its synthesis, GSA aminotransferase, increased, suggesting its crucial role in the greening process in stressful environment. The biochemical activities of Chl biosynthetic enzymes, that is, 5-aminolevulinic acid dehydratase, porphobilinogen deaminase, coproporphyrinogen III oxidase, porphyrinogen IX oxidase, Mg-chelatase and protochlorophyllide oxidoreductase, were down-regulated due to their reduced protein abundance/gene expression in water-stressed seedlings. Down-regulation of protochlorophyllide oxidoreductase resulted in impaired Shibata shift. Our results demonstrate that reduced synthesis of early intermediates, that is, GSA and 5-aminolevulinic acid, could modulate the gene expression of later enzymes of Chl biosynthesis pathway.

An Arabidopsis GluTR binding protein mediates spatial separation of 5-aminolevulinic acid synthesis in chloroplasts

5-Aminolevulinic acid (ALA) is the universal precursor for tetrapyrrole biosynthesis and is synthesized in plants in three enzymatic steps: ligation of glutamate (Glu) to tRNA(Glu) by glutamyl-tRNA synthetase, reduction of activated Glu to Glu-1-semialdehyde by glutamyl-tRNA reductase (GluTR), and transamination to ALA by Glu 1-semialdehyde aminotransferase. ALA formation controls the metabolic flow into the tetrapyrrole biosynthetic pathway. GluTR is proposed to be the key regulatory enzyme that is tightly controlled at transcriptional and posttranslational levels. We identified a GluTR binding protein (GluTRBP; previously called PROTON GRADIENT REGULATION7) that is localized in chloroplasts and part of a 300-kD protein complex in the thylakoid membrane. Although the protein does not modulate activity of ALA synthesis, the knockout of GluTRBP is lethal in Arabidopsis thaliana, whereas mutants expressing reduced levels of GluTRBP contain less heme. GluTRBP expression correlates with a function in heme biosynthesis. It is postulated that GluTRBP contributes to subcompartmentalized ALA biosynthesis by maintaining a portion of GluTR at the plastid membrane that funnels ALA into the heme biosynthetic pathway. These results regarding GluTRBP support a model of plant ALA synthesis that is organized in two separate ALA pools in the chloroplast to provide appropriate substrate amounts for balanced synthesis of heme and chlorophyll.

Characterization of three homoeologous cDNAs encoding chloroplast-targeted aminolevulinic acid dehydratase in common wheat

In the tetrapyrrole biosynthetic pathway of higher plants, 5-aminolevulinic acid (ALA) is metabolized by ALA dehydratase (ALAD). Here, we isolated ALAD1 cDNA from common wheat (Triticum aestivum L.) and its diploid progenitors, and produced transgenic tobacco plants expressing the wheat ALAD1 gene. The ALAD1 genes were highly conserved among wheat relatives, and three homoeologous loci of wheat ALAD1 (TaALAD1) were equally transcribed in common wheat. A transient expression assay of a TaALAD1-GFP (green fluorescent protein) fusion protein suggested that TaALAD1 is localized in chloroplasts. Overexpression of TaALAD1 in transgenic tobacco resulted in a significant increase in ALAD activity in leaves. Moreover, the transgenic tobacco showed vigorous growth and increased survival rate on medium containing ALA at herbicidal concentrations. These results indicate that wheat ALAD1 has catalytic activity in metabolizing ALA in plastids, and that ectopic expression of TaALAD1 in transgenic plants increases their tolerance to ALA application at high concentrations.

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

Functional assignments for the carboxyl-terminal domains of the ferrochelatase from Synechocystis PCC 6803: the CAB domain plays a regulatory role, and region II is essential for catalysis

Ferrochelatase (FeCH) catalyzes the insertion of Fe(2+) into protoporphyrin, forming protoheme. In photosynthetic organisms, FeCH and magnesium chelatase lie at a biosynthetic branch point where partitioning down the heme and chlorophyll (Chl) pathways occurs. Unlike their mammalian, yeast, and other bacterial counterparts, cyanobacterial and algal FeCHs as well as FeCH2 isoform from plants possess a carboxyl-terminal Chl a/b-binding (CAB) domain with a conserved Chl-binding motif. The CAB domain is connected to the FeCH catalytic core by a proline-rich linker sequence (region II). In order to dissect the regulatory, catalytic, and structural roles of the region II and CAB domains, we analyzed a FeCH ΔH347 mutant that retains region II but lacks the CAB domain and compared it with the ΔH324-FeCH mutant that lacks both these domains. We found that the CAB domain is not required for catalytic activity but is essential for dimerization of FeCH; its absence causes aberrant accumulation of Chl-protein complexes under high light accompanied by high levels of the Chl precursor chlorophyllide. Thus, the CAB domain appears to serve mainly a regulatory function, possibly in balancing Chl biosynthesis with the synthesis of cognate apoproteins. Region II is essential for the catalytic function of the plastid-type FeCH enzyme, although the low residual activity of the ΔH324-FeCH is more than sufficient to furnish the cellular demand for heme. We propose that the apparent surplus of FeCH activity in the wild type is critical for cell viability under high light due to a regulatory role of FeCH in the distribution of Chl into apoproteins.

