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Hiroaki Iwai

Publications and source records attributed to Hiroaki Iwai.

7 recordsLinked to original sources
PubMed2006

The gene responsible for borate cross-linking of pectin Rhamnogalacturonan-II is required for plant reproductive tissue development and fertilization.

Deficiencies in boron, a microelement that is essential for the growth and development of higher plants, often cause problems in reproductive growth. Rhamnogalacturonan-II (RG-II) in cell wall pectin acts as the sole receptor for boron in plant cells, forming a borate cross-linked RG-II dimer (dRG-II-B), but the physiological functions of dRG-II-B remain unknown. We have previously shown that the pectin glucuronyltransferase 1 gene NpGUT1, which is involved in the biosynthesis of RG-II sugar chains, is essential for the formation of the RG-II-B complex, resulting in tight intercellular attachment in meristematic tissues. Because NpGUT1 expression was found to be abundant in reproductive organs in addition to meristematic tissues, we analyzed the expression and functions of NpGUT1 in more detail in tobacco reproductive tissues. Specific NpGUT1 expression was detected in the tapetum of flower buds and in the pollen, pollen tube tips, and transmitting tissue of the pistils of flowers. Dexamethasone-induced expression of the NpGUT1 antisense gene in flower buds resulted in the formation of sterile flowers with aberrant development of pollen and transmitting tissue. Pollen tubes could not pass through pistils with aborted transmitting tissue, and expression of an NpGUT1 antisense gene in germinating pollen inhibited pollen tube elongation, accompanied by the absence of pectin RG-II and boron in the pollen tube tip. These results indicate that expression of NpGUT1 is required for the development and functions of male and female tissues.

Borates↗
PubMed2003

Analysis of sugars in squash xylem sap.

Xylem sap contains organic and inorganic compounds that might be involved in root-to-shoot communication. To clarify the physiological functions of sugars in xylem sap, we characterized the sugar compounds of the xylem sap. The 80% ethanol-soluble fraction of xylem sap contained mainly myo-inositol and oligosaccharides. The 80% ethanol precipitate was solubilized with cyclohexanediamine tetraacetate and fractionated using anion exchange chromatography. The non-bound fraction from the anion-exchange column reacted with Yariv reagent and was rich in arabinogalactan, indicating the presence of arabinogalactan proteins (AGP). The bound fraction eluted with 50 mM ammonium formate buffer and separated using size exclusion chromatography producing the pectins rhamnogaracturonan (RG)-I and RG-II with apparent molecular masses of 15000 and 11000, respectively. These results indicate that the AGP, RG-I, borate cross-linked RG-II dimer and oligosaccharides produced by root tissues are transported to above-ground organs via xylem sap.

Carbohydrate Metabolism↗
PubMed2003

Mucilage secretion regulated by sex pheromones in Closterium peracerosum-strigosum-littorale complex.

When mating-type plus (mt+) and minus (mt-) cells of the Closterium peracerosum-strigosum-littorale complex were mixed in nitrogen-depleted mating medium, secretion of mucilage containing uronic acid from cells was markedly activated and the mucilage accumulated around the cells. Substances with the ability to stimulate mucilage secretion from mt+ and mt- cells were detected in media in which mt- and mt+ cells had been separately cultured, respectively. We designated the active substances secreted from mt+ and mt- cells mucilage secretion-stimulating pheromone (MS-SP)-plus and MS-SP-minus, respectively. Activity of MS-SP-plus and MS-SP-minus decreased to 20% level by incubation at 80 degrees C for 10 min. Light was indispensable for the secretion of mucilage. The secretion of MS-SP-plus and MS-SP-minus decreased with dark treatment. MS-SP-plus eluted at around 95 k from a gel filtration column, and reacted with antibodies against two subunits of protoplast-release-inducing protein (PR-IP), which induces protoplast release from mt- cells. MS-SP-minus eluted at around 20 k from a gel filtration column, and reacted with an antibody against the PR-IP inducer, which induces the secretion of PR-IP from mt+ cells. In addition, purified PR-IP and PR-IP inducer stimulated mucilage secretion from mt- and mt+ cells, respectively. These results strongly suggested that MS-SP-plus and MS-SP-minus were the same molecules as the PR-IP and the PR-IP inducer, respectively.

