FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS

Investigating the plant response to cadmium exposure by proteomic and metabolomic approaches

Monitoring molecular dynamics of an organism upon stress is probably the best approach to decipher physiological mechanisms involved in the stress response. Quantitative analysis of proteins and metabolites is able to provide accurate information about molecular changes allowing the establishment of a range of more or less specific mechanisms, leading to the identification of major players in the considered pathways. Such tools have been successfully used to analyze the plant response to cadmium (Cd), a major pollutant capable of causing severe health issues as it accumulates in the food chain. We present a summary of proteomics and metabolomics works that contributed to a better understanding of the molecular aspects involved in the plant response to Cd. This work allowed us to provide a finer picture of general signaling, regulatory and metabolic pathways that appeared to be affected upon Cd stress. In particular, we conclude on the advantage of employing different approaches of global proteome- and metabolome-wide techniques, combined with more targeted analysis to answer molecular questions and unravel biological networks. Finally, we propose possible directions and methodologies for future prospectives in this field, as many aspects of the plant-Cd interaction remain to be discovered.

Characterization of a transport activity for long-chain peptides in barley mesophyll vacuoles

The plant vacuole is the largest compartment in a fully expanded plant cell. While only very limited metabolic activity can be observed within the vacuole, the majority of the hydrolytic activities, including proteolytic activities reside in this organelle. Since it is assumed that protein degradation by the proteasome results in the production of peptides with a size of 3-30 amino acids, we were interested to show whether the tonoplast exhibits a transport activity, which could deliver these peptides into the vacuole for final degradation. It is shown here that isolated barley mesophyll vacuoles take up peptides of 9-27 amino acids in a strictly ATP-dependent manner. Uptake is inhibited by vanadate, but not by NH(+)(4), while GTP could partially substitute for ATP. The apparent affinity for the 9 amino acid peptide was 15 μM, suggesting that peptides are efficiently transferred to the vacuole in vivo. Inhibition experiments showed that peptides with a chain length below 10 amino acids did not compete as efficiently as longer peptides for the uptake of the 9 amino acid peptide. Our results suggest that vacuoles contain at least one peptide transporter that belongs to the ABC-type transporters, which efficiently exports long-chain peptides from the cytosol into the vacuole for final degradation.

S-glutathionyl-(chloro)hydroquinone reductases: a new class of glutathione transferases functioning as oxidoreductases

Glutathione transferases (GSTs) are best known for transferring glutathione (GSH) to hydrophobic organic compounds, making the conjugates more soluble. However, the omega-class GSTs of animals and the lambda-class GSTs and dehydroascorbate reductases (DHARs) of plants have little or no activity for GSH transfer. Instead, they catalyze GSH-dependent oxidoreductions. The lambda-class GSTs reduce disulfide bonds, the DHARs reduce the disulfide bonds and dehydroascorbate, and the omega-class GSTs can reduce more substrates, including disulfide bonds, dehydroascorbate, and dimethylarsinate. Glutathionyl-(chloro)hydroquinone reductases (GS-HQRs) are the newest class of GSTs that mainly catalyze oxidoreductions. Besides the activities of the other three classes, GS-HQRs also reduce GS-hydroquinones, including GS-trichloro-p-hydroquinone, GS-dichloro-p-hydroquinone, GS-2-hydroxy-p-hydroquinone, and GS-p-hydroquinone. They are conserved and widely distributed in bacteria, fungi, protozoa, and plants, but not in animals. The four classes are phylogenetically more related to each other than to other GSTs, and they share a Cys-Pro motif at the GSH-binding site. Hydroquinones are metabolic intermediates of certain aromatic compounds. They can be auto-oxidized by O(2) to benzoquinones, which spontaneously react with GSH to form GS-hydroquinones via Michael's addition. GS-HQRs are expected to channel GS-hydroquinones, formed spontaneously or enzymatically, back to hydroquinones. When the released hydroquinones are intermediates of metabolic pathways, GS-HQRs play a maintenance role for the pathways. Further, the common presence of GS-HQRs in plants, green algae, cyanobacteria, and halobacteria suggest a beneficial role in the light-using organisms.

Gel-based and gel-free proteomic analysis of Nicotiana tabacum trichomes identifies proteins involved in secondary metabolism and in the (a)biotic stress response

Nicotiana tabacum leaves are covered by trichomes involved in the secretion of large amounts of secondary metabolites, some of which play a major role in plant defense. However, little is known about the metabolic pathways that operate in these structures. We undertook a proteomic analysis of N. tabacum trichomes in order to identify their protein complement. Efficient trichome isolation was obtained by abrading frozen leaves. After homogenization, soluble proteins and a microsomal fraction were prepared by centrifugation. Gel-based and gel-free proteomic analyses were then performed. 2-DE analysis of soluble proteins led to the identification of 1373 protein spots, which were digested and analyzed by MS/MS, leading to 680 unique identifications. Both soluble proteins and microsomal fraction were analyzed by LC MALDI-MS/MS after trypsin digestion, leading to 858 identifications, many of which had not been identified after 2-DE, indicating that the two methods complement each other. Many enzymes putatively involved in secondary metabolism were identified, including enzymes involved in the synthesis of terpenoid precursors and in acyl sugar production. Several transporters were also identified, some of which might be involved in secondary metabolite transport. Various (a)biotic stress response proteins were also detected, supporting the role of trichomes in plant defense.

Sequestration and transport of lignin monomeric precursors

Lignin is the second most abundant terrestrial biopolymer after cellulose. It is essential for the viability of vascular plants. Lignin precursors, the monolignols, are synthesized within the cytosol of the cell. Thereafter, these monomeric precursors are exported into the cell wall, where they are polymerized and integrated into the wall matrix. Accordingly, transport of monolignols across cell membranes is a critical step affecting deposition of lignin in the secondarily thickened cell wall. While the biosynthesis of monolignols is relatively well understood, our knowledge of sequestration and transport of these monomers is sketchy. In this article, we review different hypotheses on monolignol transport and summarize the recent progresses toward the understanding of the molecular mechanisms underlying monolignol sequestration and transport across membranes. Deciphering molecular mechanisms for lignin precursor transport will support a better biotechnological solution to manipulate plant lignification for more efficient agricultural and industrial applications of cell wall biomass.

Alteration of heart tissue protein profiles in acute cadmium-treated scallops Patinopecten yessoensis

Cadmium (Cd) is an extremely toxic metal that induces a wide spectrum of toxic responses in organisms in the environment. In the present study, scallops (Patinopecten yessoensis), after acclimation for 1 week in the laboratory, were subjected to acute Cd chloride (CdCl₂) toxicity, and ultramorphological and proteomic changes in their heart tissues were analyzed and compared with those of the nonexposed control group. Electron microscopy showed that ultrastructures of the cytoplasm and mitochondria in scallop hearts were badly damaged, and two-dimensional gel electrophoresis showed 32 protein spots that were differentially expressed after exposure to 10 mg/l CdCl₂ for 24 h. Of these spots, 8 were upregulated, 16 were downregulated, and 8 showed low expression. Proteins from these spots were identified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry and database searching. The results indicated that these proteins are involved in the regulation of cell structure, transport, signal transduction, and metabolism. Among other things, four proteins-identified as amino acid adenosine triphosphate (ATP)-binding cassette transporter, glycerol-3-phosphate dehydrogenase (nicotinamide adenine dinucleotide phosphate), nicotinamide adenine dinucleotide oxidase, and ATPase-were demonstrated to be especially associated with Cd toxicity. Some of the other proteins observed in this work are of particular interest in terms of their responses to Cd, which have not been reported previously. These data may provide novel biomarkers for monitoring the Cd contamination level of flowing seawater as well as provide useful insights into the mechanisms of Cd toxicity.

