Chloroplast biogenesis: Biosynthesis and accumulation of Mg-protoporphyrin IX monoester and other metalloporphyrins by isolated etioplasts and developing chloroplasts

Developing chloroplasts isolated from greening cotyledons and isolated etioplasts were capable of synthesizing and accumulating Mg-protoporphyrin IX monoester as well as longer wavelength metalloporphyrins when incubated in the dark, in the presence of air, δ-aminolevulinic acid, and cofactors (coenzyme A, glutathione, adenosine triphosphate, nicotinamide adenine dinucleotide, methyl alcohol, magnesium, potassium, and phosphate). The putative metalloporphyrins exhibited distinct fluorescence emission and excitation properties and were detected by spectrofluorometry in situ and after extraction in organic solvents. The cofactors were previously shown to be required for protochlorophyll, and chlorophyll biosynthesis and grana assembly in vitro. The putative long wavelength metalloporphyrins were suggested earlier to represent intermediates between Mg-protoporphyrin IX monomethyl ester and protochlorophyllide. The isolated plastids were similar in this aspect of their biosynthetic activity to etiolated cotyledons greening in distilled H₂O. In contrast to greening cotyledons, however, the biosynthetic activity of the isolated plastids depended on the addition of exogenous cofactors and δ-aminolevulinic acid. This was interpreted as an indication that the isolated plastids were not capable of generating their own δ-aminolevulinic acid and cofactors under the present incubation conditions. Light was not required for the conversion of added ALA to metalloporphyrins in vitro. The metalloporphyrins synthesized in vitro were more highly fluorescent in situ than those of greening cotyledons. In addition to Mg-protoporphyrin IX monoester and longer wavelength metalloporphyrins, isolated etioplasts synthesized and accumulated Zn-protoporphyrin and Zn-protoporphyrin IX monoesterlike compounds.     

Heterologous expression of the bchM gene product from Rhodobacter capsulatus and demonstration that it encodes S-adenosyl-L-methionine:Mg-protoporphyrin IX methyltransferase.

The bacteriochlorophyll biosynthesis gene, bchM, from Rhodobacter capsulatus was previously believed to code for a polypeptide involved in formation of the cyclopentone ring of protochlorophyllide from Mg-protoporphyrin IX monomethyl ester. In this study, R. capsulatus bchM was expressed in Escherichia coli and the gene product was subsequently demonstrated by enzymatic analysis to catalyze methylation of Mg-protoporphyrin IX to form Mg-protoporphyrin IX monomethyl ester. Activity required the substrates Mg-protoporphyrin IX and S-adenosyl-L-methionine. 14C-labeled product was formed in incubations containing 14C-methyl-labeled S-adenosyl-L-methionine. On the basis of these and previous results, we also conclude that the bchH gene, which was previously reported to code for Mg-protoporphyrin IX methyltransferase, is most likely involved in the Mg chelation step.     

Green or red: what stops the traffic in the tetrapyrrole pathway?

Regulation of tetrapyrrole biosynthesis is crucial to plant metabolism. The two pivotal control points are formation of the initial precursor, 5-aminolaevulinic acid (ALA), and the metal-ion insertion step: chelation of Fe(2+) into protoporphyrin IX leads to haem and phytochromobilin, whereas insertion of Mg(2+) is the first step to chlorophyll. Recent studies with mutants and transgenic plants have demonstrated that perturbation of the branch point affects ALA formation. Moreover, one of the signals that controls the expression of genes for nuclear-encoded chloroplast proteins has been shown to be Mg-protoporphyrin-IX. Here, we discuss the regulation of branch-point flux and the relative contributions of the haem and chlorophyll branches to the regulation of ALA synthesis and thus to flow through the tetrapyrrole pathway.     

