Isolation of a monocot 3-hydroxy-3-methylglutaryl coenzyme A reductase gene that is elicitor-inducible

Overexpression of PtHMGR enhances drought and salt tolerance of poplar

BACKGROUND AND AIMS

Soil salinization and aridification are swiftly engulfing the limited land resources on which humans depend, restricting agricultural production. Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is important in the biosynthesis of terpenoids, which are involved in plant growth, development and responses to environmental stresses. This study aimed to provide guidance for producing salt- and drought-resistant poplar.

METHODS

A protein expression system was used to obtain PtHMGR protein, and high-performance liquid chromatography was used to detect the activity of PtHMGR protein in vitro. In addition, a simplified version of the leaf infection method was used for transformation of 'Nanlin895' poplar (Populus×euramericana). qRT-PCR was used to identify expression levels of genes.

KEY RESULTS

PtHMGR catalysed a reaction involving HMG-CoA and NADPH to form mevalonate. Overexpression of PtHMGR in Populus × euramericana 'Nanlin895' improved drought and salinity tolerance. In the presence of NaCl and PEG6000, the rates of rooting and survival of PtHMGR-overexpressing poplars were higher than those of wild-type poplars. The transgenic lines also exhibited higher proline content and peroxidase and superoxide dismutase activities, and a lower malondialdehyde level under osmotic stress. In addition, the expression of genes related to reactive oxygen species (ROS) scavenging and formation was altered by osmotic stress. Moreover, the effect of osmotic stress on transcript levels of stress-related genes differed between the transgenic and wild-type poplars.

CONCLUSION

PtHMGR catalysed a reaction involving HMG-CoA and NADPH to form mevalonate in vitro. Overexpression of PtHMGR promoted root development, increased the expression of ROS scavenging-related genes, decreased the expression of ROS formation-related genes, and increased the activity of antioxidant enzymes in transgenic poplars, enhancing their tolerance of osmotic stress. In addition, overexpression of PtHMGR increased expression of the stress-related genes KIN1, COR15 and AAO3 and decreased that of ABI, MYB, MYC2 and RD22, enhancing the stress resistance of poplar.

Quantitative Proteomics of Potato Leaves Infected with Phytophthora infestans Provides Insights into Coordinated and Altered Protein Expression during Early and Late Disease Stages

In order to get a better understanding of protein association during Solanum tuberosum (cv. Sarpo Mira)–Phytophthora infestans incompatible interaction, we investigated the proteome dynamics of cv. Sarpo Mira, after foliar application of zoospore suspension from P. infestans isolate, at three key time-points: zero hours post inoculation (hpi) (Control), 48 hpi (EI), and 120 hpi (LI); divided into early and late disease stages by the tandem mass tagging (TMT) method. A total of 1229 differentially-expressed proteins (DEPs) were identified in cv. Sarpo Mira in a pairwise comparison of the two disease stages, including commonly shared DEPs, specific DEPs in early and late disease stages, respectively. Over 80% of the changes in protein abundance were up-regulated in the early stages of infection, whereas more DEPs (61%) were down-regulated in the later disease stage. Expression patterns, functional category, and enrichment tests highlighted significant coordination and enrichment of cell wall-associated defense response proteins during the early stage of infection. The late stage was characterized by a cellular protein modification process, membrane protein complex formation, and cell death induction. These results, together with phenotypic observations, provide further insight into the molecular mechanism of P. infestans resistance in potatos.

Dark conditions enhance aluminum tolerance in several rice cultivars via multiple modulations of membrane sterols

Aluminum-sensitive rice (Oryza sativa L.) cultivars showed increased Al tolerance under dark conditions, because less Al accumulated in the root tips (1 cm) under dark than under light conditions. Under dark conditions, the root tip concentration of total sterols, which generally reduce plasma membrane permeabilization, was higher in the most Al-sensitive japonica cultivar, Koshihikari (Ko), than in the most Al-tolerant cultivar, Rikuu-132 (R132), but the phospholipid content did not differ between the two. The Al treatment increased the proportion of stigmasterol (which has no ability to reduce membrane permeabilization) out of total sterols similarly in both cultivars under light conditions, but it decreased more in Ko under dark conditions. The carotenoid content in the root tip of Al-treated Ko was significantly lower under dark than under light conditions, indicating that isopentenyl diphosphate transport from the cytosol to plastids was decreased under dark conditions. HMG2 and HMG3 (encoding the key sterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase) transcript levels in the root tips were enhanced under dark conditions. We suggest that the following mechanisms contribute to the increase in Al tolerance under dark conditions: inhibition of stigmasterol formation to retain membrane integrity; greater partitioning of isopentenyl diphosphate for sterol biosynthesis; and enhanced expression of HMGs to increase sterol biosynthesis.