Physiological and biochemical changes in plants under waterlogging

Waterlogging usually results from overuse and/or poor management of irrigation water and is a serious constraint with damaging effects. The rapidly depleting oxygen from submerged root zone is sensed and plant adjusts expressing anaerobic proteins. Plant cells shift their metabolism towards low energy yielding anaerobic fermentation pathways in the absence of oxygen. Structural modifications are also induced as aerenchyma formation and adventitious rootings, etc. Studies at molecular and biochemical levels to facilitate early perception and subsequent responses have also been worked out to produce resistant transgenic plants. This review explores the sequential changes of plant responses at different levels regarding their defense strategies and efforts made to enhance them, tailoring crucial regulators so that they can withstand waterlogging stress.

Regulation and evolution of chlorophyll metabolism

Chlorophylls are the most abundant tetrapyrrole molecules essential for photosynthesis in photosynthetic organisms. After many years of intensive research, most of the genes encoding the enzymes for the pathway have been identified, and recently the underlying molecular mechanisms have been elucidated. These studies revealed that the regulation of chlorophyll metabolism includes all levels of control to allow a balanced metabolic flow in response to external and endogenous factors and to ensure adaptation to varying needs of chlorophyll during plant development. Furthermore, identification of biosynthetic genes from various organisms and genetic analysis of functions of identified genes enables us to predict the evolutionary scenario of chlorophyll metabolism. In this review, based on recent findings, we discuss the regulation and evolution of chlorophyll metabolism.

HEMA RNAi silencing reveals a control mechanism of ALA biosynthesis on Mg chelatase and Fe chelatase

Glutamyl-tRNA reductase (GluTR) is encoded by HEMA in higher plants and catalyzes in plastids the initial enzymatic step of tetrapyrrole biosynthesis eventually leading to heme and chlorophyll. GluTR activity is subjected to a complex regulation on multiple expression levels. An ethanol-inducible HEMA-RNA-interference (RNAi) gene construct was introduced into the tobacco genome to study the primary effects of low GluTR content on the tetrapyrrole biosynthetic pathway. During the first days of induced HEMA silencing the chlorophyll and heme contents were diminished in young leaves. HEMA mRNA and GluTR protein content were also strongly reduced. However, expression analyses revealed that none of the other tetrapyrrole biosynthesis genes were affected on the transcriptional level in a nine days period after HEMA inactivation. Previously generated transgenic tobacco lines with RNAi silenced expression of the glutamate 1-semialdehyde aminotransferase (GSA) gene did also not display changes of transcripts from selected genes of tetrapyrrole biosynthesis and photosynthesis. Although the transcript levels were not decreased after inactivation of HEMA and GSA-expression, enzyme activities for Mg chelatase and Fe chelatase were lower, which occurred in parallel to the loss of chlorophyll and heme content. Posttranslational modification of enzymes downstream of ALA-biosynthesis is proposed as a regulatory mechanism to adjust the flux through tetrapyrrole biosynthesis in balance to supply of ALA.

High light inhibits chlorophyll biosynthesis at the level of 5-aminolevulinate synthesis during de-etiolation in cucumber (Cucumis sativus) cotyledons

Using the vascular plant Cucumis sativus (cucumber) as a model, we studied the effects of high (intense and excess) light upon chlorophyll biosynthesis during de-etiolation. When illuminated with high light (1500-1600 microE/m2/s), etiolated cucumber cotyledons failed to synthesize chlorophyll entirely. However, upon transfer to low light conditions (40-45 microE/m2/s), chlorophyll biosynthesis and subsequent accumulation resumed following an initial 2-12 h delay. Duration of high light treatment negatively correlated with chlorophyll biosynthetic activity. Specifically, we found that high light severely inhibited 5-aminolevulinic acid (ALA) synthesis. This effect partly could be because of the decrease in protein level of glutamyl-tRNA reductase (GluTR) observed. Protein level of glutamate-1-semialdehyde (GSA-AT) remained unchanged. It was also found that high light did not suppress HEMA 1 expression. Therefore, we speculated that this significant inhibition of ALA synthesis might have occurred mainly because of concomitant inactivation of GluTR and/or inhibition of complex formation between GluTR and GSA-AT. Our further observation that both methyl viologen and rose bengal similarly inhibit ALA synthesis under low light conditions suggested that reactive oxygen species (ROS) could be responsible for the inhibition of ALA synthesis in cotyledons exposed to high light conditions.