Formation of Mg-Containing Chlorophyll Precursors from Protoporphyrin IX, delta-Aminolevulinic Acid, and Glutamate in Isolated, Photosynthetically Competent, Developing Chloroplasts

Intact developing chloroplasts isolated from greening cucumber (Cucumis sativus L. var Beit Alpha) cotyledons were found to contain all the enzymes necessary for the synthesis of chlorophyllide. Glutamate was converted to Mg-protoporphyrin IX (monomethyl ester) and protoclorophyllide. δ-Aminolevulinic acid and protoporphyrin IX were converted to Mg-protoporphyrin IX, Mg-protoporphyrin IX monomethyl ester, protochlorophyllide and chlorophyllide a. The conversion of δ-aminolevulinic acid or protoporphyrin IX to Mg-protoporphyrin IX (monomethyl ester) was inhibited by AMP and p-chloromercuribenzene sulfonate. Light stimulated the formation of Mg-protoporphyrin IX from all three substrates. In the case of δ-aminolevulinic acid and protoporphyrin IX, light could be replaced by exogenous ATP. In the case of glutamate, both ATP and reducing power were necessary to replace light. With all three substrates, glutamate, δ-aminolevulinic acid, and protoporphyrin IX, the stimulation of Mg-protoporphyrin IX accumulation in the light was abolished by DCMU, and this DCMU block was overcome by added ATP and reducing power.     

Biosynthesis of δ-aminolevulinic acid in Rhodopseudomonas sphaeroides

The biosynthesis of δ-aminolevulinic acid was investigated in three strains of Rhodopseudomonas sphaeroides. A wild-type strain (NCIB 8253) possessed both δ-aminolevulinic acid synthetase and γ,δ-dioxovaleric acid transaminase in the cytoplasmic and membrane cell fractions. δ-Aminolevulinic acid synthetase activities were not detected in extracts of mutant strains H5 and H5D. However, γ,δ-dioxovaleric acid transaminase was found in the cytoplasmic and membrane fractions of these latter two strains. Strain H5 required exogenously added δ-aminolevulinic acid for growth and bacteriochlorophyll synthesis. Strain H5D did not require this compound for growth and bacteriochlorophyll synthesis. γ,δ-Dioxovaleric acid added in the growth medium did not support the growth of H5, although it was actively transported into the cells. Addition of γ,δ-dioxovaleric acid to the growth medium did not enhance the growth of either the wild-type or H5D strains. These results indicate that ALA synthetase is not required for growth and bacteriochlorophyll synthesis in H5D and that γ,δ-dioxovaleric acid is probably not an intermediate in the formation of δ-aminolevulinic acid in the strains of Rhodopseudomonas sphaeroides studied. In strain H5D another pathway may function in the formation of δ-aminolevulinic acid other than that catalyzed by δ-aminolevulinic acid synthetase or γ,δ-dioxovaleric acid transaminase.     

Selenium as a novel regulator of porphyrin biosynthesis in germinating seedlings of mung bean (Phaseolus vulgaris)

5-Aminolevulinic acid, porphyrin and chlorophyll contents as well the activities of 5-aminolevulinic acid dehydratase and PBG deaminase were studied in selenium treated mung bean seedlings. Selenium had no effect on 5-aminolevulinic acid synthetic ability but inhibited 5-aminolevulinic acid dehydratase and PBG deaminase activities. Further, it was observed that selenium induced accumulation of protoporphyrin-IX and Mg-protoporphyrin ester and decreased chlorophyll levels in both light and dark-grown seedlings. The results suggest the possible regulatory role of selenium on chlorophyll synthesis by interacting with sulfhydryl containing enzymes 5-aminolevulinic acid dehydratase and porphobilinogen deaminase.     

δ-aminolevulinic acid transport and synthesis,porphyrin synthesis and heme catabolism in chick embryo liver and heart cells

Liver and heart represent two organs with markedly different needs for heme as related to their metabolic roles. To examine these diferences chick embryo heart and liver cells were compared with respect to transport of δ-aminolevulinic acid and activity of δ-aminolevulinic acid synthetase, porphyrin synthesis and heme oxygenase. Heart cells were found to have a low rate of δ-aminolevulinic acid uptake, a high resting level of δ-aminolevulinic acid synthetase activity and a lower level of heme oxygenase activity as compared with liver cells. The hepatic cell uptake of δ-aminolevulinic acid was 6–25-times that of heart cells. The embryonal heart cell appears to be a balanced autonomous system for the synthesis and degradation of heme. The embryonal liver cell represents a cell system permeable to exogenous δ-aminolevulinic acid, which is also responsive to and inducible by external stimuli.     

