Evidence for the thiamine biosynthetic pathway in higher-plant plastids and its developmental regulation

Dual function of rice OsDR8 gene in disease resistance and thiamine accumulation

The function of OsDR8, a rice disease resistance-responsive gene, was studied. Silencing of OsDR8 using an RNA interference approach resulted in phenotypic alteration of the plants. The transgenic plants with repressed expression of OsDR8 showed reduced resistance or susceptibility to Xanthomonas oryzae pv. oryzae and Magnaporthe grisea causing bacterial blight and blast, which are two of the most devastating diseases in rice worldwide, respectively. The putative product of OsDR8 was highly homologous to an enzyme involved in the biosynthesis of the thiazole precursor of thiamine. Transgenic plants showing repressed expression of OsDR8 and reduced resistance had significantly lower levels of thiamine than the control plants. Exogenous application of thiamine could complement the compromised defense of the OsDR8-silenced plants. The expression level of several defense-responsive genes including the earlier functional genes of defense transduction pathway, OsPOX and OsPAL, and the downstream genes of the pathway, OsPR1a, OsPR1b, OsPR4, OsPR5 and OsPR10, was also decreased in the OsDR8-silenced plants. These results suggest that the impact of OsDR8 on disease resistance in rice may be through the regulation of expression of other defense-responsive genes and the site of OsDR8 function is on the upstream of the signal transduction pathway. In addition, the accumulation of thiamine may be essential for bacterial blight resistance and blast resistance.

Functional characterization of the thi1 promoter region from Arabidopsis thaliana

The Arabidopsis thaliana THI1 protein is involved in thiamine biosynthesis and is targeted to both chloroplasts and mitochondria by N-terminal control regions. To investigate thi1 expression, a series of thi1 promoter deletions were fused to the beta-glucuronidase (GUS) reporter gene. Transgenic plants were generated and expression patterns obtained under different environmental conditions. The results show that expression derived from the thi1 promoter is detected early on during development and continues throughout the plant's life cycle. High levels of GUS expression are observed in both shoots and roots during vegetative growth although, in roots, expression is restricted to the vascular system. Deletion analysis of the thi1 promoter region identified a region that is responsive to light. The smallest fragment (designated Pthi322) encompasses 306 bp and possesses all the essential signals for tissue specificity, as well as responsiveness to stress conditions such as sugar deprivation, high salinity, and hypoxia.

Systemic response to aphid infestation by Myzus persicae in the phloem of Apium graveolens

Little is known about the molecular processes involved in the phloem response to aphid feeding. We investigated molecular responses to aphid feeding on celery (Apium graveolenscv. Dulce) plants infested with the aphid Myzus persicae, as a means of identifying changes in phloem function. We used celery as our model species as it is easy to separate the phloem from the surrounding tissues in the petioles of mature leaves of this species. We generated a total of 1187 expressed sequence tags (ESTs), corresponding to 891 non-redundant genes. We analysed these ESTs in silico after cDNA macroarray hybridisation. Aphid feeding led to significant increase in RNA accumulation for 126 different genes. Different patterns of deregulation were observed, including transitory or stable induction 3 or 7 days after infestation. The genes affected belonged to various functional categories and were induced systemically in the phloem after infestation. In particular, genes involved in cell wall modification, water transport, vitamin biosynthesis, photosynthesis, carbon assimilation and nitrogen and carbon mobilisation were up-regulated in the phloem. Further analysis of the response in the phloem or xylem suggested that a component of the response was developed more specifically in the phloem. However, this component was different from the stress responses in the phloem driven by pathogen infection. Our results indicate that the phloem is actively involved in multiple adjustments, recruiting metabolic pathways and in structural changes far from aphid feeding sites. However, they also suggest that the phloem displays specific mechanisms that may not be induced in other tissues.

