Salt and genotype impact on antioxidative enzymes and lipid peroxidation in two rice cultivars during de-etiolation

Crop yield is severely affected by soil salinity, as salt levels that are harmful to plant growth occur in large terrestrial areas of the world. The present investigation describes the studies of enzymatic activities, in-gel assays, gene expression of some of the major antioxidative enzymes, tocopherol accumulation, lipid peroxidation, ascorbate and dehydroascorbate contents in a salt-sensitive rice genotype PB1, and a relatively salt-tolerant cultivar CSR10 in response to 200 mM NaCl. Salt solution was added to the roots of hydroponically grown 5-day-old etiolated rice seedlings, 12 h prior to transfer to cool white fluorescent + incandescent light (100 μmol photons m(-2) s(-1)). Total tocopherol and ascorbate contents declined in salt-stressed rice seedlings. Among antioxidative enzymes, an increase in the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), ascorbate peroxidase (EC 1.11.1.11), glutathione reductase (EC 1.6.4.2), and their gene expression was observed in both cultivars in response to salt stress. The salt-tolerant cultivar CSR10 resisted stress due to its early preparedness to combat oxidative stress via upregulation of gene expression and enzymatic activities of antioxidative enzymes and a higher redox status of the antioxidant ascorbate even in a non-stressed environment.

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.

Light intensity-dependent modulation of chlorophyll b biosynthesis and photosynthesis by overexpression of chlorophyllide a oxygenase in tobacco

Chlorophyll b is synthesized by the oxidation of a methyl group on the B ring of a tetrapyrrole molecule to a formyl group by chlorophyllide a oxygenase (CAO). The full-length CAO from Arabidopsis (Arabidopsis thaliana) was overexpressed in tobacco (Nicotiana tabacum) that grows well at light intensities much higher than those tolerated by Arabidopsis. This resulted in an increased synthesis of glutamate semialdehyde, 5-aminolevulinic acid, magnesium-porphyrins, and chlorophylls. Overexpression of CAO resulted in increased chlorophyll b synthesis and a decreased chlorophyll a/b ratio in low light-grown as well as high light-grown tobacco plants; this effect, however, was more pronounced in high light. The increased potential of the protochlorophyllide oxidoreductase activity and chlorophyll biosynthesis compensated for the usual loss of chlorophylls in high light. Increased chlorophyll b synthesis in CAO-overexpressed plants was accompanied not only by an increased abundance of light-harvesting chlorophyll proteins but also of other proteins of the electron transport chain, which led to an increase in the capture of light as well as enhanced (40%-80%) electron transport rates of photosystems I and II at both limiting and saturating light intensities. Although the quantum yield of carbon dioxide fixation remained unchanged, the light-saturated photosynthetic carbon assimilation, starch content, and dry matter accumulation increased in CAO-overexpressed plants grown in both low- and high-light regimes. These results demonstrate that controlled up-regulation of chlorophyll b biosynthesis comodulates the expression of several thylakoid membrane proteins that increase both the antenna size and the electron transport rates and enhance carbon dioxide assimilation, starch content, and dry matter accumulation.

Overexpression of protochlorophyllide oxidoreductase C regulates oxidative stress in Arabidopsis

