Making light of it: the role of plant haem oxygenases in phytochrome chromophore synthesis.

The haem oxygenase (HO) enzyme catalyses the oxidation of haem to biliverdin IX alpha, CO and Fe(2+), and performs a wide variety of roles in Nature, including degradation of haem from haemoglobin, iron acquisition and phycobilin biosynthesis. In plants, HOs are required for the synthesis of the chromophore of the phytochrome family of photoreceptors. There are four HO genes in the Arabidopsis genome. Analysis of a mutant deficient in HO1 (the hy1 mutant) has demonstrated that this plastid-localized protein is the major HO in the phytochrome chromophore synthesis pathway. HO2 may also have a minor role in this pathway, but our understanding of the divergent roles of this small gene family is still far from complete.

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.

Biliverdin reductase-induced phytochrome chromophore deficiency in transgenic tobacco.

Targeted expression of mammalian biliverdin IXalpha reductase (BVR), an enzyme that metabolically inactivates linear tetrapyrrole precursors of the phytochrome chromophore, was used to examine the physiological functions of phytochromes in the qualitative short-day tobacco (Nicotiana tabacum cv Maryland Mammoth) plant. Comparative phenotypic and photobiological analyses of plastid- and cytosol-targeted BVR lines showed that multiple phytochrome-regulated processes, such as hypocotyl and internode elongation, anthocyanin synthesis, and photoperiodic regulation of flowering, were altered in all lines examined. The phytochrome-mediated processes of carotenoid and chlorophyll accumulation were strongly impaired in plastid-targeted lines, but were relatively unaffected in cytosol-targeted lines. Under certain growth conditions, plastid-targeted BVR expression was found to nearly abolish the qualitative inhibition of flowering by long-day photoperiods. The distinct phenotypes of the plastid-targeted BVR lines implicate a regulatory role for bilins in plastid development or, alternatively, reflect the consequence of altered tetrapyrrole metabolism in plastids due to bilin depletion.

Altered etioplast development in phytochrome chromophore-deficient mutants.

Inhibition of chromophore synthesis in the phytochrome-deficient aurea (au) and yellow-green-2 (yg-2) mutants of tomato (Solanum lycopersicum L.) results in a severe reduction of protochlorophyllide (Pchlide) accumulation in dark-grown hypocotyls. Experiments with apophytochrome-deficient mutants indicate that the inhibition of Pchlide accumulation results from two separate effects: one dependent on the activity of phytochromes A and B1 and one phytochrome-independent effect that is attributed to a feedback inhibition of the tetrapyrrole biosynthesis pathway. Cotyledons only show phytochrome-independent inhibition of Pchlide synthesis. Analysis of NADPH:protochlorophyllide oxidoreductase levels by western blotting showed that the reduction in Pchlide in au and yg-2 is accompanied by a correlative, but less substantial, decrease in NADPH:protochlorophyllide oxidoreductase. Consistent with this result, in vivo fluorescence spectra demonstrate that both mutants are primarily deficient in non-phototransformable Pchlide. Analysis of etioplast structure indicates that plastid development in au and yg-2 is retarded in hypocotyls and partially impaired in cotyledons, again correlating with the reduction in Pchlide. Since Pchlide synthesis is also reduced in chromophore-deficient mutants of pea (Pisum sativum L.) and Arabidopsis thaliana (L.) Heynh. (Landsberg erecta) these results may be significant for explaining aspects of the phenotype of this mutant class that are independent of the loss of phytochrome.

The aurea and yellow-green-2 mutants of tomato are deficient in phytochrome chromophore synthesis.

The phytochrome-deficient aurea mutant of tomato has been widely used for the study of both phytochrome function and the role of other photoreceptors in the control of development in higher plants. To date the exact nature of the aurea mutation has remained unknown, though this information is clearly important for the interpretation of these studies. It has been proposed that aurea and yellow-green-2, another mutant of tomato that has a similar phenotype to aurea, could be deficient in phytochrome chromophore synthesis. We have examined this hypothesis by measuring the activity of the enzymes committed to phytochrome chromophore synthesis in these mutants. The approach takes advantage of a recently developed high pressure liquid chromatography-based assay for the synthesis of the free phytochrome chromophore, phytochromobilin from its immediate precursors biliverdin IXalpha and heme. Isolated etioplasts from aurea and yellow-green-2 seedlings were specifically unable to convert biliverdin IXalpha to 3Z-phytochromobilin and heme to biliverdin IXalpha, respectively. In addition, the level of total noncovalently bound heme in the mutants was the same as in wild type seedlings. Together, these results identify both aurea and yellow-green-2 as mutants that are deficient in phytochrome chromophore synthesis.

