Analysis of secondary cell wall formation in Arabidopsis

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ABSTRACT A genetic approach has been taken to study secondary cell wall formation in Arabidopsis . Xylem elements that fail to form a secondary cell wall are unable to withstand the negative pressure generated during water transport and collapse inwards. We have named plants that exhibit these collapsed xylem phenotype irregular xylem (irx) mutants. To date, of the 5 irx complementation groups identified, one ( irx4 ) is deficient in lignin deposition and the remaining 4 are deficient in cellulose deposition. Although the secondary cell walls of irx4 plants have wild type levels of cellulose and xylan, they are greatly expanded, demonstrating the importance of lignin in cross-linking components of the secondary cell wall. The gene defective in irx4 is one of 10 genes identified in Arabidopsis that appear to encode Cinnamoyl CoA Reductase. We have demonstrated that irx3 is caused by a mutation in a member of the CesA gene family (AtCesA7). Furthermore, it has been shown that mutations in irx1 are caused by a defect in another member of the same gene family. We have shown that both of these gene products are essential for cellulose synthesis in the same cell types and that IRX1 and IRX3 interact directly as components of the same cellulose synthase complex. More recently, we have demonstrated that irx5 is caused by a defect in AtCesA4. We are currently studying the reasons why IRX1, IRX3 and IRX5 are all essential for cellulose synthesis and how different CesA family members are organised within the cellulose synthase complex.