Light absorbed by colored intermediates of chlorophyll biosynthesis is not utilized in photosynthesis; instead, it is transferred to molecular oxygen, generating singlet oxygen (¹O₂). As there is no enzymatic detoxification mechanism available in plants to destroy ¹O₂, its generation should be minimized. We manipulated the concentration of a major chlorophyll biosynthetic intermediate i.e., protochlorophyllide in Arabidopsis by overexpressing the light-inducible protochlorophyllide oxidoreductase C (PORC) that effectively phototransforms endogenous protochlorophyllide to chlorophyllide leading to minimal accumulation of the photosensitizer protochlorophyllide in light-grown plants. In PORC overexpressing (PORCx) plants exposed to high-light, the ¹O₂ generation and consequent malonedialdehyde production was minimal and the maximum quantum efficiency of photosystem II remained unaffected demonstrating that their photosynthetic apparatus and cellular organization were intact. Further, PORCx plants treated with 5-aminolevulinicacid when exposed to light, photo-converted over-accumulated protochlorophyllide to chlorophyllide, reduced the generation of ¹O₂ and malonedialdehyde production and reduced plasma membrane damage. So PORCx plants survived and bolted whereas, the 5-aminolevulinicacid-treated wild-type plants perished. Thus, overexpression of PORC could be biotechnologically exploited in crop plants for tolerance to ¹O₂-induced oxidative stress, paving the use of 5-aminolevulinicacid as a selective commercial light-activated biodegradable herbicide. Reduced protochlorophyllide content in PORCx plants released the protochlorophyllide-mediated feed-back inhibition of 5-aminolevulinicacid biosynthesis that resulted in higher 5-aminolevulinicacid production. Increase of 5-aminolevulinicacid synthesis upregulated the gene and protein expression of several downstream chlorophyll biosynthetic enzymes elucidating a regulatory net work of expression of genes involved in 5-aminolevulinicacid and tetrapyrrole biosynthesis.

Early and late plastid development in response to chill stress and heat stress in wheat seedlings

Five-day-old etiolated wheat (Triticum aestivum L.) seedlings were transferred to 7°C (chill stress), 25°C (control), and 42°C (heat stress) and were kept in the dark or light for different time periods. Plastids were isolated from the control and stressed seedlings, and their low-temperature (77 K) fluorescence emission spectra were monitored. Most of the Protochlorophyllide (Pchlide) present in heat-stressed etiolated seedlings were in nonphototransformable form. The phototransformable Pchlide (F657) rapidly decreased when 5-day-old etiolated seedlings were transferred to 42°C in the dark for 24 h. A flash illumination of 0.2 s given to etiolated heat-stressed seedlings resulted in substantial arrest of Shibata shift, while in chill-stress conditions, it was only partially affected. In high temperature, due to disaggregation of polymeric Pchlide-Pchlide oxidoreductase (POR)-nicotinamide adenine dinucleotide phosphate (NADPH) molecules, the conversion of nonphototransformable Pchlide to its phototransformable form is substantially delayed resulting in impaired Shibata shift and belated development of the core antenna CP47 Photosystem II (PSII). Chill stress, however, did not disaggregate the polymeric Pchlide-POR-NADPH molecule-suppressed Pchlide and Chl synthesis and impaired of the assembly of PSII core antenna CP47 that emits F695 and PSI that emits F735. The decreased gene/protein expression and reduced posttranslational import of plastidic proteins, importantly POR in temperature-stressed plants, may be responsible for the delay in conversion of nonphototransformable to phototransformable form of Pchlide and plastid biogenesis.

Genome-wide transcriptome and proteome analyses of tobacco psaA and psbA deletion mutants

Photosynthesis in higher land plants is a complex process involving several proteins encoded by both nuclear and chloroplast genomes that require a highly coordinated gene expression. Significant changes in plastid differentiation and biochemical processes are associated with the deletion of chloroplast genes. In this study we report the genome-wide responses caused by the deletion of tobacco psaA and psbA genes coding core components of photosystem I (PSI) and photosystem II (PSII), respectively, generated through a chloroplast genetic engineering approach. Transcriptomic and quantitative proteomic analysis showed the down regulation of specific groups of nuclear and chloroplast genes involved in photosynthesis, energy metabolism and chloroplast biogenesis. Moreover, our data show simultaneous activation of several defense and stress responsive genes including those involved in reactive oxygen species (ROS) scavenging mechanisms. A major finding is the differential transcription of the plastome of deletion mutants: genes known to be transcribed by the plastid encoded polymerase (PEP) were generally down regulated while those transcribed by the nuclear encoded polymerase (NEP) were up regulated, indicating simultaneous activation of multiple signaling pathways in response to disruption of PSI and PSII complexes. The genome wide transcriptomic and proteomic analysis of the ∆psaA and ∆psbA deletion mutants revealed a simultaneous up and down regulation of the specific groups of genes located in nucleus and chloroplasts suggesting a complex circuitry involving both retrograde and anterograde signaling mechanisms responsible for the coordinated expression of nuclear and chloroplast genomes.

