An ethylene-responsive flower senescence-related gene from carnation encodes a protein homologous to glutathione S-transferases

Allies or Enemies: The Role of Reactive Oxygen Species in Developmental Processes of Black Cottonwood (Populus trichocarpa)

In contrast to aboveground organs (stems and leaves), developmental events and their regulation in underground organs, such as pioneer and fine roots, are quite poorly understood. The objective of the current study was to achieve a better understanding of the physiological and molecular role of reactive oxygen species (ROS) and ROS-related enzymes in the process of stem and pioneer root development in black cottonwood (Populus trichocarpa), as well as in the senescence of leaves and fine roots. Results of a transcriptomic analysis revealed that primary/secondary growth and senescence are accompanied by substantial changes in the expression of genes related to oxidative stress metabolism. We observed that some mechanisms common for above- and under-ground organs, e.g., the expression of superoxide dismutase (SOD) genes and SOD activity, declined during stems' and pioneer roots' development. Moreover, the localization of hydrogen peroxide (H₂O₂) and superoxide (O₂•^-) in the primary and secondary xylem of stems and pioneer roots confirms their involvement in xylem cell wall lignification and the induction of programmed cell death (PCD). H₂O₂ and O₂•^- in senescing fine roots were present in the same locations as demonstrated previously for ATG8 (AuTophaGy-related) proteins, implying their participation in cell degradation during senescence, while O₂•^- in older leaves was also localized similarly to ATG8 in chloroplasts, suggesting their role in chlorophagy. ROS and ROS-related enzymes play an integral role in the lignification of xylem cell walls in Populus trichocarpa, as well as the induction of PCD during xylogenesis and senescence.

Magnesium Hydride-Mediated Sustainable Hydrogen Supply Prolongs the Vase Life of Cut Carnation Flowers via Hydrogen Sulfide

Magnesium hydride (MgH₂) is a promising solid-state hydrogen source with high storage capacity (7.6 wt%). Although it is recently established that MgH₂ has potential applications in medicine because it sustainably supplies hydrogen gas (H₂), the biological functions of MgH₂ in plants have not been observed yet. Also, the slow reaction kinetics restricts its practical applications. In this report, MgH₂ (98% purity; 0.5-25 μm size) was firstly used as a hydrogen generation source for postharvest preservation of flowers. Compared with the direct hydrolysis of MgH₂ in water, the efficiency of hydrogen production from MgH₂ hydrolysis could be greatly improved when the citrate buffer solution is introduced. These results were further confirmed in the flower vase experiment by showing higher efficiency in increasing the production and the residence time of H₂ in solution, compared with hydrogen-rich water. Mimicking the response of hydrogen-rich water and sodium hydrosulfide (a hydrogen sulfide donor), subsequent experiments discovered that MgH₂-citrate buffer solution not only stimulated hydrogen sulfide (H₂S) synthesis but also significantly prolonged the vase life of cut carnation flowers. Meanwhile, redox homeostasis was reestablished, and the increased transcripts of representative senescence-associated genes, including DcbGal and DcGST1, were partly abolished. By contrast, the discussed responses were obviously blocked by the inhibition of endogenous H₂S with hypotaurine, an H₂S scavenger. These results clearly revealed that MgH₂-supplying H₂ could prolong the vase life of cut carnation flowers via H₂S signaling, and our results, therefore, open a new window for the possible application of hydrogen-releasing materials in agriculture.

Molecular characterization of zeta class glutathione S-transferases from Pinus brutia Ten

Glutathione transferases (GSTs; EC 2.5.1.18) play important roles in stress tolerance and metabolic detoxification in plants.In higher plants, studies on GSTs have focussed largely on agricultural plants. There is restricted information about molecular characterization of GSTs in gymnosperms. To date, only tau class GST enzymes have been characterized from some pinus species. For the first time, the present study reports cloning and molecular characterization of two zeta class GST genes, namely PbGSTZ1 and PbGSTZ2 from Pinus brutia Ten., which is an economically important pine native to the eastern Mediterranean region and have to cope with several environmental stress conditions. The PbGSTZ1 gene was isolated from cDNA, whereas PbGSTZ2 was isolated from genomic DNA. Sequence analysis of PbGSTZ1 and PbGSTZ2 revealed the presence of an open reading frame of 226 amino acids with typical consensus sequences of the zeta class plant GSTs. Protein and secondary structure prediction analysis of two zeta class PbGSTZs have shared common features of other plant zeta class GSTs. Genomic clone, PbGSTZ2 gene, is unexpectedly intronless. Extensive sequence analysis of PbGSTZ2, with cDNA clone, PbGSTZ1, revealed 87% identity at nucleotide and 81% identity at amino acid levels with 41 amino acids differences suggesting that genomic PbGSTZ2 gene might be an allelic or a paralogue version of PbGSTZ1.

Two pear glutathione S-transferases genes are regulated during fruit development and involved in response to salicylic acid, auxin, and glucose signaling

Two genes encoding putative glutathione S-transferase proteins were isolated from pear (Pyrus pyrifolia) and designated PpGST1 and PpGST2. The deduced PpGST1 and PpGST2 proteins contain conserved Glutathione S-transferase N-terminal domain (GST_N) and Glutathione S-transferase, C-terminal domain (GST_C). Using PCR amplification technique, the genomic clones corresponding to PpGST1 and PpGST2 were isolated and shown to contain two introns and a singal intron respectively with typical GT/AG boundaries defining the splice junctions. Phylogenetic analysis clearly demonstrated that PpGST1 belonged to Phi class of GST superfamilies and had high homology with apple MdGST, while PpGST2 was classified into the Tau class of GST superfamilies. The expression of PpGST1 and PpGST2 genes was developmentally regulated in fruit. Further study demonstrated that PpGST1 and PpGST2 expression was remarkably induced by glucose, salicylic acid (SA) and indole-3-aceticacid (IAA) treatments in pear fruit, and in diseased fruit. These data suggested that PpGST1 and PpGST2 might be involved in response to sugar, SA, and IAA signaling during fruit development of pear.

The Antioxidants Changes in Ornamental Flowers during Development and Senescence

The concentration of antioxidant compounds is constitutive and variable from species to species and is also variable considering the development of the plant tissue. In this review, we take into consideration the antioxidant changes and the physiological, biochemical and molecular factors that are able to modulate the accumulation of antioxidant compounds in ornamental flowers during the whole development process until the senescence. Many ornamental flowers are natural sources of very important bioactive compounds with benefit to the human health and their possible role as dietary components has been reported. The most part of antioxidants are flower pigments such as carotenoids and polyphenols, often present in higher concentration compared with the most common fruits and vegetables. The antioxidants content changes during development and during senescence many biochemical systems and molecular mechanisms are activated to counteract the increase of reactive oxygen species and free radicals. There is a tight correlation between antioxidants and senescence processes and this aspect is detailed and appropriately discussed.