Induction of isoforms of tetrapyrrole biosynthetic enzymes, AtHEMA2 and AtFC1, under stress conditions and their physiological functions in Arabidopsis

In the tetrapyrrole biosynthetic pathway, isoforms of glutamyl-tRNA reductase (HEMA2) and ferrochelatase1 (FC1) are mainly expressed in nonphotosynthetic tissues. Here, using promoter-beta-glucuronidase constructs, we showed that the expressions of Arabidopsis (Arabidopsis thaliana) HEMA2 (AtHEMA2) and FC1 (AtFC1) were induced in photosynthetic tissues by oxidative stresses such as wounding. Transcript levels and beta-glucronidase activity were rapidly induced within 30 min, specifically in the wound area in a jasmonate-independent manner. Transcriptome analysis of wound-specific early inducible genes showed that AtHEMA2 and AtFC1 were coinduced with hemoproteins outside plastids, which are related to defense responses. Ozone fumigation or reagents generating reactive oxygen species induced the expression of both genes in photosynthetic tissues, suggesting that reactive oxygen species is involved in the induction. Since cycloheximide or puromycin induced the expression of both genes, inhibition of cytosolic protein synthesis is involved in the induction of these genes in photosynthetic tissues. The physiological functions of AtHEMA2 and AtFC1 were investigated using insertional knockout mutants of each gene. Heme contents of the roots of both mutants were about half of that of the respective wild types. In wild-type plants, heme contents were increased by ozone exposure. In both mutants, reduction of the ozone-induced increase in heme content was observed. These results suggest the existence of the tetrapyrrole biosynthetic pathway controlled by AtHEMA2 and AtFC1, which normally functions for heme biosynthesis in nonphotosynthetic tissues, but is induced in photosynthetic tissues under oxidative conditions to supply heme for defensive hemoproteins outside plastids.

Clp protease controls chlorophyll b synthesis by regulating the level of chlorophyllide a oxygenase

Chlorophyll b is one of the major light-harvesting pigments in green plants and it is essential for optimal light harvesting. Chlorophyll b is synthesized from chlorophyll a by chlorophyllide a oxygenase (CAO) which consists of A, B and C domains. Previously, we demonstrated that the C domain alone has a catalytic function, while the A and B domains control the level of CAO protein in response to chlorophyll b accumulation. We hypothesized that the accumulation of chlorophyll b triggers the proteolytic degradation of CAO. In this study, in order to gain further insight into this regulatory mechanism we screened for mutants that have defects in the control of CAO accumulation. Seeds from a transgenic line of Arabidopsis which overexpressed a CAO-GFP fusion were mutagenized and their progenies were screened by laser-scanning confocal microscopy for mutants showing an elevated level of GFP fluorescence. One particular mutant (dca1) exhibited stronger GFP fluorescence and accumulated a GFP-CAO fusion protein at a higher level. Concomitantly, the chlorophyll a to b ratio decreased in this mutant. The mutation in the dca1 mutant was mapped to the ClpC1 gene, thereby indicating that a chloroplast Clp protease is involved in regulating chlorophyll b biosynthesis through the destabilization of CAO protein in response to the accumulation of chlorophyll b.

Tetrapyrrole biosynthesis in higher plants

Tetrapyrroles play vital roles in various biological processes, including photosynthesis and respiration. Higher plants contain four classes of tetrapyrroles, namely, chlorophyll, heme, siroheme, and phytochromobilin. All of the tetrapyrroles are derived from a common biosynthetic pathway. Here we review recent progress in the research of tetrapyrrole biosynthesis from a cellular biological view. The progress consists of biochemical, structural, and genetic analyses, which contribute to our understanding of how the flow and the synthesis of tetrapyrrole molecules are regulated and how the potentially toxic intermediates of tetrapyrrole synthesis are maintained at low levels. We also describe interactions of tetrapyrrole biosynthesis and other cellular processes including the stay-green events, the cell-death program, and the plastid-to-nucleus signal transduction. Finally, we present several reports on attempts for agricultural and horticultural applications in which the tetrapyrrole biosynthesis pathway was genetically modified.

Mg-protoporphyrin IX and heme control HEMA, the gene encoding the first specific step of tetrapyrrole biosynthesis, in Chlamydomonas reinhardtii

HEMA encodes glutamyl-tRNA reductase (GluTR), which catalyzes the first step specific for tetrapyrrole biosynthesis in plants, archaea, and most eubacteria. In higher plants, GluTR is feedback inhibited by heme and intermediates of chlorophyll biosynthesis. It plays a key role in controlling flux through the tetrapyrrole biosynthetic pathway. This enzyme, which in Chlamydomonas reinhardtii is encoded by a single gene (HEMA), exhibits homology to GluTRs of higher plants and cyanobacteria. HEMA mRNA accumulation was inducible not only by light but also by treatment of dark-adapted cells with Mg-protoporphyrin IX (MgProto) or hemin. The specificity of these tetrapyrroles as inducers was demonstrated by the absence of induction observed upon the feeding of protoporphyrin IX, the precursor of both heme and MgProto, or chlorophyllide. The HEMA mRNA accumulation following treatment of cells with light and hemin was accompanied by increased amounts of GluTR. However, the feeding of MgProto did not suggest a role for Mg-tetrapyrroles in posttranscriptional regulation. The induction by light but not that by the tetrapyrroles was prevented by inhibition of cytoplasmic protein synthesis. Since MgProto is synthesized exclusively in plastids and heme is synthesized in plastids and mitochondria, the data suggest a role of these compounds as organellar signals that control expression of the nuclear HEMA gene.