Signaling events leading to red-light-induced suppression of photomorphogenesis in wheat (Triticum aestivum)

Perception of red light (400 μmol photon m²/s) by the shoot bottom turned off the greening process in wheat. To understand the signaling cascade leading to this photomorphogenic response, certain signaling components were probed in seedlings grown in different light regimes. Upon analysis the gene expression of heterotrimeric Gα and Gβ were severely down-regulated in seedlings grown without vermiculite and having their shoot bottom exposed to red light (R/V-) and was similar to that of dark-grown seedlings. Supplementing the red-light-grown V- seedlings with blue light resulted in up-regulation of both Gα and Gβ expression, suggesting that blue light is able to modulate G protein expression. Treatment of cytokinin analog benzyladenine to cytokinin-deficient red-light-grown R/V- seedlings resulted in up-regulation of gene expression of both Gα and Gβ. To probe further, modulators of signal transduction pathway--AlF₃ (G protein activator), LaCl₃ (Ca(2+) channel blocker), NaF (nonspecific phosphatase inhibitor), or calmodulin (CaM) antagonists trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-nafthalene-sulfonamide (W-7)--were added along with Hoagland solution to the roots of 4-day-old etiolated seedlings, grown on germination paper and transferred to red light. AlF₃, LaCl₃, NaF failed to elicit any photomorphogenic response. However, CaM antagonists TFP and W-7 significantly reversed the red-light-induced suppression of photomorphogenesis. Phosphorylation of proteins assayed in the absence or presence of CaM antagonist TFP revealed respective up-regulation or down-regulation of phosphorylation of several plastidic proteins in R/V- seedlings. These suggest that signal transduction of red light perceived by the shoot bottom to suppress photomorphogenesis is mediated by CaM-dependent protein kinases.

Siroheme: an essential component for life on earth

Life on earth is dependent on sulphur (S) and nitrogen (N). In plants, the second step in the reduction of sulphate and nitrate are mediated by the enzymes sulphite and nitrite reductases, which contain the iron (Fe)-containing siroheme as a cofactor. It is synthesized from the tetrapyrrole primogenitor uroporphyrinogen III in the plastids via three enzymatic reactions, methylation, oxidation and ferrochelatation. Without siroheme biosynthesis, there would be no life on earth. Limitations in siroheme should have an enormous effect on the S- and N-metabolism, plant growth, development, fitness and reproduction, biotic and abiotic stresses including growth under S, N and Fe limitations, and the response to pathogens and beneficial interaction partners. Furthermore, the vast majority of redox-reactions in plants depend on S-components, and S-containing compounds are also involved in the detoxification of heavy metals and other chemical toxins. Disturbance of siroheme biosynthesis may cause the accumulation of light-sensitive intermediates and reactive oxygen species, which are harmful, or they can function as signaling molecules and participate in interorganellar signaling processes. This review highlights the role of siroheme in these scenarios.

Role of temperature stress on chloroplast biogenesis and protein import in pea

Modulation of photosynthesis and chloroplast biogenesis, by low and high temperatures, was studied in 12-d-old pea (Pisum sativum) plants grown at 25 degrees C and subsequently exposed to 7 degrees C or 40 degrees C up to 48 h. The decline in variable chlorophyll a fluorescence/maximum chlorophyll a fluorescence and estimated electron transport rate in temperature-stressed plants was substantially restored when they were transferred to room temperature. The ATP-driven import of precursor of small subunit of Rubisco (pRSS) into plastids was down-regulated by 67% and 49% in heat-stressed and chill-stressed plants, respectively. Reduction in binding of the pRSS to the chloroplast envelope membranes in heat-stressed plants could be due to the down-regulation of Toc159 gene/protein expression. In addition to impaired binding, reduced protein import into chloroplast in heat-stressed plants was likely due to decreased gene/protein expression of certain components of the TOC complex (Toc75), the TIC complex (Tic20, Tic32, Tic55, and Tic62), stromal Hsp93, and stromal processing peptidase. In chill-stressed plants, the gene/protein expression of most of the components of protein import apparatus other than Tic110 and Tic40 were not affected, suggesting the central role of Tic110 and Tic40 in inhibition of protein import at low temperature. Heating of intact chloroplasts at 35 degrees C for 10 min inhibited protein import, implying a low thermal stability of the protein import apparatus. Results demonstrate that in addition to decreased gene and protein expression, down-regulation of photosynthesis in temperature-stressed plants is caused by reduced posttranslational import of plastidic proteins required for the replacement of impaired proteins coded by nuclear genome.