Neurospora crassa CyPBP37: a cytosolic stress protein that is able to replace yeast Thi4p function in the synthesis of vitamin B1

Recently, we identified CyPBP37 of Neurospora crassa as a binding partner of cyclophilin41. CyPBP37 function had not yet been described, although orthologs in other organisms have been implicated in the biosynthesis of the thiazole moiety of thiamine (vitamin B1) and/or stress-related pathways. Here, CyPBP37 is characterized as an abundant cytosolic protein with a functional NAD-binding site. Saccharomyces cerevisiae mutants lacking Thi4p (the CyPBP37 ortholog) are auxotrophic for vitamin B1 (thiamine) but can grow in the presence of the thiazole moiety of thiamine, suggesting a role for Thi4p in the biosynthesis of thiazole. N.crassa CyPBP37 is able to functionally replace Thi4p in yeast thiazole synthesis. Cellular fractionation studies revealed that Thi4p is a cytosolic protein in S.cerevisiae, like its ortholog CyPBP37 in N.crassa. This implies that thiamine synthesis takes place in the cytosol of both organisms and not in the mitochondria, as suggested. The expression of CyPBP37 and Thi4p is repressed by thiamine but not by thiazole in the growth medium. In addition to its function in thiazole synthesis, CyPBP37 is a stress-inducible protein. N.crassa cyclophilin41 can chaperone the folding of CyPBP37, its own binding partner.

Light control of nuclear gene mRNA abundance and translation in tobacco

Photosynthetic signals modulate expression of nuclear genes at the levels of mRNA transcription, mRNA stability, and translation. In transgenic tobacco (Nicotiana tabacum), the pea (Pisum sativum) Ferredoxin 1 (Fed-1) mRNA dissociates from polyribosomes and becomes destabilized when photosynthesis is inhibited by photosynthetic electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. We used polymerase chain reaction suppressive-subtractive hybridization to identify similarly regulated endogenous tobacco genes. This screen identified 14 nuclear-encoded tobacco mRNAs whose light-induced increase in abundance is suppressed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Sequence analysis of the cognate cDNAs revealed that nine of the mRNAs encode putative chloroplast-targeted proteins. We asked whether the abundance of these mRNAs was regulated transcriptionally or posttranscriptionally. Of the five mRNAs with sufficient abundance to detect using nuclear run-on assays, we observed transcriptional regulation of alpha-tubulin, thiazole biosynthetic enzyme, and pSKA10 (an unknown gene). Photosystem A subunit L and, to a lesser extent, alpha-tubulin and pSKA10 mRNAs, may also be stabilized in the light. In contrast, Rubisco small subunit mRNA abundance appears to be transcriptionally up-regulated but posttranscriptionally down-regulated in the light. To determine whether, like Fed-1 mRNA, the mRNAs identified in this screen were translationally responsive to light, we characterized the polyribosome association of these mRNAs in the light and after a 15-min dark treatment. A subset of the mRNAs showed dramatic dark-induced polyribosome dissociation, similar to Fed-1 mRNA, and all of the mRNAs showed at least slight polyribosome dissociation. Thus, both posttranscriptional and translational regulation appear to be important mechanisms regulating the expression of many nuclear-encoded mRNAs encoding proteins involved in photosynthesis.

Towards deciphering phloem: a transcriptome analysis of the phloem of Apium graveolens

Events occurring in the phloem tissue are key to understanding a wide range of developmental and physiological processes in vascular plants. While a considerable amount of molecular information on phloem proteins has emerged in the past decade, a unified picture of the molecular mechanisms involved in phloem differentiation and function is still lacking. New models to increase our understanding of this complex tissue can be created by the development of global approaches such as genomic analysis. In order to obtain a comprehensive overview of the molecular biology of the phloem tissue, we developed a genomic approach using Apium graveolens as a model. cDNA libraries were constructed from mRNAs extracted from isolated phloem of petioles. Expression data obtained from the analysis of 989 expressed sequence tags (ESTs) and the transcript profile deduced from a cDNA macroarray of 1326 clones were combined to identify genes showing distinct expression patterns in the vascular tissues. Comparisons of expression profiles obtained from the phloem, xylem and storage parenchyma tissues uncovered tissue-specific differential expression patterns for given sets of genes. The major classes of mRNAs predominantly found in the phloem encode proteins related to phloem structure, metal homeostasis or distribution, stress responses and degradation or turnover of proteins. Of great interest for future studies are the genes we found to be specifically expressed in the phloem but for which the function is still unknown, and also those genes described in previous reports to be up or downregulated by specific interactions. From a broader prospective, our results also clearly demonstrate that cDNA macroarray technology can be used to identify the key genes involved in various physiological and developmental processes in the phloem.