Cellular levels of glutamyl-tRNA reductase and glutamate-1-semialdehyde aminotransferase do not control chlorophyll synthesis in Chlamydomonas reinhardtii

5-Aminolevulinic acid (ALA) is the first committed universal precursor in the tetrapyrrole biosynthesis pathway. In plants, algae, and most bacteria, ALA is generated from glutamate. First, glutamyl-tRNA synthetase activates glutamate by ligating it to tRNA(Glu). Activated glutamate is then converted to glutamate 1-semialdehyde (GSA) by glutamyl-tRNA reductase (GTR). Finally, GSA is rearranged to ALA by GSA aminotransferase (GSAT). In the unicellular green alga Chlamydomonas reinhardtii, GTR and GSAT were found in the chloroplasts and were not detected in the mitochondria by immunoblotting. The levels of both proteins (assayed by immunoblotting) and their mRNAs (assayed by RNA blotting) were approximately equally abundant in cells growing in continuous dark or continuous light (fluorescent tubes, 80 micromol photons s(-1) m(-2)), consistent with the ability of the cells to form chlorophyll under both conditions. In cells synchronized to a 12-h-light/12-h-dark cycle, chlorophyll accumulated only during the light phase. However, GTR and GSAT were present at all phases of the cycle. The GTR mRNA level increased in the light and peaked about 2-fold at 2 h into the light phase, and GTR protein levels also increased and peaked 2-fold at 4 to 6 h into the light phase. In contrast, although the GSAT mRNA level increased severalfold at 2 h into the light phase, the level of GSAT protein remained approximately constant in the light and dark phases. Under all growth conditions, the cells contained significantly more GSAT than GTR on a molar basis. Our results indicate that the rate of chlorophyll synthesis in C. reinhardtii is not directly controlled by the expression levels of the mRNAs for GTR or GSAT, or by the cellular abundance of these enzyme proteins.

The Chlamydomonas reinhardtii gtr gene encoding the tetrapyrrole biosynthetic enzyme glutamyl-trna reductase: structure of the gene and properties of the expressed enzyme

Plants, algae, cyanobacteria and many other bacteria synthesize the tetrapyrrole precursor, delta-aminolevulinic acid (ALA), from glutamate by means of a tRNAGlu-mediated pathway. The enzyme glutamyl-tRNA reductase (GTR) catalyzes the first committed step in this pathway, which is the reduction of tRNA-bound glutamate to produce glutamate 1-semialdehyde. Chlamydomonas reinhardtii mRNA encoding gtr was sequenced from a cDNA and genomic libraries. The 3179-bp gtr cDNA contains a 1566-bp open reading frame that encodes a 522-amino acid polypeptide. After removal of the predicted transit peptide, the mature 480-residue GTR has a calculated molecular weight of 52,502. The deduced C. reinhardtii mature GTR amino acid sequence has more than 55% identity to a GTR sequence of Arabidopsis thaliana, and significant similarity to GTR proteins of other plants and prokaryotes. Southern blot analysis of C. reinhardtii genomic DNA indicates that C. reinhardtii has only one gtr gene. Genomic DNA sequencing revealed the presence of a small intron near the putative transit peptide cleavage site. Expression constructs for the full-length initial gtr translation product, the mature protein after transit peptide removal, and the coding sequence of the second exon were cloned into expression vector that also introduced a C-terminal His6 tag. All of these constructs were expressed in E. coli, and both the mature protein and the exon 2 translation product complemented a hemA mutation. The expressed proteins were purified by Ni-affinity column chromatography to yield active GTR. Purified mature GTR was not inhibited by heme, but heme inhibition was restored upon addition of C. reinhardtii soluble proteins.

Chlamydomonas reinhardtii in the landscape of pigments

This review focuses on the biosynthesis of pigments in the unicellular alga Chlamydomonas reinhardtii and their physiological and regulatory functions in the context of information gathered from studies of other photosynthetic organisms. C. reinhardtii is serving as an important model organism for studies of photosynthesis and the pigments associated with the photosynthetic apparatus. Despite extensive information pertaining to the biosynthetic pathways critical for making chlorophylls and carotenoids, we are just beginning to understand the control of these pathways, the coordination between pigment and apoprotein synthesis, and the interactions between the activities of these pathways and those for other important cellular metabolites branching from these pathways. Other exciting areas relating to pigment function are also emerging: the role of intermediates of pigment biosynthesis as messengers that coordinate metabolism in the chloroplast with nuclear gene activity, and the identification of photoreceptors and their participation in critical cellular processes including phototaxis, gametogenesis, and the biogenesis of the photosynthetic machinery. These areas of research have become especially attractive for intensive development with the application of potent molecular and genomic tools currently being applied to studies of C. reinhardtii.

