Conserved Function of the Plant ABC Transporter Comatose (ABCD) in the Control of Peroxisomal Beta-Oxidation.

The Arabidopsis COMATOSE (ABCD) gene was originally identified in a genetic screen for loci that promote germination. ABCD is a single-copy gene encoding a full-length ATP-binding cassette transporter that is required for the import of several biologically important molecules into the peroxisome. Severe mutant alleles of ABCD demonstrate an inability to complete germination, suggesting a role for ABCD in the dormancy to radicle protrusion transition, and protein profiles of cts seeds resemble those of the dormant wild type. We have obtained and characterized mutants of the barley orthologue of ABCD. Barley has two ABCD genes and there are two locations where ß-oxidation occurs in germinating grain. In barley the breakdown of starch in the endosperm is the main source of energy during seedling establishment. However, the embryo and the aleurone have significant amounts of stored lipid, and ß-oxidation occurs in both locations. The auxin precursors, 2,4-dichlorophe- noxybutyric acid (2,4-DB) and indole butyric acid (IBA), which are converted by peroxisomal (-oxidation to 2,4-D and IAA, respectively, have been used to isolate mutants in this pathway by virtue of their long root phenotype when grown on 2,4-DB and IBA. Previous works showed that two alleles containing mutations in NBD1 exhibited resistance to auxin precursors 2,4-DB and IBA. The aim of this project is to apply this knowledge to a crop of commercial significance (barley) to ascertain the role of ABCD in barley physiology and development. We used Arabidopsis thaliana and rice sequences to identify the barley orthologue of the AtABCD gene, and we used barley genetic resources to analyze the comparative function of this gene. Using the SCRI barley TILLING resource we have identified mutations in the 2 barley ABCD orthologous genes HvABCD1 and HvABCD2, and are looking for phenotypic effects of these mutations. We are also analyzing HvABCD RNAi lines obtained by transformation. Extending knowledge of candidate genes obtained from the model Arabidopsis into commercial crops such as barley may provide a mechanism to improve malting quality and/or the dormancy characteristics of this crop.