3-Hydroxy-3-methylglutaryl coenzyme A reductase 1 (HMG1) is highly associated with the cell division during the early stage of fruit development which determines the final fruit size in Litchi chinensis

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC: 1.1.1.34), an enzyme catalyzing the first committed step in the mevalonic acid (MVA) pathway for the biosynthesis of isoprenoids, has been reported to be involved in the fruit size determination through the regulation of early cell division. In litchi, the cell number achieved by this early cell division determines the final fruit size, but whether HMGR plays any role in this process was unknown. In this study, we set out to address this question with gene cloning and expression analysis in fruits of different pheno- or genotypes. We found that the litchi genome includes two HMGR homologues, denoted as LcHMG1 and LcHMG2. Despite 70% sequence identity at the amino acid level, they exhibited distinct expression patterns during litchi fruit development. LcHMG1 expression was highest in the early stage of fruit development, correlated with the high level of cell division. Absolute levels of LcHMG1 expression varied among fruits of different pheno- or genotypes, with expression in large-fruited types reaching higher levels for longer duration compared to that in small-fruited types. The expression patterns for LcHMG1 strongly suggest that this gene is involved in early cell division and fruit size determination in litchi. In contrast, LcHMG2 was most highly expressed in the late stage of fruit development, in association with biosynthesis of isoprenoid compounds required for later cell enlargement. These findings provided new insights on the function of HMGR genes during fruit development.

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

Characterization of two NADPH: cytochrome P450 reductases from cotton (Gossypium hirsutum)

Cytochrome P450 monooxygenases (P450s) are commonly involved in biosynthesis of endogenous compounds and catabolism of xenobiotics, and their activities rely on a partner enzyme, cytochrome P450 reductase (CPR, E.C.1.6.2.4). Two CPR cDNAs, GhCPR1 and GhCPR2, were isolated from cotton (Gossypium hirsutum). They are 71% identical to each other at the amino acid sequence level and belong to the Class I and II of dicotyledonous CPRs, respectively. The recombinant enzymes reduced cytochrome c, ferricyanide and dichlorophenolindophenol (DCPIP) in an NADPH-dependent manner, and supported the activity of CYP73A25, a cinnamate 4-hydroxylase of cotton. Both GhCPR genes were widely expressed in cotton tissues, with a reduced expression level of GhCPR2 in the glandless cotton cultivar. Expression of GhCPR2, but not GhCPR1, was inducible by mechanical wounding and elicitation, indicating that the GhCPR2 is more related to defense reactions, including biosynthesis of secondary metabolites.

Metabolomic approaches reveal that phosphatidic and phosphatidyl glycerol phospholipids are major discriminatory non-polar metabolites in responses by Brachypodium distachyon to challenge by Magnaporthe grisea

Metabolomic approaches were used to elucidate some key metabolite changes occurring during interactions of Magnaporthe grisea--the cause of rice blast disease--with an alternate host, Brachypodium distachyon. Fourier-transform infrared (FT-IR) spectroscopy provided a high-throughput metabolic fingerprint of M. grisea interacting with the B. distachyon accessions ABR1 (susceptible) and ABR5 (resistant). Principal component-discriminant function analysis (PC-DFA) allowed the differentiation between developing disease symptoms and host resistance. Alignment of projected 'test-set' on to 'training-set' data indicated that our experimental approach produced highly reproducible data. Examination of PC-DFA loading plots indicated that fatty acids were one chemical group that discriminated between responses by ABR1 and ABR5 to M. grisea. To identify these, non-polar extracts of M. grisea-challenged B. distachyon were directly infused into an electrospray ionization mass spectrometer (ESI-MS). PC-DFA indicated that M. grisea-challenged ABR1 and ABR5 were differentially clustered away from healthy material. Subtraction spectra and PC-DFA loadings plots revealed discriminatory analytes (m/z) between each interaction and seven metabolites were subsequently identified as phospholipids (PLs) by ESI-MS-MS. Phosphatidyl glycerol (PG) PLs were suppressed during both resistant and susceptible responses. By contrast, different phosphatidic acid PLs either increased or were reduced during resistance or during disease development. This suggests considerable and differential PL processing of membrane lipids during each interaction which may be associated with the elaboration/suppression of defence mechanisms or developing disease symptoms.