Cold sensitivity in rice (Oryza sativa L.) is strongly correlated with a naturally occurring I99V mutation in the multifunctional glutathione transferase isoenzyme GSTZ2

GSTZs [Zeta class GSTs (glutathione transferases)] are multifunctional enzymes that belong to a highly conserved subfamily of soluble GSTs found in species ranging from fungi and plants to animals. GSTZs are known to function as MAAIs [MAA (maleylacetoacetate) isomerases], which play a role in tyrosine catabolism by catalysing the isomerization of MAA to FAA (fumarylacetoacetate). As tyrosine metabolism in plants differs from animals, the significance of GSTZ/MAAI is unclear. In rice (Oryza sativa L.), a major QTL (quantitative trait locus) for seedling cold tolerance has been fine mapped to a region containing the genes OsGSTZ1 and OsGSTZ2. Sequencing of tolerant (ssp. japonica cv. M-202) and sensitive (ssp. indica cv. IR50) cultivars revealed two SNPs (single nucleotide polymorphisms) in OsGSTZ2 that result in amino acid differences (I99V and N184I). Recombinant OsGSTZ2 containing the Val99 residue found in IR50 had significantly reduced activity on MAA and DCA (dichloroacetic acid), but the Ile184 residue had no effect. The distribution of the SNP (c.295A>G) among various rice accessions indicates a significant association with chilling sensitivity in rice seedlings. The results of the present study show that naturally occurring OsGSTZ2 isoforms differ in their enzymatic properties, which may contribute to the differential response to chilling stress generally exhibited by the two major rice subspecies.

A genomic approach to study anthocyanin synthesis and flower pigmentation in passionflowers

Most of the plant pigments ranging from red to purple colors belong to the anthocyanin group of flavonoids. The flowers of plants belonging to the genus Passiflora (passionflowers) show a wide range of floral adaptations to diverse pollinating agents, including variation in the pigmentation of floral parts ranging from white to red and purple colors. Exploring a database of expressed sequence tags obtained from flower buds of two divergent Passiflora species, we obtained assembled sequences potentially corresponding to 15 different genes of the anthocyanin biosynthesis pathway in these species. The obtained sequences code for putative enzymes are involved in the production of flavonoid precursors, as well as those involved in the formation of particular ("decorated") anthocyanin molecules. We also obtained sequences encoding regulatory factors that control the expression of structural genes and regulate the spatial and temporal accumulation of pigments. The identification of some of the putative Passiflora anthocyanin biosynthesis pathway genes provides novel resources for research on secondary metabolism in passionflowers, especially on the elucidation of the processes involved in floral pigmentation, which will allow future studies on the role of pigmentation in pollinator preferences in a molecular level.

A specific group of genes respond to cold dehydration stress in cut Alstroemeria flowers whereas ambient dehydration stress accelerates developmental senescence expression patterns

Petal development and senescence entails a normally irreversible process. It starts with petal expansion and pigment production, and ends with nutrient remobilization and ultimately cell death. In many species this is accompanied by petal abscission. Post-harvest stress is an important factor in limiting petal longevity in cut flowers and accelerates some of the processes of senescence such as petal wilting and abscission. However, some of the effects of moderate stress in young flowers are reversible with appropriate treatments. Transcriptomic studies have shown that distinct gene sets are expressed during petal development and senescence. Despite this, the overlap in gene expression between developmental and stress-induced senescence in petals has not been fully investigated in any species. Here a custom-made cDNA microarray from Alstroemeria petals was used to investigate the overlap in gene expression between developmental changes (bud to first sign of senescence) and typical post-harvest stress treatments. Young flowers were stressed by cold or ambient temperatures without water followed by a recovery and rehydration period. Stressed flowers were still at the bud stage after stress treatments. Microarray analysis showed that ambient dehydration stress accelerates many of the changes in gene expression patterns that would normally occur during developmental senescence. However, a higher proportion of gene expression changes in response to cold stress were specific to this stimulus and not senescence related. The expression of 21 transcription factors was characterized, showing that overlapping sets of regulatory genes are activated during developmental senescence and by different stresses.

GST profile expression study in some selected plants: in silico approach

Glutathione acts as a protein disulphide reductant, which detoxifies herbicides by conjugation, either spontaneously or by the activity of one of a number of glutathione-S-transferases (GSTs), and regulates gene expression in response to environmental stress and pathogen attack. GSTs play roles in both normal cellular metabolisms as well as in the detoxification of a wide variety of xenobiotic compounds, and they have been intensively studied with regard to herbicide detoxification in plants. A newly discovered plant GST subclass has been implicated in numerous stress responses, including those arising from pathogen attack, oxidative stress and heavy-metal toxicity. In addition, plants GSTs play a role in the cellular response to auxins and during the normal metabolism of plant secondary products like anthocyanins and cinnamic acid. The present work involves two in silico analytical approaches-general secondary structure prediction studies of the proteins and detailed signature pattern studies of some selected GST classes in Arabdiopsis thaliana, mustard, maize and bread wheat by standard Bioinformatics tools; structure prediction tools; signature pattern tools; and the evolutionary trends were analyzed by ClustalW. For this purpose, sequences were obtained from standard databases. The work reveals that these proteins are mainly alpha helical in nature with specific signature pattern similar to phosphokinase C, tyrosine kinase and casein kinase II proteins, which are closely related to plant oxidative stress. This study aims to comprehend the relationship of GST gene family and plant oxidative stress with respect to certain specific conserved motifs, which may help in future studies for screening of biomodulators involved in plant stress metabolism.

A comparison of leaf and petal senescence in wallflower reveals common and distinct patterns of gene expression and physiology

Petals and leaves share common evolutionary origins but perform very different functions. However, few studies have compared leaf and petal senescence within the same species. Wallflower (Erysimum linifolium), an ornamental species closely related to Arabidopsis (Arabidopsis thaliana), provide a good species in which to study these processes. Physiological parameters were used to define stages of development and senescence in leaves and petals and to align these stages in the two organs. Treatment with silver thiosulfate confirmed that petal senescence in wallflower is ethylene dependent, and treatment with exogenous cytokinin and 6-methyl purine, an inhibitor of cytokinin oxidase, suggests a role for cytokinins in this process. Subtractive libraries were created, enriched for wallflower genes whose expression is up-regulated during leaf or petal senescence, and used to create a microarray, together with 91 senescence-related Arabidopsis probes. Several microarray hybridization classes were observed demonstrating similarities and differences in gene expression profiles of these two organs. Putative functions were ascribed to 170 sequenced DNA fragments from the libraries. Notable similarities between leaf and petal senescence include a large proportion of remobilization-related genes, such as the cysteine protease gene SENESCENCE-ASSOCIATED GENE12 that was up-regulated in both tissues with age. Interesting differences included the up-regulation of chitinase and glutathione S-transferase genes in senescing petals while their expression remained constant or fell with age in leaves. Semiquantitative reverse transcription-polymerase chain reaction of selected genes from the suppression subtractive hybridization libraries revealed more complex patterns of expression compared with the array data.

Integrated signaling in flower senescence: an overview

Flower senescence is the terminal phase of developmental processes that lead to the death of flower, which include, flower wilting, shedding of flower parts and fading of blossoms. Since it is a rapid process as compared to the senescence of other parts of the plant it therefore provides excellent model system for the study of senescence. During flower senescence, developmental and environmental stimuli enhance the upregulation of catabolic processes causing breakdown and remobilization of cellular constituents. Ethylene is well known to play regulatory role in ethylene-sensitive flowers while in ethylene-insensitive flowers abscisic acid (ABA) is thought to be primary regulator. Subsequent to perception of flower senescence signal, death of petals is accompanied by the loss of membrane permeability, increase in oxidative and decreased level of protective enzymes. The last stages of senescence involve the loss of of nucleic acids (DNA and RNA), proteins and organelles, which is achieved by activation of several nucleases, proteases and wall modifiers. Environmental stimuli such as pollination, drought and other stresses also affect senescence by hormonal imbalance. In this article we have covered the following: perception mechanism and specificity of flower senescence, flower senescence-associated events, like degradation of cell membranes, proteins and nucleic acids, environmental/external factors affecting senescence, like pollination and abiotic stress, hormonal and non-hormonal regulation of flower/petal senescence and finally the senescence associated genes (SAGs) have also been described.