Adhesives↗
PubMed2002

A pectin glucuronyltransferase gene is essential for intercellular attachment in the plant meristem.

Intercellular attachment is an essential process in the morphogenesis of multicellular organisms. A unique mutant, nolac-H18 (nonorganogenic callus with loosely attached cells), generated by T-DNA transformation using leaf-disk cultures of haploid Nicotiana plumbaginifolia, lost the ability to form tight intercellular attachments and adventitious shoots. The gene tagged with T-DNA, named NpGUT1 (glucuronyltransferase 1), was similar to the gene for the catalytic domains of animal glucuronyltransferases and was expressed predominantly in shoot and root apical meristems. The transformation of NpGUT1 complemented the nolac-H18 mutation, and the expression of antisense NpGUT1 RNA produced crumbled shoots. The mutation caused defects in the glucuronic acid of rhamnogalacturonan II of pectin, which drastically reduced the formation of borate cross-linking of rhamnogalacturonan II. NpGUT1, which encodes a unique glucuronyltransferase, is a glycosyltransferase gene identified in pectin biosynthesis and is essential for intercellular attachment in plant meristems and tissues.

Amino Acid Sequence↗
PubMed2002

Germanium does not substitute for boron in cross-linking of rhamnogalacturonan II in pumpkin cell walls.

Boron (B)-deficient pumpkin (Cucurbita moschata Duchesne) plants exhibit reduced growth, and their tissues are brittle. The leaf cell walls of these plants contain less than one-half the amount of borate cross-linked rhamnogalacturonan II (RG-II) dimer than normal plants. Supplying germanium (Ge), which has been reported to substitute for B, to B-deficient plants does not restore growth or reduce tissue brittleness. Nevertheless, the leaf cell walls of the Ge-treated plants accumulated considerable amounts of Ge. Dimeric RG-II (dRG-II) accounted for between 20% and 35% of the total RG-II in the cell walls of the second to fourth leaves from Ge-treated plants, but only 2% to 7% of the RG-II was cross-linked by germanate (dRG-II-Ge). The ability of RG-II to form a dimer is not reduced by Ge treatment because approximately 95% of the monomeric RG-II generated from the walls of Ge-treated plants is converted to dRG-II-Ge in vitro in the presence of germanium oxide and lead acetate. However, dRG-II-Ge is unstable and is converted to monomeric RG-II when the Ge is removed. Therefore, the content of dRG-II-Ge and dRG-II-B described above may not reflect the actual ratio of these in muro. (10)B-Enriched boric acid and Ge are incorporated into the cell wall within 10 min after their foliar application to B-deficient plants. Foliar application of (10)B but not Ge results in an increase in the proportion of dRG-II in the leaf cell wall. Taken together, our results suggest that Ge does not restore the growth of B-deficient plants.

Boric Acids↗
PubMed2002

Gibberellin produced in the cotyledon is required for cell division during tissue reunion in the cortex of cut cucumber and tomato hypocotyls.

Cucumber (Cucumis sativus) hypocotyls were cut to one-half of their diameter transversely, and morphological and histochemical analyses of the process of tissue reunion in the cortex were performed. Cell division in the cortex commenced 3 d after cutting, and the cortex was nearly fully united within 7 d. 4',6-Diamidino-2-phenylindole staining and 5-bromo-2'-deoxyuridine labeling experiments indicate that nDNA synthesis occurred during this process. In addition, specific accumulation of pectic substances was observed in the cell wall of attached cells in the reunion region of the cortex. Cell division during tissue reunion was strongly inhibited when the cotyledon was removed. This inhibition was reversed by applying gibberellin (GA, 10(-4) M GA3) to the apical tip of the cotyledon-less plant. Supporting this observation, cell division in the cortex was inhibited by treatment of the cotyledon with 10(-4) M uniconazole-P (an inhibitor of GA biosynthesis), and this inhibition was also reversed by simultaneous application of GA. In contrast to the essential role of cotyledon, normal tissue reunion in cut hypocotyls was still observed when the shoot apex was removed. The requirement of GA for tissue reunion in cut hypocotyls was also evident in the GA-deficient gib-1 mutant of tomato (Lycopersicon esculentum). Our results suggest that GA, possibly produced in cotyledons, is essential for cell division in reuniting cortex of cut hypocotyls.

Bromodeoxyuridine↗