ABC transporters involved in the transport of plant secondary metabolites

Functional definition and global regulation of Zur, a zinc uptake regulator in a Streptococcus suis serotype 2 strain causing streptococcal toxic shock syndrome

Zinc is an essential trace element for all living organisms and plays pivotal roles in various cellular processes. However, an excess of zinc is extremely deleterious to cells. Bacteria have evolved complex machineries (such as efflux/influx systems) to control the concentration at levels appropriate for the maintenance of zinc homeostasis in cells and adaptation to the environment. The Zur (zinc uptake regulator) protein is one of these functional members involved in the precise control of zinc homeostasis. Here we identified a zur homologue designated 310 from Streptococcus suis serotype 2, strain 05ZYH33, a highly invasive isolate causing streptococcal toxic shock syndrome. Biochemical analysis revealed that the protein product of gene 310 exists as a dimer form and carries zinc ions. An isogenic gene replacement mutant of gene 310, the Delta310 mutant, was obtained by homologous recombination. Physiological tests demonstrated that the Delta310 mutant is specifically sensitive to Zn(2+), while functional complementation of the Delta310 mutant can restore its duration capability, suggesting that 310 is a functional member of the Zur family. Two-dimensional electrophoresis indicated that nine proteins in the Delta310 mutant are overexpressed in comparison with those in the wild type. DNA microarray analyses suggested that 121 genes in the Delta310 mutant are affected, of which 72 genes are upregulated and 49 are downregulated. The transcriptome of S. suis serotype 2 with high Zn(2+) concentrations also showed 117 differentially expressed genes, with 71 upregulated and 46 downregulated. Surprisingly, more than 70% of the genes differentially expressed in the Delta310 mutant were the same as those in S. suis serotype 2 that were differentially expressed in response to high Zn(2+) concentration, consistent with the notion that 310 is involved in zinc homeostasis. We thus report for the first time a novel zinc-responsive regulator, Zur, from Streptococcus suis serotype 2.

Tracking monolignols during wood development in lodgepole pine

Secondary xylem (wood) formation in gymnosperms requires that the tracheid protoplasts first build an elaborate secondary cell wall from an array of polysaccharides and then reinforce it with lignin, an amorphous, three-dimensional product of the random radical coupling of monolignols. The objective of this study was to track the spatial distribution of monolignols during development as they move from symplasm to apoplasm. This was done by feeding [(3)H]phenylalanine ([(3)H]Phe) to dissected cambium/developing wood from lodgepole pine (Pinus contorta var latifolia) seedlings, allowing uptake and metabolism, then rapidly freezing the cells and performing autoradiography to detect the locations of the monolignols responsible for lignification. Parallel experiments showed that radioactivity was incorporated into polymeric lignin and a methanol-soluble pool that was characterized by high-performance liquid chromatography. [(3)H]Phe was incorporated into expected lignin precursors, such as coniferyl alcohol and p-coumaryl alcohol, as well as pinoresinol. Coniferin, the glucoside of coniferyl alcohol, was detected by high-performance liquid chromatography but was not radioactively labeled. With light microscopy, radiolabeled phenylpropanoids were detected in the rays as well as the tracheids, with the two cell types showing differential sensitivity to inhibitors of protein translation and phenylpropanoid metabolism. Secondary cell walls of developing tracheids were heavily labeled when incubated with [(3)H]Phe. Inside the cell, cytoplasm was most strongly labeled followed by Golgi and low-vacuole label. Inhibitor studies suggest that the Golgi signal could be attributed to protein, rather than phenylpropanoid, origins. These data, produced with the best microscopy tools that are available today, support a model in which unknown membrane transporters, rather than Golgi vesicles, export monolignols.

Transcript profiling in Vitis riparia during chilling requirement fulfillment reveals coordination of gene expression patterns with optimized bud break

Endodormant grapevine buds require a period of chilling before they break and begin to grow. Custom Vitis bud cDNA microarrays (9,216 features) were used to examine gene expression patterns in overwintering Vitis riparia buds during 2,000 h of 4 degrees C chilling. Three-node cuttings collected concurrently with buds were monitored to determine dormancy status. Chilling requirement was fulfilled after 1,500 h of chilling; however, 2,000 h of chilling significantly increased the rate of bud break. Microarray analysis identified 1,469 significantly differentially expressed (p value < 0.05) array features when 1,000, 1,500, and 2,000 h of chilling were compared to 500 h of chilling. Functional classification revealed that the majority of genes were involved in metabolism, cell defense/stress response, and genetic information processing. The number of significantly differentially expressed genes increased with chilling hour accumulation. The expression of a group of 130 genes constantly decreased during the chilling period. Up-regulated genes were not detected until the later stages of chilling accumulation. Hierarchical clustering of non-redundant expressed sequence tags revealed inhibition of genes involved in carbohydrate and energy metabolism and activation of genes involved in signaling and cell growth. Clusters with expression patterns associated with increased chilling and bud break were identified, indicating several candidate genes that may serve as indicators of bud chilling requirement fulfillment.