Plant glutathione transferases — a decade falls short

The glutathione transferase (GST) superfamily in plants has been subdivided into eight classes, seven of which (phi, tau, zeta, theta, lambda, dehydroascorbate reductase, and tetrachlorohydroquinone dehalogenase) are soluble and one is microsomal. Since their identification in plants in 1970, these enzymes have been well established as phase II detoxification enzymes that perform several other essential functions in plant growth and development. These enzymes catalyze nucleophilic conjugation of the reduced form of the tripeptide glutathione to a wide variety of hydrophobic, electrophilic, and usually cytotoxic substrates. In plants, the conjugated product is either sequestered in the vacuole or transferred to the apoplast. The GSTs of phi and tau classes, which are plant-specific and the most abundant, are chiefly involved in xenobiotic metabolism. Zeta- and theta-class GSTs have very restricted activities towards xenobiotics. Theta-class GSTs are glutathione peroxidases and are involved in oxidative-stress metabolism, whereas zeta-class GSTs act as glutathione-dependent isomerases and catalyze the glutathione-dependent conversion of maleylacetoacetate to fumarylacetoacetate. Zeta-class GSTs participate in tyrosine catabolism. Dehydroascorbate reductase- and lambda-class GSTs function as thioltransferases. Microsomal-class GSTs are members of the MAPEG (membrane-associated proteins in eicosanoid and glutathione metabolism) superfamily. A plethora of studies utilizing both proteomics and genomics approaches have greatly helped in revealing the functional diversity exhibited by these enzymes. The three-dimensional structure of some of the members of the family has been described and this has helped in elucidating the mechanism of action and active-site amino-acid residues of these enzymes. Although a large amount of information is available on this complex enzyme superfamily, more research is necessary to answer additional questions such as, why are phi- and tau-class GSTs more abundant than GSTs from other classes? What functions do phi- and tau-class GSTs perform in plant taxa other than angiosperms? Do more GST classes exist? Future studies on GSTs should focus on these aspects.

Sucrose prevents up-regulation of senescence-associated genes in carnation petals

cDNA microarrays were used to characterize senescence-associated gene expression in petals of cut carnation (Dianthus caryophyllus) flowers, sampled from anthesis to the first senescence symptoms. The population of PCR fragments spotted on these microarrays was enriched for flower-specific and senescence-specific genes, using subtractive hybridization. About 90% of the transcripts showed a large increase in quantity, approximately 25% transiently, and about 65% throughout the 7 d experiment. Treatment with silver thiosulphate (STS), which blocks the ethylene receptor and prevented the normal senescence symptoms, prevented the up-regulation of almost all of these genes. Sucrose treatment also considerably delayed visible senescence. Its effect on gene expression was very similar to that of STS, suggesting that soluble sugars act as a repressor of ethylene signal transduction. Two fragments that encoded a carnation EIN3-like (EIL) protein were isolated, some of which are key transcription factors that control ethylene response genes. One of these (Dc-EIL3) was up-regulated during senescence. Its up-regulation was delayed by STS and prevented by sucrose. Sucrose, therefore, seems to repress ethylene signalling, in part, by preventing up-regulation of Dc-EIL3. Some other transcription factors displayed an early increase in transcript abundance: a MYB-like DNA binding protein, a MYC protein, a MADS-box factor, and a zinc finger protein. Genes suggesting a role in senescence of hormones other than ethylene encoded an Aux/IAA protein, which regulate transcription of auxin-induced genes, and a cytokinin oxidase/dehydrogenase, which degrades cytokinin. Taken together, the results suggest a master switch during senescence, controlling the co-ordinated up-regulation of numerous ethylene response genes. Dc-EIL3 might be (part of) this master switch.

Enzymes of tyrosine catabolism in Arabidopsis thaliana

Tyrosine catabolism is an essential pathway in animals, but its role in plants is unclear. The first steps of tyrosine degradation lead to the formation of homogentisate. In animals this is then sequentially acted on by homogentisate dioxygenase (HGO), maleylacetoacetate isomerase (MAAI) and fumarylacetoacetate hydrolase (FAH) to generate fumarate and acetoacetate. In plants, homogentisate is used to generate the essential redox metabolites tocopherol and plastoquinone, which effectively act as an alternative metabolic fate for tyrosine. Having determined that a zeta class glutathione transferase from Arabidopsis thaliana is a functional MAAI, we have looked for evidence that the mammalian degradation pathway could also operate in plants. Based on array and quantitative PCR experiments, the A. thaliana homologues AtHGO, AtMAAI and AtFAH could be shown to be expressed, with AtHGO and AtMAAI showing evidence of co-regulation. cDNAs encoding AtHGO, AtMAAI and AtFAH were cloned in Escherichia coli and shown to represent a fully functional catabolic pathway when combined in vitro. The significance of this pathway, including increased transcription of the associated enzymes in senescing tissue, compartmentalisation and impact on flux into synthesis of Vitamin E and other tocopherols of biotechnological interest is discussed.

Regulatory and functional interactions of plant growth regulators and plant glutathione S-transferases (GSTs)

Plant glutathioneS-transferases (GSTs) are a heterogeneous superfamily of multifunctional proteins, grouped into six classes. The tau (GSTU) and phi (GSTF) class GSTs are the most represented ones and are plant-specific, whereas the smaller theta (GSTT) and zeta (GSTZ) classes are also found in animals. The lambda GSTs (GSTL) and the dehydroascorbate reductases (DHARs) are more distantly related. Plant GSTs perform a variety of pivotal catalytic and non-enzymatic functions in normal plant development and plant stress responses, roles that are only emerging. Catalytic functions include glutathione (GSH)-conjugation in the metabolic detoxification of herbicides and natural products. GSTs can also catalyze GSH-dependent peroxidase reactions that scavenge toxic organic hydroperoxides and protect from oxidative damage. GSTs can furthermore catalyze GSH-dependent isomerizations in endogenous metabolism, exhibit GSH-dependent thioltransferase safeguarding protein function from oxidative damage and DHAR activity functioning in redox homeostasis. Plant GSTs can also function as ligandins or binding proteins for phytohormones (i.e., auxins and cytokinins) or anthocyanins, thereby facilitating their distribution and transport. Finally, GSTs are also indirectly involved in the regulation of apoptosis and possibly also in stress signaling. Plant GST genes exhibit a diversity of expression patterns during biotic and abiotic stresses. Stress-induced plant growth regulators (i.e., jasmonic acid [JA], salicylic acid [SA], ethylene [ETH], and nitric oxide [NO] differentially activate GST gene expression. It is becoming increasingly evident that unique combinations of multiple, often interactive signaling pathways from various phytohormones and reactive oxygen species or antioxidants render the distinct transcriptional activation patterns of individual GSTs during stress. Underestimated post-transcriptional regulations of individual GSTs are becoming increasingly evident and roles for phytohormones (i.e., ABA and JA) in these processes are being anticipated as well. Finally, indications are emerging that NO may regulate the activity of specific plant GSTs. In this review, the current knowledge on the regulatory and functional interactions of phytohormones and plant GSTs are covered. We refer to a previous extensive review on plant GSTs (Marrs, 1996) for most earlier work. An introduction on the classification and roles of plant GSTs is included here, but these topics are more extensively discussed in other reviews (Dixon et al., 2002a; Edwards et al., 2000; Frova, 2003).

Upregulation of a tonoplast-localized cytochrome P450 during petal senescence in Petunia inflata

BACKGROUND

Gene expression in Petunia inflata petals undergoes major changes following compatible pollination. Severe flower wilting occurs reproducibly within 36 hours, providing an excellent model for investigation of petal senescence and programmed cell death. Expression of a number of genes and various enzyme activities involved in the degradation and remobilization of macromolecules have been found to be upregulated during the early stages of petal senescence.