Transcriptome analysis of the barley-deoxynivalenol interaction: evidence for a role of glutathione in deoxynivalenol detoxification

Trichothecenes are a major group of toxins produced by phytopathogenic fungi, including Fusarium graminearum. Trichothecenes inhibit protein synthesis in eukaryotic cells and are toxicologically relevant mycotoxins for humans and animals. Because they promote plant disease, the role of host responses to trichothecene accumulation is considered to be an important aspect of plant defense and resistance to fungal infection. Our overall objective was to examine the barley response to application of the type B trichothecene deoxynivalenol (DON). We found that DON is diluted by movement from the application site to acropetal and basipetal florets. A susceptible barley genotype converted DON to DON-3-O-glucoside, indicating that UDP-glucosyltransferases capable of detoxifying DON must exist in barley. RNA profiling of DON-treated barley spikes revealed strong upregulation of gene transcripts encoding ABC transporters, UDP-glucosyltransferases, cytochrome P450s, and glutathione-S-transferases. We noted that transcripts encoding cysteine synthases were dramatically induced by DON, and that toxin-sensitive yeast on glutathione- or cysteine-supplemented media or carrying a gene that encodes a cysteine biosynthetic enzyme exhibit DON resistance, suggesting that preventing glutathione depletion by increasing cysteine supply could play a role in ameliorating the impact of DON. Evidence for nonenzymatic formation of DON-glutathione adducts in vitro was found using both liquid chromatography-mass spectrometry and nuclear magnetic resonance analysis, indicating that the formation of DON-glutathione conjugates in vivo may reduce the impact of trichothecenes. Our results indicate that barley exhibits multiple defense mechanisms against trichothecenes.

A novel role for Arabidopsis mitochondrial ABC transporter ATM3 in molybdenum cofactor biosynthesis

The molybdenum cofactor (Moco) is a prosthetic group required by a number of enzymes, such as nitrate reductase, sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. Its biosynthesis in eukaryotes can be divided into four steps, of which the last three are proposed to occur in the cytosol. Here, we report that the mitochondrial ABC transporter ATM3, previously implicated in the maturation of extramitochondrial iron-sulfur proteins, has a crucial role also in Moco biosynthesis. In ATM3 insertion mutants of Arabidopsis thaliana, the activities of nitrate reductase and sulfite oxidase were decreased to approximately 50%, whereas the activities of xanthine dehydrogenase and aldehyde oxidase, whose activities also depend on iron-sulfur clusters, were virtually undetectable. Moreover, atm3 mutants accumulated cyclic pyranopterin monophosphate, the first intermediate of Moco biosynthesis, but showed decreased amounts of Moco. Specific antibodies against the Moco biosynthesis proteins CNX2 and CNX3 showed that the first step of Moco biosynthesis is localized in the mitochondrial matrix. Together with the observation that cyclic pyranopterin monophosphate accumulated in purified mitochondria, particularly in atm3 mutants, our data suggest that mitochondria and the ABC transporter ATM3 have a novel role in the biosynthesis of Moco.

Response of ATP sulfurylase and serine acetyltransferase towards cadmium in hyperaccumulator Sedum alfredii Hance

We studied the responses of the activities of adenosine-triphosphate (ATP) sulfurylase (ATPS) and serine acetyltransferase (SAT) to cadmium (Cd) levels and treatment time in hyperaccumulating ecotype (HE) Sedum alfredii Hance, as compared with its non-hyperaccumulating ecotype (NHE). The results show that plant growth was inhibited in NHE but promoted in HE when exposed to high Cd level. Cd concentrations in leaves and shoots rapidly increased in HE rather than in NHE, and they became much higher in HE than in NHE along with increasing treatment time and Cd supply levels. ATPS activity was higher in HE than in NHE in all Cd treatments, and increased with increasing Cd supply levels in both HE and NHE when exposed to Cd treatment within 8 h. However, a marked difference of ATPS activity between HE and NHE was found with Cd treatment for 168 h, where ATPS activity increased in HE but decreased in NHE. Similarly, SAT activity was higher in HE than in NHE at all Cd treatments, but was more sensitive in NHE than in HE. Both ATPS and SAT activities in NHE leaves tended to decrease with increasing treatment time after 8 h at all Cd levels. The results reveal the different responses in sulfur assimilation enzymes and Cd accumulation between HE and NHE. With increasing Cd stress, the activities of sulfur assimilation enzymes (ATPS and SAT) were induced in HE, which may contribute to Cd accumulation in the hyperaccumulator Sedum alfredii Hance.

Plant Vacuolar ATP-binding Cassette Transporters That Translocate Folates and Antifolates in Vitro and Contribute to Antifolate Tolerance in Vivo

The vacuoles of pea (Pisum sativum) leaves and red beet (Beta vulgaris) storage root are major sites for the intracellular compartmentation of folates. In the light of these findings and preliminary experiments indicating that some plant multidrug resistance-associated protein (MRP) subfamily ATP-binding cassette transporters are able to transport compounds of this type, the Arabidopsis thaliana vacuolar MRP, AtMRP1 (AtABCC1), and its functional equivalent(s) in vacuolar membrane vesicles purified from red beet storage root were studied. In so doing, it has been determined that heterologously expressed AtMRP1 and its equivalents in red beet vacuolar membranes are not only competent in the transport of glutathione conjugates but also folate monoglutamates and antifolates as exemplified by pteroyl-l-glutamic acid and methotrexate (MTX), respectively. In agreement with the results of these in vitro transport measurements, analyses of atmrp1 T-DNA insertion mutants of Arabidopsis ecotypes Wassilewskia and Columbia disclose an MTX-hypersensitive phenotype. atmrp1 knock-out mutants are more sensitive than wild-type plants to growth retardation by nanomolar concentrations of MTX, and this is associated with impaired vacuolar antifolate sequestration. The vacuoles of protoplasts isolated from the leaves of Wassilewskia atmrp1 mutants accumulate 50% less [(3)H]MTX than the vacuoles of protoplasts from wild-type plants when incubated in media containing nanomolar concentrations of this antifolate, and vacuolar membrane-enriched vesicles purified from the mutant catalyze MgATP-dependent [(3)H]MTX uptake at only 40% of the capacity of the equivalent membrane fraction from wild-type plants. AtMRP1 and its counterparts in other plant species therefore have the potential for participating in the vacuolar accumulation of folates and related compounds.

