Multiple heme oxygenase family members contribute to the biosynthesis of the phytochrome chromophore in Arabidopsis

The oxidative cleavage of heme by heme oxygenases (HOs) to form biliverdin IXalpha (BV) is the committed step in the biosynthesis of the phytochrome (phy) chromophore and thus essential for proper photomorphogenesis in plants. Arabidopsis (Arabidopsis thaliana) contains four possible HO genes (HY1, HO2-4). Genetic analysis of the HY1 locus showed previously that it is the major source of BV with hy1 mutant plants displaying long hypocotyls and decreased chlorophyll accumulation consistent with a substantial deficiency in photochemically active phys. More recent analysis of HO2 suggested that it also plays a role in phy assembly and photomorphogenesis but the ho2 mutant phenotype is more subtle than that of hy1 mutants. Here, we define the functions of HO3 and HO4 in Arabidopsis. Like HY1, the HO3 and HO4 proteins have the capacity to synthesize BV from heme. Through a phenotypic analysis of T-DNA insertion mutants affecting HO3 and HO4 in combination with mutants affecting HY1 or HO2, we demonstrate that both of the encoded proteins also have roles in photomorphogenesis, especially in the absence of HY1. Disruption of HO3 and HO4 in the hy1 background further desensitizes seedlings to red and far-red light and accelerates flowering time, with the triple mutant strongly resembling seedlings deficient in the synthesis of multiple phy apoproteins. The hy1/ho3/ho4 mutant can be rescued phenotypically and for the accumulation of holo-phy by feeding seedlings BV. Taken together, we conclude that multiple members of the Arabidopsis HO family are important for synthesizing the bilin chromophore used to assemble photochemically active phys.     

Expression and biochemical properties of a ferredoxin-dependent heme oxygenase required for phytochrome chromophore synthesis

The HY1 gene of Arabidopsis encodes a plastid heme oxygenase (AtHO1) required for the synthesis of the chromophore of the phytochrome family of plant photoreceptors. To determine the enzymatic properties of plant heme oxygenases, we have expressed the HY1 gene (without the plastid transit peptide) in Escherichia coli to produce an amino terminal fusion protein between AtHO1 and glutathione S-transferase. The fusion protein was soluble and expressed at high levels. Purified recombinant AtHO1, after glutathione S-transferase cleavage, is a hemoprotein that forms a 1:1 complex with heme. In the presence of reduced ferredoxin, AtHO1 catalyzed the formation of biliverdin IXalpha from heme with the concomitant production of carbon monoxide. Heme oxygenase activity could also be reconstituted using photoreduced ferredoxin generated through light irradiation of isolated thylakoid membranes, suggesting that ferredoxin may be the electron donor in vivo. In addition, AtHO1 required an iron chelator and second reductant, such as ascorbate, for full activity. These results show that the basic mechanism of heme cleavage has been conserved between plants and other organisms even though the function, subcellular localization, and cofactor requirements of heme oxygenases differ substantially.     

The phytochrome-deficient pcd2 mutant of pea is unable to convert biliverdin IXα to 3(Z)-phytochromobilin

During screening of ethylmethane sulphonate-mutagenized pea ( Pisum sativum L.) seedlings under far-red light a mutant line, AF130, was isolated which showed a reduction in both red and far-red light-induced de-etiolation responses. The photomorphogenic phenotype of AF130 results from a single recessive mutation which is not allelic with the previously described phytochrome chromophore biosynthesis mutant pcd1 . This new mutant has been designated pcd2 , for p hytochrome c hromophore d eficient 2. Like pcd1 , etiolated pcd2 seedlings are severely deficient in spectrally active phytochrome and contain wild-type levels of phytochrome A apoprotein which is not substantially depleted by red light treatment. Etioplast preparations from pcd2 seedlings can metabolize heme to biliverdin (BV) IXα, but are unable to convert BV IXα to the phytochrome chromophore, phytochromobilin. The PCD1 and PCD2 genes therefore control consecutive steps in phytochromobilin synthesis. Despite a similarly severe impairment of photomorphogenic responses, pcd2 mutant seedlings do not display the strongly chlorotic phenotype of pcd1 , suggesting that this characteristic of pcd1 does not result from phytochrome deficiency per se , but is a specific effect of the pcd1 mutation. A double mutant between pcd1 and pcd2 was constructed. This mutant is paler than pcd1 and less responsive to red light than either single mutant, but retains a strong response to blue light.     

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.     

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.     