Global patterns of gene expression in rice cultivars undergoing a susceptible or resistant interaction with the parasitic plant Striga hermonthica

Striga hermonthica is a root hemiparasite of cereals that causes devastating loss of yield. Recently, a rice cultivar, Nipponbare, was discovered, which exhibits post-attachment resistance to this parasite and quantitative trait loci (QTL) associated with the resistance were identified. Changes in gene expression in susceptible (IAC 165) and resistant (Nipponbare) rice cultivars were profiled using rice whole-genome microarrays. In addition to a functional categorization of changes in gene expression, genes that were significantly up-regulated within resistance QTL were identified. The resistance reaction was characterized by up-regulation of defence genes, including pathogenesis-related proteins, pleiotropic drug resistance ABC transporters, genes involved in phenylpropanoid metabolism and WRKY transcription factors. These changes in gene expression resemble those associated with resistance to microbial pathogens. Three genes encoding proteins of unknown function, within a major resistance QTL on chromosome 12, were highly up-regulated and are excellent candidate resistance genes. The susceptible interaction was characterized by large-scale down-regulation of gene expression, particularly within the functional categories plant growth regulator signalling and metabolism, biogenesis of cellular components and cell division. Up-regulated genes included nutrient transporters, enzymes of amino acid metabolism and some abiotic stress genes.

Gene expression and metabolism in tomato fruit surface tissues

The cuticle, covering the surface of all primary plant organs, plays important roles in plant development and protection against the biotic and abiotic environment. In contrast to vegetative organs, very little molecular information has been obtained regarding the surfaces of reproductive organs such as fleshy fruit. To broaden our knowledge related to fruit surface, comparative transcriptome and metabolome analyses were carried out on peel and flesh tissues during tomato (Solanum lycopersicum) fruit development. Out of 574 peel-associated transcripts, 17% were classified as putatively belonging to metabolic pathways generating cuticular components, such as wax, cutin, and phenylpropanoids. Orthologs of the Arabidopsis (Arabidopsis thaliana) SHINE2 and MIXTA-LIKE regulatory factors, activating cutin and wax biosynthesis and fruit epidermal cell differentiation, respectively, were also predominantly expressed in the peel. Ultra-performance liquid chromatography coupled to a quadrupole time-of-flight mass spectrometer and gas chromatography-mass spectrometry using a flame ionization detector identified 100 metabolites that are enriched in the peel tissue during development. These included flavonoids, glycoalkaloids, and amyrin-type pentacyclic triterpenoids as well as polar metabolites associated with cuticle and cell wall metabolism and protection against photooxidative stress. Combined results at both transcript and metabolite levels revealed that the formation of cuticular lipids precedes phenylpropanoid and flavonoid biosynthesis. Expression patterns of reporter genes driven by the upstream region of the wax-associated SlCER6 gene indicated progressive activity of this wax biosynthetic gene in both fruit exocarp and endocarp. Peel-associated genes identified in our study, together with comparative analysis of genes enriched in surface tissues of various other plant species, establish a springboard for future investigations of plant surface biology.

SalK/SalR, a two-component signal transduction system, is essential for full virulence of highly invasive Streptococcus suis serotype 2

Background

Streptococcus suis serotype 2 (S. suis 2, SS2) has evolved into a highly infectious entity, which caused the two recent large-scale outbreaks of human SS2 epidemic in China, and is characterized by a toxic shock-like syndrome. However, the molecular pathogenesis of this new emerging pathogen is still poorly understood.

Methodology/Principal Findings

89K is a newly predicted pathogenicity island (PAI) which is specific to Chinese epidemic strains isolated from these two SS2 outbreaks. Further bioinformatics analysis revealed a unique two-component signal transduction system (TCSTS) located in the candidate 89K PAI, which is orthologous to the SalK/SalR regulatory system of Streptococcus salivarius. Knockout of salKR eliminated the lethality of SS2 in experimental infection of piglets. Functional complementation of salKR into the isogenic mutant ΔsalKR restored its soaring pathogenicity. Colonization experiments showed that the ΔsalKR mutant could not colonize any susceptible tissue of piglets when administered alone. Bactericidal assays demonstrated that resistance of the mutant to polymorphonuclear leukocyte (PMN)-mediated killing was greatly decreased. Expression microarray analysis exhibited a transcription profile alteration of 26 various genes down-regulated in the ΔsalKR mutant.