The Phytochrome-Deficient pcd1 Mutant of Pea Is Unable to Convert Heme to Biliverdin IX[alpha].

We isolated a new pea mutant that was selected on the basis of pale color and elongated internodes in a screen under white light. The mutant was designated pcd1 for phytochrome chromophore deficient. Light-grown pcd1 plants have yellow-green foliage with a reduced chlorophyll (Chl) content and an abnormally high Chl a/Chl b ratio. Etiolated pcd1 seedlings are developmentally insensitive to far-red light, show a reduced response to red light, and have no spectrophotometrically detectable phytochrome. The phytochrome A apoprotein is present at the wild-type level in etiolated pcd1 seedlings but is not depleted by red light treatment. Crude phytochrome preparations from etiolated pcd1 tissue also lack spectral activity but can be assembled with phycocyanobilin, an analog of the endogenous phytochrome chromophore phytochromobilin, to yield a difference spectrum characteristic of an apophytochrome-phycocyanobilin adduct. These results indicate that the pcd1-conferred phenotype results from a deficiency in phytochrome chromophore synthesis. Furthermore, etioplast preparations from pcd1 seedlings can metabolize biliverdin (BV) IX[alpha] but not heme to phytochromobilin, indicating that pcd1 plants are severely impaired in their ability to convert heme to BV IX[alpha]. This provides clear evidence that the conversion of heme to BV IX[alpha] is an enzymatic process in higher plants and that it is required for synthesis of the phytochrome chromophore and hence for normal photomorphogenesis.

(3Z)- and (3E)-phytochromobilin are intermediates in the biosynthesis of the phytochrome chromophore.

Using a high performance liquid chromatography (HPLC)-based assay, we have demonstrated that isolated oat etioplasts convert the linear tetrapyrrole biliverdin IX alpha to (3E)-phytochromobilin, the proposed precursor of the chromophore of the plant photoreceptor phytochrome. In addition to (3E)-phytochromobilin, the synthesis of a second phytochromobilin was detected by its ability to functionally assemble with recombinant oat apophytochrome A. The structure of this new pigment has been determined to be the 3Z isomer of phytochromobilin by absorption and 1H NMR spectroscopy. Like (3E)-phytochromobilin, assembly of HPLC-purified (3Z)-phytochromobilin with apophytochrome yielded a holoprotein that is spectrally indistinguishable from native oat phytochrome A. However, the postchromatographic conversion of (3Z)- to (3E)-phytochromobilin appears to be responsible for this result. Kinetic HPLC analyses have demonstrated that (3Z)-phytochromobilin is synthesized prior to the 3E isomer by oat etioplasts. We therefore propose that (3Z)-phytochromobilin is the immediate product of biliverdin IX alpha reduction by the enzyme phytochromobilin synthase. This implicates the presence of an isomerase that catalyzes the conversion of (3Z)- to (3E)-phytochromobilin, the immediate precursor of the phytochrome A chromophore.

Inactivation of phytochrome- and phycobiliprotein-chromophore precursors by rat liver biliverdin reductase.

The phytochrome chromophore precursor, 3E-phytochromobilin, and the phycobiliprotein chromophore precursors, 3E-phycocyanobilin and 3E-phycoerythrobilin, are enzymatically converted to novel rubinoid products by purified rat liver biliverdin reductase. Phytochromobilin and phycocyanobilin are particularly good substrates for biliverdin reductase with Km and Vmax values very similar to those of the natural substrate, biliverdin IX alpha. Phycoerythrobilin is the least preferred of the three bilin substrates. 1H NMR spectroscopy of phycocyanorubin, the product of phycocyanobilin catalysis by biliverdin reductase, and comparison of absorption spectra of all three rubinoid products reveal that the C10 methine bridge is selectively reduced by biliverdin reductase without altering the A-ring ethylidene substituent. In vitro phytochrome assembly experiments demonstrate that the phytorubin products do not form photoactive adducts with recombinant apophytochrome. These results suggest that ectopic expression of biliverdin reductase in plants will prevent assembly of the functional photoreceptor and thus will potentially alter light-mediated plant growth and development.

