Kentaro Nakaminami

Tissue-Specific Localization of an Abscisic Acid Biosynthetic Enzyme, AAO3, in Arabidopsis

Arabidopsis aldehyde oxidase 3 (AAO3) is an enzyme involved in abscisic acid (ABA) biosynthesis in response to drought stress. Since the enzyme catalyzes the last step of the pathway, ABA production sites may be determined by the presence of AAO3. Here, AAO3 localization was investigated using AAO3 promoter:AAO3-GFP transgenic plants and by an immunohistochemical technique. AAO3-GFP protein exhibited an activity to produce ABA from abscisic aldehyde, and the transgene restored the wilty phenotype of the aao3 mutant. GFP-fluorescence was detected in the root tips, vascular bundles of roots, hypocotyls and inflorescence stems, and along the leaf veins. Intense immunofluorescence signals were localized in phloem companion cells and xylem parenchyma cells. Faint but significant GFP- and immuno-fluorescence signals were observed in the leaf guard cells. In situ hybridization with antisense AAO3 mRNA showed AAO3 mRNA expression in the guard cells of dehydrated leaves. These results indicate that the ABA synthesized in vascular systems is transported to various target tissues and cells, and also that the guard cells themselves are able to synthesize ABA.

A Cold-regulated Nucleic Acid-binding Protein of Winter Wheat Shares a Domain with Bacterial Cold Shock Proteins

The molecular mechanisms of cold acclimation are still largely unknown; however, it has been established that overwintering plants such as winter wheat increases freeze tolerance during cold treatments. In prokaryotes, cold shock proteins are induced by temperature downshifts and have been proposed to function as RNA chaperones. A wheat cDNA encoding a putative nucleic acid-binding protein, WCSP1, was isolated and found to be homologous to the predominant CspA of Escherichia coli. The putative WCSP1 protein contains a three-domain structure consisting of an N-terminal cold shock domain with two internal conserved consensus RNA binding domains and an internal glycine-rich region, which is interspersed with three C-terminal CX 2CX 4HX 4C (CCHC) zinc fingers. Each domain has been described independently within several nucleotide-binding proteins. Northern and Western blot analyses showed that WCSP1mRNA and protein levels steadily increased during cold acclimation, respectively. WCSP1 induction was cold-specific because neither abscisic acid treatment, drought, salinity, nor heat stress induced WCSP1 expression. Nucleotide binding assays determined that WCSP1 binds ssDNA, dsDNA, and RNA homopolymers. The capacity to bind dsDNA was nearly eliminated in a mutant protein lacking C-terminal zinc fingers. Structural and expression similarities to E. coli CspA suggest that WCSP1 may be involved in gene regulation during cold acclimation.

Cloning of a Gene Cluster Responsible for the Biosynthesis of Diterpene Aphidicolin, a Specific Inhibitor of DNA Polymerase α

The fungal diterpene, aphidicolin, is a well-known specific inhibitor of DNA polymerase α. Terpenoids are an important class of natural products. However, identification of the biosynthetic gene cluster in terpenoids is relatively rare compared with another important class of natural products, polyketides. To explore a reliable identification method for the biosynthetic gene cluster in fungal diterpenoids, cloning of the biosynthetic gene cluster of aphidicolin was employed. The application of a simple PCR method for genome walking based on the sequence of cDNA encoding aphidicolan-16β-ol synthase (ACS) allowed us to analyze a 15.6-kb region of the Phoma betae genomic DNA. Six ORFs, PbGGS, ACS, PbP450-1, PbP450-2, PbTP, and PbTF were found in this region, and respectively expected to encode geranylgeranyl diphosphate synthase, diterpene synthase, two cytochrome P-450s, the transporter and transcription factor. Their amino acid sequences and introns were deduced by a corresponding cDNA analysis. This study shows that simple PCR-based genome walking without constructing a genomic DNA library is useful for identification of a small gene cluster. We propose a general strategy for the cloning the biosynthetic genes of fungal diterpenoids by using fungal GGS.

Germination of photoblastic lettuce seeds is regulated via the control of endogenous physiologically active gibberellin content, rather than of gibberellin responsiveness

Phytochrome regulates lettuce (Lactuca sativa L. cv. Grand Rapids) seed germination via the control of the endogenous level of bioactive gibberellin (GA). In addition to the previously identified LsGA20ox1, LsGA20ox2, LsGA3ox1, LsGA3ox2, LsGA2ox1, and LsGA2ox2, five cDNAs were isolated from lettuce seeds: LsCPS, LsKS, LsKO1, LsKO2, and LsKAO. Using an Escherichia coli expression system and functional assays, it is shown that LsCPS and LsKS encode ent-copalyl diphosphate synthase and ent-kaurene synthase, respectively. Using a Pichia pastoris system, it was found that LsKO1 and LsKO2 encode ent-kaurene oxidases and LsKAO encodes ent-kaurenoic acid oxidase. A comprehensive expression analysis of GA metabolism genes using the quantitative reverse transcription polymerase chain reaction suggested that transcripts of LsGA3ox1 and LsGA3ox2, both of which encode GA 3-oxidase for GA activation, were primarily expressed in the hypocotyl end of lettuce seeds, were expressed at much lower levels than the other genes tested, and were potently up-regulated by phytochrome. Furthermore, LsDELLA1 and LsDELLA2 cDNAs that encode DELLA proteins, which act as negative regulators in the GA signalling pathway, were isolated from lettuce seeds. The transcript levels of these two genes were little affected by light. Lettuce seeds in which de novo GA biosynthesis was suppressed responded almost identically to exogenously applied GA, irrespective of the light conditions, suggesting that GA responsiveness is not significantly affected by light in lettuce seeds. It is proposed that lettuce seed germination is regulated mainly via the control of the endogenous content of bioactive GA, rather than the control of GA responsiveness.