Conclusions/Significance

These findings suggest that SalK/SalR is requisite for the full virulence of ethnic Chinese isolates of highly pathogenic SS2, thus providing experimental evidence for the validity of this bioinformatically predicted PAI.

Proteomic analysis on a high salt tolerance introgression strain of Triticum aestivum/Thinopyrum ponticum

Soil salinity is a major abiotic constraint to agricultural productivity. We successfully bred a new common wheat (Triticum aestivum L.) introgression variety (Shanrong No. 3) with high salt-tolerance via asymmetric somatic hybridization between common wheat cultivar (Jinan 177) and UV-irradiated Agropyron elongatum (Thinopyrum ponticum Podp). We report here a comparative proteomic analysis to investigate variety-specific and salt-responsive proteins between seedling-roots of Shanrong No. 3 and Jinan 177. In total, 114 spots reproducibly presented differential expression patterns on 2-DE maps. Of them, 34 were variety-specific and 49 were salt-responsive. We identified 110 spots by MALDI-TOF MS and partially confirmed by MALDI-TOF-TOF MS, and functionally classified them into signal transduction, transcription and translation, transporting, chaperones, proteolysis and detoxification, etc. Meanwhile, we also found the alteration of protein expression of Shanrong No. 3 through inhibition of old proteins and production of novel ones, change in abundance and sensitivity of some nonsalt-responsive and salt-responsive proteins, as well as PTMs. Furthermore, comparison between proteome and transcripteome using cDNA microarray showed that there were only 20 proteins with abundances correlative to signal densities of corresponding EST probes. This study gives us a global insight into proteomic difference between Shanrong No. 3 and Jinan 177 in constitute and to salt-response.

Plant ABC proteins--a unified nomenclature and updated inventory

The ABC superfamily comprises both membrane-bound transporters and soluble proteins involved in a broad range of processes, many of which are of considerable agricultural, biotechnological and medical potential. Completion of the Arabidopsis and rice genome sequences has revealed a particularly large and diverse complement of plant ABC proteins in comparison with other organisms. Forward and reverse genetics, together with heterologous expression, have uncovered many novel roles for plant ABC proteins, but this progress has been accompanied by a confusing proliferation of names for plant ABC genes and their products. A consolidated nomenclature will provide much-needed clarity and a framework for future research.

The Arabidopsis DESPERADO/AtWBC11 transporter is required for cutin and wax secretion

The cuticle fulfills multiple roles in the plant life cycle, including protection from environmental stresses and the regulation of organ fusion. It is largely composed of cutin, which consists of C(16-18) fatty acids. While cutin composition and biosynthesis have been studied, the export of cutin monomers out of the epidermis has remained elusive. Here, we show that DESPERADO (AtWBC11) (abbreviated DSO), encoding a plasma membrane-localized ATP-binding cassette transporter, is required for cutin transport to the extracellular matrix. The dso mutant exhibits an array of surface defects suggesting an abnormally functioning cuticle. This was accompanied by dramatic alterations in the levels of cutin monomers. Moreover, electron microscopy revealed unusual lipidic cytoplasmatic inclusions in epidermal cells, disappearance of the cuticle in postgenital fusion areas, and altered morphology of trichomes and pavement cells. We also found that DSO is induced by salt, abscisic acid, and wounding stresses and its loss of function results in plants that are highly susceptible to salt and display reduced root branching. Thus, DSO is not only essential for developmental plasticity but also plays a vital role in stress responses.

Cytological and biochemical analysis of COF1, an Arabidopsis mutant of an ABC transporter gene

In transposon-tagged lines of Arabidopsis we found two new mutants, cof1-1 and cof1-2 (cuticular defect and organ fusion), that show the phenotype of wilting when grown in soil, organ fusion of rosette leaves and infertility. Toluidine blue testing and scanning electron microscopy observation revealed that these mutants had cuticular defects in the stems and adult leaves, but not in cotyledones. Transmission electron microscopy observation revealed thinner cuticle layers in the mutants, and cuticular materials interspersed between the two fused epidermal layers were observed in the mutant rosette leaves. These two mutants had a transposon insertion in the coding regions of WBC11, which was classified as a member of ABC transporter genes in Arabidopsis. WBC11 showed high sequence similarity to CER5 (also called WBC12), which was involved in cuticular lipid export. Gas chromatographic analysis revealed that C29 alkane extracted from the stem surface of cof1 mutants was reduced whereas C29 ketone was accumulated, which was different from the case of cer5 mutants. While cer5 mutants had fairly normal morphology, cof1 mutants had pleiotropic phenotypes so that COF1/WBC11 could have important roles different from those of CER5/WBC12. We also found that C29 alkane was accumulated in the intracellular extract of cof1 mutants, suggesting a function for WBC11 in the direct transport of lipid molecules. Pollen observation showed that mutant pollen grains were irregularly shaped. The function of COF1/WBC11 in lipid transport for the construction of cuticle layers and pollen coats for normal organ formation is discussed.