Enhanced resistance to blast fungus and bacterial blight in transgenic rice constitutively expressing OsSBP, a rice homologue of mammalian selenium-binding proteins

The rice Oryza sativa selenium-binding protein homologue (OsSBP) gene encodes a homologue of mammalian selenium-binding proteins, and it has been isolated as one of the genes induced by treating a plant with a cerebroside elicitor from rice blast fungus. The possible role of OsSBP in plant defense was evaluated by using a transgenic approach. Plants overexpressing OsSBP showed enhanced resistance to a virulent strain of rice blast fungus as well as to rice bacterial blight. The expression of defense-related genes and the accumulation of phytoalexin after infection by rice blast fungus were accelerated in the OsSBP overexpressors. A higher level of H(2)O(2) accumulation and reduced activity of such scavenging enzymes as ascorbate peroxidase and catalase were seen when the OsSBP-overexpressing plants were treated with the protein phosphatase 1 inhibitor, calyculin A. These results suggest that the upregulation of OsSBP expression conferred enhanced tolerance to different pathogens, possibly by increasing plant sensitivity to endogenous defense responses. Additionally, the OsSBP protein might have a role in modulating the defense mechanism to biotic stress in rice.

Tourist C transposable elements are closely associated with genes expressed in flowers of rice (Oryza sativa)

Tourist elements comprise a group of transposable elements in plants. One of these elements, Tourist-OsaCatA(a Tourist C element), has been found in the 5; flanking region of a catalase gene, CatA, in rice (Oryza sativa). Using reverse transcriptase-PCR (RT-PCR) analyses of leaves, roots, flowers and developing seeds of rice, we assessed the transcription levels of ten known genes containing Tourist C elements, and of three additional putative genes for which expressed sequence tags (ESTs) including Tourist C elements have been isolated. We found that nine of the ten known genes and two of the three represented by ESTs were expressed in at least one of the organs we analyzed, and all of the genes detected were expressed in flowers, usually in stamens or pistils. We also assessed the expression of the 29 Tourist C-containing hypothetical coding sequences (CDSs) obtained so far by high-throughput genomic sequencing. We found that CDSs of all 11 genes whose transcripts were detectable by RT-PCR were expressed in flowers, especially in stamens or pistils. In contrast, RT-PCR analyses of genes or CDSs associated with other miniature inverted-repeat transposable elements (MITEs), such as Tourist D, Gaijin, Explorer, and Castaway, showed that some of them were expressed only minimally or not at all in flowers. Therefore, compared with other MITEs, Tourist C elements seem to show a strong association with genes that are expressed in the flowers of rice.

Molecular characterization of three 3-hydroxy-3-methylglutaryl-CoA reductase genes including pathogen-induced Hmg2 from pepper (Capsicum annuum)

Sesquiterpene phytoalexins, a class of plant defense metabolites, are synthesized from the cytosolic acetate/mevalonate pathway in isoprenoids biosynthetic system of plants. The 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the synthesis of mevalonate, which is the specific precursor of this pathway, as a multi gene family. Three kinds of cDNA clones encoding HMGR were isolated from Korean red pepper (Capsicum annuum L. cv. NocKwang) and the HMGR2 gene (Hmg2) was especially obtained from a cDNA library constructed with Phytophthora capsici-infected pepper root RNAs. The Hmg2 encoding a 604-amino-acid peptide had typical features as an elicitor-induced isoform among HMGRs on its gene structure and had a predicted amino acid sequence homology. In addition, the expression of Hmg2 was rapidly induced within 1 h in response to a fungal pathogen and continuously increased up to 48 h. Together with sesquiterpene cyclase gene that was strongly induced 24 h after pathogen-infection, the Hmg2 and farnesyl pyrophosphate synthase gene were coordinately and sequentially regulated for the biosynthesis of defense-related sesquiterpene phytoalexins in pepper.