Glucose and δ-Aminolevulinic Acid Stimulate the Dark Chlorophyll Synthesis of Rice Seedlings

This research was to examine if rice (Oryza sativa L.), a monocotyledon of angiosperm, was able to synthesize chlorophyll (Chl) in complete darkness. Five-cm-tall etiolated seedlings of rice were used as starting materials and treated with or without various concentrations of glucose and/or δ-aminolevulinic acid (ALA) in the dark. Leaves harvested at the indicated time were determined for their contents of Chl, protoporphyrin Ⅸ(Proto), Mg-protoporphyrin Ⅸ(Mg-Proto) and protochlorophyllide (Pchlide). The mole percentage of porphyrin was calculated. The Chl content in the etiolated rice seedlings slightly increased from about 2.5 μg/g to 7.5 μg/g within 12 d in the dark, but the total Chl of dark-grown rice increased from 0.36 μg/g to 3.6 μg/g. While the mole percentages of Proto, Mg-Proto and Pchlide in the dark-grown seedlings without any treatment were about 65%, 27.5% and 7.5% at the beginning, respectively, those in the light-grown seedlings were about 42.5%, 35% and 22.5%, respectively. The mole percentage of porphyrin of etiolated seedlings resumed its normal ratio within 2 d after treatment with glucose. While the Chl content of etiolated seedlings grown in culture solution with 3% and 6% glucose increased 2.5 and 4.0 folds, respectively, those with 3% and 6% glucose and 1 mmol/L ALA increased 22 and 24 folds, respectively. It is concluded that angiosperm might be able to synthesize a small amount of Chl in complete darkness, that either glucose or ALA could stimulate dark Chl synthesis in angiosperm, and that a combination of glucose and ALA exhibited an additional effect. It is still unknown and remains to be further explored what is the mechanism of the effect of glucose and ALA on the Chl synthesis of rice in the dark. Key words: angiosperm; rice; dark chlorophyll synthesis; glucose; δ-aminolevulinic acid; protoporphyrin Ⅸ; Mg-protoporphyrin Ⅸ; protochlorophyllide     

Porphyrogenic effects and induction of heme oxygenase in vivo by δ-aminolevulinic acid

The effects of single large doses of the porphyrin-heme precursor ?d-aminolevulinic acid on tissue porphyrins and on δ-aminolevulinate synthase and heme oxygenase, the rate-living enzymes of liver heme synthesis and degradation respectively, were studied in the chick embryo in ovo, in the mouse and in the rat. δ-Aminolevulinic acid treatment produced a distinctive pattern characterized by extensive tissue porphyrin accumulation and alterations in these rate-limiting enzymes in the liver. Repression of basal or allylisopropylacetamide-induced liver δ-aminolevulinate synthase was observed and, in the mouse and the rat, induction of liver heme oxygenase after δ-aminolevulinic acid treatment, in a manner similar to the known effects of hemin on these enzymes. In the chick embryo liver in ovo heme oxygenase was substantially higher than in rat and mouse liver, and was not significantly induced by δ-aminolevulinic acid or other compounds, including hemin, CS₂ and CoCl₂. Levulinic acid, an analogue of δ-aminolevulinic acid, did not induce heme oxygenase in mouse liver. δ-Aminolevunilic acid treatment did not impair ferrochelatase activity but was associated with slight and variable decreases in liver cytochrome P-450. Treatment of chick embryos with a small ‘priming’ dose of 1,4-dihydro-3,5-dicarbethoxycollidine, which impairs liver ferrochelatase activity, accentuated porphyrin accumulation after δ-aminolevulinic acid in the liver. These observations indicate that exogenous δ-aminolevulinic acid is metabolized to porphyrins in a number of tissues and, at least in the liver, to a physiologically significant amount of heme, thereby producing an increase in the size of one or more of the heme pools that regulate both heme systhesis and degradation. It is also possible than when δ-aminolevulinic acid is markedly overproduced in vivo it may be transported to many tissues and re-enter the heme pathway and alter porphyrin-heme metabolism in cells and tissues other than those in which its overproduction primarily occurs.     