Flavonoids are differentially taken up and transported long distances in Arabidopsis

Flavonoids are synthesized in response to developmental and environmental signals and perform many functions in plants. Arabidopsis (Arabidopsis thaliana) roots grown in complete darkness do not accumulate flavonoids since the expression of genes encoding enzymes of flavonoid biosynthesis is light dependent. Yet, flavonoids accumulate in root tips of plants with light-grown shoots and light-shielded roots, consistent with shoot-to-root flavonoid movement. Using fluorescence microscopy, a selective flavonoid stain, and localized aglycone application to transparent testa mutants, we showed that flavonoids accumulated in tissues distal to the application site, indicating uptake and movement systems. This was confirmed by time-course fluorescence experiments and high-performance liquid chromatography. Flavonoid applications to root tips resulted in basipetal movement in epidermal layers, with subsequent fluorescence detected 1 cm from application sites after 1 h. Flavonoid application to midroot or cotyledons showed movement of flavonoids toward the root tip mainly in vascular tissue. Naringenin, dihydrokaempferol, and dihydroquercetin were taken up at the root tip, midroot, or cotyledons and traveled long distances via cell-to-cell movement to distal tissues, followed by conversion to quercetin and kaempferol. In contrast, kaempferol and quercetin were only taken up at the root tip. Using ATP-binding cassette (ABC) transporter and H(+)-ATPase inhibitors suggested that a multidrug resistance-associated protein ABCC transporter facilitated flavonoid movement away from the application site.

Novel insight into the regulation of GSH biosynthesis in higher plants

In higher plants, the redox-active tripeptide glutathione (GSH) fulfills a plethora of functions. These include its pivotal role for maintaining the cellular redox poise and its involvement in detoxification of heavy metals and xenobiotics. Intimately linked to these functions, GSH also acts as a cellular signal, mediating control of enzyme and/or regulatory protein activities, either directly or via glutaredoxins. The redox potential of the GSH/GSSG couple is not only affected by the GSH/GSSG ratio but also by changes in GSH synthesis and/or degradation. As this couple operates as redox buffer in several cellular compartments, the regulation of GSH biosynthesis and transport (both intra- and intercellularly) are fundamental to the maintenance of cellular redox homeostasis during plant development and, even more so, when plants are exposed to biotic or abiotic stress. This review highlights novel aspects of GSH biosynthesis and transport with a focus on the regulation of the GSH1 (= gamma-glutamylcysteine synthetase) enzyme. Interestingly, GSH1 appears to be exclusively confined to the plastids, whereas the second biosynthetic enzyme, GSH2, is predominantly localized in the cytosol. GSH1 expression and enzyme activity are under multiple controls, extending from transcriptional regulation to post-translational redox control. Now that the plant GSH1 protein structure has been solved, the molecular basis of GSH1 function and redox regulation can be addressed. The review concludes with a discussion of the simultaneous changes observed for GSH synthesis, transport, and metabolism during Cd-induced phytochelatin accumulation.

Mitochondrial ATP-binding cassette proteins

The family of ATP-binding cassette (ABC) proteins is among the largest and most diverse in biology. Members of this family are transmembrane proteins found in all organisms and all biologic membranes from the plasma membrane to intracellular organelles such as the Golgi apparatus, lysosomes, peroxisomes, endoplasmic reticulum, and mitochondria. These proteins are very abundant in bacteria, and given the generally accepted origin of mitochondria from an alpha-proteobacterium, it is logical to assume the mitochondria would also contain these proteins. Mitochondria, however, have surprisingly few ABC proteins and they are dissimilar from those of bacteria. Despite their relative paucity, mitochondrial ABC proteins are believed to play a very important role in cellular homeostasis across very diverse species, including yeast, higher plants, mice, and humans. The yeast protein Atm1p plays a critical role in the transport of Fe/S clusters to the cytosol, and a similar function has been attributed to the homologous human proteins MTABC3 and ABC7. Another yeast protein Mdl1p is a high copy suppressor of ATM1, and regulates cellular resistance to oxidative stress and may be involved in peptide transport across the mitochondrial membrane. The human protein mABC1 has recently been identified to be involved in protection of myocardial cells against oxidative stress. Despite their low numbers, mitochondrial ABC proteins are intricately involved in mitochondrial and cellular homeostasis and may be important mediators of cell survival. In this review, we will discuss the structure, function, physiology, and pathophysiology of these mitochondrial ABC proteins.