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.

Gene expression profiling of the tetrapyrrole metabolic pathway in Arabidopsis with a mini-array system

Tetrapyrrole compounds, such as chlorophylls, hemes, and phycobilins, are synthesized in many enzymatic steps. For regulation of the tetrapyrrole metabolic pathway, it is generally considered that several specific isoforms catalyzing particular enzymatic steps control the flow of tetrapyrrole intermediates by differential regulation of gene expression depending on environmental and developmental factors. However, the coordination of such regulatory steps and orchestration of the overall tetrapyrrole metabolic pathway are still poorly understood. In this study, we developed an original mini-array system, which enables the expression profiling of each gene involved in tetrapyrrole biosynthesis simultaneously with high sensitivity. With this system, we performed a transcriptome analysis of Arabidopsis seedlings in terms of the onset of greening, endogenous rhythm, and developmental control. Data presented here clearly showed that based on their expression profiles at the onset of greening, genes involved in tetrapyrrole biosynthesis can be classified into four categories, in which genes are coordinately regulated to control the biosynthesis. Moreover, genes in the same group were similarly controlled in an endogenous rhythmic manner but also by a developmental program. The physiological significance of these gene clusters is discussed.

Subcellular localization of two types of ferrochelatase in cucumber

It is widely believed that ferrochelatase (protoheme ferrolyase, EC 4.99.1.1), which catalyzes the insertion of ferrous ion into protoporphyrin IX to form protoheme, exists in both plastids and mitochondria of higher plants. By in vitro import assay with isolated pea (Pisum sativum L.) organelles, it has been proposed that one of two isoforms of ferrochelatase (type 1) is dual-targeted into both plastids and mitochondria, and functions for heme biosynthesis in the both organelles. Recently, however, mitochondrial targeting of ferrochelatase is being disputed since pea mitochondria appeared to accept a variety of chloroplast proteins including the type-1 ferrochelatase of Arabidopsis thaliana (L.) Heynh. To clarify the precise subcellular localization of ferrochelatase in higher plants, here we investigated the subcellular localization of two types of ferrochelatase (CsFeC1 and CsFeC2) in cucumber (Cucumis sativus L.). In cotyledons, a significant level of specific ferrochelatase activity was detected in thylakoid membranes, but only a trace level of activity was detectable in mitochondria. Western blot analysis with specific antibodies showed that anti-CsFeC2 antiserum cross-reacted with plastids in photosynthetic and non-photosynthetic tissues. Anti-CsFeC1 did not cross-react with mitochondria, but CsFeC1 was clearly detectable in plastids from non-photosynthetic tissues. In situ transient-expression assays using green fluorescent protein demonstrated that, as well as CsFeC2, the N-terminal transit peptide of CsFeC1 targeted the fusion protein solely into plastids, but not into mitochondria. These results demonstrated that in cucumber both CsFeC1 and CsFeC2 are solely targeted into plastids, but not into mitochondria. Screening of a cucumber genomic or cDNA library did not allow any other ferrochelatase homologous gene to be isolated. The data presented here imply the reconsideration of mitochondrial heme biosynthesis in higher plants.

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

The barley line albostrians exhibits a severe block in chloroplast development as a result of a mutationally induced lack of plastid ribosomes. White leaves of this mutant contain undifferentiated plastids, possess only traces of chlorophyll (Chl), and are photosynthetically inactive. Chl deficiency, combined with a continuous heme requirement, should lead to drastic changes in the tetrapyrrole metabolism in white versus green leaves. We analyzed the extent to which the synthesis rate of the pathway and the porphyrin distribution toward the Chl- and heme-synthesizing bifurcation is altered in the white tissue of albostrians. Expression and activity of several distinctively regulated enzymes, such as glutamyl-tRNAglu reductase, glutamate 1-semialdehyde aminotransferase, Mg- and Fe-chelatase, and Chl synthetase, were altered in white mutant leaves in comparison to control leaves. A drastic loss in the rate-limiting formation of 5-aminolevulinate and in the Mg-chelatase and Mg-protoporphyrin IX methyltransferase activity, as well as an increase in Fe-chelatase activity, accounts for a decrease in the metabolic flux and the re-direction of metabolites. It is proposed that the tightly balanced control of activities in the pathway functions by different metabolic feedback loops and in response to developmental state and physiological requirements. This data supports the idea that the initial steps of Mg-porphyrin synthesis contribute to plastid-derived signaling toward the nucleus. The barley mutant albostrians proved to be a valuable system for studying regulation of tetrapyrrole biosynthesis and their involvement in the bi-directional communication between plastids and nucleus.