Point mutation is responsible for Arabidopsis tz-201 mutant phenotype affecting thiamin biosynthesis

A point mutation in the thi1 gene, involved in the synthesis of thiamin, has been identified in a tz-201 mutant line of Arabidopsis thaliana. The mutation occurs in a conserved protein domain and prevents the mutant plants from synthesizing thiamin. Complementation assays in yeast thi4 mutant confirm that this mutation hinders thiamin synthesis and, thus, is responsible for the tz phenotype. Northern blot analyses indicate that, in contrast to the yeast homologue, thi1 expression is not influenced by the presence of thiamin; however, reduced transcription of the gene is observed in roots and dark grown plants.

Molecular interactions between the specialist herbivore Manduca sexta (lepidoptera, sphingidae) and its natural host Nicotiana attenuata: V. microarray analysis and further characterization of large-scale changes in herbivore-induced mRNAs

We extend our analysis of the transcriptional reorganization that occurs when the native tobacco, Nicotiana attenuata, is attacked by Manduca sexta larvae by cloning 115 transcripts by mRNA differential display reverse transcription-polymerase chain reaction and subtractive hybridization using magnetic beads (SHMB) from the M. sexta-responsive transcriptome. These transcripts were spotted as cDNA with eight others, previously confirmed to be differentially regulated by northern analysis on glass slide microarrays, and hybridized with Cy3- and Cy5-labeled probes derived from plants after 2, 6, 12, and 24 h of continuous attack. Microarray analysis proved to be a powerful means of verifying differential expression; 73 of the cloned genes (63%) were differentially regulated (in equal proportions from differential display reverse transcription-polymerase chain reaction and SHMB procedures), and of these, 24 (32%) had similarity to known genes or putative proteins (more from SHMB). The analysis provided insights into the signaling and transcriptional basis of direct and indirect defenses used against herbivores, suggesting simultaneous activation of salicylic acid-, ethylene-, cytokinin-, WRKY-, MYB-, and oxylipin-signaling pathways and implicating terpenoid-, pathogen-, and cell wall-related transcripts in defense responses. These defense responses require resources that could be made available by decreases in four photosynthetic-related transcripts, increases in transcripts associated with protein and nucleotide turnover, and increases in transcripts associated with carbohydrate metabolism. This putative up-regulation of defense-associated and down-regulation of growth-associated transcripts occur against a backdrop of altered transcripts for RNA-binding proteins, putative ATP/ADP translocators, chaperonins, histones, and water channel proteins, responses consistent with a major metabolic reconfiguration that underscores the complexity of response to herbivore attack.

Enhanced selenium tolerance and accumulation in transgenic Arabidopsis expressing a mouse selenocysteine lyase

Selenium (Se) toxicity is thought to be due to nonspecific incorporation of selenocysteine (Se-Cys) into proteins, replacing Cys. In an attempt to direct Se flow away from incorporation into proteins, a mouse (Mus musculus) Se-Cys lyase (SL) was expressed in the cytosol or chloroplasts of Arabidopsis. This enzyme specifically catalyzes the decomposition of Se-Cys into elemental Se and alanine. The resulting SL transgenics were shown to express the mouse enzyme in the expected intracellular location, and to have SL activities up to 2-fold (cytosolic lines) or 6-fold (chloroplastic lines) higher than wild-type plants. Se incorporation into proteins was reduced 2-fold in both types of SL transgenics, indicating that the approach successfully redirected Se flow in the plant. Both the cytosolic and chloroplastic SL plants showed enhanced shoot Se concentrations, up to 1.5-fold compared with wild type. The cytosolic SL plants showed enhanced tolerance to Se, presumably because of their reduced protein Se levels. Surprisingly, the chloroplastic SL transgenics were less tolerant to Se, indicating that (over) production of elemental Se in the chloroplast is toxic. Expression of SL in the cytosol may be a useful approach for the creation of plants with enhanced Se phytoremediation capacity.

Differential usage of two in-frame translational start codons regulates subcellular localization of Arabidopsis thaliana THI1

Arabidopsis thaliana THI1 is encoded by a single nuclear gene and directed simultaneously to mitochondria and chloroplasts from a single major transcript. In vitro transcription/translation experiments revealed the presence of two translational products by the differential usage of two in-frame translational start codons. The coupling site-specific mutations on the THI1 encoding sequence with green fluorescent protein (GFP) gene fusions showed that translation initiation at the first AUG directs translocation of THI1 to chloroplasts. However, when translation starts from the second AUG, THI1 is addressed to mitochondria. Analysis of the translation efficiency of thi1 mRNA revealed that the best context for translation initiation is to use the first AUG. In addition, a suboptimal context in the vicinity of the second AUG initiation codon, next to a stable stem-and-loop structure that is likely to slow translation, has been noted. The fact that translation preferentially occurs in the first AUG of this protein suggests a high requirement for TH1 in chloroplasts. Although the frequency of upstream AUG translation is higher, according to the first AUG rule, initiation at the second AUG deviates significantly from Kozak's consensus. It suggests leaky ribosomal scanning, reinitiation or the internal entry of ribosomes to assure mitochondrial protein import.

