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Characterization of xtcl mutant with reduced cuticular wax accumulation Wang, Qian


Cuticular wax is a component of the plant cuticle, the lipid barrier which covers the surface of above ground primary plant organs and plays important protective roles. The isolation of wax deficient mutants from Arabidopsis and other plants resulted in identification and isolation of genes required for wax deposition, and broadened our knowledge of this process in plants. To identify additional components involved in cuticular wax production, I investigated the role of the XTC1 gene, defective in the xtcl (extra cotyledon 1) mutant. This mutant was reported to have reduced levels of cuticular wax on its inflorescence stems and accumulate a large number of oil bodies in the primordia of its extra cotyledons. Stem wax extraction and gas chromatography analysis showed that the total xtcl stem wax is decreased 3-fold in comparison to the wild type, and that all wax components were reduced to a similar extent. Compositional analyses of leaf and seed fatty acids demonstrated that saturated fatty acid content was decreased by around 55%, and unsaturated fatty acid content was approximately 20% lower in xtcl mutants. A detailed examination of xtcl seeds revealed seed deformities, altered seed coat permeability and defective seed mucilage extrusion. Positional cloning of the XTC1 gene resulted in the discovery that it is identical to FATB, an already characterized gene known to encode the fatty acid thioesterase B. The FATB enzyme releases saturated free fatty acids (C16:0and C18:0) from ACP in the plastid and allows their export across the plastid envelope. Analysis of FATB gene expression pattern showed that FATB is transcribed ubiquitously in all tissues and in different development stages. It is therefore not surprising that FATB disruption results in multiple lipid associated phenotypes, including decreased cuticular wax amounts and altered fatty acid compositions of leaves and seeds. Additional phenotypes caused by mutations in FATB that affect embryo and seed development and lead to appearance of extra cotyledons, altered permeability of the seed coat and defective seed mucilage extrusion are difficult to explain at present.

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