Heat stable ssDNA/RNA-binding activity of a wheat cold shock domain protein

The cold‐induced wheat WCSP1 protein belongs to the cold shock domain (CSD) protein family. In prokaryotes and eukaryotes, the CSD functions as a nucleic acid‐binding domain. Here, we demonstrated that purified recombinant WCSP1 is boiling soluble and binds ss/dsDNA and mRNA. Furthermore, boiled‐WCSP1 retained its characteristic nucleic acid‐binding activity. A WCSP1 deletion mutant, containing only a CSD, lost ssDNA/RNA‐binding activity; while a mutant containing the CSD and the first glycine‐rich region (GR) displayed the activity. These data indicated that the first GR of WCSP1 is necessary for the binding activity but is not for the heat stability of the protein.

Gibberellin Is Essentially Required for Carrot (Daucus carota L.) Somatic Embryogenesis: Dynamic Regulation of Gibberellin 3-Oxidase Gene Expressions

A GA biosynthesis inhibitor, uniconazole, caused many shrunken embryos when it was supplied to cultured carrot (Daucus carota L.) cells at the induction of somatic embryos. The abnormality was prevented by exogenous GA1 or GA4. To analyze the status of GA biosynthesis during somatic embryogenesis, expression patterns of newly isolated genes encoding GA biosynthetic enzymes, two GA 20-oxidases, three GA 3-oxidases, and two GA 2-oxidases were observed by using a semi-quantitative reverse-transcription-polymerase chain reaction with gene-specific primers. Transcript levels of GA 20-oxidases and GA 2-oxidases did not change greatly during development of the somatic embryo. On the other hand, drastic changes were found in three GA 3-oxidase genes. Strikingly, expression of a GA 3-oxidase gene, DcGA3ox2, was elevated once in somatic embryogenesis, but not in the non-induced suspension cells. The enzymatic functions of these gene products were also confirmed using recombinant proteins expressed in Escherichia coli. Our results indicate that GA biosynthesis is required for carrot somatic embryogenesis.

Oligouridylate Binding Protein 1b Plays an Integral Role in Plant Heat Stress Tolerance.

Stress granules (SGs), which are formed in the plant cytoplasm under stress conditions, are transient dynamic sites (particles) for mRNA storage. SGs are actively involved in protecting mRNAs from degradation. Oligouridylate binding protein 1b (UBP1b) is a component of SGs. The formation of microscopically visible cytoplasmic foci, referred to as UBP1b SG, was induced by heat treatment in UBP1b-overexpressing Arabidopsis plants (UBP1b-ox). A detailed understanding of the function of UBP1b, however, is still not clear. UBP1b-ox plants displayed increased heat tolerance, relative to control plants, while ubp1b mutants were more sensitive to heat stress than control plants. Microarray analysis identified 117 genes whose expression was heat-inducible and higher in the UBP1b-ox plants. RNA decay analysis was performed using cordycepin, a transcriptional inhibitor. In order to determine if those genes serve as targets of UBP1b, the rate of RNA degradation of a DnaJ heat shock protein and a stress-associated protein (AtSAP3) in UBP1b-ox plants was slower than in control plants; indicating that the mRNAs of these genes were protected within the UBP1b SG granule. Collectively, these data demonstrate that UBP1b plays an integral role in heat stress tolerance in plants.

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Significance Hormone-like peptides derived from small coding genes (<100 amino acids) have not been extensively characterized in relation to abiotic stress tolerance. Focusing on 17 salinity stress-inducible small coding genes in Arabidopsis, we showed that four genes conferred increased salinity stress tolerance when overexpressed in transgenic plants. One of the four genes (AtPROPEP3) was found to induce salinity stress tolerance by treatment with a 13-peptide (KPTPSSGKGGKHN) fragment, providing unique functional evidence for enhanced salinity stress tolerance in plants in response to a peptide treatment. Although the 13-peptide fragment shares homology with known peptides associated with immune response, the other peptides may encode unique hormone-like peptides associated with salinity stress tolerance. Peptides encoded by small coding genes play an important role in plant development, acting in a similar manner as phytohormones. Few hormone-like peptides, however, have been shown to play a role in abiotic stress tolerance. In the current study, 17 Arabidopsis genes coding for small peptides were found to be up-regulated in response to salinity stress. To identify peptides leading salinity stress tolerance, we generated transgenic Arabidopsis plants overexpressing these small coding genes and assessed survivability and root growth under salinity stress conditions. Results indicated that 4 of the 17 overexpressed genes increased salinity stress tolerance. Further studies focused on AtPROPEP3, which was the most highly up-regulated gene under salinity stress. Treatment of plants with synthetic peptides encoded by AtPROPEP3 revealed that a C-terminal peptide fragment (AtPep3) inhibited the salt-induced bleaching of chlorophyll in seedlings. Conversely, knockdown AtPROPEP3 transgenic plants exhibited a hypersensitive phenotype under salinity stress, which was complemented by the AtPep3 peptide. This functional AtPep3 peptide region overlaps with an AtPep3 elicitor peptide that is related to the immune response of plants. Functional analyses with a receptor mutant of AtPep3 revealed that AtPep3 was recognized by the PEPR1 receptor and that it functions to increase salinity stress tolerance in plants. Collectively, these data indicate that AtPep3 plays a significant role in both salinity stress tolerance and immune response in Arabidopsis.