The AtNFS2 gene from Arabidopsis thaliana encodes a NifS-like plastidial cysteine desulphurase

NifS-like proteins are cysteine desulphurases required for the mobilization of sulphur from cysteine. They are present in all organisms, where they are involved in iron-sulphur (Fe-S) cluster biosynthesis. In eukaryotes, these enzymes are present in mitochondria, which are the major site for Fe-S cluster assembly. The genome of the model plant Arabidopsis thaliana contains two putative NifS-like proteins. A cDNA corresponding to one of them was cloned by reverse-transcription PCR, and named AtNFS2. The corresponding transcript is expressed in many plant tissues. It encodes a protein highly related (75% similarity) to the slr0077-gene product from Synechocystis PCC 6803, and is predicted to be targeted to plastids. Indeed, a chimaeric AtNFS2-GFP fusion protein, containing one-third of AtNFS2 from its N-terminal end, was addressed to chloroplasts. Overproduction in Escherichia coli and purification of recombinant AtNFS2 protein enabled one to demonstrate that it bears a pyridoxal 5'-phosphate-dependent cysteine desulphurase activity in vitro, thus being the first NifS homologue characterized to date in plants. The putative physiological functions of this gene are discussed, including the attractive hypothesis of a possible role in Fe-S cluster assembly in plastids.

Simultaneous detection of thiamine and its phosphate esters from microalgae by HPLC

We present an easy and sensitive method for measuring thiamine and its phosphate esters in small biological samples of microalgae (Amphidinium carterae Hulburt and Nitzschia microcephala Grun). The method consists of extraction of thiamine and its derivatives in acid solution, followed by liquid chromatography with fluorescence detection. The detection limit is as low as 15 fmol of thiamine. For comparison to microalgae, the method has been applied to evaluate thiamine levels in the crustacean Artemia salina Leach and is suitable for nutritional studies of the food web of the Baltic salmon, which suffers from thiamine deficiency. This method of HPLC analysis can be readily utilized to follow uptake and interconversion of thiamine and its phosphate esters in many micro- and macroalgae.

Nutritional genomics: manipulating plant micronutrients to improve human health

The nutritional health and well-being of humans are entirely dependent on plant foods either directly or indirectly when plants are consumed by animals. Plant foods provide almost all essential vitamins and minerals and a number of other health-promoting phytochemicals. Because micronutrient concentrations are often low in staple crops, research is under way to understand and manipulate synthesis of micronutrients in order to improve crop nutritional quality. Genome sequencing projects are providing novel approaches for identifying plant biosynthetic genes of nutritional importance. The term "nutritional genomics" is used to describe work at the interface of plant biochemistry, genomics, and human nutrition.

IMPROVING THE NUTRIENT COMPOSITION OF PLANTS TO ENHANCE HUMAN NUTRITION AND HEALTH

Plant foods contain almost all of the mineral and organic nutrients established as essential for human nutrition, as well as a number of unique organic phytochemicals that have been linked to the promotion of good health. Because the concentrations of many of these dietary constituents are often low in edible plant sources, research is under way to understand the physiological, biochemical, and molecular mechanisms that contribute to their transport, synthesis and accumulation in plants. This knowledge can be used to develop strategies with which to manipulate crop plants, and thereby improve their nutritional quality. Improvement strategies will differ between various nutrients, but generalizations can be made for mineral or organic nutrients. This review focuses on the plant nutritional physiology and biochemistry of two essential human nutrients, iron and vitamin E, to provide examples of the type of information that is needed, and the strategies that can be used, to improve the mineral or organic nutrient composition of plants.

