We previously showed that the VASCULAR-RELATED NAC-DOMAIN6 (VND6) and VND7 genes, which encode NAM/ ATAF/CUC domain protein transcription factors, act as key regulators of xylem vessel differentiation. Here, we report a glucocorticoid-mediated posttranslational induction system of VND6 and VND7. In this system, VND6 or VND7 is expressed as a fused protein with the activation domain of the herpes virus VP16 protein and hormone-binding domain of the animal glucocorticoid receptor, and the protein's activity is induced by treatment with dexamethasone (DEX), a glucocorticoid derivative. Upon DEX treatment, transgenic Arabidopsis (Arabidopsis thaliana) plants carrying the chimeric gene exhibited transdifferentiation of various types of cells into xylem vessel elements, and the plants died. Many genes involved in xylem vessel differentiation, such as secondary wall biosynthesis and programmed cell death, were up-regulated in these plants after DEX treatment. Chemical analysis showed that xylan, a major hemicellulose component of the dicot secondary cell wall, was increased in the transgenic plants after DEX treatment. This induction system worked in poplar (Populus tremula 3 tremuloides) trees and in suspension cultures of cells from Arabidopsis and tobacco (Nicotiana tabacum); more than 90% of the tobacco BY-2 cells expressing VND7-VP16-GR transdifferentiated into xylem vessel elements after DEX treatment. These data demonstrate that the induction systems controlling VND6 and VND7 activities can be used as powerful tools for understanding xylem cell differentiation.Secondary cell wall formation is one of the characteristic features of plant cells. This structure is found in several types of plant cells, such as xylem vessels and fiber cells, as additional thick layers between the primary cell wall and plasma membrane. The major components of primary cell wall are cellulose, hemicellulose, and pectin. Secondary wall contains a wide range of additional compounds, e.g. a complex phenolic polymer, lignin, in xylem cells, which are believed to modify mechanical properties and permeability of walls.Secondary wall formation has been intensively studied during xylem cell differentiation. Recently, several NAM/ATAF/CUC (NAC) domain protein transcription factors were shown to play crucial roles in specification into distinct xylem cells (Demura and Fukuda, 2007;. The genes for VASCULAR-RELATED NAC-DOMAIN1 (VND1) through VND7 are preferentially expressed in differentiating xylem vessels (Kubo et al., 2005;Yamaguchi et al., 2008), and the overexpression of VND6 and VND7 can induce the ectopic differentiation of metaxylem-like vessels and protoxylem-like
A temperature-sensitive, elongation-deficient mutant of Arabidopsis thaliana was isolated. At the non-permissive temperature of 31 degrees C, the mutation impaired tissue elongation; otherwise, tissue development was normal. Hypocotyl cells that had established cell walls at 21 degrees C under light-dark cycles ceased elongation and swelled when the mutant was shifted to 31 degrees C and darkness, indicating that the affected gene is essential for cell elongation. Analysis of the cell walls of mutant plants grown at 31 degrees C revealed that the cellulose content was reduced to 40% and the pectin content was increased to 162% of the corresponding values for the wild type grown at the same temperature. The increased amounts of pectin in the mutant were bound tightly to cellulose microfibrils. No change in the content of hemicellulose was apparent in the 31 degrees C-adapted mutant. Field emission-scanning electron microscopy suggested that the structure of cellulose bundles was affected by the mutation; X-ray diffraction, however, revealed no change in the crystallite size of cellulose microfibrils. The regeneration of cellulose microfibrils from naked mutant protoplasts was substantially delayed at 31 degrees C. The recessive mutation was mapped to chromosome V, and map-based cloning identified it as a single G-->A transition (resulting in a Gly(429)-->Arg substitution) in KORRIGAN, which encodes a putative membrane-bound endo-1,4-beta-glucanase. These results demonstrate that the product of this gene is required for cellulose synthesis.
;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 anionexchange 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 15,000 and 11,000, 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.
Feruloyl-CoA:arabinoxylan-trisaccharide O-hydroxycinnamoyl transferase, which catalyzes the transfer of ferulic acid from Fer-CoA to arabinoxylan-trisaccharide in the formation of feruloyl arabinoxylan-trisaccharide (Fer-AXX), has been found in an ionically bound fraction and a cytosol fraction of suspension-cultured rice (Oriza sativa L. cv. Nipponbare) cells. Analysis of reaction products by high-performance liquid chromatography showed the formation of product A, which is one of the transfer products having the same retention time as authentic Fer-AXX. Product A was purified by reverse-phase chromatographies to characterize its structure. The isolated product A showed the same ultraviolet spectrum and molecular weight on fast atom bombardment mass spectrometric analysis as those of authentic Fer-AXX. Alakaline saponification of product A released ferulic acid and oligosaccharide. The released oligosaccharide consisted of arabinose and xylose in a molar ratio of 1:2. These results support the identity of product A as feruloylated arabinoxylan-trisaccharide and show the existence of a feruloyltransferase catalyzing the feruloylation of a hemicellulosic fragment.
We examined concentrations of boron (B) and dimerization of rhamnogalacturonan II (RG‐II), a B‐binding polysaccharide, in the cell wall of a low‐B sensitive mutant of Arabidopsis thaliana, bor1‐1, to investigate possible effects of the bor1‐1 mutation on the biochemical form of pectins in the cell wall. In the bor1‐1 mutant, B concentrations in the cell wall from shoots were lower than those in the wild type at low B supply, whereas they were similar at sufficient B supply. The amount of B present as borate ester of the RG‐II dimer (dRG‐II‐B) in the bor1‐1 mutant was lower than that in the wild type at low B supply. In the wild type, about 90 % of RG‐II was present as dRG‐II‐B, both, at low and sufficient B supply. In the bor1‐1 mutant, about 60 % of RG‐II was in its monomeric form (mRG‐II) at low B supply, whereas more than 85 % of it was present as dRG‐II‐B at sufficient B supply. However, similar as the wild type, mRG‐II derived from the bor1‐1 mutant was able to form dRG‐II‐B in vitro in the presence of borate and lead. Sugar composition of cell wall fractions was similar in both genotypes. These results suggest that the polysaccharide composition in the cell wall was not strongly affected by the bor1‐1 mutation. The observed difference in dimerization of RG‐II at low B supply is most likely due to a reduced B concentration in the shoots of the bor1‐1 mutant.
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