RESULTS

By performing differential display of cDNAs during Petunia inflata petal senescence, a highly upregulated gene encoding a cytochrome P450 was identified. Analysis of the complete cDNA sequence revealed that the predicted protein is a member of the CYP74C family (CYP74C9) and is highly similar to a tomato CYP74C allene oxide synthase (AOS) that is known to be active on 9-hydroperoxides. Cloning of the petunia genomic DNA revealed an intronless gene with a promoter region that carries signals found in stress-responsive genes and potential binding sites for Myb transcription factors. Transcripts were present at detectable levels in root and stem, but were 40 times more abundant in flowers 36 hours after pollination. Ethylene and jasmonate treatment resulted in transitory increases in expression in detached flowers. A protein fusion of the CYP74C coding region to a C-terminal GFP was found to be located in the tonoplast.

CONCLUSION

Though oxylipins, particularly jasmonates, are known to be involved in stress responses, the role of other products of CYP74 enzymes is less well understood. The identification of a CYP74C family member as a highly upregulated gene during petal senescence suggests that additional products of fatty acid metabolism may play important roles during programmed cell death. In contrast to the chloroplast localization of AOS proteins in the CYP74A subfamily, GFP fusion data indicates that the petunia CYP74C9 enzyme is in the tonoplast. This result suggests that the highly similar CYP74C enzymes that have been identified in two other Solanaceous plants may also be associated with the vacuole, an organelle known to have a prominent role in programmed cell death.

Genomic structure and differential expression of two tandem-arranged GSTZ genes in rice

Glutathione S-transferases (GSTs) are scavenging enzymes that detoxify cellular xenobiotics and toxins by catalyzing the conjugation of these substrates with a tripeptide glutathione. GSTs are classified depending on gene organization and sequence similarity. The sequence analysis of genomic DNA for zeta class GST (GSTZ) locus in rice indicated that two homologous GSTZ genes lay in a tandem orientation with a short (0.4 kb) intergenic spacer. The upstream OsGSTZ1 and downstream OsGSTZ2 spanned 3.5 and 3.2 kb with nine coding exons, respectively. The transcript of OsGSTZ1 had a long 3' untranslated region (3' UTR) that was mostly encoded by a 10th noncoding exon, whereas OsGSTZ2 mRNA contained a long 5' UTR. Northern blot analysis showed that OsGSTZ1/2 messages were strongly expressed in leaf blades, while transcripts from roots were low level. Because OsGSTZ1/2 messages in leaf tissues were strongly induced only by water treatment, it was difficult to assay for the induction of OsGSTZ1/2 transcripts by various stress treatments. Thus, using rice culture cells, we analyzed the respective responses of OsGSTZ1 and OsGSTZ2 genes against various treatments by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The results showed that OsGSTZ1 was expressed at a level ca. 1000-fold higher than OsGSTZ2 in suspension cells without stress treatment. OsGSTZ1 was expressed constitutively under various stress conditions. In contrast, the expression of OsGSTZ2 gene was strongly enhanced to 30-fold by treatment with jasmonic acid. These observations suggested that the expression of OsGSTZ1 and OsGSTZ2 genes are differentially regulated in the culture cell of rice.

Overexpression of enzymes involved in glutathione synthesis enhances tolerance to organic pollutants in Brassica juncea

Transgenic Indian mustard (Brassica juncea) overexpressing y-glutamylcysteine synthetase (ECS) or glutathione synthetase (GS) were shown previously to have two-fold higher levels of glutathione and total nonprotein thiols, as well as enhanced cadmium tolerance and accumulation. Here, the hypothesis was tested that these transgenics have enhanced tolerance to organic pollutants, based on the reasoning that many organic xenobiotics are detoxified via conjugation to glutathione. Both the ECS and GS transgenics showed enhanced tolerance to atrazine: while root growth of wildtype seedlings was inhibited 50% by 100 mg L(-1) atrazine, ECS and GS root growth was inhibited 20-30% (P < 0.05). The tolerance of the transgenics to CDNB (1-chloro-2,4-dinitrobenzene). metolachlor, and phenanthrene was also somewhat higher than wild type, but these differences were not as pronounced. Each of the organics treatments significantly enhanced total nonprotein thiol levels in all plant types (2 to 12-fold). Overall, these results suggest that GSH biosynthesis is limiting for atrazine detoxification in Indian mustard and that overexpression of enzymes involved in GSH biosynthesis offers a promising approach to create plants with the enhanced capacity to tolerate not only heavy metals, but also certain organics.

Large-scale identification of leaf senescence-associated genes

Leaf senescence is a form of programmed cell death, and is believed to involve preferential expression of a specific set of "senescence-associated genes" (SAGs). To decipher the molecular mechanisms and the predicted complex network of regulatory pathways involved in the senescence program, we have carried out a large-scale gene identification study in a reference plant, Arabidopsis thaliana. Using suppression subtractive hybridization, we isolated approximately 800 cDNA clones representing SAGs expressed in senescing leaves. Differential expression was confirmed by Northern blot analysis for 130 non-redundant genes. Over 70 of the identified genes have not previously been shown to participate in the senescence process. SAG-encoded proteins are likely to participate in macromolecule degradation, detoxification of oxidative metabolites, induction of defense mechanisms, and signaling and regulatory events. Temporal expression profiles of selected genes displayed several distinct patterns, from expression at a very early stage, to the terminal phase of the senescence syndrome. Expression of some of the novel SAGs, in response to age, leaf detachment, darkness, and ethylene and cytokinin treatment was compared. The large repertoire of SAGs identified here provides global insights about regulatory, biochemical and cellular events occurring during leaf senescence.

The rapid induction of glutathione S-transferases AtGSTF2 and AtGSTF6 by avirulent Pseudomonas syringae is the result of combined salicylic acid and ethylene signaling

The expression of two members of the glutathione S-transferase (GST) multigene family was studied in Arabidopsis plants inoculated with an avirulent strain of Pseudomonas syringae pv. tomato (Pst). Accumulation of AtGSTF2 and AtGSTF6 transcripts started 4 and 2 h after inoculation, respectively, and clearly preceded the induction of the pathogenesis-related PR-1 gene. The aim of this work was to find the reason for the faster induction of the two GSTs compared with classical salicylic acid (SA)-regulated PR-proteins. Expression studies in Pst-inoculated SA-signaling mutants NahG and npr1 revealed that induction of both GSTs was SA-dependent and partially NPR1-independent. The induction of AtGSTF2 by Pst was also strongly repressed in the ethylene insensitive etr1 mutant. Both GSTs were induced by low amounts of SA (0.1 mM) and ethylene (0.1 ppm) while PR-1 gene expression was unaffected by ethylene. Interestingly, ethylene was about 50-fold less effective in NahG compared with wild-type plants thus suggesting a potentiation effect of SA on ethylene-induced accumulation of AtGST transcripts. Increased AtGST expression in plants inoculated with Pst correlated with increased production of SA and ethylene. However, the initial phase of AtGSTF6 induction was independent of SA- and ethylene-signaling. The jasmonate (JA)-insensitive mutant jar1 showed normal induction kinetics for both GSTs. Our data support the hypothesis that full expression of the pathogen-induced AtGSTF2 and, to a lesser extent AtGSTF6, is the result of combined SA- and ethylene-signaling and that early AtGSTF6 expression depends on additional unknown signaling mechanisms.