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Significance Hormone-like peptides derived from small coding genes (<100 amino acids) have not been extensively characterized in relation to abiotic stress tolerance. Focusing on 17 salinity stress-inducible small coding genes in Arabidopsis, we showed that four genes conferred increased salinity stress tolerance when overexpressed in transgenic plants. One of the four genes (AtPROPEP3) was found to induce salinity stress tolerance by treatment with a 13-peptide (KPTPSSGKGGKHN) fragment, providing unique functional evidence for enhanced salinity stress tolerance in plants in response to a peptide treatment. Although the 13-peptide fragment shares homology with known peptides associated with immune response, the other peptides may encode unique hormone-like peptides associated with salinity stress tolerance. Peptides encoded by small coding genes play an important role in plant development, acting in a similar manner as phytohormones. Few hormone-like peptides, however, have been shown to play a role in abiotic stress tolerance. In the current study, 17 Arabidopsis genes coding for small peptides were found to be up-regulated in response to salinity stress. To identify peptides leading salinity stress tolerance, we generated transgenic Arabidopsis plants overexpressing these small coding genes and assessed survivability and root growth under salinity stress conditions. Results indicated that 4 of the 17 overexpressed genes increased salinity stress tolerance. Further studies focused on AtPROPEP3, which was the most highly up-regulated gene under salinity stress. Treatment of plants with synthetic peptides encoded by AtPROPEP3 revealed that a C-terminal peptide fragment (AtPep3) inhibited the salt-induced bleaching of chlorophyll in seedlings. Conversely, knockdown AtPROPEP3 transgenic plants exhibited a hypersensitive phenotype under salinity stress, which was complemented by the AtPep3 peptide. This functional AtPep3 peptide region overlaps with an AtPep3 elicitor peptide that is related to the immune response of plants. Functional analyses with a receptor mutant of AtPep3 revealed that AtPep3 was recognized by the PEPR1 receptor and that it functions to increase salinity stress tolerance in plants. Collectively, these data indicate that AtPep3 plays a significant role in both salinity stress tolerance and immune response in Arabidopsis.

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Significance Hormone-like peptides derived from small coding genes (<100 amino acids) have not been extensively characterized in relation to abiotic stress tolerance. Focusing on 17 salinity stress-inducible small coding genes in Arabidopsis, we showed that four genes conferred increased salinity stress tolerance when overexpressed in transgenic plants. One of the four genes (AtPROPEP3) was found to induce salinity stress tolerance by treatment with a 13-peptide (KPTPSSGKGGKHN) fragment, providing unique functional evidence for enhanced salinity stress tolerance in plants in response to a peptide treatment. Although the 13-peptide fragment shares homology with known peptides associated with immune response, the other peptides may encode unique hormone-like peptides associated with salinity stress tolerance. Peptides encoded by small coding genes play an important role in plant development, acting in a similar manner as phytohormones. Few hormone-like peptides, however, have been shown to play a role in abiotic stress tolerance. In the current study, 17 Arabidopsis genes coding for small peptides were found to be up-regulated in response to salinity stress. To identify peptides leading salinity stress tolerance, we generated transgenic Arabidopsis plants overexpressing these small coding genes and assessed survivability and root growth under salinity stress conditions. Results indicated that 4 of the 17 overexpressed genes increased salinity stress tolerance. Further studies focused on AtPROPEP3, which was the most highly up-regulated gene under salinity stress. Treatment of plants with synthetic peptides encoded by AtPROPEP3 revealed that a C-terminal peptide fragment (AtPep3) inhibited the salt-induced bleaching of chlorophyll in seedlings. Conversely, knockdown AtPROPEP3 transgenic plants exhibited a hypersensitive phenotype under salinity stress, which was complemented by the AtPep3 peptide. This functional AtPep3 peptide region overlaps with an AtPep3 elicitor peptide that is related to the immune response of plants. Functional analyses with a receptor mutant of AtPep3 revealed that AtPep3 was recognized by the PEPR1 receptor and that it functions to increase salinity stress tolerance in plants. Collectively, these data indicate that AtPep3 plays a significant role in both salinity stress tolerance and immune response in Arabidopsis.