The effect of simulated microgravity on formation of the pigment apparatus in etiolated barley seedlings

The effect of horizontal clinorotation on the dynamics of the accumulation of the main photosynthetic pigments in the greening of 6-day-old etiolated barley seedlings has been studied. The content of protochlorophillide, the direct precursor of chlorophyll a, in clinorotated seedlings in the dark was 9–20% lower than in the control group. After exposure of barley seedlings to light for 12 h under clinorotation, chlorophyll accumulation lagged behind the control by 45% and reached the control value after 48–72 h. The total content of carotenoids increased many fold during greening; at the first stage the carotenoid level in clinorotated seedlings was less than in the control. The synthesis rates of δ-aminolevulinic acid and δ-aminolevulinate dehydratase activity in clinorotated seedlings were slower than in the control after 24 h of greening and after 72 h of greening reaching the control values. The activity of Mg-protoporphyrin IX chelatase catalyzing the incorporation of Mg ions in the structure of chlorophyll a, did not change when exposed to clinorotation. The results we obtained show inhibition of the initial stages of chlorophyll biosynthesis in the conditions of simulated microgravity. The light, to a certain extent, decreases the negative effect of microgravity on the formation of the photosynthetic apparatus in plants.     

Separation of Mg-Protoporphyrin IX and Mg-Protoporphyrin IX Monomethyl Ester Synthesized de novo by Developing Cucumber Etioplasts

High pressure liquid chromatography was used to demonstrate that chelation of Mg²⁺ into protoporphyrin IX precedes methylation in isolated greening etioplasts from cucumber (Cucumis sativus L. var. Beit Alpha) cotyledons. Mg-protoporphyrin IX synthesized in vitro from protoporphyrin IX, Mg²⁺, and ATP or exogenous Mg-protoporphyrin IX could serve as substrates for the methylation step. In either case, S-adenosylmethionine was the methyl donor and could not be replaced by ATP plus methionine.     

Chloroplast Biogenesis: XXIV. Intrachloroplastic Localization of the Biosynthesis and Accumulation of Protoporphyrin IX, Magnesium-Protoporphyrin Monoester, and Longer Wavelength Metalloporphyrins during Greening

The intraplastidic localization of the endogenous metabolic pools from protoporphyrin to protochlorophyll was determined in Cucumis sativus. The endogenous protoporphyrin, Mg-protoporphyrin monoester + longer wavelength metalloporphyrins, protochlorophyllide and protochlorophyllide ester were membrane-bound. Protoporphyrin was synthesized in the stroma and subsequently became associated with the membranes. The membrane-associated protoporphyrin was then converted into Mg-protoporphyrin monoester + longer wavelength metalloporphyrins by membrane-bound enzymes. Although lysed plastids were capable of converting exogenous δ-aminolevulinic acid to protochlorophyllide, the net synthesis of protochlorophyllide from exogenous δ-aminolevulinic acid was lost upon segregating the lysed plastids into stromal and membrane fractions and then recombining the stromal and membrane fraction prior to incubation. The segregated membrane fraction was still capable of converting protoporphyrin into Mg-protoporphyrin monoester + longer wavelength metalloporphyrins in the presence or absence of the stromal fraction. These results indicated that although the reactions from protoporphyrin to Mg-protoporphyrin monoester and longer wavelength metalloporphyrins could survive a considerable degree of plastid disruption, the reactions from Mg-protoporphyrin monoester and longer wavelength metalloporphyrins to protochlorophyllide were more sensitive to structural disorganization.     

Hemin feedback inhibition at reticulocyte δ-aminolevulinic acid synthetase and δ-aminolevulinic acid dehydratase

Addition of hemin (5–200 μM) to a rabbit reticulocyte iron-free incubation medium, resulted in a progressive inhibition of heme synthesis as measured by incorporation of (¹⁴C)-glycine. In contrast when (¹⁴C) δ-aminolevulinic acid incorporation into heme was studied, significant inhibition below that of the (¹⁴C)-glycine control only occurred with hemin concentrations greater than 100 μM. Hemin progressively inhibited cellular and mitochondrialδ-aminolevulinic acid synthetase activity, as well as cellular δ-aminolevulinic acid dehydratase activity. The results indicated that elevated levels of hemin initially control heme synthesis by feedback inhibition at the rate-limiting enzyme of heme synthesis, δ-aminolevulinic acid synthetase. Hemin inhibition of δ-aminolevulinic acid dehydratase is only significant for the entrire heme synthetic pathway when greater than one-third of this enzyme's activity is inhibited.     