Transgenic plants for phytoremediation of herbicides

Herbicides are economically important, but the non-point pollution that they cause may disrupt the surrounding environment. Phytoremediation of herbicides has been well studied using conventional plants. Transgenic plants produced for metabolizing herbicides and long-persisting pollutants can be used for phytoremediation of foreign chemicals in contaminated soil and water. The genes involved in the metabolism of chemical compounds can be isolated from various organisms, including bacteria, fungi, plants, and animals, and these genes are then introduced into candidate plants. Transgenic plants expressing mammalian P450s and the other enzymes showed tolerance and phytoremediation activity toward target herbicides. Transgenic plants can also enhance the absorption and detoxification of pollutants, thereby aiding the phytoremediation of contaminated environments.

Phytochelatins: peptides involved in heavy metal detoxification

Phytochelatins (PCs) are enzymatically synthesized peptides known to involve in heavy metal detoxification and accumulation, which have been measured in plants grown at high heavy metal concentrations, but few studies have examined the response of plants even at lower environmentally relevant metal concentrations. Recently, genes encoding the enzyme PC synthase have been identified in plants and other species enabling molecular biological studies to untangle the mechanisms underlying PC synthesis and its regulation. The present paper embodies review on recent advances in structure of PCs, their biosynthetic regulation, roles in heavy metal detoxification and/or accumulation, and PC synthase gene expression for better understanding of mechanism involved and to improve phytoremediation efficiency of plants for wider application.

Natural variation in an ABC transporter gene associated with seed size evolution in tomato species

Seed size is a key determinant of evolutionary fitness in plants and is a trait that often undergoes tremendous changes during crop domestication. Seed size is most often quantitatively inherited, and it has been shown that Sw4.1 is one of the most significant quantitative trait loci (QTLs) underlying the evolution of seed size in the genus Solanum—especially in species related to the cultivated tomato. Using a combination of genetic, developmental, molecular, and transgenic techniques, we have pinpointed the cause of the Sw4.1 QTL to a gene encoding an ABC transporter gene. This gene exerts its control on seed size, not through the maternal plant, but rather via gene expression in the developing zygote. Phenotypic effects of allelic variation at Sw4.1 are manifested early in seed development at stages corresponding to the rapid deposition of starch and lipids into the endospermic cells. Through synteny, we have identified the Arabidopsis Sw4.1 ortholog. Mutagenesis has revealed that this ortholog is associated with seed length variation and fatty acid deposition in seeds, raising the possibility that the ABC transporter may modulate seed size variation in other species. Transcription studies show that the ABC transporter gene is expressed not only in seeds, but also in other tissues (leaves and roots) and, thus, may perform functions in parts of the plants other than developing seeds. Cloning and characterization of the Sw4.1 QTL gives new insight into how plants change seed during evolution and may open future opportunities for modulating seed size in crop plants for human purposes.

Given fixed resources, plants have a choice whether to produce many small seeds or a few large seeds. In terms of reproductive fitness, there are costs and benefits to both strategies. As a result, plant species vary more than 100,000-fold in both seed size and seed output. The current study focuses on understanding the molecular and developmental basis of a single genetic locus (or quantitative trait locus) that determines seed size between the cultivated tomato and its wild relatives. We show that the cause of size variation can be traced to a gene encoding an ABC transporter protein. The gene apparently exercises its control on seed size through expression in the developing seeds and not the mother plant that nurtures those seeds. A comparison with the model plant Arabidopsis thaliana suggests that the ABC transporter identified in tomato may also control seed size in other plants, opening research opportunities for understanding plant adaptation and for potentially modulating seed size in crop plants for human purposes.

Glucosinolate metabolites required for an Arabidopsis innate immune response

The perception of pathogen or microbe-associated molecular pattern molecules by plants triggers a basal defense response analogous to animal innate immunity and is defined partly by the deposition of the glucan polymer callose at the cell wall at the site of pathogen contact. Transcriptional and metabolic profiling in Arabidopsis mutants, coupled with the monitoring of pathogen-triggered callose deposition, have identified major roles in pathogen response for the plant hormone ethylene and the secondary metabolite 4-methoxy-indol-3-ylmethylglucosinolate. Two genes, PEN2 and PEN3, are also necessary for resistance to pathogens and are required for both callose deposition and glucosinolate activation, suggesting that the pathogen-triggered callose response is required for resistance to microbial pathogens. Our study shows that well-studied plant metabolites, previously identified as important in avoiding damage by herbivores, are also required as a component of the plant defense response against microbial pathogens.

Binding and glutathione conjugation of porphyrinogens by plant glutathione transferases

Overexpression in Escherichia coli of a tau (U) class glutathione transferase (GST) from maize (Zea mays L.), termed ZmGSTU1, caused a reduction in heme levels and an accumulation of porphyrin precursors. This disruption was highly specific, with the expression of the closely related ZmGSTU2 or other maize GSTs having little effect. Expression in E. coli of a series of chimeric ZmGSTU1/ZmGSTU2 proteins identified domains responsible for disrupting porphyrin metabolism. In addition to known heme precursors, expression of ZmGSTU1 led to the accumulation of a novel glutathione conjugate of harderoporphyrin(ogen) (2,7,12,18-tetramethyl-3-vinylporphyrin-8,13,17-tripropionic acid). Using the related protoporphyrinogen as a substrate, conjugation could be shown to occur on one vinyl group and was actively catalyzed by the ZmGSTU. In plant transgenesis studies, the ZmGSTUs did not perturb porphyrin metabolism when expressed in the cytosol of Arabidopsis or tobacco. However, expression of a ZmGSTU1-ZmGSTU2 chimera in the chloroplasts of tobacco resulted in the accumulation of the harderoporphyrin(ogen)-glutathione conjugate observed in the expression studies in bacteria. Our results show that the well known ability of GSTs to act as ligand binding (ligandin) proteins of porphyrins in vitro results in highly specific interactions with porphyrinogen intermediates, which can be demonstrated in both plants and bacteria in vivo.

Rice-arsenate interactions in hydroponics: whole genome transcriptional analysis

Rice (Oryza sativa) varieties that are arsenate-tolerant (Bala) and -sensitive (Azucena) were used to conduct a transcriptome analysis of the response of rice seedlings to sodium arsenate (AsV) in hydroponic solution. RNA extracted from the roots of three replicate experiments of plants grown for 1 week in phosphate-free nutrient with or without 13.3 muM AsV was used to challenge the Affymetrix (52K) GeneChip Rice Genome array. A total of 576 probe sets were significantly up-regulated at least 2-fold in both varieties, whereas 622 were down-regulated. Ontological classification is presented. As expected, a large number of transcription factors, stress proteins, and transporters demonstrated differential expression. Striking is the lack of response of classic oxidative stress-responsive genes or phytochelatin synthases/synthatases. However, the large number of responses from genes involved in glutathione synthesis, metabolism, and transport suggests that glutathione conjugation and arsenate methylation may be important biochemical responses to arsenate challenge. In this report, no attempt is made to dissect differences in the response of the tolerant and sensitive variety, but analysis in a companion article will link gene expression to the known tolerance loci available in the BalaxAzucena mapping population.

