NADPH : protochlorophyllide oxidoreductases in white pine (Pines strobes) and loblolly pine (P. taeda)

Photoprotective role of NADPH:protochlorophyllide oxidoreductase A

A homology model of NADPH:protochlorophyllide (Pchlide) oxidoreductase A (POR; E.C. 1.3.33.1) of barley is developed and verified by site-directed mutagenesis. PORA is considered a globular protein consisting of nine alpha-helices and seven beta-strands. The model predicts the presence of two functionally distinctive Pchlide binding sites where the pigment is coordinated by cysteine residues. The pigment bound to the first, high-affinity Pchlide binding site is used for the formation of the photoactive state of the enzyme. The pigment bound to the second, low-affinity Pchlide binding site is involved in the PORA:PORB interaction, allowing for resonance energy transfer between the neighboring PORs in the complex. In the in vitro reconstituted light-harvesting POR:Pchlide complex (LHPP), light absorbed by PORA-bound Pchlide b is transferred to PORB-bound Pchlide a. That induces the conversion of Pchlide a to chlorophyllide (Chlide) a. This energy transfer eliminates the possibility of Pchlide b photoreduction and prevents that excited triplet states of either Pchlides a or b accumulate and provoke singlet oxygen production. Together, our results provide a photoprotective role of PORA during greening.

Protochlorophyllide reduction: mechanisms and evolutions

Protochlorophyllide (Pchlide) reductases are key enzymes in the process of chlorophyll biosynthesis. In this review, current knowledge on the molecular organization, substrate specificity and assembly of the light-dependent reduced nicotinamide adenine dinucleotide phosphate:Pchlide oxidoreductases are discussed. Characteristics of light-independent enzymes are also described briefly, and the possible reasons for the selection of light-dependent enzymes during the course of evolution are discussed.

An Arabidopsis porB porC double mutant lacking light-dependent NADPH:protochlorophyllide oxidoreductases B and C is highly chlorophyll-deficient and developmentally arrested

A key reaction in the biosynthesis of chlorophylls (Chls) a and b from cyanobacteria through higher plants is the strictly light-dependent reduction of protochlorophyllide (Pchlide) a to chlorophyllide (Chlide) a. Angiosperms, unlike other photosynthetic organisms, rely exclusively upon this mechanism to reduce Pchlide and hence require light to green. In Arabidopsis, light-dependent Pchlide reduction is mediated by three structurally related but differentially regulated NADPH:Pchlide oxidoreductases, denoted as PORA, PORB, and PORC. The PORA and PORB genes, but not PORC, are strongly expressed early in seedling development. In contrast, expression of PORB and PORC, but not PORA, is observed in older seedlings and adult plants. We have tested the hypothesis that PORB and PORC govern light-dependent Chl biosynthesis throughout most of the plant development by identifying porB and porC mutants of Arabidopsis, the first higher plant por mutants characterized. The porB-1 and porC-1 mutants lack the respective POR transcripts and specific POR isoforms because of the interruption of the corresponding genes by a derivative of the maize Dissociation (Ds) transposable element. Single por mutants, grown photoperiodically, display no obvious phenotypes at the whole plant or chloroplast ultrastructural levels, although the porB-1 mutant has less extensive etioplast inner membranes. However, a light-grown porB-1 porC-1 double mutant develops a seedling-lethal xantha phenotype at the cotyledon stage, contains only small amounts of Chl a, and possesses chloroplasts with mostly unstacked thylakoid membranes. PORB and PORC thus seem to play redundant roles in maintaining light-dependent Chl biosynthesis in green plants, and are together essential for growth and development.

Photosynthetic electron transport regulates the stability of the transcript for the protochlorophyllide oxidoreductase gene in the liverwort, Marchantia paleacea var. diptera

The transfer of Marchantia paleacea var. diptera cells to darkness caused a reversible repression in the accumulation of transcript for a gene, por, encoding the NADPH: protochlorophyllide oxidoreductase (EC 1.3.1.33). The photosynthetic inhibitor DCMU and DBMIB repressed the accumulation in light. In the presence of transcription inhibitor cordycepin, not only incubation in the dark but also addition of DCMU or DBMIB in light stimulated the degradation of the por transcript. These findings suggest that photosynthetic electron transport is involved in regulating the stability of the por transcript.