A BchD (magnesium chelatase) mutant of rhodobacter sphaeroides synthesizes zinc bacteriochlorophyll through novel zinc-containing intermediates

Heme and bacteriochlorophyll a (BChl) biosyntheses share the same pathway to protoporphyrin IX, which then branches as follows. Fe(2+) chelation into the macrocycle by ferrochelatase results in heme formation, and Mg(2+) addition by Mg-chelatase commits the porphyrin to BChl synthesis. It was recently discovered that a bchD (Mg-chelatase) mutant of Rhodobacter sphaeroides produces an alternative BChl in which Mg(2+) is substituted by Zn(2+). Zn-BChl has been found in only one other organism before, the acidophilic Acidiphilium rubrum. Our objectives in this work on the bchD mutant were to 1) elucidate the Zn-BChl biosynthetic pathway in this organism and 2) understand causes for the low amounts of Zn-BChl produced. The bchD mutant was found to contain a Zn-protoporphyrin IX pool, analogous to the Mg-protoporphyrin IX pool found in the wild type strain. Inhibition of ferrochelatase with N-methylprotoporphyrin IX caused Zn-protoporphyrin IX and Zn-BChl levels to decline by 80-90% in the bchD mutant, whereas in the wild type strain, Mg-protoporphyrin IX and Mg-BChl levels increased by 170-240%. Two early metabolites of the Zn-BChl pathway were isolated from the bchD mutant and identified as Zn-protoporphyrin IX monomethyl ester and divinyl-Zn-protochlorophyllide. Our data support a model in which ferrochelatase synthesizes Zn-protoporphyrin IX, and this metabolite is acted on by enzymes of the BChl pathway to produce Zn-BChl. Finally, the low amounts of Zn-BChl in the bchD mutant may be due, at least in part, to a bottleneck upstream of the step where divinyl-Zn-protochlorophyllide is converted to monovinyl-Zn-protochlorophyllide.     

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.     

ATP requirement for mg chelatase in developing chloroplasts

The synthesis of Mg-protoporphyrin-IX from exogenous protoporphyrin-IX, in a crude plastid pellet extracted from greening cucumber cotyledons was found to require l-glutamate as a cofactor. It has now been shown that glutamate acts in the presence of contaminating mitochondria to provide an ATP regenerating system. With purified plastids, Mg chelatase is not stimulated by glutamate; instead, it requires a high concentration of ATP and is greatly stimulated by added phosphoenolpyruvate and pyruvate kinase. GTP, UTP, CTP, and ITP will not substitute for ATP. ADP in the absence of an ATP generating system is completely ineffective, whereas it is slightly inhibitory in the presence of 10 mm ATP. AMP is strongly inhibitory in the reaction; 50% inhibition is obtained at approximately 3.5 mm AMP in the presence of 10 mm ATP.     

Effects of Iron and Oxygen on Chlorophyll Biosynthesis : I. IN VIVO OBSERVATIONS ON IRON AND OXYGEN-DEFICIENT PLANTS

Corn (Zea mays, L.), bean (Phaseolus vulgaris L.), barley (Hordeum vulgare L.), spinach (Spinacia oleracea L.), and sugarbeet (Beta vulgaris L.) grown under iron deficiency, and Potamogeton pectinatus L, and Potamogeton nodosus Poir. grown under oxygen deficiency, contained less chlorophyll than the controls, but accumulated Mg-protoporphyrin IX and/or Mg-protoporphyrin IX monomethyl ester. No significant accumulation of these intermediates was detected in the controls or in the tissue of plants stressed by S, Mg, N deficiency, or by prolonged dark treatment. Treatment of normal plant tissue with δ-aminolevulinic acid in the dark resulted in the accumulation of protochlorophyllide. If this treatment was carried out under conditions of iron or oxygen deficiency, less protochlorophyllide was formed, but a significant amount of Mg-protoporphyrin IX and Mg-protoporphyrin IX monomethyl ester accumulated.     