Expression of the novel wheat gene TM20 confers enhanced cadmium tolerance to bakers' yeast

Cadmium causes the generation of reactive oxygen species, which in turn causes cell damage. We isolated a novel gene from a wheat root cDNA library, which conferred Cd(II)-specific tolerance when expressed in yeast (Saccharomyces cerevisiae). The gene, which we called TaTM20, for Triticum aestivum transmembrane 20, encodes a putative hydrophobic polypeptide of 889 amino acids, containing 20 transmembrane domains arranged as a 5-fold internal repeating unit of 4 transmembrane domains each. Expression of TaTM20 in yeast cells stimulated Cd(II) efflux resulting in a decrease in the content of yeast intracellular cadmium. TaTM20-induced Cd(II) tolerance was maintained in yeast even under conditions of reduced GSH. These results demonstrate that TaTM20 enhances Cd(II) tolerance in yeast through the stimulation of Cd(II) efflux from the cell, partially independent of GSH. Treatment of wheat seedlings with Cd(II) induced their expression of TaTM20, decreasing subsequent root Cd(II) accumulation and suggesting a possible role for TaTM20 in Cd(II) tolerance in wheat.

Comparative mutant analysis of Arabidopsis ABCC-type ABC transporters: AtMRP2 contributes to detoxification, vacuolar organic anion transport and chlorophyll degradation

The enormous metabolic plasticity of plants allows detoxification of many harmful compounds that are generated during biosynthetic processes or are present as biotic or abiotic toxins in their environment. Derivatives of toxic compounds such as glutathione conjugates are moved into the central vacuole via ATP-binding cassette (ABC)-type transporters of the multidrug resistance-associated protein (MRP) subfamily. The Arabidopsis genome contains 15 AtMRP isogenes, four of which (AtMRP1, 2, 11 and 12) cluster together in one of two major phylogenetic clades. We isolated T-DNA knockout alleles in all four highly homologous AtMRP genes of this clade and subjected them to physiological analysis to assess the function of each AtMRP of this group. None of the single atmrp mutants displayed visible phenotypes under control conditions. In spite of the fact that AtMRP1 and AtMRP2 had been described as efficient ATP-dependent organic anion transporters in heterologous expression experiments, the contribution of three of the AtMRP genes (1, 11 and 12) to detoxification is marginal. Only knockouts in AtMRP2 exhibited a reduced sensitivity towards 1-chloro-2,4-dinitrobenzene, but not towards other herbicides. AtMRP2 but not AtMRP1, 11 and 12 is involved in chlorophyll degradation since ethylene-treated rosettes of atmrp2 showed reduced senescence, and AtMRP2 expression is induced during senescence. This suggests that AtMRP2 is involved in vacuolar transport of chlorophyll catabolites. Vacuolar uptake studies demonstrated that transport of typical MRP substrates was reduced in atmrp2. We conclude that within clade I, only AtMRP2 contributes significantly to overall organic anion pump activity in vivo.

Expression differences for genes involved in lignin, glutathione and sulphate metabolism in response to cadmium in Arabidopsis thaliana and the related Zn/Cd-hyperaccumulator Thlaspi caerulescens

Cadmium (Cd) is a widespread, naturally occurring element present in soil, rock, water, plants and animals. Cd is a non-essential element for plants and is toxic at higher concentrations. Transcript profiles of roots of Arabidopsis thaliana (Arabidopsis) and Thlaspi caerulescens plants exposed to Cd and zinc (Zn) are examined, with the main aim to determine the differences in gene expression between the Cd-tolerant Zn-hyperaccumulator T. caerulescens and the Cd-sensitive non-accumulator Arabidopsis. This comparative transcriptional analysis emphasized the role of genes involved in lignin, glutathione and sulphate metabolism. Furthermore the transcription factors MYB72 and bHLH100 were studied for their involvement in metal homeostasis, as they showed an altered expression after exposure to Cd. The Arabidopsis myb72 knockout mutant was more sensitive to excess Zn or iron (Fe) deficiency than wild type, while Arabidopsis transformants overexpressing bHLH100 showed increased tolerance to high Zn and nickel (Ni) compared to wild-type plants, confirming their role in metal homeostasis in Arabidopsis.

Metabolic profiling using Fourier-transform ion-cyclotron-resonance mass spectrometry

With the aid of the extreme resolving power of Fourier-transform ion-cyclotron-resonance mass spectrometry (FT-ICR/MS), we have developed a metabolomics platform for high-throughput metabolic profiling and metabolite candidate identification integrating a data-processing system, the Dr.DMASS program ( http://kanaya.naist.jp/DrDMASS/ ), and a metabolite-species database, KNApSAcK ( http://kanaya.aist-nara.ac.jp/KNApSAcK/ ). We discuss the potential of this FT-ICR/MS-based metabolic profiling scheme as a general metabolomics tool by clarification of plant metabolic disorders and specific metabolite accumulation patterns caused by herbicidal enzyme inhibitors.

L-Rhamnose transport is sugar kinase (RhaK) dependent in Rhizobium leguminosarum bv. trifolii

Strains of Rhizobium leguminosarum which are unable to catabolize l-rhamnose, a methyl-pentose sugar, are compromised in the ability to compete for nodule occupancy versus wild-type strains. Previous characterization of the 11-kb region necessary for the utilization of rhamnose identified a locus carrying catabolic genes and genes encoding the components of an ABC transporter. Genetic evidence suggested that the putative kinase RhaK carried out the first step in the catabolism of rhamnose. Characterization of this kinase led to the observation that strains carrying rhamnose kinase mutations were unable to transport rhamnose into the cell. The absence of a functional rhamnose kinase did not stop the transcription and translation of the ABC transporter components. By developing an in vitro assay for RhaK activity, we have been able to show that (i) RhaK activity is consistent with RhaK phosphorylating rhamnose and (ii) biochemical activity of RhaK is necessary for rhamnose transport.

Facts and fiction about sulfur metabolism in relation to plant-pathogen interactions

Sulfur deficiency developed into a widespread nutrient disorder in the 1980s because of the drastic decrease of SO(2) emissions in western Europe after Clean Air Acts came into force. It was observed that not only the yield and quality of agricultural crops were negatively affected by sulfur deficiency but also their health status. Since the mid 1990s the physiological background of this latter phenomenon in the sulfur metabolism has been studied by different researchers. From 2001 until 2006, field trials with different varieties of oilseed rape were conducted in Germany, and also from 2001 until 2003 in Scotland, to investigate the underlying mechanisms of sulfur-induced resistance and to develop fertiliser strategies which increase the health status of crops and minimise the requirement for chemical fungicides. A comprehensive disease assessment was conducted and a range of different sulfur-containing metabolites and enzymes were analysed in relation to sulfur nutrition and fungal diseases. H2S emissions from field-grown crops under different sulfur nutritional status were studied for the first time and a positive relationship was observed. Besides S fertilisation, fungal infection increased H2S emissions, too. The studies deliver new insight into the complex of sulfur-induced resistance but many questions still remain open. This contribution will show different possible strategies to solve some of the open questions.