Two distinct isopentenyl diphosphate isomerases in cytosol and plastid are differentially induced by environmental stresses in tobacco

Two distinct cDNA clones (IPI1 and IPI2) encoding IPI were isolated from Nicotiana tabacum. In situ expression of isopentenyl diphosphate isomerase-1 (IPI1)- and IPI2-green fluorescent protein fusion constructs revealed that IPI1 and IPI2 were localized in chloroplast and cytosol, respectively. The level of IPI1 mRNA was increased under high-salt and high-light stress conditions, while that of IPI2 mRNA was increased under high-salt and cold stress conditions. Both IPI transcripts were increased in an abscisic acid-independent manner. This is the first report of a cytosolic IPI. The results indicated that two distinct IPIs were differentially induced in response to stress.

Identification of defense-related rice genes by suppression subtractive hybridization and differential screening

Identification of host genes involved in defense responses is one of most critical steps leading to the elucidation of disease resistance mechanisms in plants. In this study, two different cloning strategies were employed to identify defense-related genes from a tropical japonica rice cultivar (Oryza sativa cv. Drew). With the use of bacterial colony arrays, differential screening of a blast fungus (Pyricularia grisea)-induced rice cDNA library led to the isolation of 22 distinct rice genes that are expressed differentially in response to blast infection. Sequence analysis indicates that most of them are full-length cDNAs encoding pathogenesis-related proteins or other relatively abundant proteins. In combination with treatments of cycloheximide plus jasmonic acid (JA) or benzothiadiazole (BTH) in rice seedlings, the polymerase chain reaction-based suppression subtractive hybridization also was conducted to search for immediate early (IE) defense-related genes whose transcription is independent of de novo protein synthesis. The initial screening of only 768 subtracted clones resulted in the identification of 34 distinct IE genes that are induced by JA, BTH, and/or blast infection. Database searches revealed that these IE genes encode putative mitogen-activated protein kinase, diacylglycerol kinase, zinc finger protein, RelA-SpoT protein, ankyrin-containing protein, ABC transporter, beta-ketoacyl-CoA synthase, and other potential defense-signaling components. Further characterization of these novel IE genes will likely facilitate the elucidation of defense signal transduction in rice plants.

Expression of Brassica juncea 3-hydroxy-3-methylglutaryl CoA synthase is developmentally regulated and stress-responsive

3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) is an enzyme in mevalonate biosynthesis. In plants, investigations have focused on HMG CoA reductase (HMGR) and less is known of the preceding enzyme, HMGS. To understand the regulation of HMGS, we have isolated a Brassica juncea cDNA encoding HMGS, BjHMGS1, for use as a hybridization probe in Northern blot analyses. BjHMGS is expressed in all plant organs and shows developmental regulation in flower, seed and seedling, with highest expression in early development. In seedlings, expression is highest in young hypocotyls and is induced during the greening of etiolated cotyledons. BjHMGS is down-regulated by abscisic acid, osmotic stress and dehydration, the effects of which arrested seedling growth. Thus BjHMGS expression shows correlation with rapid cell division and growth, like HMGR. This is not unexpected, as mevalonate is the precursor to many essential isoprenoid compounds, including sterols for membrane biogenesis. Wounding, methyl jasmonate or salicylic acid induce BjHMGS expression, suggesting that, like HMGR, HMGS is involved in defence. As in animals, coordinated regulation of HMGS with HMGR occurred in B. juncea upon germination and in response to salicylic acid. HMGS assays confirmed that Escherichia coli-expressed recombinant BjHMGS1 shows HMGS activity that is inhibited by F244, a specific inhibitor of HMGS. Southern blot analysis revealed gene families encoding HMGS in Brassica species and a summation of homologous genes in the fusion amphidiploid genome of B. juncea, a bi-parental species derived from diploids B. nigra and B. campestris.

Molecular, functional and evolutionary characterization of the gene encoding HMG-CoA reductase in the fission yeast, Schizosaccharomyces pombe