Differential distribution of chlorophyll biosynthetic intermediates in stroma, envelope and thylakoid membranes in Beta vulgaris

Stroma, envelope and thylakoid membranes were prepared from chloroplasts isolated from leaves of Beta vulgaris. Out of total plastidic protochlorophyllide, envelope membranes contained 1.5%, thylakoids had the maximum 98.48% and stroma had a trace fraction of 0.02%. Distribution of the Mg-protoporphyrin IX and its monoester was 89.0% in thylakoids, 10.0% in stroma and 1.0% in envelope. A substantial fraction (33.77%) of plastidic protoporphyrin IX was partitioned into stroma. Envelope contained 0.66% and thylakoids had 65.57% of the total plastidic protoporphyrin IX pool. The proportion of monovinyl and divinyl forms of protochlorophyllide was almost similar in intact plastid, thylakoids, and outer and inner envelope membranes suggesting a tight regulation of vinyl reductase enzyme. The significance of differential distribution of chlorophyll biosynthetic intermediates among thylakoids, envelope and stroma is discussed.

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen ((1)O(2))-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F(v)/F(m) (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH(2)OH failed to restore the F(v)/F(m) ratio suggesting that the reduction of F(v)/F(m) reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q(A)(-) with S(2)) and B (due to recombination of Q(B)(-) with S(2)/S(3)) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F(695) band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by (1)O(2) produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.

Light and dark modulation of chlorophyll biosynthetic genes in response to temperature

Temperature and light significantly influence chloroplast development and chlorophyll biosynthesis. To understand the mechanism of the modulation of chlorophyll biosynthesis, the levels of transcripts and proteins of many enzymatic steps of tetrapyrrole biosynthesis in wheat and cucumber were simultaneously examined. The effect of low (chill-stress) as well as high (heat-stress) temperatures on dark- and light-grown seedlings was monitored. The protochlorophyllide oxidoreductase (POR) content was greatly reduced in response to light in control and heat-stressed seedlings. However, the POR level was not reduced in light-exposed chill-stressed seedlings. The genes for glutamate semialdehyde aminotransferase (gsa; cucumber), glutamyl-tRNA reductase (GluTR; cucumber), 5-aminolevulinic acid dehydratase (Ala D; cucumber and wheat) and for a subunit of Mg-chelatase (Chl I; wheat) showed a reduced expression in cold stress compared to controls and heat-stress conditions. Although expression of the ferrochelatase gene (Fch) and geranylgeranyl reductase gene (Chl P) was upregulated in light, they were downregulated by both chill- and heat-stress. Interestingly, gsa and uroporphyrinogen decarboxylase gene (UroD) and gene product abundance was stimulated by light and heat-stress implying the presence of both light and heat-inducible elements in their promoters. This observation corroborates with the previous report of increased enzymatic activity of UroD in heat-stressed cucumber seedlings. The gsa and Uro D may play an important role in tolerance of the greening process of plants to heat-stress.