Novel tonoplast transporters identified using a proteomic approach with vacuoles isolated from cauliflower buds

Young meristematic plant cells contain a large number of small vacuoles, while the largest part of the vacuome in mature cells is composed by a large central vacuole, occupying 80% to 90% of the cell volume. Thus far, only a limited number of vacuolar membrane proteins have been identified and characterized. The proteomic approach is a powerful tool to identify new vacuolar membrane proteins. To analyze vacuoles from growing tissues we isolated vacuoles from cauliflower (Brassica oleracea) buds, which are constituted by a large amount of small cells but also contain cells in expansion as well as fully expanded cells. Here we show that using purified cauliflower vacuoles and different extraction procedures such as saline, NaOH, acetone, and chloroform/methanol and analyzing the data against the Arabidopsis (Arabidopsis thaliana) database 102 cauliflower integral proteins and 214 peripheral proteins could be identified. The vacuolar pyrophosphatase was the most prominent protein. From the 102 identified proteins 45 proteins were already described. Nine of these, corresponding to 46% of peptides detected, are known vacuolar proteins. We identified 57 proteins (55.9%) containing at least one membrane spanning domain with unknown subcellular localization. A comparison of the newly identified proteins with expression profiles from in silico data revealed that most of them are highly expressed in young, developing tissues. To verify whether the newly identified proteins were indeed localized in the vacuole we constructed and expressed green fluorescence protein fusion proteins for five putative vacuolar membrane proteins exhibiting three to 11 transmembrane domains. Four of them, a putative organic cation transporter, a nodulin N21 family protein, a membrane protein of unknown function, and a senescence related membrane protein were localized in the vacuolar membrane, while a white-brown ATP-binding cassette transporter homolog was shown to reside in the plasma membrane. These results demonstrate that proteomic analysis of highly purified vacuoles from specific tissues allows the identification of new vacuolar proteins and provides an additional view of tonoplastic proteins.

gamma-Glutamyl transpeptidase GGT4 initiates vacuolar degradation of glutathione S-conjugates in Arabidopsis

The xenobiotic monochlorobimane is conjugated to glutathione in the cytosol of Arabidopsis thaliana, transported to the vacuole, and hydrolyzed to cysteine S-bimane [Grzam, A., Tennstedt, P., Clemens, S., Hell, R. and Meyer, A.J. (2006) Vacuolar sequestration of glutathione S-conjugates outcompetes a possible degradation of the glutathione moiety by phytochelatin synthase. FEBS Lett. 580, 6384-6390]. The work here identifies gamma-glutamyl transpeptidase 4 (At4g29210, GGT4) as the first step of vacuolar degradation of glutathione conjugates. Hydrolysis of glutathione S-bimane is blocked in ggt4 null mutants of A. thaliana. Accumulation of glutathione S-bimane in mutants and in wild-type plants treated with the high affinity GGT inhibitor acivicin shows that GGT4 is required to initiate the two step hydrolysis sequence. GGT4:green fluorescent protein fusions were used to demonstrate that GGT4 is localized in the lumen of the vacuole.

Role of a Carboxylesterase in Herbicide Bioactivation in Arabidopsis thaliana

Arabidopsis thaliana contains multiple carboxyesterases (AtCXEs) with activities toward xenobiotics, including herbicide esters that are activated to their phytotoxic acids upon hydrolysis. On the basis of their susceptibility to inhibition by organophosphates, these AtCXEs are all serine hydrolases. Using a trifunctional probe bearing a fluorophosphonate together with biotin and rhodamine to facilitate detection and recovery, four dominant serine hydrolases were identified in the proteome of Arabidopsis. Using a combination of protein purification, capture with the trifunctional probe and proteomics, one of these hydrolases, AtCXE12, was shown to be the major carboxyesterase responsible for hydrolyzing the pro-herbicide methyl-2,4-dichlorophenoxyacetate (2,4-D-methyl) to the phytotoxic acid 2,4-dichlorophenoxyacetic acid. Recombinant expression of the other identified hydrolases showed that AtCXE12 was unique in hydrolyzing 2,4-D-methyl. To determine the importance of AtCXE12 in herbicide metabolism and efficacy, the respective tDNA knock-out (atcxe12) plants were characterized and shown to lack expression of AtCXE12 and have greatly reduced levels of 2,4-D-methyl-hydrolyzing activity. Young atcxe12 seedlings were less sensitive than wild type plants to 2,4-D-methyl, confirming a role for the enzyme in herbicide bioactivation in Arabidopsis.

Differential expression of phenylpropanoid and related genes in brown-midrib bm1, bm2, bm3, and bm4 young near-isogenic maize plants

The expression of phenylpropanoid and related genes was investigated in bm1, bm2, bm3, and bm4 near-isogenic maize plants at the 4-5 leaf stage using a gene-specific cell wall macro-array. The bm3 mutant, which is mutated in the caffeic acid O-methyltransferase (COMT) gene, exhibited the lowest number of differentially expressed genes. Although no other phenylpropanoid gene had an altered expression, two distinct OMT and two cytochrome P450 genes were overexpressed suggesting the activation of alternative hydroxylation/methylation pathways. The bm1 mutant had the highest number of differentially expressed genes, all of which were under-expressed. Bm1 mutant plants were affected not only in cinnamyl alcohol dehydrogenase (bm1 related CAD) gene expression as expected, but also in the expression of other CAD/SAD gene family members and several regulatory genes including MYB, ARGONAUTE and HDZip. As originally believed, the bm1 mutation could be localized at the CAD locus, but more probably in a gene that regulates the expression of the CAD gene family. The profile of under-expressed genes in the bm2 mutant is nearly similar to that of bm1. These genes fell under several functional categories including phenylpropanoid metabolism, transport and trafficking, transcription factors and regulatory genes. As the bm2 mutant exhibited a lower guaiacyl (G) unit lignin content, the bm2 mutation could affect a regulatory gene involved, perhaps indirectly, in the regulation, conjugation or transport of coniferaldehyde, or the establishment of G-rich maize tissues. The pattern of gene expression in bm4 plants, characterized by the over-expression of phenylpropanoid and methylation genes, suggests that the bm4 mutation likely also affects a gene involved in the regulation of lignification.

Identification of a cerulenin resistance gene from Monascus pilosus

Detoxification is essential for the fungal growth in the drug stress environments, and the multidrug transporters play an important role in this process. Here a cerulenin transporter gene (MpMdt, AB206476) was identified from Monascus pilosus. MpMdt mRNA contains 1951 bp and encodes a protein of 559 amino acid residues with 11 trans-membrane domains; and there is no difference in the sequence of MpMdt mRNA between the wild type M. pilosus IFO4520 and its cerulenin resistant mutant MK-1. Up-expression of MpMdt renders the cerulenin resistance of the mutant MK-1. Over-expression of MpMdt could also increase the cerulenin tolerance in the transgenic M. pilosus IFO4520. These results suggested that MpMdt is able to efflux-transport the anti-fungal antibiotic cerulenin and increase the cerulenin resistance of M. pilosus.