PORPHOBILINOGEN DEAMINASE Deficiency Alters Vegetative and Reproductive Development and Causes Lesions in Arabidopsis

The Arabidopsis rugosa1 (rug1) mutant has irregularly shaped leaves and reduced growth. In the absence of pathogens, leaves of rug1 plants have spontaneous lesions reminiscent of those seen in lesion-mimic mutants; rug1 plants also express cytological and molecular markers associated with defence against pathogens. These rug1 phenotypes are made stronger by dark/light transitions. The rug1 mutant also has delayed flowering time, upregulation of the floral repressor FLOWERING LOCUS C (FLC) and downregulation of the flowering promoters FT and SOC1/AGL20. Vernalization suppresses the late flowering phenotype of rug1 by repressing FLC. Microarray analysis revealed that 280 nuclear genes are differentially expressed between rug1 and wild type; almost a quarter of these genes are involved in plant defence. In rug1, the auxin response is also affected and several auxin-responsive genes are downregulated. We identified the RUG1 gene by map-based cloning and found that it encodes porphobilinogen deaminase (PBGD), also known as hydroxymethylbilane synthase, an enzyme of the tetrapyrrole biosynthesis pathway, which produces chlorophyll, heme, siroheme and phytochromobilin in plants. PBGD activity is reduced in rug1 plants, which accumulate porphobilinogen. Our results indicate that Arabidopsis PBGD deficiency impairs the porphyrin pathway and triggers constitutive activation of plant defence mechanisms leading to leaf lesions and affecting vegetative and reproductive development.     

Two dominant photomorphogenic mutations of Arabidopsis thaliana identified as suppressor mutations of hy2

By screening suppressor mutants of the hy2 mutation of Arabidopsis thaliana , two dominant photomorphogenic mutants, shy1-1D and shy2-1D , for two genetic loci designated as SHY1 and SHY2 ( s uppressor of hy 2 mutation) have been isolated. Both of these non-allelic, extragenic suppressor mutations of hy2 are located on chromosome 1 of the Arabidopsis genome. Both mutations suppress the elongated hypocotyl phenotype of hy2 by light-independent inhibition of hypocotyl growth as well as by increasing the effectiveness of light inhibition of hypocotyl elongation. The shy1-1D mutation is partially photomorphogenic in darkness with apical hook opening and reduced hypocotyl elongation. The shy2-1D mutant displays highly photomorphogenic characteristics in darkness such as true leaf development, cotyledon expansion, and extremely reduced hypocotyl growth. In regard to hypocotyl elongation, however, the shy2-1D mutation is still light sensitive. Examination of red/far-red light responses shows that the shy1-1D mutation suppresses the hypocotyl elongation of the hy2 mutation effectively in red light but not effectively in far-red light. The shy2-1D suppresses hypocotyl elongation of the hy2 mutation effectively in both red and far-red light. Both mutations can also suppress the early-flowering phenotype of hy2 and have a distinct pleiotropic effect on leaf development such as upward leaf rolling. The data obtained suggest that SHY1 and SHY2 represent a novel class of components involved in the photomorphogenic pathways of Arabidopsis . This is the first report on the identification of dominant mutations in the light signal transduction pathway of plants.     

The tomato photomorphogenetic mutant, aurea, is deficient in phytochromobilin synthase for phytochrome chromophore biosynthesis

The aurea mutants of tomato have been widely used as phytochrome-deficient mutants for photomorphogenetic and photobiological studies. By expressed sequence tag (EST)-based screening of sequence databases, we found a tomato gene that encodes a protein homologous to Arabidopsis HY2 for phytochromobilin synthase catalyzing the last step of phytochrome chromophore biosynthesis. The tomato protein expressed in Escherichia coli showed phytochromobilin synthase activity. The corresponding loci in all aurea mutants tested have nucleotide substitutions, deletions or DNA rearrangements. These results indicate that aurea is a mutant of phytochromobilin synthase in tomato. We also discuss a phylogenetic analysis of phytochromobilin synthases in the bilin reductase family.     

A complement of ten essential and pleiotropic arabidopsis COP/DET/FUS genes is necessary for repression of photomorphogenesis in darkness.

Two genetic screens, one for mutations resulting in photomorphogenic development in darkness and the other for mutants with fusca phenotype, have thus far identified six pleiotropic Arabidopsis COP/DET/FUS genes. Here, we characterized representative mutants that define four additional pleiotropic photomorphogenic loci and a null mutant allele of the previously defined DET1 locus. Dark-grown seedlings homozygous for these recessive mutations exhibit short hypocotyls and expanded cotyledons and are lethal before reaching reproductive development. Dark-grown mutant seedlings also display characteristic photomorphogenic cellular differentiation and elevated expression of light-inducible genes. In addition, analyses of plastids from dark-grown mutants reveal partial chloroplast differentiation and absence of etioplast development. Root vascular bundle cells of light-grown mutant seedlings develop chloroplasts, suggesting that these FUS gene products are important for suppression of chloroplast differentiation in light-grown roots. Double-mutant analyses indicate that these pleiotropic cop/det/fus mutations are epistatic to mutations in phytochromes, a blue-light photoreceptor, and a downstream regulatory component, HY5. Therefore, there is a complement of at least 10 essential and pleiotropic Arabidopsis genes that are necessary for repression of photomorphogenic development.     