Purification of a phi-type glutathione S-transferase from pumpkin flowers, and molecular cloning of its cDNA

A major species of glutathione S-transferase (GST), Pugf, was highly purified from pumpkin flowers. Two-dimensional electrophoresis of the purified enzyme gave two adjacent protein spots. The specific activity of the purified enzyme was 2.4 micromol min(-1) mg(-1) protein for 1-chloro-2,4-dinitrobenzene. This value is one to two orders of magnitude lower than that of pumpkin tau-type GSTs. The expression pattern of Pugf in healthy pumpkin plants and responses to various stresses were examined by western blotting. Pugf was found in high concentrations in petioles, stems, and roots as well as flowers, and was more abundant in expanding young organs than in fully expanded mature organs. Dehydration caused a slight increase in its concentration, but high and low temperatures, salty stress, and 2,4-dichlorophenoxyacetic acid seemed to have no effects. A cDNA encoding Pugf was cloned and sequenced. Sequence comparison with other plant GSTs suggested that it should be classified as a phi-type GST.

Structure, function and evolution of glutathione transferases: implications for classification of non-mammalian members of an ancient enzyme superfamily

The glutathione transferases (GSTs; also known as glutathione S-transferases) are major phase II detoxification enzymes found mainly in the cytosol. In addition to their role in catalysing the conjugation of electrophilic substrates to glutathione (GSH), these enzymes also carry out a range of other functions. They have peroxidase and isomerase activities, they can inhibit the Jun N-terminal kinase (thus protecting cells against H(2)O(2)-induced cell death), and they are able to bind non-catalytically a wide range of endogenous and exogenous ligands. Cytosolic GSTs of mammals have been particularly well characterized, and were originally classified into Alpha, Mu, Pi and Theta classes on the basis of a combination of criteria such as substrate/inhibitor specificity, primary and tertiary structure similarities and immunological identity. Non-mammalian GSTs have been much less well characterized, but have provided a disproportionately large number of three-dimensional structures, thus extending our structure-function knowledge of the superfamily as a whole. Moreover, several novel classes identified in non-mammalian species have been subsequently identified in mammals, sometimes carrying out functions not previously associated with GSTs. These studies have revealed that the GSTs comprise a widespread and highly versatile superfamily which show similarities to non-GST stress-related proteins. Independent classification systems have arisen for groups of organisms such as plants and insects. This review surveys the classification of GSTs in non-mammalian sources, such as bacteria, fungi, plants, insects and helminths, and attempts to relate them to the more mainstream classification system for mammalian enzymes. The implications of this classification with regard to the evolution of GSTs are discussed.

The structure of a zeta class glutathione S-transferase from Arabidopsis thaliana: characterisation of a GST with novel active-site architecture and a putative role in tyrosine catabolism

The cis-trans isomerisation of maleylacetoacetate to fumarylacetoacetate is the penultimate step in the tyrosine/phenylalanine catabolic pathway and has recently been shown to be catalysed by glutathione S-transferase enzymes belonging to the zeta class. Given this primary metabolic role it is unsurprising that zeta class glutathione S-transferases are well conserved over a considerable period of evolution, being found in vertebrates, plants, insects and fungi. The structure of this glutathione S-transferase, cloned from Arabidopsis thaliana, has been solved by single isomorphous replacement with anomalous scattering and refined to a final crystallographic R-factor of 19.6% using data from 25.0 A to 1.65 A. The zeta class enzyme adopts the canonical glutathione S-transferase fold and forms a homodimer with each subunit consisting of 221 residues. In agreement with structures of glutathione S-transferases from the theta and phi classes, a serine residue (Ser17) is present in the active site, at a position that would allow it to stabilise the thiolate anion of glutathione. Site-directed mutagenesis of this residue confirms its importance in catalysis. In addition, the role of a highly conserved cysteine residue (Cys19) present in the active site of the zeta class glutathione S-transferase enzymes is discussed.

An early ethylene up-regulated gene encoding a calmodulin-binding protein involved in plant senescence and death

35S-Labeled calmodulin (CaM) was used to screen a tobacco anther cDNA library. A positive clone (NtER1) with high homology to an early ethylene-up-regulated gene (ER66) in tomato, and an Arabidopsis homolog was isolated and characterized. Based on the helical wheel projection, a 25-mer peptide corresponding to the predicted CaM-binding region of NtER1 (amino acids 796-820) was synthesized. The gel-mobility shift assay showed that the peptide formed a stable complex with CaM only in the presence of Ca(2+). CaM binds to NtER1 with high affinity (K(d) approximately 12 nm) in a calcium-dependent manner. Tobacco flowers at different stages of development were treated with ethylene or with 1-methylcyclopropene for 2 h before treating with ethylene. Northern analysis showed that the NtER1 was rapidly induced after 15 min of exposure to ethylene. However, the 2-h 1-methylcyclopropene treatment totally blocked NtER1 expression in flowers at all stages of development, suggesting that NtER1 is an early ethylene-up-regulated gene. The senescing leaves and petals had significantly increased NtER1 induction as compared with young leaves and petals, implying that NtER1 is developmentally regulated and acts as a trigger for senescence and death. This is the first documented evidence for the involvement of Ca(2+)/CaM-mediated signaling in ethylene action.

Characterisation of a Zeta Class Glutathione Transferase from Arabidopsis thaliana with a Putative Role in Tyrosine Catabolism

A glutathione transferase (GST) similar to zeta GSTs in animals and fungi has been cloned from Arabidopsis thaliana using RT-PCR. The Arabidopsis zeta GST (AtGSTZ1) was expressed in Escherichia coli as his-tagged polypeptides, which associated together to form the 50-kDa AtGSTZ1-1 homodimer. Following purification, AtGSTZ1-1 was assayed for a range of activities and compared with other purified recombinant plant GSTs from the phi, tau, and theta classes. AtGSTZ1-1 differed from the other GSTs in showing no glutathione conjugating activity toward xenobiotics and no glutathione peroxidase activity toward organic hydroperoxides. Uniquely among the plant GSTs, AtGSTZ1-1 showed activity as a maleylacetone isomerase (MAI). This glutathione-dependent reaction is analogous to the cis-trans isomerization of maleylacetoacetate to fumarylacetoacetate, which occurs in the course of tyrosine catabolism to acetoacetate and fumarate. Thus, rather than functioning as a conventional GST, AtGSTZ1-1 appears to be involved in tyrosine degradation. In addition to the MAI activity, the AtGSTZ1-1 also catalyzed the glutathione-dependent dehalogenation of dichloroacetic acid to glyoxylic acid. This latter activity was used to demonstrate the presence of functional AtGSTZ1-1 inplanta.

A genomics approach to the comprehensive analysis of the glutathione S-transferase gene family in soybean and maize

By BLAST searching a large expressed sequence tag database for glutathione S-transferase (GST) sequences we have identified 25 soybean (Glycine max) and 42 maize (Zea mays) clones and obtained accurate full-length GST sequences. These clones probably represent the majority of members of the GST multigene family in these species. Plant GSTs are divided according to sequence similarity into three categories: types I, II, and III. Among these GSTs only the active site serine, as well as another serine and arginine in or near the "G-site" are conserved throughout. Type III GSTs have four conserved sequence patches mapping to distinct structural features. Expression analysis reveals the distribution of GSTs in different tissues and treatments: Maize GSTI is overall the most highly expressed in maize, whereas the previously unknown GmGST 8 is most abundant in soybean. Using DNA microarray analysis we observed increased expression among the type III GSTs after inducer treatment of maize shoots, with different genes responding to different treatments. Protein activity for a subset of GSTs varied widely with seven substrates, and any GST exhibiting greater than marginal activity with chloro-2,4 dinitrobenzene activity also exhibited significant activity with all other substrates, suggesting broad individual enzyme substrate specificity.