The synthesis of mevalonate, a molecule required for both sterol and isoprene biosynthesis in eukaryotes, is catalysed by 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Using a gene dosage approach, we have isolated the gene encoding HMG-CoA reductase hmgl+, from the fission yeast Schizosaccharomyces pombe (Accession Number L76979). Specifically, hmgl+ was isolated on the basis of its ability to confer resistance to lovastatin, a competitive inhibitor of HMG-CoA reductase. Gene disruption analysis showed that hmgl+ was an essential gene. This result provided evidence that, unlike Saccharomyces cerevisiae, S. pombe contained only a single functional HMG-CoA reductase gene. The presence of a single HMG-CoA reductase gene was confirmed by genomic hybridization analysis. As observed for the S. cerevisiae HMGlp, the hmgl+ protein induced membrane proliferations known as karmellae. A previously undescribed 'feed-forward' regulation was observed in which elevated levels of HMG-CoA synthase, the enzyme catalysing the synthesis of the HMG-CoA reductase substrate, induced elevated levels of hmgl+ protein in the cell and conferred partial resistance to lovastatin. The amino acid sequences of yeast and human HMG-CoA reductase were highly divergent in the membrane domains, but were extensively conserved in the catalytic domains. We tested whether the gene duplication that produced the two functional genes in S. cerevisiae occurred before or after S. pombe and S. cerevisiae diverged by comparing the log likelihoods of trees specified by these hypotheses. We found that the tree specifying post-divergence duplication had significantly higher likelihood. Moreover, phylogenetic analyses of available HMG-CoA reductase sequences also suggested that the lineages of S. pombe and S. cerevisiae diverged approximately 420 million years ago but that the duplication event that produced two HMG-CoA reductase genes in the budding yeast occurred only approximately 56 million years ago. To date, S. pombe is the only unicellular eukaryote that has been found to contain a single HMG-CoA reductase gene. Consequently, S. pombe may provide important opportunities to study aspects of the regulation of sterol biosynthesis that have been difficult to address in other organisms and serve as a test organism to identify novel therapies for modulating cholesterol synthesis.

A computer-based systematic survey reveals the predominance of small inverted-repeat elements in wild-type rice genes

Several recent reports indicate that mobile elements are frequently found in and flanking many wild-type plant genes. To determine the extent of this association, we performed computer-based systematic searches to identify mobile elements in the genes of two "model" plants, Oryza sativa (domesticated rice) and Arabidopsis thaliana. Whereas 32 common sequences belonging to nine putative mobile element families were found in the noncoding regions of rice genes, none were found in Arabidopsis genes. Five of the nine families (Gaijin, Castaway, Ditto, Wanderer, and Explorer) are first described in this report, while the other four were described previously (Tourist, Stowaway, p-SINE1, and Amy/LTP). Sequence similarity, structural similarity, and documentation of past mobility strongly suggests that many of the rice common sequences are bona fide mobile elements. Members of four of the new rice mobile element families are similar in some respects to members of the previously identified inverted-repeat element families, Tourist and Stowaway. Together these elements are the most prevalent type of transposons found in the rice genes surveyed and form a unique collection of inverted-repeat transposons we refer to as miniature inverted-repeat transposable elements or MITEs. The sequence and structure of MITEs are clearly distinct from short or long interspersed nuclear elements (SINEs or LINEs), the most common transposable elements associated with mammalian nuclear genes. Mobile elements, therefore, are associated with both animal and plant genes, but the identity of these elements is strikingly different.

The N-terminal domain of tomato 3-hydroxy-3-methylglutaryl-CoA reductases. Sequence, microsomal targeting, and glycosylation

The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the conversion of 3-hydroxy-3-methylglutaryl-CoA to mevalonic acid, considered the rate-limiting step in isoprenoid biosynthesis. In plants, isoprenoid compounds play important roles in mediating plant growth and development, electron transport, photosynthesis, and disease resistance. Sequence comparisons of plant HMGR proteins with those from yeast and mammalian systems reveal high levels of sequence identity within the catalytic domain but significant divergence in the membrane domain. Mammalian HMGRs are integral membrane proteins of the endoplasmic reticulum with eight membrane-spanning regions. In contrast, the membrane domain of plant HMGRs is predicted to contain only one to two transmembrane spans. We have isolated and sequenced a clone (pCD4) encoding exon 1 of tomato hmg1. The membrane domain structures of two differentially regulated tomato HMGR isoforms, HMG1 and HMG2, were analyzed using in vitro transcription and translation systems. Microsomal membrane insertion of the tomato HMGRs is co-translational and does not involve cleavage of an N-terminal targeting peptide. HMGR membrane topography was established by protease protection studies of the HMG1 membrane domain and an analogous region of HMG2 engineered to contain a c-myc epitope tag. The data indicate that both tomato HMGRs span the membrane two times with both the C and N termini located in the cytosol. Lumenal localization of the short peptide predicted to lie within the endoplasmic reticulum was further confirmed by in vitro glycosylation of an asparagine-linked glycosylation site present in HMG2.