Microinjection of heme oxygenase genes rescues phytochrome-chromophore-deficient mutants of the moss Ceratodon purpureus

 In protonemal tip cells of the moss Ceratodon purpureus (Hedw.) Brid., phototropism and chlorophyll accumulation are regulated by the photoreceptor phytochrome. The mutant ptr116 lacks both responses as a result of a defect in the biosynthesis of phytochromobilin, the chromophore of phytochrome, at the point of biliverdin formation. The rescue of the phototropic response and of chlorophyll synthesis were tested by injecting different substances into tip cells of ptr116. Microinjection was first optimised with the use of fluorescent dyes and an expression plasmid containing a green fluorescent protein (GFP) gene. Injected phycocyanobilin, which substitutes for phytochromobilin, rescued both the phototropic response and light-induced chlorophyll accumulation in ptr116. The same results were obtained when expression plasmids with heme oxygenase genes of rat (HO-1) and Arabidopsis thaliana (L.) Heynh. (HY1) were injected. Heme oxygenase catalyses the conversion of heme into biliverdin. Whereas HY1 has a plastid target sequence and is presumably transferred to plastids, HO-1 is proposed to be cytosolic. The data show that ptr116 lacks heme oxygenase enzyme activity and indicate that heme oxygenases of various origin are active in Ceratodon bilin synthesis. In addition, it can be inferred from the data that the intracellular localisation of the expressed heme oxygenase is not important since the plastid enzyme can be replaced by a cytosolic one. Received: 8 March 1999 / Accepted: 30 July 1999     

Heme oxygenase-2 is activated by calcium-calmodulin

The heme oxygenase family of enzymes catalyzes the metabolism of heme to biliverdin, ferrous iron, and carbon monoxide (CO). At least two isoforms exist, heme oxygenase-1 (HO1) and heme oxygenase-2 (HO2), which are encoded by separate genes. HO2 is selectively enriched in neurons, and substantial evidence suggests that HO2-derived CO functions as a neurotransmitter/neuromodulator. However, a molecular mechanism for the rapid activation of HO2 during neuronal activity has not been described. Through a yeast two-hybrid screen we identified calmodulin as a potential regulator of HO2 activity. Calmodulin binds with nanomolar affinity to HO2 in a calcium-dependent manner via a canonical 1-10 motif, resulting in a 3-fold increase in catalytic activity. Mutations within this motif block calmodulin binding and calcium-dependent stimulation of enzyme activity in vitro and in intact cells. The calcium mobilizing agents ionomycin and glutamate stimulate endogenous HO2 activity in primary cortical cultures, establishing in vivo relevance. Calcium-calmodulin provides a mechanism for rapid and transient activation of HO2 during neuronal activity.     

shygrl1 is a mutant affected in multiple aspects of photomorphogenesis

We have used a counter-selection strategy based on aberrant phytochrome regulation of an Lhcb gene to isolate an Arabidopsis mutant designated shygrl1 (shg1). shg1 seedlings have reduced phytochrome-mediated induction of the Lhcb gene family, but normal phytochrome-mediated induction of several other genes, including the rbcS1a gene. Additional phenotypes observed in shg1 plants include reduced chlorophyll in leaves and additional photomorphogenic abnormalities when the seedlings are grown on medium containing sucrose. Mutations in the TATA-proximal region of the Lhcb1*3 promoter that are known to be important for phytochrome regulation affected reporter gene expression in a manner similar to the shg1 mutation. Our results are consistent with the possibility that the mutation either leads to defective chloroplast development or to aberrant phytochrome regulation. They also add to the evidence of complex interactions between light- and sucrose-regulated pathways.     

NAD(H)-dependent glutamate dehydrogenase is essential for the survival of Arabidopsis thaliana during dark-induced carbon starvation

Interconversion between glutamate and 2-oxoglutarate, which can be catalysed by glutamate dehydrogenase (GDH), is a key reaction in plant carbon (C) and nitrogen (N) metabolism. However, the physiological role of plant GDH has been a controversial issue for several decades. To elucidate the function of GDH, the expression of GDH in various tissues of Arabidopsis thaliana was studied. Results suggested that the expression of two Arabidopsis GDH genes was differently regulated depending on the organ/tissue types and cellular C availability. Moreover, Arabidopsis mutants defective in GDH genes were identified and characterized. The two isolated mutants, gdh1-2 and gdh2-1, were crossed to make a double knockout mutant, gdh1-2/gdh2-1, which contained negligible levels of NAD(H)-dependent GDH activity. Phenotypic analysis on these mutants revealed an increased susceptibility of gdh1-2/gdh2-1 plants to C-deficient conditions. This conditional phenotype of the double knockout mutant supports the catabolic role of GDH and its role in fuelling the TCA cycle during C starvation. The reduced rate of glutamate catabolism in the gdh2-1 and gdh1-2/gdh2-1 plants was also evident by the growth retardation of these mutants when glutamate was supplied as the alternative N source. Furthermore, amino acid profiles during prolonged dark conditions were significantly different between WT and the gdh mutant plants. For instance, glutamate levels increased in WT plants but decreased in gdh1-2/gdh2-1 plants, and aberrant accumulation of several amino acids was detected in the gdh1-2/gdh2-1 plants. These results suggest that GDH plays a central role in amino acid breakdown under C-deficient conditions.