Neuroprotective and Anti-ageing Effects of Curcumin in Aged Rat Brain Regions

This study investigated the influence of chronically administered curcumin on normal ageing-related parameters: lipid peroxidation, lipofuscin concentration and intraneuronal lipofuscin accumulation, activities of the enzymes superoxide dismutase (SOD), glutathione peroxidase (GPx), and Na(+), K(+), -adenosine triphosphatase (Na(+), K(+), -ATPase) in different brain regions (cerebral cortex, hippocampus, cerebellum and medulla) of 6- and 24-month-old rats. In normal ageing, lipid peroxidation and lipofuscin concentration were found to increase with ageing, the activities of SOD, GPx and Na(+), K(+), -ATPase, however, decreased with ageing. Chronic curcumin treatment of both 6 and 24 months old rats resulted in significant decreases in lipid peroxide and the lipofuscin contents in brain regions, the activities of SOD, GPx and Na(+), K(+), -ATPase however, showed significant increase in various brain regions. The present study, thus, demonstrated the antioxidative, antilipofusinogenesic and anti-ageing effects of curcumin in the brain.

Microgravity effects on thylakoid, single leaf, and whole canopy photosynthesis of dwarf wheat

The concept of using higher plants to maintain a sustainable life support system for humans during long-duration space missions is dependent upon photosynthesis. The effects of extended exposure to microgravity on the development and functioning of photosynthesis at the leaf and stand levels were examined onboard the International Space Station (ISS). The PESTO (Photosynthesis Experiment Systems Testing and Operations) experiment was the first long-term replicated test to obtain direct measurements of canopy photosynthesis from space under well-controlled conditions. The PESTO experiment consisted of a series of 21-24 day growth cycles of Triticum aestivum L. cv. USU Apogee onboard ISS. Single leaf measurements showed no differences in photosynthetic activity at the moderate (up to 600 micromol m(-2) s(-1)) light levels, but reductions in whole chain electron transport, PSII, and PSI activities were measured under saturating light (>2,000 micromol m(-2) s(-1)) and CO(2) (4000 micromol mol(-1)) conditions in the microgravity-grown plants. Canopy level photosynthetic rates of plants developing in microgravity at approximately 280 micromol m(-2) s(-1) were not different from ground controls. The wheat canopy had apparently adapted to the microgravity environment since the CO(2) compensation (121 vs. 118 micromol mol(-1)) and PPF compensation (85 vs. 81 micromol m(-2) s(-1)) of the flight and ground treatments were similar. The reduction in whole chain electron transport (13%), PSII (13%), and PSI (16%) activities observed under saturating light conditions suggests that microgravity-induced responses at the canopy level may occur at higher PPF intensity.

Photoregulation of the greening process of wheat seedlings grown in red light*

Wheat seedling grown with their shoot bottom exposed to red light (400 micromol m(-2) s(-1)) either with constant illumination or light-dark cycles did not accumulate chlorophyll. This near-etiolation response was manifested by a critical threshold intensity of red light and did not need continuous illumination. The inhibition of the greening process resulted from reduced synthesis of glutamate-1-semialdehyde and consequent reduction in tetrapyrrole precursor 5-aminolevulinic acid. Red light perceived by the shoot bottom down regulated the protein and/or gene expression of enzymes involved in the biosynthesis of tetrapyrroles. The contents of endogenous cytokinins, i.e., isopentenyl-adenosine and dihydrozeatinriboside, were reduced in seedlings grown in red light having their shoot bottom exposed. Application of exogenous cytokinin and its analogue to roots of seedlings grown in red light reversed the down regulation of the greening process. The reversal of red-light-induced near-etiolation morphogenesis by far-red (200 micromol m(-2) s(-1)) or blue (25 mumol m(-2) s(-1)) light suggests that it could be a very high red-irradiance response of phytochrome, in the meristematic layers of the shoot bottom, that works in concert with blue light receptor(s).

Light-dependent regulation of chlorophyll b biosynthesis in chlorophyllide a oxygenase overexpressing tobacco plants

Chlorophyllide a oxygenase (CAO) that converts chlorophyllide a to chlorophyllide b was overexpressed in tobacco to increase chlorophyll (Chl) b biosynthesis and alter the Chl a/b ratio. Transgenic plants along with their wild-type cultivars were grown in low and high light intensities. In low light there was 20% increase in chlorophyll b contents in transgenic plants, which resulted in 16% reduction in the Chl a/b ratio. In high light, total Chl contents were 31% higher in transgenic plants than those of wild type. The increase in Chl a was 19% and that of Chl b was 72% leading to 31% decline of Chl a/b ratio. The increase in Chl b contents was accompanied by enhanced CAO expression that was highly pronounced in low light. As compared to low light, in high light Lhcb1 and Chl a/b transcripts abundance was significantly increased in transgenic plants suggesting a close relationship between Chl b synthesis and cab gene expression. However, there was a small increase in expression of LHCII proteins, which did not correspond to 72% increase in Chl b content in transgenic line, implying that LHCPII has the ability to bind more Chl b molecules.