Interaction of FLU, a negative regulator of tetrapyrrole biosynthesis, with the glutamyl-tRNA reductase requires the tetratricopeptide repeat domain of FLU

Regulation of tetrapyrrole biosynthesis in plants has been attributed to feedback control of glutamyl-tRNA reductase (GLU-TR) by heme. Recently, another negative regulator, the FLU protein, has been discovered that operates independently of heme. A truncated form of FLU that contains two domains implicated in protein-protein interaction was co-expressed in yeast with either GLU-TR or glutamate-1-semialdehyde-2-1-aminotransferase (GSA-AT), the second enzyme involved in delta-aminolevulinic acid (ALA) biosynthesis. FLU interacts strongly with GLU-TR but not with GSA-AT. Two variants of FLU that carry single amino acid exchanges within their coiled coil and tetratricopeptide repeat (TPR) domains, respectively, were also tested. Only the FLU variant with the mutated TPR motif lost the capacity to interact with GLU-TR.

Divergent regulation of the HEMA gene family encoding glutamyl-tRNA reductase in Arabidopsis thaliana: expression of HEMA2 is regulated by sugars, but is independent of light and plastid signalling

The synthesis of 5-aminolevulinic acid (ALA) is a key regulatory step for the production of hemes and chlorophyll via the tetrapyrrole synthesis pathway. The first enzyme committed to ALA synthesis is glutamyl-tRNA reductase encoded in Arabidopsis by a small family of nuclear-encoded HEMA genes. To better understand the regulation of the tetrapyrrole synthesis pathway we have made a detailed study of HEMA2 expression with transgenic Arabidopsis thaliana L. Col. plants carrying chimeric HEMA2 promoter:gusA fusion constructs. Our results show that the HEMA2 promoter directs expression predominantly to roots and flowers, but that HEMA2 is also expressed at low levels in photosynthetic tissues. Deletion analysis of the HEMA2 promoter indicates that a ca. 850 bp fragment immediately upstream of the HEMA2 coding region is sufficient to drive regulated gusA expression. In contrast to HEMA1, HEMA2 is not up-regulated by red, far-red, blue, UV or white light. In addition, elimination of a promotive plastid signal by Norflurazon-induced photobleaching of plastids had no effect on HEMA2 expression while being required for normal white-light induction of HEMA1. HEMA2 expression in the cotyledons is inhibited by the presence of sucrose or glucose, but not fructose, and this response is light-independent. HEMA1 expression in cotyledons is also inhibited by sugars, but in a strictly light-dependent manner. The roles of HEMA1 and HEMA2 in meeting cellular tetrapyrrole requirements are discussed.

Identification of a soybean protein that interacts with GAGA element dinucleotide repeat DNA

Dinucleotide repeat DNA with the pattern (GA)(n)/(TC)(n), so-called GAGA elements, control gene expression in animals, and are recognized by a specific regulatory protein. Here, a yeast one-hybrid screen was used to isolate soybean (Glycine max) cDNA encoding a GAGA-binding protein (GBP) that binds to (GA)(n)/(CT)(n) DNA. Soybean GBP was dissimilar from the GAGA factor of Drosophila melanogaster. Recombinant GBP protein did not bind to dinucleotide repeat sequences other than (GA)(n)/(CT)(n). GBP bound to the promoter of the heme and chlorophyll synthesis gene Gsa1, which contains a GAGA element. Removal of that GAGA element abrogated binding of GBP to the promoter. Furthermore, insertion of the GAGA element to a nonspecific DNA conferred GBP-binding activity on that DNA. Thus, the GAGA element of the Gsa1 promoter is both necessary and sufficient for GBP binding. Gbp mRNA was expressed in leaves and was induced in symbiotic root nodules elicited by the bacterium Bradyrhizobium japonicum. In addition, Gbp transcripts were much higher in leaves of dark-treated etiolated plantlets than in those exposed to light for 24 h. Homologs of GBP were found in other dicots and in the monocot rice (Oryza sativa), as well. We suggest that interaction between GAGA elements and GBP-like proteins is a regulatory feature in plants.

Regulation of the tetrapyrrole biosynthetic pathway leading to heme and chlorophyll in plants and cyanobacteria

Photosynthetic organisms synthesize chlorophylls, hemes, and bilin pigments via a common tetrapyrrole biosynthetic pathway. This review summarizes current knowledge about the regulation of this pathway in plants, algae, and cyanobacteria. Particular emphasis is placed on the regulation of glutamate-1-semialdehyde formation and on the channelling of protoporphyrin IX into the heme and chlorophyll branches. The potential role of chlorophyll molecules that are not bound to photosynthetic pigment-protein complexes ('free chlorophylls') or of other Mg-containing porphyrins in regulation of tetrapyrrole synthesis is also discussed.

