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      Cloning and characterization of irregular xylem4 (irx4): a severely lignin-deficient mutant of Arabidopsis.

      The Plant Journal
      Aldehyde Oxidoreductases, genetics, Alleles, Arabidopsis, metabolism, Biomechanical Phenomena, Cell Wall, chemistry, ultrastructure, Cellulose, biosynthesis, Chromosome Mapping, Cloning, Molecular, Genetic Complementation Test, Lignin, Molecular Sequence Data, Monosaccharides, analysis, Mutation, Phenotype, Phylogeny, Plant Proteins, Plant Stems, Plants, Genetically Modified, Polysaccharides, RNA Splice Sites, RNA, Messenger, RNA, Plant

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          Abstract

          A severe lignin mutant, irx4, has been identified in Arabidopsis thaliana as a result of its collapsed xylem phenotype. In contrast to previously described irx mutants, irx4 plants have 50% less lignin than wild-type plants, whilst the cellulose and hemicellulose content remained unchanged. These alterations in the composition of irx4 secondary cell walls had a dramatic effect on the morphology and architecture of the walls, which expand to fill most of the cell, and also on the physical properties of irx4 stems. Further analysis indicated that the irx4 mutation occurred in a cinnamoyl-CoA reductase (CCR) gene within a highly conserved intron splice site sequence of intron 2. As a result, CCR mRNA transcripts were incorrectly spliced. Transgenic plants expressing an IRX3 promoter-CCR cDNA construct were used to generate a series of plants with varying degrees of lignin content in order to assess the role of lignin content in determining the physical properties of Arabidopsis stems.

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          A simple and rapid preparation of alditol acetates for monosaccharide analysis

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            Lignin: occurrence, biogenesis and biodegradation.

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              Collapsed xylem phenotype of Arabidopsis identifies mutants deficient in cellulose deposition in the secondary cell wall.

              Recessive mutations at three loci cause the collapse of mature xylem cells in inflorescence stems of Arabidopsis. These irregular xylem (irx) mutations were identified by screening plants from a mutagenized population by microscopic examination of stem sections. The xylem cell defect was associated with an up to eightfold reduction in the total amount of cellulose in mature inflorescence stems. The amounts of cell wall-associated phenolics and polysaccharides were unaffected by the mutations. Examination of the cell walls by using electron microscopy demonstrated that the decreases in cellulose content of irx lines resulted in an alteration of the spatial organization of cell wall material. This suggests that a normal pattern of cellulose deposition may be required for assembly of lignin or polysaccharides. The reduced cellulose content of the stems also resulted in a decrease in stiffness of the stem material. This is consistent with the irregular xylem phenotype and suggests that the walls of irx plants are not resistant to compressive forces. Because lignin was implicated previously as a major factor in resistance to compressive forces, these results suggest either that cellulose has a direct role in providing resistance to compressive forces or that it is required for the development of normal lignin structure. The irx plants had a slight reduction in growth rate and stature but were otherwise normal in appearance. The mutations should be useful in facilitating the identification of factors that control the synthesis and deposition of cellulose and other cell wall components.
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