Programmed cell death during pollination-induced petal senescence in petunia

Petal senescence, one type of programmed cell death (PCD) in plants, is a genetically controlled sequence of events comprising its final developmental stage. We characterized the pollination-induced petal senescence process in Petunia inflata using a number of cell performance markers, including fresh/dry weight, protein amount, RNA amount, RNase activity, and cellular membrane leakage. Membrane disruption and DNA fragmentation with preferential oligonucleosomal cleavage, events characteristic of PCD, were found to be present in the advanced stage of petal senescence, indicating that plant and animal cell death phenomena share one of the molecular events in the execution phase. As in apoptosis in animals, both single-stranded DNase and double-stranded DNase activities are induced during petal cell death and are enhanced by Ca(2+). In contrast, the release of cytochrome c from mitochondria, one commitment step in signaling of apoptosis in animal cells, was found to be dispensable in petal cell death. Some components of the signal transduction pathway for PCD in plants are likely to differ from those in animal cells.

Isolation of a zeta class wheat glutathione S-transferase gene

A new Zeta class glutathione S-transferases (GST) gene, pGST, has been cloned from wheat for the first time by the differential display PCR (DD-PCR) method. The genomic sequence of pGST, TA-GSTZ1, contains nine exons that encode a polypeptide of 213 amino acids and eight introns. The deduced amino acid sequence of TA-GSTZ1 as well as the exon:intron placement are more similar to the GSTs of the Zeta class than to the two wheat GSTs reported earlier. The pGST cDNA gene product expressed in Escherichia coli and purified by affinity chromatography showed typical Zeta class GST and glutathione peroxidase activities. Sequence polymorphism in the 3' untranslated region (UTR) of TA-GSTZ1 gene in wheat has been discovered. In this study, an 89 bp sequence is present in the 3' UTR of TA-GSTZ1gene in 16 wheat cultivars but absent in the other five. Although the biological importance of this polymorphism is unknown, it can be useful as a genetic marker in wheat breeding.

Ethylene-regulated gene expression in tomato fruit: characterization of novel ethylene-responsive and ripening-related genes isolated by differential display

Differential display was used to isolate early ethylene-regulated genes from late immature green tomato fruit in order to obtain a broader understanding of the molecular basis by which ethylene coordinates the ripening process. Nineteen novel ethylene-responsive (ER) cDNA clones were isolated that fell into three classes: (i) ethylene up-regulated (ii) ethylene down-regulated, and (iii) transiently induced. Expression analysis revealed that ethylene-dependent changes in mRNA accumulation occurred rapidly (15 min) for most of the ER clones. The predicted proteins encoded by the ER genes are putatively involved in processes as diverse as primary metabolism, hormone signalling and stress responses. Although a number of the isolated ER clones correspond to genes already documented in other species, their responsiveness to ethylene is described here for the first time. Among the ER clones sharing high homology with regulatory genes, ER43, a putative GTP-binding protein, and ER50, a CTR1-like clone, are potentially involved in signal transduction. ER24 is homologous to the multi-protein bridging factor MBF1 involved in transcriptional activation, and finally, two clones are homologous to genes involved in post-transcriptional regulation: ER49, a putative translational elongation factor, and ER68, a mRNA helicase-like gene. Six ER clones correspond to as yet unidentified genes. The expression studies indicated that all the ER genes are ripening-regulated, and, depending on the clone, show changes in transcript accumulation either at the breaker, turning, or red stage. Analysis of transcript accumulation in different organs indicated a strong bias towards expression in the fruit for many of the clones. The potential roles for some of the ER clones in propagating the ethylene response and regulating fruit ripening are discussed.

Expression and inducibility of alpha, pi, and Mu glutathione S-transferase protein and mRNA in murine lung

This investigation sought to establish the cellular expression and distribution of the alpha, pi, and mu classes of glutathione S-transferase (GST) enzymes in murine lung under control conditions and after treatment with tert-butyl-4-hydroxyanisole (BHA). Immunohistochemical and in situ hybridization studies were used to identify lung cells that were labeled for the GST subunits Yp, Ya, and Yb1. Immunoblotting of cytosolic proteins produced single bands of 28, 29, and 31 kD for Ya, Yp, and Yb1, respectively, in samples from untreated and BHA-treated mice. Treatment with BHA increased Ya and Yp reactivity, but this was not as marked for Yb1. Immunohistochemical staining for the Yp, Ya, and Yb1 subunits was localized in bronchioles and parenchyma of untreated and BHA-treated mice. Bronchiolar Clara and alveolar type II cells were stained to the greatest extent for all of the GST subunits. BHA treatment produced increased staining that was most pronounced in the bronchiolar epithelium. Ya and Yp were localized in the cytoplasm and nucleus, whereas Yb1 was found mainly in the cytoplasm. Immunoblots of extracted nuclear proteins revealed a band of 29 kD for Ya, with increased immunoreactivity in BHA-treated mice. In situ hybridization done with oligonucleotide probes showed abundant silver grains representing Ya, Yp, and Yb1 messenger RNA (mRNA) transcripts in the bronchioles. Grains were also localized in alveolar septa, and were most numerous in type II cells. Quantitative image analysis confirmed good agreement between relative levels of protein and mRNA transcripts. Quantities of mRNA transcripts for all subunits were increased in the parenchyma by BHA treatment, but the magnitudes of induction were most striking for Ya and Yp in the bronchioles. These results demonstrated that Ya, Yp, and Yb1 reside in specific lung areas and cells, and that in induced states, their increased expression is accompanied by increased mRNA.

Glutathione S-transferases of the yeast Yarrowia lipolytica have unusually large molecular mass

Two similar glutathione S-transferases (GSTs), which do not bind to glutathione- or S-hexylglutathione-agarose affinity resins, have been purified from the yeast Yarrowia lipolytica. An approx. 400-fold purification was obtained by a combination of DEAE-Sephadex, phenyl-Sepharose, hydroxyapatite and Mono-Q anion-exchange chromatography. The native molecular mass of both proteins was estimated as approx. 110 kDa by both Superose-12 gel-filtration chromatography and non-denaturing electrophoresis. SDS/PAGE indicated a subunit mass of 50 kDa. Reverse-phase HPLC of purified proteins gave a single, well-resolved, peak, suggesting that the proteins are homodimers. Identical behaviour on HPLC, native electrophoresis and SDS/PAGE, N-terminal sequencing, sensitivity to a panel of inhibitors and identical specific activities with 1-chloro-2,4-dinitrobenzene as substrate suggest that the two isoenzymes are very similar. The enzymes do not immunoblot with antisera to any of the main GST classes, and N-terminal sequencing suggests no clear relationship with previously characterized enzymes, such as that of the fungus, Phanerochaete chrysosporium [Dowd, Buckley and Sheehan (1997) Biochem. J. 324, 243-248]. It is possible that the two isoenzymes arise as a result of post-translational modification of a single GST isoenzyme.

Zeta, a novel class of glutathione transferases in a range of species from plants to humans

Sequence alignment and phylogenetic analysis has identified a new subgroup of glutathione S-transferase (GST)-like proteins from a range of species extending from plants to humans. This group has been termed the Zeta class. An atomic model of the N-terminal domain suggests that the members of the Zeta class have a similar structure to that of other GSTs, binding glutathione in a similar orientation in the G site. Recombinant human GSTZ1-1 has been expressed in Escherichia coli and characterized. The protein is a dimer composed of 24.2 kDa subunits and has minimal glutathione-conjugating activity with ethacrynic acid and 7-chloro-4-nitrobenz-2-oxa-1, 3-diazole. Although low in comparison with other GSTs, GSTZ1-1 has glutathione peroxidase activity with t-butyl and cumene hydroperoxides. The members of the Zeta class have been conserved over a long evolutionary period, suggesting that they might have a role in the metabolism of a compound that is common in many living cells.