Phytochrome assembly. The structure and biological activity of 2(R),3(E)-phytochromobilin derived from phycobiliproteins.

The unicellular rhodophyte, Porphyridium cruentum, and the filamentous cyanobacterium, Calothrix sp. PCC 7601, contain phycobiliproteins that have covalently bound phycobilin chromophores. Overnight incubation of solvent-extracted cells at 40 degrees C with methanol liberates free phycobilins that are derived from the protein-bound bilins by methanolytic cleavage of the thioether linkages between bilin and apoprotein. Two of the free bilins were identified as 3(E)-phycocyanobilin and 3(E)-phycoerythrombilin by comparative spectrophotometry and high pressure liquid chromatography. Methanolysis also yields a third bilin free acid whose absorption and 1H NMR spectra support the assignment of the 3(E)-phytochromobilin structure. This novel bilin is the major pigment isolated from cells that are pre-extracted with acetone-containing solvents. Since phytochrome- or phytochromobilin-containing proteins are not present in either organism, the 3(E)-phytochromobilin must arise by oxidation of phycobilin chromophores. This pigment is not obtained by similar treatment of a cyanobacterium and a rhodophyte that lack phycoerythrin. Therefore, 3(E)-phytochromobilin appears to be derived from phycoerythrobilin-containing proteins. Comparative CD spectroscopy of 3(E)-phytochrombilin and 3(E)-phycocyanobilin suggests that the two bilins share the R stereochemistry at the 2-position in the reduced pyrrole ring. Incubation of 2(R),3(E)-phytochromobilin with recombinant oat apophytochrome yields a covalent bilin adduct that is photoactive and spectrally indistinguishable from native oat phytochrome isolated from etiolated seedlings. These results establish that the phycobiliprotein-derived 2(R),3(E)-phytochromobilin is a biologically active phytochrome chromophore precursor.

Holophytochrome assembly. Coupled assay for phytochromobilin synthase in organello.

Utilizing an in vitro coupled assay system, we show that isolated plastids from cucumber cotyledons convert the linear tetrapyrrole biliverdin IX alpha to the free phytochrome chromophore, phytochromobilin, which assembles with oat apophytochrome to yield photoactive holoprotein. The spectral properties of this synthetic phytochrome are indistinguishable from those of the natural photoreceptor. The plastid-dependent biliverdin conversion activity is strongly stimulated by both NADPH and ATP. Substitution of the nonnatural XIII alpha isomer of biliverdin for the IX alpha isomer affords a synthetic holophytochrome adduct with blue-shifted difference spectra. These results, together with experiments using boiled plastids, indicate that phytochromobilin synthesis from biliverdin is enzyme-mediated. Experiments where NADPH (and ATP) levels in intact developing chloroplasts are manipulated by feeding the metabolites 3-phosphoglycerate, dihydroxyacetone phosphate, and glucose 6-phosphate or by illumination with white light, support the hypothesis that the enzyme that accomplishes this conversion, phytochromobilin synthase, is plastid-localized. It is therefore likely that all of the enzymes of the phytochrome chromophore biosynthetic pathway reside in the plastid.

Hypogammaglobulinaemia in foals: prevalence on Victorian studs and simple methods for detection and correction in the field.

The prevalence of hypogammaglobulinaemia in 82 young foals was determined. Twelve foals were considered clinically abnormal at birth and ten died within two weeks. All of these foals were hypogammaglobulinaemic. Seven (10%) of the other 70 apparently normal foals were hypogammaglobulinaemic despite having suckled normally. Three of these foals developed significant disease and one died at one month of age. Rapid detection of foals with low serum immunoglobulin levels was achieved by adapting the zinc sulphate turbidity test to partially evacuated blood collection tubes. This permitted test to be conducted on the stud or in the veterinarian's own laboratory. Plasma concentrated twofold by a freeze thaw technique was administered intravenously to supplement the immunoglobulin levels of two colostrum deprived foals. The simplicity of the concentration procedure eliminated the need for laboratory preparation of equine immunoglobulin.