Inhibition of Photosystem I and Photosystem II in Wheat Seedlings with their Root-shoot Transition Zones Exposed to Red Light

The present study was conducted to observe the role of the root-shoot transition zone in the development of PS I and PS II in red light. The development of PS II and PS I was severely inhibited when root-shoot transition zones of wheat seedlings were exposed to red light (670 nm) of intensity 500 micromol m(-2) s(-1). Chlorophyll biosynthesis was also inhibited in these seedlings. Most of the PS I and PS II proteins (D1, LHCPII, CP47, OEC33) and their transcript levels were severely inhibited but cyt b6f complex proteins were only partially inhibited. Protein and transcript levels of Rubisco large subunit and protochlorophyllide (Pchlide) biosynthesis were also severely inhibited in these seedlings. When incubated in the dark with or without the precursor of chlorophyll biosynthesis ALA, these plants accumulated most of the Pchlide, as non-phototransformable Pchlide, suggesting low activity of NADPH:protochlorophyllide oxidoreductase (EC 1.6.99.1) in these plants. These effects were not observed when the seedlings were grown in red light with their root-shoot transition zones covered. These results suggest that the root-shoot transition zone plays an important role in the overall greening process involving transcription and translation of photosynthetic genes.

Developmental changes in sub-plastidic distribution of chlorophyll biosynthetic intermediates in cucumber (Cucumis sativus L.)

Four-day-old etiolated cucumber seedlings (Cucumis sativus L.) were transferred to cool-white-fluorescent light (15 mumol m-2 s-1) for 1 h and 24 hours and etiochloroplasts and chloroplasts were isolated from developing cotyledons. Plastids were fractionated to stroma, envelope and thylakoid or inner membranes and the pigment contents of all these different fractions were analysed. In intact cucumber chloroplast protochlorophylide was present in significant amounts whereas protoporphyrin IX and Mg-protoporphyrin plus its monoester were present only in very small quantities. Out of the total chloroplastic protochlorophylide pool 1.0% was partitioned to envelope membranes and 99.0% was partitioned to thylakoids. Stroma had only trace amounts of protochlorophylide. In contrast to chloroplasts, etiochloroplasts had, besides protochlorophylide, significant amounts of other chlorophyll biosynthetic intermediates. In etiochloroplasts, protoporphyrin IX primarily partitioned to inner membranes (59.1%) followed by stroma (37.7%) and envelope (3.21%). The content of Mg-protoporphyrin IX plus its monoester in different subplastidic fractions was 74.4% for inner membranes, 22.58% for stroma and 3.02% for envelope. Protochlorophyllide primarily partitioned to inner membranes (95.79%), followed by envelope (4.15%) and, to a negligible extent (0.06%), into stroma. The sub-plastidic distribution of chlorophyll biosynthetic intermediates in etiochloroplasts was, therefore, different than that of chloroplasts. The significance of differential distribution of chlorophyll biosynthetic intermediates among thylakoids, envelope and stroma in developing and mature plastids is discussed in relation to chloroplast biogenesis.

Detection of protoporphyrin IX in envelope membranes of pea chloroplasts

Envelope membranes were prepared from mature pea chloroplasts. The tetrapyrrole contents of envelope membranes were analysed. The envelope membranes of pea chloroplasts contained substantial amounts of protoporphyrin IX and trace amounts of Mg-protoporphyrin IX and its monoester in addition to protochlorophyllide. The protoporphyrin IX content of envelope membranes was 89.25 pmol (mg protein)(-1). Its content in pea envelope membrane was higher than that of protochlorophyllide. The proportion of monovinyl and divinyl forms of protochlorophyllide present in pea chloroplast envelope membrane was 3:7. The significance of the presence of protoporphyrin IX in the envelope membrane is discussed in relation to plastidic Chl biosynthesis.