Two types of ferrochelatase in photosynthetic and nonphotosynthetic tissues of cucumber: their difference in phylogeny, gene expression, and localization

Ferrochelatase catalyzes the insertion of Fe(2+) into protoporphyrin IX to generate protoheme. In higher plants, there is evidence for two isoforms of this enzyme that fulfill different roles. Here, we describe the isolation of a second ferrochelatase cDNA from cucumber (CsFeC2) that was less similar to a previously isolated isoform (CsFeC1) than it was to some ferrochelatases from other higher plants. In in vitro import experiments, the two cucumber isoforms showed characteristics similar to their respective ferrochelatase counterparts of Arabidopsis thaliana. The C-terminal region of CsFeC2 but not CsFeC1 contained a conserved motif found in light-harvesting chlorophyll proteins, and CsFeC2 belonged to a phylogenetic group of plant ferrochelatases containing this conserved motif. We demonstrate that CsFeC2 was localized predominantly in thylakoid membranes as an intrinsic protein, and forming complexes probably with the C-terminal conserved motif, but a minor portion was also detected in envelope membranes. CsFeC2 mRNA was detected in all tissues and was light-responsive in cotyledons, whereas CsFeC1 mRNA was detected in nonphotosynthetic tissues and was not light-responsive. Interestingly, tissue-, light-, and cycloheximide-dependent expressions of the two isoforms of ferrochelatase were similar to those of two glutamyl-tRNA reductase isoforms involved in the early step of tetrapyrrole biosynthesis, suggesting the existence of distinctly controlled tetrapyrrole biosynthetic pathways in photosynthetic and nonphotosynthetic tissues.

Elimination of POR expression correlates with red leaf formation in Amaranthus tricolor

Amaranthus tricolor L. tricolor cv. Earlysplendor, an ornamental amaranth, generates red leaves instead of green leaves in late summer to early autumn. Red leaf formation was promoted under short-day conditions and delayed by night-break treatments. Red leaves were characterized by lower levels of chlorophyll accumulation rather than higher levels of red pigment (betacyanin) accumulation. However, the metabolic activity toward the production of Mg-protoporphyrin, an intermediate in the biosynthesis pathway for chlorophyll, was detected in red leaves as well as in green leaves. RNA gel blot analysis was performed to assess the expression of nine genes encoding eight enzymes involved in chlorophyll biosynthesis. Among these enzymes, red-leaf-specific reduction of gene expression was observed only for NADPH-protochlorophyllide oxidoreductase (POR), a key enzyme catalyzing a later step of chlorophyll biosynthesis. In addition, immunoblot analysis showed no accumulation of POR protein(s) in red leaves. These data indicate that the repression of POR gene expression and resultant loss of chlorophyll synthesis activity plays a role in red leaf formation of A. tricolor.

Regulation of HEMA1 expression by phytochrome and a plastid signal during de-etiolation in Arabidopsis thaliana

The synthesis of 5-aminolevulinic acid (ALA) is the rate-limiting step for the formation of all plant tetrapyrroles, including chlorophyll and heme, and regulation of ALA synthesis is therefore critical to plant development. Glutamyl-tRNA reductase (GluTR) is the first committed enzyme of this pathway and is encoded by a small family of nuclear HEMA genes. Here, we have used transgenic Arabidopsis (Arabidopsis thaliana L. Col) lines expressing chimeric HEMA1 promoter:gusA fusion genes, combined with RNA gel blot analyses, to characterise the light-mediated regulation of the Arabidopsis HEMA1 gene during de-etiolation. HEMA1 was expressed strongly, but not exclusively, in photosynthetic tissues and was shown to be light regulated at the transcriptional level by the phytochrome family of photoreceptors acting in both the far-red high irradiance and low fluence response modes. The HEMA2 gene, which is expressed only in roots of seedlings, was not light regulated. Analysis of truncated HEMA1 promoter constructs demonstrated that a -199/+252 promoter fragment was sufficient to confer full light-responsiveness to gusA expression. This fragment contained GT-1/I-box and CCA-1 binding sites that are implicated as the light-responsive cis elements. Both the full-length and truncated HEMA1 promoters required the presence of intact chloroplasts for full expression, consistent with previous indications that light and plastid factor signals converge to co-ordinately regulate expression of photosynthesis-related nuclear genes. These results provide the most comprehensive analysis to date of the light-regulation of a tetrapyrrole biosynthetic gene and support a direct link between regulation of HEMA1 transcription and chlorophyll accumulation during seedling de-etiolation.

Antisense HEMA1 RNA expression inhibits heme and chlorophyll biosynthesis in arabidopsis

5-aminolevulinic acid (ALA) is a precursor in the biosynthesis of tetrapyrroles including chlorophylls and heme. The formation of ALA involves two enzymatic steps which take place in the chloroplast in plants. The first enzyme, glutamyl-tRNA reductase, and the second enzyme, glutamate-1-semialdehyde-2,1-aminomutase, are encoded by the nuclear HEMA and GSA genes, respectively. To assess the significance of the HEMA gene for chlorophyll and heme synthesis, transgenic Arabidopsis plants that expressed antisense HEMA1 mRNA from the constitutive cauliflower mosaic virus 35S promoter were generated. These plants exhibited varying degrees of chlorophyll deficiency, ranging from patchy yellow to total yellow. Analysis indicated that these plants had decreased levels of chlorophyll, non-covalently bound hemes, and ALA; their levels were proportional to the level of glutamyl-tRNA reductase expression and were inversely related to the levels of antisense HEMA transcripts. Plants that lacked chlorophyll failed to survive under normal growth conditions, indicating that HEMA gene expression is essential for growth.