Molecular phylogeny of glutathione-S-transferases

The glutathione-S-transferase (GST) protein superfamily is currently composed of nearly 100 sequences. This study documents a greater phylogenetic diversity of GSTs than previously realized. Parsimony and distance phylogenetic methods of GST amino acid sequences yielded virtually the same results. There appear to be at least 25 groups (families) of GST-like proteins, as different from one another as are the currently recognized classes. This diversity will require the design of a new nomenclature for this large protein superfamily. There is one well-supported large clade containing the mammalian mu, pi, and alpha classes as well as GSTs from molluscs, helminths, nematodes, and arthropods.

Molecular characterization of Fdx1, a putidaredoxin-type [2Fe-2S] ferredoxin able to transfer electrons to the dioxin dioxygenase of Sphingomonas sp. RW1

Bacterium Sphingomonas sp. strain RW1 is, under aerobic conditions, able to degrade dibenzofuran and dibenzo-p-dioxin. The first step of the pathway is performed by a ring-dihydroxylating enzyme. Bunz and Cook have reported the purification and characterization of this dioxin dioxygenase and a ferredoxin able to transfer electrons to the dioxygenase [Bunz, P. V. & Cook, A. M. (1993) J. Bacteriol. 175, 6467-6475]. The gene encoding this [2Fe-2S] ferredoxin was identified by screening a genomic library constructed in pLAFR3 with a probe generated by a nested-PCR amplification. Primers for the amplification were designed based on the N-terminus sequence of the purified ferredoxin and on sequence comparisons with related proteins. Several cosmids were obtained and the ferredoxin gene, fdx1, was subcloned from one of them. The nucleotide sequence of a 4.6-kb DNA fragment encompassing the ferredoxin gene was determined. While in the case of all known multi-component dioxygenases, genes encoding the alpha and beta subunits are found to be contiguous with the gene of the specific electron carrier, the fdx1 gene in Sphingomonas sp. RW1 does not appear to be directly linked with the dioxin dioxygenase genes. Rather, it is clustered with genes apparently encoding two atypical decarboxylases/isomerases and a glutathione S-transferase. The ferredoxin gene was hyperexpressed and the recombinant ferredoxin was purified. Spectroscopic characterization of Fdx1 demonstrated the presence of a putidaredoxin-type [2Fe-2S] cluster in this protein. Its redox potential was determined to be -245 (+/- 5) mV versus the normal hydrogen electrode at 25 degrees C, pH 8.0. Therefore, the protein is closely related to [2Fe-2S] ferredoxins known to be electron donors to monooxygenases involved in hydroxylation of aromatic compounds. Thus, this report provides clear evidence that a putidaredoxin-type [2Fe-2S] ferredoxin, namely Fdx1, is able to transfer electrons to the dioxin dioxygenase of Sphingomonas sp. RW1.

POLLINATION REGULATION OF FLOWER DEVELOPMENT

▪ Abstract  Pollination regulates a syndrome of developmental responses that contributes to successful sexual reproduction in higher plants. Pollination-regulated developmental events collectively prepare the flower for fertilization and embryogenesis while bringing about the loss of floral organs that have completed their function in pollen dispersal and reception. Components of this process include changes in flower pigmentation, senescence and abscission of floral organs, growth and development of the ovary, and, in certain cases, pollination also triggers ovule and female gametophyte development in anticipation of fertilization. Pollination-regulated development is initiated by the primary pollination event at the stigma surface, but because developmental processes occur in distal floral organs, the activity of interorgan signals that amplify and transmit the primary pollination signal to floral organs is implicated. Interorgan signaling and signal amplification involves the regulation of ethylene biosynthetic gene expression and interorgan transport of hormones and their precursors. The coordination of pollination- regulated flower development including gametophyte, embryo, and ovary development; pollination signaling; the molecular regulation of ethylene biosynthesis; and interorgan communication are presented.

Glutathione S-transferases from the white-rot fungus, Phanerochaete chrysosporium

A glutathione S-transferase (GST) was purified to homogeneity from the white-rot fungus, Phanerochaete chrysosporium, by affinity chromatography on glutathione-agarose followed by Mono-Q ion-exchange FPLC. This protein immunoblotted with antisera to rat Theta class GST 5-5 and also showed N-terminal sequence similarity to the Theta class, including the presence of a conserved serine residue that has been specifically implicated in catalysis in this class [Wilce, Board, Feil and Parker (1995) EMBO J. 14, 2133-2143] and other residues conserved in plant sequences. Catalytic activity was found to be highly labile in the purified protein, although preliminary evidence for activity (approx. 120 m-units/mg) with 1,2-epoxy-3-(p-nitrophenoxy)propane was obtained in some preparations. The enzyme seems to be a dimer with a subunit molecular mass of 25 kDa by SDS/PAGE. The native molecular masses estimated by non-denaturing electrophoresis and by Superose-12 gel filtration were 58 and 45 kDa respectively. A second protein purified in this study also gave low level of activity with 1,2-epoxy-3-(p-nitrophenoxy)propane and had a subunit molecular mass of 28 kDa (native size 62-63 kDa), but did not immunoblot with any GST class and seemed to be N-terminally blocked.

Analysis of a Rhizobium leguminosarum gene encoding a protein homologous to glutathione S-transferases

A novel Rhizobium leguminosarum gene, gstA, the sequence of which indicated that it was a member of the gene family of glutathione S-transferases (GSTs), was identified. The homology was greatest to the GST enzymes of higher plants. The Rhizobium gstA gene was normally expressed at a very low level. The product of gstA was over-expressed and purified from Escherichia coli. It was shown to bind to the affinity matrix glutathione-Sepharose, but no enzymic GST activity with 1-chloro-2,4-dinitrobenzene as substrate was detected. gstA encoded a polypeptide of 203 amino acid residues with a calculated molecular mass of 21990 Da. Transcribed divergently from gstA is another gene, gstR, which was similar in sequence to the LysR family of bacterial transcriptional regulators. A mutation in gstR had no effect on the transcription of itself or gstA under the growth conditions used here. Mutations in gstA and gstR caused no obvious phenotypic defect and the biological functions of these genes remain to be determined.

A cDNA coding for glutathione S-transferase from the unicellular green algae Coccomyxa sp

A cDNA coding for glutathione S-transferase (GST) was cloned and sequenced from the unicellular green algae Coccomyxa sp. The predicted 215 amino acid polypeptide (23.9 kDa, pI 5.3) is most similar to the theta-type GSTs found in a variety of different eukaryotic organisms. Within this sub-class, the Coccomyxa GST is 42% identical (63% similar) to the flatfish Pleuronectes platessa homologue, and 24 to 35% (49-57%) to other theta-type GST's.

Environmental and developmental signals modulate proline homeostasis: evidence for a negative transcriptional regulator

In many plants, osmotic stress induces a rapid accumulation of proline through de novo synthesis from glutamate. This response is thought to play a pivotal role in osmotic stress tolerance [Kishor, P. B. K., Hong, Z., Miao, G.-H., Hu, C.-A. A. and Verma, D. P. S. (1995) Plant Physiol. 108, 1387-1394]. During recovery from osmotic stress, accumulated proline is rapidly oxidized to glutamate and the first step of this process is catalyzed by proline oxidase. We have isolated a full-length cDNA from Arabidopsis thaliana, At-POX, which maps to a single locus on chromosome 3 and that encodes a predicted polypeptide of 499 amino acids showing significant similarity with proline oxidase sequences from Drosophila and Saccharomyces cerevisiae (55.5% and 45.1%, respectively). The predicted location of the encoded polypeptide is the inner mitochondrial membrane. RNA gel blot analysis revealed that At-POX mRNA levels declined rapidly upon osmotic stress and this decline preceded proline accumulation. On the other hand, At-POX mRNA levels rapidly increased during recovery. Free proline, exogenously added to plants, was found to be an effective inducer of At-POX expression; indeed, At-POX was highly expressed in flowers and mature seeds where the proline level is higher relative to other organs of Arabidopsis. Our results indicate that stress- and developmentally derived signals interact to determine proline homeostasis in Arabidopsis.