Gabaculine does not inhibit cytokinin-stimulated biosynthesis of chlorophyll in Pinus nigra seedlings in the dark

Chlorophyll (Chl) accumulation was monitored during black pine (Pinus nigra L.) seed germination for 14 days in the light and in the dark in the presence of gabaculine (GAB) and cytokinin in order to elucidate the regulation of gymnosperm seedling greening in the dark, primarily at the level of aminolevulinic acid formation. In the light, GAB inhibited chlorophyll accumulation in a manner dependent on concentration and developmental stage, and in the dark it showed no effect. Cytokinin, 10(-5) M benzyl adenine (BA) partly overcame GAB-induced inhibition in the light, mainly during earlier developmental stages. In the seedlings grown in the dark, an equal quantity of Chl accumulated in the presence of cytokinin with and without GAB and it was approximately 20-40% higher than in the control seedlings or in the seedlings grown only in the presence of GAB. 5-Amino-levulinic acid (ALA) synthesis was equal in the light and in the dark in seedlings of the same age and seedlings treated with GAB grown in the dark. In the light, GAB inhibited ALA synthetic activity. The results indicate that ALA synthesis is not a rate-limiting step within Chl biosynthesis in pine seedlings grown in the dark.

Chlorophyll synthesis in dark-grown pine primary needles

The pigment content of dark-grown primary needles of Pinus jeffreyi L. and Pinus sylvestris L. was determined by high-performance liquid chromatography. The state of protochlorophyllide a and of chlorophylls during dark growth were analyzed by in situ 77 K fluorescence spectroscopy. Both measurements unambiguously demonstrated that pine primary needles are able to synthesize chlorophyll in the dark. Norflurazon strongly inhibited both carotenoid and chlorophyll synthesis. Needles of plants treated with this inhibitor had low chlorophyll content, contained only traces of xanthophylls, and accumulated carotenoid precursors. The first form of chlorophyll detected in young pine needles grown in darkness had an emission maximum at 678 nm. Chlorophyll-protein complexes with in situ spectroscopic properties similar to those of fully green needles (685, 695, and 735 nm) later accumulated in untreated plants, whereas in norflurazon-treated plants the photosystem I emission at 735 nm was completely lacking. To better characterize the light-dependent chlorophyll biosynthetic pathway in pine needles, the 77 K fluorescence properties of in situ protochlorophyllide a spectral forms were studied. Photoactive and nonphotoactive protochlorophyllide a forms with emission properties similar to those reported for dark-grown angiosperms were found, but excitation spectra were substantially red shifted. Because of their lower chlorophyll content, norflurazon-treated plants were used to study the protochlorophyllide a photoreduction process triggered by one light flash. The first stable chlorophyllide photoproduct was a chlorophyllide a form emitting at 688 nm as in angiosperms. Further chlorophyllide a shifts usually observed in angiosperms were not detected. The rapid regeneration of photoactive protochlorophyllide a from nonphotoactive protochlorophyllide after one flash was demonstrated.

Genetic analysis of chlorophyll biosynthesis

During this decade, there have been major advancements in the understanding of genetic loci involved in synthesis of the family of Mg-tetrapyrroles known as chlorophylls and bacteriochlorophylls. Molecular genetic analysis of Mg-tetrapyrrole biosynthesis was initiated by the performance of detailed sequence and mutational analysis of the photosynthesis gene cluster from Rhodobacter capsulatus. These studies provided the first detailed understanding of genes involved in bacteriochlorophyll a biosynthesis. In the short time since these studies were initiated, most of the chlorophyll biosynthesis genes have been identified by virtue of their ability to complement bacteriochlorophyll a biosynthesis mutants as well as by sequence homology comparisons. This review is centered on a discussion of our current understanding of bacterial, algal, and plant genes that code for enzymes in the Mg-branch of the tetrapyrrole biosynthetic pathway that are responsible for synthesis of chlorophylls and bacteriochlorophylls.

