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

A severe lignin mutant, irx4, has been identified in Arabidopsis thaliana a s a result of its collapsed xylem phenotype. In contrast to previously desc ribed irx mutants, irx4 plants have 50% less lignin than wildtype plants, w hilst the cellulose and hemicellulose content remained unchanged. These alt erations in the composition of irx4 secondary cell walls had a dramatic eff ect on the morphology and architecture of the walls, which expand to fill m ost of the cell, and also on the physical properties of irx4 stems. Further analysis indicated that the irx4 mutation occurred in a cinnamoyl-CoA redu ctase (CCR) gene within a highly conserved intron splice site sequence of i ntron 2. As a result, CCR mRNA transcripts were incorrectly spliced. Transg enic plants expressing an IRX3 promoter-CCR cDNA construct were used to gen erate a series of plants with varying degrees of lignin content in order to assess the role of lignin content in determining the physical properties o f Arabidopsis stems.