THE FUNCTIONS AND REGULATION OF GLUTATHIONE S-TRANSFERASES IN PLANTS

Glutathione S-transferases (GSTs) play roles in both normal cellular metabolism as well as in the detoxification of a wide variety of xenobiotic compounds, and they have been intensively studied with regard to herbicide detoxification in plants. A newly discovered plant GST subclass has been implicated in numerous stress responses, including those arising from pathogen attack, oxidative stress, and heavy-metal toxicity. In addition, plant GSTs play a role in the cellular response to auxins and during the normal metabolism of plant secondary products like anthocyanins and cinnamic acid. This review presents the current knowledge about the functions of GSTs in regard to both herbicides and endogenous substrates. The catalytic mechanism of GST activity as well as the fate of glutathione S-conjugates are reviewed. Finally, a summary of what is known about the gene structure and regulation of plant GSTs is presented.

Three-dimensional structure of glutathione S-transferase from Arabidopsis thaliana at 2.2 A resolution: structural characterization of herbicide-conjugating plant glutathione S-transferases and a novel active site architecture

Glutathione S-transferases (GST) are a family of multifunctional enzymes involved in the metabolization of a broad variety of xenobiotics and reactive endogenous compounds. The interest in plant glutathione S-transferases may be attributed to their agronomic value, since it has been demonstrated that glutathione conjugation for a variety of herbicides is the major resistance and selectivity factor in plants. The three-dimensional structure of glutathione S-transferase from the plant Arabidopsis thaliana has been solved by multiple isomorphous replacement and multiwavelength anomalous dispersion techniques at 3 A resolution and refined to a final crystallographic R-factor of 17.5% using data from 8 to 2.2 A resolution. The enzyme forms a dimer of two identical subunits each consisting of 211 residues. Each subunit is characterized by the GST-typical modular structure with two spatially distinct domains. Domain I consists of a central four-stranded beta-sheet flanked on one side by two alpha-helices and on the other side by an irregular segment containing three short 3(10)-helices, while domain II is entirely helical. The dimeric molecule is globular with a prominent large cavity formed between the two subunits. The active site is located in a cleft situated between domains I and II and each subunit binds two molecules of a competitive inhibitor S-hexylglutathione. Both hexyl moieties are oriented parallel and fill the H-subsite of the enzyme's active site. The glutathione peptide of one inhibitor, termed productive binding, occupies the G-subsite with multiple interactions similar to those observed for other glutathione S-transferases, while the glutathione backbone of the second inhibitor, termed unproductive binding, exhibits only weak interactions mediated by two polar contacts. A most striking difference from the mammalian glutathione S-transferases, which share a conserved catalytic tyrosine residue, is the lack of this tyrosine in the active site of the plant glutathione S-transferase.

Amino acid sequencing, molecular cloning and modelling of the chick liver class-theta glutathione S-transferase CL1

Glutathione S-transferase CL1-2 heterodimers purified from 1-day-old chick livers were digested with Achromobacter proteinase I. The resulting fragments were separated for amino acid sequence analysis. Oligonucleotide probes were constructed based on sequence similarity to class-Theta glutathione S-transferases for PCR using a chicken liver cDNA library as template. A full-length clone (1725 bp) encoding a polypeptide comprising 261 amino acids was isolated. Including conservative substitutions, this protein has 70-73% sequence similarity with other mammalian class-Theta glutathione S-transferases. Based on known X-ray crystal structures of class-Alpha, -Mu and -Pi glutathione S-transferases, a model is constructed for the N-terminal 232 residues of CL1.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

Molecular analysis of pentachlorophenol degradation

A limited number of microorganisms have been described for their ability to partially degrade pentachlorophenol (PCP), or to completely mineralize it. Several years ago we chose one of these microorganisms, Flavobacterium sp. strain ATCC 39723, for use in a detailed molecular analysis of the catabolism of PCP. This strain was chosen because it had previously been studied in great detail for its growth characteristics in relation to degradation of PCP. In this paper we provide an overview of the degradation pathway of PCP to 2,6-dichloro-p-hydroquinone by Flavobacterium. The specific biochemical reactions and the genes encoding the enzymes are reviewed. The successful transformation and site specific mutagenesis of Flavobacterium, as well as the discovery of two new pcp alleles is also presented.

Cloning and characterization of maize herbicide safener-induced cDNAs encoding subunits of glutathione S-transferase isoforms I, II and IV

Several GSTs have been characterised in maize. GST I is a homodimer of 29 kDa subunits, GST II a hetrodimer of 27 kDa and 29 kDa subunits and GST IV a homodimer of 27 kDa subunits. We report the isolation and characterization of a herbicide-safener inducible cDNA clone, GST-27. Based on partial amino acid sequence, GST-27 encodes the 27 kDa subunit present in both glutathione S-transferase isoforms GST II and IV. Northern blotting was used to compare the expression patterns of GST-27 with that of GST-29. Transcripts corresponding to GST-27 were found to be constitutively expressed in RNA isolated from the root, but no expression was detected in RNA isolated from aerial parts of the plant. The application of herbicide safener caused a dramatic increase in the expression of GST-27 in all aerial plant parts tested. GST-29 was found to be constitutively expressed in RNA isolated from a number of maize tissues. The basal level of GST-29 expression showed a minimal increase upon herbicide safener treatment. Although a range of hormonal, environmental and physiological stimuli failed to elevate GST-27 levels, some increase in GST-27 mRNA was observed in the late stages of leaf senescence and after treatments resulting in phytotoxic effects.

An ethylene-responsive enhancer element is involved in the senescence-related expression of the carnation glutathione-S-transferase (GST1) gene

The increased production of ethylene during carnation petal senescence regulates the transcription of the GST1 gene encoding a subunit of glutathione-S-transferase. We have investigated the molecular basis for this ethylene-responsive transcription by examining the cis elements and trans-acting factors involved in the expression of the GST1 gene. Transient expression assays following delivery of GST1 5' flanking DNA fused to a beta-glucuronidase receptor gene were used to functionally define sequences responsible for ethylene-responsive expression. Deletion analysis of the 5' flanking sequences of GST1 identified a single positive regulatory element of 197 bp between -667 and -470 necessary for ethylene-responsive expression. The sequences within this ethylene-responsive region were further localized to 126 bp between -596 and -470. The ethylene-responsive element (ERE) within this region conferred ethylene-regulated expression upon a minimal cauliflower mosaic virus-35S TATA-box promoter in an orientation-independent manner. Gel electrophoresis mobility-shift assays and DNase I footprinting were used to identify proteins that bind to sequences within the ERE. Nuclear proteins from carnation petals were shown to specifically interact with the 126-bp ERE and the presence and binding of these proteins were independent of ethylene or petal senescence. DNase I footprinting defined DNA sequences between -510 and -488 within the ERE specifically protected by bound protein. An 8-bp sequence (ATTTCAAA) within the protected region shares significant homology with promoter sequences required for ethylene responsiveness from the tomato fruit-ripening E4 gene.