Catalytic function of a novel protein protochlorophyllide oxidoreductase C of Arabidopsis thaliana

In Arabidopsis thaliana Por C has been identified only on sequence homology to that of por A and por B. To demonstrate its catalytic function Arabidopsis thaliana protochlorophyllide oxidoreductase C gene (por c) that codes for the mature part of POR C protein having 335 amino acids was expressed in Escherchia coli cells. The POR C enzyme in the presence of NADPH and protochlorophyllide when incubated in dark formed a ternary complex. When it was excited at 433 nm, it had a fluorescence emission peak at 636 nm. After illumination with actinic cool white fluorescent light, a peak at 673 nm due to chlorophyllide gradually increased with concomitant decrease of 636 nm emission, demonstrating the gradual phototransformation of protochlorophyllide to chlorophyllide. The significance of differential por gene expression in light and dark among different species is discussed.

Oxidative stress in cucumber (Cucumis sativus L) seedlings treated with acifluorfen

Treatment of diphenyl ether herbicide acifluorfen-Na (AF-Na) to intact cucumber (Cucumis sativus L cv Poinsette) seedlings induced overaccumulation of protoporphyrin IX in light (75 mumole m-2 s-1). The extra-plastidic protoporphyrin IX accumulated during the light exposure disappeared within two hours of transfer of acifluorofen-treated seedlings to darkness. The dark disappearance was due to re-entry of migrated protoporphyrin IX into the plastid and its subsequent conversion to protochlorophyllide. In light, protoporphyrin IX acted as a photosensitizer and caused generation of active oxygen species. The latter caused damage to the cellular membranes by peroxidation of membrane lipids that resulted in production of malondialdehyde. Damage to the plastidic membranes resulted in damage to photosystem I and photosystem II reactions. Dark-incubation of herbicide-sprayed plants before their exposure to light enhanced photodynamic damage due to diffusion of the herbicide to the site of action. Compared to control, in treated samples the cation-induced increases in variable fluorescence/maximum fluorescence ratio and increase in photosystem II activity was lower due to reduced grana stacking in herbicide-treated and light-exposed plants.

Regulation of protoporphyrin IX biosynthesis by intraplastidic compartmentalization and adenosine triphosphate

Subplastidic preparations from cotyledons of cucumber (Cucumis sativus L.) were tested for their ability to synthesize protoporphyrin IX from the substrate 5-aminolevulinic acid. Envelope or thylakoid membranes failed to synthesize protoporphyrin IX from the substrate 5-aminolevulinic acid. Stromal preparations synthesized a very low amount of protoporphyrin IX. In a reconstitution experiment using stroma + envelope membranes, protoporphyrin IX synthesis from 5-aminolevulinic acid was enhanced by 660% over that of stroma alone. However, when thylakoids were added to the stroma + envelope mixture, protoporphyrin IX synthesis from 5-aminolevulinic acid was completely inhibited. In the reconstituted stroma + envelope membrane mixture, the reducing agent dithiothreitol enhanced the protoporphyrin IX-synthesizing ability and completely abolished the inhibition of protoporphyrin IX synthesis by thylakoids. This suggested that the oxidizing agents usually associated with the thylakoid membranes inhibited protoporphyrin IX biosynthesis and the inhibition was alleviated by the reducing power of dithiothreitol. This study exposes the weakness of in vitro reconstitution experiments in mimicking the in vivo-conditions. Addition of ATP stimulated protoporphyrin IX synthesis by 50% in the supernatant fraction of chloroplast lysate. This ATP-induced stimulation of protoporphyrin IX synthesis was due to the enhancement of the activities of uroporphyrinogen decarboxylase and protoporphyrinogen oxidase, involved in tetrapyrrole biosynthesis. The ATP-induced stimulation of porphyrinogen oxidase activity was an energy-dependent reaction.