Thiamin metabolism and thiamin diphosphate-dependent enzymes in the yeast Saccharomyces cerevisiae: genetic regulation

The yeast Saccharomyces cerevisiae utilises external thiamin for the production of thiamin diphosphate (ThDP) or can synthesise the cofactor itself. Prior to uptake into the cell thiamin phosphates are first hydrolysed and thiamin is taken up as free vitamin which is then pyrophosphorylated by a pyrophosphokinase. Synthesis of ThDP starts with the production of hydroxyethylthiazole and hydroxymethylpyrimidine. Those are linked to yield thiamin phosphate which is hydrolysed to thiamin and subsequently pyrophosphorylated. The THI genes encoding the enzymes of these final steps of ThDP production and of thiamin utilisation have been identified. Their expression is controlled by the level of thiamin and a number of regulatory proteins involved in regulated expression of the THI genes are known. However, the molecular details of the regulatory circuits need to be deciphered. Since the nucleotide sequence of the entire yeast genome is known we can predict the number of ThDP-dependent enzymes in S. cerevisiae. Eleven such proteins have been found: pyruvate decarboxylase (Pdc, three isoforms), acetolactate synthase, a putative alpha-ketoisocaproate decarboxylase with a regulatory role in ThDP synthesis and two proteins of unknown function form the group of Pdc related enzymes. In addition there are two isoforms for transketolase as well as the E1 subunits of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase. Expression of most of these genes is either induced or repressed by glucose. Surprisingly, it has been found recently that expression of one of the genes for Pdc is repressed by thiamin. In addition, the regulatory protein Pdc2p was shown to be required for high level expression of both the THI and the PDC genes. Apparently, the production of ThDP and of the enzymes using this cofactor is coordinately regulated. Future research will focus on the elucidation of the molecular mechanisms of this novel type of regulation.

Dual role for the yeast THI4 gene in thiamine biosynthesis and DNA damage tolerance

The THI4 gene of Saccharomyces cerevisiae encodes an enzyme of the thiamine biosynthetic pathway. The plant homolog thi1, from Arabidopsis thaliana, is also involved in thiamine biosynthesis; but was originally cloned due to its capacity to complement DNA repair deficient phenotypes in Escherichia coli. Here, the behavior of a thi4 disrupted strain was examined for increased sensitivity to treatment with the DNA damaging agents ultraviolet radiation (UV, 254 nm) and methyl methanesulfonate (MMS). Although the thi4 null mutant showed a similar level of survival as the wild-type strain, a higher frequency of respiratory mutants was induced by the two treatments. A similar phenotype was seen with wild-type strains expressing an antisense THI4 construct. Further analysis of respiratory mutants revealed that these were due to mutations of mitochondrial DNA (mtDNA) rather than nuclear DNA, consisting of rho-petite mutants. Moreover, the frequency of mutations was unaffected by the presence or absence of thiamine in the growth medium, and the defect leading to induction of petites in the thi4 mutant was corrected by expression of the Arabidopsis thi1 gene. Thus, Thi4 and its plant homolog appear to be dual functional proteins with roles in thiamine biosynthesis and mitochondrial DNA damage tolerance.

Thi1, a thiamine biosynthetic gene in Arabidopsis thaliana, complements bacterial defects in DNA repair

An Arabidopsis thaliana cDNA was isolated by complementation of the Escherichia coli mutant strain BW535 (xth, nfo, nth), which is defective in DNA base excision repair pathways. This cDNA partially complements the methyl methane sulfonate (MMS) sensitive phenotype of BW535. It also partially corrects the UV-sensitive phenotype of E. coli AB1886 (uvrA) and restores its ability to reactivate UV-irradiated lambda phage. It has an insert of ca. 1.3 kb with an open reading frame of 1047 bp (predicting a protein with a molecular mass of 36 kDa). This cDNA presents a high homology to a stress related gene from two species of Fusarium (sti35) and to genes whose products participate in the thiamine biosynthesis pathway, THI4, from Saccharomyces cerevisiae and nmt2 from Schizosaccharomyces pombe. The Arabidopsis predicted polypeptide has homology to several protein motifs: amino-terminal chloroplast transit peptide, dinucleotide binding site, DNA binding and bacterial DNA polymerases. The auxotrophy for thiamine in the yeast thi4::URA3 disruption strain is complemented by the Arabidopsis gene. Thus, the cloned gene, named thi1, is likely to function in the biosynthesis of thiamine in plants. The data presented in this work indicate that thi1 may also be involved in DNA damage tolerance in plant cells.