Glutathione conjugates in the vacuole are degraded by gamma-glutamyl transpeptidase GGT3 in Arabidopsis

gamma-Glutamyl transpeptidase (GGT) is the enzyme responsible for breaking the gamma-glutamyl bond between Glu and Cys in glutathione (GSH). We are using this gene family to study GSH degradation in plants. There are four putative GGT genes in Arabidopsis, and one of them, GGT3 (At4g29210), is analyzed in this study. GGT3 is localized to the vacuole based on organelle-targeting programs, subcellular distribution of GFP fusion proteins during transient expression in onion (Allium cepa) epidermal tissues, and its ability to metabolize vacuolar substrates in Arabidopsis plants. While Northern blots and promoter:GUS expression patterns have suggested that GGT3 is transcribed at relatively high levels in all parts of the plant, a comparison of enzyme activities in different organs of wild-type and a ggt3 knockout mutant showed that GGT3 was a major contributor to total GGT activity in roots, but a relatively minor contributor in other tissues. Wild-type Arabidopsis plants treated with monobromobimane (mBB) form a fluorescent GSH-mBB conjugate that is moved into the vacuole and then metabolized to Cys-Gly-mBB and Cys-mBB in that order. The first step is catalyzed by GGT3, and GSH-mBB metabolism is completely blocked in the roots of ggt3 knockout plants. In ggt3 leaves, some GSH-mBB metabolism still proceeds using the apoplastic GGT1. This identifies GGT3 as catalyzing the obligate initial step in GSH conjugate metabolism, and suggests that it has an important role in protecting plants from some xenobiotic chemicals.

Transport of primary metabolites across the plant vacuolar membrane

Mesophyll cells and most types of storage cells harbor large central vacuoles representing the main cellular store for sugars and other primary metabolites like carboxylic- or and amino acids. The general biochemical characteristics of sugar transport across the vacuolar membrane are already known since a couple of years but only recently the first tonoplast sugar carriers have been identified on the molecular level. A candidate sucrose carrier has been identified in a proteomic approach. In Arabidopsis, the tonoplast monosaccharide transporters (TMT) represent a small protein family comprising only three members, which reside in the vacuolar membrane. Two of three tmt genes are induced upon cold, drought or salt stress and tmt knock out mutants exhibit altered monosaccharide levels upon cold induction. These observations indicate that TMT proteins represent the first examples of tonoplast sugar carriers involved in the cellular response upon osmotic stress stimuli.

Function of phytochelatin synthase in catabolism of glutathione-conjugates

Detoxification of xenobiotic compounds and heavy metals is a pivotal capacity of organisms, in which glutathione (GSH) plays an important role. In plants, electrophilic herbicides are conjugated to the thiol group of GSH, and heavy metal ions form complexes as thiolates with GSH-derived phytochelatins (PCs). In both detoxification processes of plants, phytochelatin synthase (PCS) emerges as a key player. The enzyme is activated by heavy metal ions and catalyzes PC formation from GSH by transferring glutamylcysteinyl residues (gamma-EC) onto GSH. In this study with Arabidopsis, we show that PCS plays a role in the plant-specific catabolism of glutathione conjugates (GS-conjugates). In contrast to animals, breakdown of GS-conjugates in plants can be initiated by cleavage of the carboxyterminal glycine residue that leads to the generation of the corresponding gamma-EC-conjugate. We used the xenobiotic bimane in order to follow GS-conjugate turnover. Functional knockout of the two PCS of Arabidopsis, AtPCS1 and AtPCS2, revealed that AtPCS1 provides a major activity responsible for conversion of the fluorescent bimane-GS-conjugate (GS-bimane) into gamma-EC-bimane. AtPCS1 deficiency resulted in a gamma-EC-bimane deficiency. Transfection of PCS-deficient cells with AtPCS1 recovered gamma-EC-bimane levels. The level of the gamma-EC-bimane conjugate was enhanced several-fold in the presence of Cd2+ ions in the wild type, but not in the PCS-deficient double mutant, consistent with a PCS-catalyzed GS-conjugate turnover. Thus AtPCS1 has two cellular functions: mediating both heavy metal tolerance and GS-conjugate degradation.

Designing and transgenic expression of melanin gene in tobacco trichome and cotton fiber

In Streptomyces antibioticus, there are two genes TYRA and ORF438 required for the melanin biogenesis. To investigate whether expression of these two genes in cotton can change cotton fiber colour, we modified the TYRA and ORF438 genes to make their codon usage closer to the codon preference of cotton fiber genes. The resulting versions of these two genes were referred to as DTYRA and DORF438, respectively. Vacuolar targeting signals were also added to their ends. Under the cotton fiber specific LTP3 promoter, DORF438 and DTYRA were first transformed into model plant tobacco (Nicotiana tabacum). Molecular analyses showed that both the DORF438 and DTYRA genes were successfully expressed in transgenic plants, and the melanin deposition was observed in the trichomes of transgenic tobacco. Excitedly, when the same DORF438 and DTYRA expression cassettes were transformed into cotton (Gossypium hirsutum L.) by pollen tube pathway, the colour of cotton fiber changed from white to brown. Molecular analyses confirmed that both genes were transformed into cotton and expressed successfully. All these results indicate that the synthesized DOFR438 and DTYRA genes can work well in tobacco and cotton. Our study may provide a potential method for modifying the colour of cotton fiber.

Plant ATP-binding cassette transporters

The ATP-binding cassette (ABC) protein superfamily is one of the largest known, with over 120 members in both Arabidopsis thaliana and rice (Oryza sativa). Most, but not all, ABC proteins are modularly organized membrane proteins ("ABC transporters") that mediate MgATP-energized transmembrane transport and/or regulate other transporters. The range of processes in which members of the various subclasses of plant ABC transporters have been implicated encompasses polar auxin transport, lipid catabolism, xenobiotic detoxification, disease resistance, and stomatal function. Although it is often possible to predict the likely function of a plant ABC transporter on the basis of its subfamily membership, there are many whose capabilities deviate from what would be predicted from the properties of even their most sequence-related counterparts. When taking account of this and the disparate processes in which the few that have been characterized participate, it is likely that elucidation of the mechanistic basis of any given plant process will necessitate consideration of at least one ABC transporter.

Vacuolar sequestration of glutathione S-conjugates outcompetes a possible degradation of the glutathione moiety by phytochelatin synthase

Monochlorobimane was used as a model xenobiotic for Arabidopsis to directly monitor the compartmentation of glutathione-bimane conjugates in situ and to quantify degradation intermediates in vitro. Vacuolar sequestration of the conjugate was very fast and outcompeted carboxypeptidation to the gamma-glutamylcysteine-bimane intermediate (gamma-EC-B) by phytochelatin synthase (PCS) in the cytosol. Following vacuolar sequestration, degradation proceeded to cysteine-bimane without intermediate. Only co-infiltration of monochlorobimane with Cd2+ and Cu2+ increased gamma-EC-B formation to 4% and 25%, respectively, within 60 min. The role of PCS under simultaneous heavy metal stress was confirmed by investigation of different pcs1 null-mutants. In the absence of elevated heavy metal concentrations glutathione-conjugates are therefore first sequestered to the vacuole and subsequently degraded with the initial breakdown step being rate-limiting.