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

The pc-1 phenotype of Chlamydomonas reinhardtii results from a deletion mutation in the nuclear gene for NADPH:protochlorophyllide oxidoreductase

The pc-1 mutant of Chlamydomonas reinhardtii has been shown to be incapable of protochlorophyllide photoconversion in vivo and is thought to be defective in light-dependent NADPH:protochlorophyllide oxidoreductase activity. We have isolated and characterized the nuclear genes encoding this enzyme from wild-type and pc-1 mutant Chlamydomonas cells. The wild-type CRlpcr-1 gene encodes a 397 amino acid polypeptide of which the N-terminal 57 residues comprise the chloroplast transit sequence. The Chlamydomonas protochlorophyllide reductase has 66-70% identity (79-82% similarity) to the higher plant enzymes. Transcripts encoding protochlorophyllide reductase are abundant in dark-grown wild-type cells, but absent or at very low levels in cells grown in the light. Similarly, immunoreactive protochlorophyllide reductase protein is also present to a greater extent in dark- versus light-grown wild-type cells. Both pc-1 and pc-1 y-7 cells lack CRlpcr-1 mRNA and the major (36 kDa) immunodetectable form of protochlorophyllide reductase consistent with their inability to photoreduce protochlorophyllide. DNA sequence analysis revealed that the lpcr gene in pc-1 y-7 cells contains a two-nucleotide deletion within the fourth and fifth codons of the protochlorophyllide reductase precursor that causes a shift in the reading frame and results in premature termination of translation. The absence of protochlorophyllide reductase message in pc-1 and pc-1 y-7 cells is likely the consequence of this frameshift mutation in the lpcr gene. Introduction of the CRlpcr-1 gene into pc-1 y-7 cells by nuclear transformation was sufficient to restore the wild-type phenotype. Transformants contained both protochlorophyllide reductase mRNA and immunodetectable enzyme protein. These studies demonstrate that pc-1 was in fact a defect in protochlorophyllide reductase activity and provide the first in vivo molecular evidence that the lpcr gene product is essential for light-dependent protochlorophyllide reduction.

The in vitro assembly of the NADPH-protochlorophyllide oxidoreductase in pea chloroplasts

The NADPH-protochlorophyllide oxidoreductase (pchlide reductase, EC 1.6.99.1) is the major protein in the prolamellar bodies (PLBs) of etioplasts, where it catalyzes the light-dependent reduction of protochlorophyllide to chlorophyllide during chlorophyll synthesis in higher plants. The suborganellar location in chloroplasts of light-grown plants is less clear. In vitro assays were performed to characterize the assembly process of the pchlide reductase protein in pea chloroplasts. Import reactions employing radiolabelled precursor protein of the pchlide reductase showed that the protein was efficiently imported into fully matured green chloroplasts of pea. Fractionation assays following an import reaction revealed that imported protein was targeted to the thylakoid membranes. No radiolabelled protein could be detected in the stromal or envelope compartments upon import. Assembly reactions performed in chloroplast lysates showed that maximum amount of radiolabelled protein was associated to the thylakoid membranes in a thermolysin-resistant conformation when the assays were performed in the presence of hydrolyzable ATP and NADPH, but not in the presence of NADH. Furthermore, membrane assembly was optimal at pH 7.5 and at 25 degrees C. However, further treatment of the thylakoids with NaOH after an assembly reaction removed most of the membrane-associated protein. Assembly assays performed with the mature form of the pchlide reductase, lacking the transit peptide, showed that the pre-sequence was not required for membrane assembly. These results indicate that the pchlide reductase is a peripheral protein located on the stromal side of the membrane, and that both the precursor and the mature form of the protein can act as substrates for membrane assembly.

Genetic analyses of photopigment biosynthesis in eubacteria: a guiding light for algae and plants

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