Novel inhibitors of glutamyl-tRNA(Glu) reductase identified through cell-based screening of the heme/chlorophyll biosynthetic pathway

The metabolite 5-aminolevulinic acid (ALA) is an early committed intermediate in the biosynthetic pathway of heme and chlorophyll formation. In plants, 5-aminolevulinic acid is synthesized via a two-step pathway in which glutamyl-tRNA(Glu) is reduced by glutamyl-tRNA(Glu) reductase (GluTR) to glutamate 1-semialdehyde, followed by transformation to 5-aminolevulinic acid catalyzed by glutamate 1-semialdehyde aminotransferase. Using an Escherichia coli cell-based high-throughput assay to screen small molecule libraries, we identified several chemical classes that specifically inhibit heme/chlorophyll biosynthesis at this point by demonstrating that the observed cell growth inhibition is reversed by supplementing the medium with 5-aminolevulinic acid. These compounds were further tested in vitro for inhibition of the purified enzymes GluTR and glutamate 1-semialdehyde aminotransferase as confirmation of the specificity and site of action. Several promising compounds were identified from the high-throughput screen that inhibit GluTR with an I(0.5) of less than 10 microM. Our results demonstrate the efficacy of cell-based high-throughput screening for identifying inhibitors of 5-aminolevulinic acid biosynthesis, thus representing the first report of exogenous inhibitors of this enzyme.

Expression of a soybean gene encoding the tetrapyrrole-synthesis enzyme glutamyl-tRNA reductase in symbiotic root nodules

Heme and chlorophyll accumulate to high levels in legume root nodules and in photosynthetic tissues, respectively, and they are both derived from the universal tetrapyrrole precursor delta-aminolevulinic acid (ALA). The first committed step in ALA and tetrapyrrole synthesis is catalyzed by glutamyl-tRNA reductase (GTR) in plants. A soybean (Glycine max) root-nodule cDNA encoding GTR was isolated by complementation of an Escherichia coli GTR-defective mutant for restoration of ALA prototrophy. Gtr mRNA was very low in uninfected roots but accumulated to high levels in root nodules. The induction of Gtr mRNA in developing nodules was subsequent to that of the gene Enod2 (early nodule) and coincided with leghemoglobin mRNA accumulation. Genomic analysis revealed two Gtr genes, Gtr1 and a 3' portion of Gtr2, which were isolated from the soybean genome. RNase-protection analysis using probes specific to Gtr1 and Gtr2 showed that both genes were expressed, but Gtr1 mRNA accumulated to significantly higher levels. In addition, the qualitative patterns of expression of Gtr1 and Gtr2 were similar to each other and to total Gtr mRNA in leaves and nodules of mature plants and etiolated plantlets. The data indicate that Gtr1 is universal for tetrapyrrole synthesis and that a Gtr gene specific for a tissue or tetrapyrrole is unlikely. We suggest that ALA synthesis in specialized root nodules involves an altered spatial expression of genes that are otherwise induced strongly only in photosynthetic tissues of uninfected plants.

Novel insights in the control of tetrapyrrole metabolism of higher plants

Tetrapyrrole biosynthesis has been the subject of numerous studies over several decades. In recent years scientific interest in plant tetrapyrrole biosynthesis has included both genetical and biochemical elucidation of almost every enzymatic step of the pathway and has focused to an increasing extent on the regulatory mechanism of the entire metabolic pathway, but in particular of key steps, such as synthesis of 5-aminolevulinate, magnesium chelatase or protochlorophyllide oxidoreductase.

Barley glutamyl tRNAGlu reductase: mutations affecting haem inhibition and enzyme activity

Glutamyl tRNA(Glu) reductase converts glutamate molecules that are ligated at their alpha-carboxyl groups to tRNA(Glu) into glutamate 1-semialdehyde, an intermediate in the synthesis of 5-aminolevulinate, chlorophyll and haem. The mature plant enzymes contain a highly conserved extension of 31-34 amino acids at the N-terminus not present in bacterial enzymes. It is shown that barley glutamyl tRNAGlu reductases with a deletion of the 30 N-terminal amino acids have the same high specific activity as the untruncated enzymes, but are highly resistant to feed-back inhibition by haem. This peptide domain thus interacts directly or indirectly with haem and the toxicity of the 30 amino acid peptide for Escherichia coli experienced in mutant rescue and overexpression experiments can be explained by extensive haem removal from the metabolic pools that cannot be tolerated by the cell. Induced missense mutations identify nine amino acids in the 451 residue long C-terminal part of the barley glutamyl tRNA(Glu) reductase which upon substitution curtail drastically, but do not eliminate entirely the catalytic activity of the enzyme. These amino acids are thus important for the catalytic reaction or tRNA binding.