30-year progress of membrane transport in plants

In the past 30 years enormous progress was made in plant membrane biology and transport physiology, a fact reflected in the appearance of textbooks. The first book dedicated to 'Membrane Transport in Plants' was published on the occasion of the 'International Workshop on Membrane Transport in Plants' held at the Nuclear Research Center, Jülich, Germany [Zimmermann and Dainty (eds) 1974] and was followed in 1976 by a related volume 'Transport in plants II' in the 'Encyclopedia of plant physiology' [Lüttge and Pitman (eds) 1976]. A broad spectrum of topics including thermodynamics of transport processes, water relations, primary reactions of photosynthesis, as well as more conventional aspects of membrane transport was presented. The aim of the editors of the first book was to bring advanced thermodynamical concepts to the attention of biologists and to show physical chemists and biophysicist what the more complex biological systems were like. To bundle known data on membrane transport in plants and relevant fields for mutual understanding, interdisciplinary research and clarification of problems were considered highly important for further progress in this scientific area of plant physiology. The present review will critically evaluate the progress in research in membrane transport in plants that was achieved during the past. How did 'Membrane Transport in Plants' progress within the 30 years between the publication of the first book about this topic (Zimmermann and Dainty 1974), a recent one with the same title (Blatt 2004), and today?

The apical conjugate efflux pump ABCC2 (MRP2)

ABCC2 is a member of the multidrug resistance protein subfamily localized exclusively to the apical membrane domain of polarized cells, such as hepatocytes, renal proximal tubule epithelia, and intestinal epithelia. This localization supports the function of ABCC2 in the terminal excretion and detoxification of endogenous and xenobiotic organic anions, particularly in the unidirectional efflux of substances conjugated with glutathione, glucuronate, or sulfate, as exemplified by leukotriene C(4), bilirubin glucuronosides, and some steroid sulfates. The hepatic ABCC2 pump contributes to the driving forces of bile flow. Acquired or hereditary deficiency of ABCC2, the latter known as Dubin-Johnson syndrome in humans, causes an increased concentration of bilirubin glucuronosides in blood because of their efflux from hepatocytes via the basolateral ABCC3, which compensates for the deficiency in ABCC2-mediated apical efflux. In this article we provide an overview on the molecular characteristics of ABCC2 and its expression in various tissues and species. We discuss the transcriptional and posttranscriptional regulation of ABCC2 and review approaches to the functional analysis providing information on its substrate specificity. A comprehensive list of sequence variants in the human ABCC2 gene summarizes predicted and proven functional consequences, including variants leading to Dubin-Johnson syndrome.

The early responses of Arabidopsis thaliana cells to cadmium exposure explored by protein and metabolite profiling analyses

To get more insight into plant cell response to cadmium (Cd) stress, both proteomic and metabolomic "differential display" analyses were performed on Arabidopsis thaliana cells exposed to different concentrations of the toxic chemical. After a 24 h treatment, soluble proteins extracted from untreated and treated cells were separated by 2-D-PAGE and image analyses were performed to quantify and compare protein levels. Proteins up- and down-regulated in response to Cd were identified by MS and mapped into specific metabolic pathways and cellular processes, highlighting probable activation of the carbon, nitrogen, and sulfur metabolic pathways. For some of these proteins, Northern blot and RT-PCR analyses were performed to test transcript accumulation in response to Cd. In parallel, metabolite profiling analyses by LC coupled to ESI MS were initiated to better characterize the metabolic adaptation to the chemical stress. This study revealed that the main variation at the metabolite level came from the presence of six different families of phytochelatins, in A. thaliana cells treated with Cd, whose accumulation increases with Cd concentrations. Taken together these data provide an overview of the molecular and cellular changes elicited by Cd exposure.

AtATM3 is involved in heavy metal resistance in Arabidopsis

AtATM3, an ATP-binding cassette transporter of Arabidopsis (Arabidopsis thaliana), is a mitochondrial protein involved in the biogenesis of iron-sulfur clusters and iron homeostasis in plants. Our gene expression analysis showed that AtATM3 is up-regulated in roots of plants treated with cadmium [Cd(II)] or lead (II); hence, we investigated whether this gene is involved in heavy metal tolerance. We found that AtATM3-overexpressing plants were enhanced in resistance to Cd, whereas atatm3 mutant plants were more sensitive to Cd than their wild-type controls. Moreover, atatm3 mutant plants expressing 35S promoter-driven AtATM3 were more resistant to Cd than wild-type plants. Since previous reports often showed that the cytosolic glutathione level is positively correlated with heavy metal resistance, we measured nonprotein thiols (NPSH) in these mutant plants. Surprisingly, we found that atatm3 contained more NPSH than the wild type under normal conditions. AtATM3-overexpressing plants did not differ under normal conditions, but contained less NPSH than wild-type plants when exposed to Cd(II). These results suggest a role for AtATM3 in regulating cellular NPSH level, a hypothesis that was further supported by our gene expression study. Genetic or pharmacological inhibition of glutathione biosynthesis led to the elevated expression of AtATM3, whereas expression of the glutathione synthase gene GSH1 was increased under Cd(II) stress and in the atatm3 mutant. Because the closest homolog of AtATM3 in fission yeast (Schizosaccharomyces pombe), HMT1, is a vacuolar membrane-localized phytochelatin-Cd transporter, it is tempting to speculate that glutathione-Cd(II) complexes formed in the mitochondria are exported by AtATM3. In conclusion, our data show that AtATM3 contributes to Cd resistance and suggest that it may mediate transport of glutamine synthetase-conjugated Cd(II) across the mitochondrial membrane.

Expression analyses of Arabidopsis oligopeptide transporters during seed germination, vegetative growth and reproduction

AtOPT promoter-GUS fusions were constructed for six of the nine known, putative oligopeptide transporters (OPTs) in Arabidopsis thaliana and used to examine AtOPT expression at various stages of plant development. AtOPT1, AtOPT3, AtOPT4, AtOPT6 and AtOPT7 were expressed in the embryonic cotyledons prior to root radicle emergence. Except for AtOPT8, which gave weak expression, all AtOPTs were strongly expressed in post-germinative seedlings with strongest expression in vascular tissues of cotyledons and hypocotyls. Preferential expression of AtOPTs in vascular tissues was also observed in cotyledons, leaves, hypocotyls, roots, flowers, siliques, and seed funiculi of seedlings and adult plants. Differential tissue-specific expression was observed for specific AtOPTs. For example, AtOPT1, AtOPT3 and AtOPT8 were uniquely expressed in pollen. Only AtOPT1 was expressed in growing pollen tubes, while only AtOPT6 was observed in ovules. AtOPT8 was transiently expressed in seeds during early stages of embryogenesis. Iron limitation was found to enhance expression of AtOPT3. These data suggest distinct cellular roles for specific AtOPTs including nitrogen mobilization during germination and senescence, pollen tube growth, pollen and ovule development, seed formation and metal transport.