镁螯合酶的结构与功能

镁螯合酶（magnesium chelatase）是叶绿素合成过程中的关键酶,催化原卟啉IX与Mg2+螯合形成镁原卟啉IX。镁螯合酶由催化亚基H与AAA+亚基I、D组成。通过这3种亚基的协调配合,在ATP驱动下实现Mg2+与原卟啉IX的螯合,推动叶绿素的合成。在这一过程中,基因组解偶联基因4（GUN4）蛋白对其发挥重要的正调控作用。自上世纪90年代以来,镁螯合酶独特的结构及其作用机制一直吸引着研究者们的兴趣。本文结合最新的研究进展,阐述镁螯合酶的结构、酶促反应动力学及其催化机制等。另外,对于GUN4蛋白对镁螯合酶的调控也进行了概述。     

Chloroplast biogenesis. Biosynthesis and accumulation of protochlorophyll by isolated etioplasts and developing chloroplasts.

Etioplasts and developing chloroplasts were isolated from etiolated Cucumis cotyledons that were irradiated with white fluorescent light for various periods of time. The endogenous porphyrins and phorbins of the isolated plastids were partitioned between hexane, hexane-extracted aqueous acetone and a lipoprotein precipitate. Spectrofluorometric determinations were performed on these fractions without further fractionation. For quantitative determinations, the fluorescence amplitudes of the various fluorescent components were corrected for fluorescence emission overlap by sets of simultaneous equations. Developing chloroplasts contained endogenous amounts of the following metabolites: Protochlorophyllide, protochlorophyllide ester, Mg-protoporphyrin monoester + longer-wavelength metalloporphyrins and protoporphyrin. The protochlorophyll pool consisted mainly of protochlorophyllide. The latter was heterogeneous and consisted of at least two chemically related protochlorophyllides. In contrast to developing chloroplasts, irradiated etioplasts contained mostly protochlorophyllide ester and smaller amounts of protochlorophyllide. Upon incubation of developing chloroplasts and irradiated etioplasts with δ-aminolevulinic acid and cofactors (coenzyme A, glutathione, adenosine triphosphate, nicotinamide adenine dinucleotide, methyl alcohol, magnesium, potassium and phosphate), a net synthesis and accumulation of protochlorophyllide, Mg-protoporphyrin monoester + longer-wavelength metalloporphyrins, protoporphyrin, coproporphyrin and uroporphyrin were observed. Small amounts of zinc-coproporphyrin and zinc-uroporphyrin were also formed. In some experiments a net synthesis of protochlorophyllide ester was also observed. This report represents the first account of the unambiguous net synthesis of protochlorophyll in vitro.     

Influence of photodynamic processes induced by 2,2′-dipyridyl on the enzymatic system of chlorophyll biosynthesis

The influence of 2,2′-dipyridyl (2,2′-DP) on the activity of one of the enzymes at the initial stages of chlorophyll (Chl) biosynthesis, δ-aminolevulinic acid dehydratase (ALAD; δ-aminolevulinate hydro-lyase, EC 4.2.1.24), as well as on δ-aminolevulinic acid (ALA) accumulation was investigated in green barley (Hordeum vulgare L.) leaves. In seven-day-old green leaves treated with 3 mM 2,2′-DP for 17 h in darkness and subsequently irradiated with "white light" (15 W m-2) for 4, 8, and 24 h the ALAD activity was 51 % as compared to that in untreated leaves. At the same time, the ALA forming system was most sensitive to the photodynamic processes caused by 2,2′-DP. After 8 h of irradiation, ALA synthesis was entirely inhibited. After the treatment the leaves accumulated exceptionally high amounts of Chl precursors such as protoporphyrin IX (Proto), Mg-protoporphyrin IX (Mg-Proto), its monomethyl ester, and protochlorophyllide (Pchlide) that are photosensitizers of photodynamic processes in plants. A comparatively low Chl and carotenoid (Car) destruction was registered during the subsequent 4 and 8 h of irradiation. At the same time, the content of Chl precursors was negligible. The low photodestruction of Chl and Car included in pigment-protein complexes, against the background of fast porphyrin disappearance, and fast decrease of enzymatic activities at the initial stages of Chl production could mean that the photodynamic effect induced by porphyrins accumulated in the presence of 2,2′-DP affected first the Chl enzymatic system and did not change the pool of already synthesized photosynthetic pigments.