ABC transporters involved in the transport of plant secondary metabolites

Plants produce a large number of secondary metabolites, such as alkaloids, terpenoids, polyphenols, quinones and many further compounds having combined structures of those groups. Physiological roles of those metabolites for plants are still under investigation, but they play, at least in part, important functions as protectants for plant bodies against herbivores and pathogens, as well as from physical stresses like ultraviolet light and heat. In order to accomplish these functions, biosyntheses and accumulation of secondary metabolites are highly regulated in a temporal and spatial manner in plant organs, where they can appropriately accumulate. In this mini-review, I introduce the mechanism of accumulation and membrane transport of these metabolites, in particular, focusing on ATP-binding cassette transporters involved.

The multidrug resistance-associated protein (MRP/ABCC) subfamily of ATP-binding cassette transporters in plants

In many different plant species, genes belonging to the multidrug resistance-associated protein (MRP, ABCC) subfamily of ABC transporters have been identified. Following the discovery of vacuolar transport systems for xenobiotic or plant-produced conjugated organic anions, plant MRPs were originally proposed to be primarily involved in the vacuolar sequestration of potentially toxic metabolites. Indeed, heterologous expression of different Arabidopsis MRPs in yeast demonstrates their activity as ATP-driven pumps for structurally diverse substrates. Recent analysis of protein-protein interactions and the characterization of knockout mutants in Arabidopsis suggests that apart from transport functions plant MRPs play additional roles including the control of plant transpiration through the stomata. Here, we review and discuss the diverse functions of plant MRP-type ABC transporters and present an organ-related and developmental analysis of the expression of Arabidopsis MRPs using the publicly available full-genome chip data.

Glutathione homeostasis and redox-regulation by sulfhydryl groups

Continuous control of metabolism and developmental processes is a key feature of live cells. Cysteine thiol residues of proteins are both exceptionally useful in terms of structural and regulatory aspects, but at the same time exceptionally vulnerable to oxidation. Conserved cysteines thus are highly important for the function of metabolic enzymes and for signaling processes underlying responses to environmental factors. The underlying mechanism for the central role of thiol-mediated redox control in cellular metabolism is the ability of the cysteine-thiols to reversibly change their redox state followed by changes of structural, catalytic or regulatory functions. The cellular glutathione/glutathione disulfide redox buffer is present in cells at millimolar concentrations and forms one major basis of redox homeostasis by which protein thiols can maintain their redox state or oxidized protein thiols can be reverted to their reduced state. Besides acting as redox buffer, glutathione also acts as an electron donor for both scavenging of reactive oxygen, e.g. from photosynthesis and respiration, and metabolic reactions such as reduction of hydroperoxides and lipidperoxides or sulfate assimilation. The central role of glutathione is further emphasized by its involvement in signaling processes and the crosstalk of redox signaling processes with other means of signaling including protein glutathionylation and control of transcription factors. The present review aims at highlighting the key functions of glutathione in thiol-mediated redox control and its interplay with other protein-thiol-based redox systems.

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).

PGP4, an ATP binding cassette P-glycoprotein, catalyzes auxin transport in Arabidopsis thaliana roots

Members of the ABC (for ATP binding cassette) superfamily of integral membrane transporters function in cellular detoxification, cell-to-cell signaling, and channel regulation. More recently, members of the multidrug resistance P-glycoprotein (MDR/PGP) subfamily of ABC transporters have been shown to function in the transport of the phytohormone auxin in both monocots and dicots. Here, we report that the Arabidopsis thaliana MDR/PGP PGP4 functions in the basipetal redirection of auxin from the root tip. Reporter gene studies showed that PGP4 was strongly expressed in root cap and epidermal cells. PGP4 exhibits apolar plasma membrane localization in the root cap and polar localization in tissues above. Root gravitropic bending and elongation as well as lateral root formation were reduced in pgp4 mutants compared with the wild type. pgp4 exhibited reduced basipetal auxin transport in roots and a small decrease in shoot-to-root transport consistent with a partial loss of the redirective auxin sink in the root. Seedlings overexpressing PGP4 exhibited increased shoot-to-root auxin transport. Heterologous expression of PGP4 in mammalian cells resulted in 1-N-naphthylthalamic acid-reversible net uptake of [3H]indole-3-acetic acid. These results indicate that PGP4 functions primarily in the uptake of redirected or newly synthesized auxin in epidermal root cells.

Profiling candidate genes involved in wax biosynthesis in Arabidopsis thaliana by microarray analysis

Plant epidermal wax forms a hydrophobic layer covering aerial plant organs which constitutes a barrier against uncontrolled water loss and biotic stresses. Wax biosynthesis requires the coordinated activity of a large number of enzymes for the formation of saturated very-long-chain fatty acids and their further transformation in several aliphatic compounds. We found in the available database 282 candidate genes that may play a role in wax synthesis, regulation and transport. To identify the most interesting candidates, we measured the level of expression of 204 genes in the aerial parts of 15-day-old Arabidopsis seedlings by performing microarray experiments. We showed that only 25% of the putative candidates were expressed to significant levels in our samples, thus significantly reducing the number of genes which will be worth studying using reverse genetics to demonstrate their involvement in wax accumulation. We identified a beta-keto acyl-CoA synthase gene, At5g43760, which is co-regulated with the wax gene CER6 in a number of conditions and organs. By contrast, we showed that neither the fatty acyl-CoA reductase genes nor the wax synthase genes were expressed in 15-day-old leaves and stems, raising questions about the identity of the enzymes involved in the acyl-reduction pathway that accounts for 20% of the total wax amount.

Characterization of electrogenic bromosulfophthalein transport in carnation petal microsomes and its inhibition by antibodies against bilitranslocase

Bilitranslocase is a rat liver plasma membrane carrier, displaying a high-affinity binding site for bilirubin. It is competitively inhibited by grape anthocyanins, including aglycones and their mono- and di-glycosylated derivatives. In plant cells, anthocyanins are synthesized in the cytoplasm and then translocated into the central vacuole, by mechanisms yet to be fully characterized. The aim of this work was to determine whether a homologue of rat liver bilitranslocase is expressed in carnation petals, where it might play a role in the membrane transport of anthocyanins. The bromosulfophthalein-based assay of rat liver bilitranslocase transport activity was implemented in subcellular membrane fractions, leading to the identification of a bromosulfophthalein carrier (K(M) = 5.3 microm), which is competitively inhibited by cyanidine 3-glucoside (Ki = 51.6 microm) and mainly noncompetitively by cyanidin (Ki = 88.3 microm). Two antisequence antibodies against bilitranslocase inhibited this carrier. In analogy to liver bilitranslocase, one antibody identified a bilirubin-binding site (Kd = 1.7 nm) in the carnation carrier. The other antibody identified a high-affinity binding site for cyanidine 3-glucoside (Kd = 1.7 microm) on the carnation carrier only, and a high-affinity bilirubin-binding site (Kd = 0.33 nm) on the liver carrier only. Immunoblots showed a putative homologue of rat liver bilitranslocase in both plasma membrane and tonoplast fractions, isolated from carnation petals. Furthermore, only epidermal cells were immunolabeled in petal sections examined by microscopy. In conclusion, carnation petals express a homologue of rat liver bilitranslocase, with a putative function in the membrane transport of secondary metabolites.