| The plant hormone abscisic acid (ABA) influences late embryogenesis, seed dormancy and stomatal conductance (Davies, 1995). In Arabidopsis thaliana, genes involved in ABA signal transduction have been identified in screens for mutants with altered sensitivity to ABA in germination. The ENHANCED RESPONSE TO ABSCIS1C ACID1 (ERA1) and ABSCISIC ACID INSENSITVE3 (ABI3) genes were isolated in such screens. ERA1 encodes the beta-subunit of a farnesyl transferase while ABI3 encodes a B3 domain family transcription factor. However, mutations in ERA1 and ABI3 differ in their mutant phenotypes when compared to other ABA signaling and biosynthetic genes, particularly in terms of their developmental abnormalities. ABI3 was proposed to act as an instructive regulator of late embryogenesis, as ectopic expression of ABI3 in Arabidopsis vegetative tissue induces seed-specific genetic programmes in the presence of ABA. ERA1 is positioned genetically upstream of ABI3, and ERA1 was shown to negatively regulate ABI3 expression.; This thesis addresses the role of ABI3 as a developmental regulator outside of the seed, and describes the identification of regulators of ABI3 expression. In addition to modulating sensitivity to ABA in the seed, farnesylation and ABI3 regulate auxin sensitivity in the root. In turn, ABI3 is an auxin-responsive regulator of lateral root development. A bioinformatic analysis of the ABI3 promoter identified 21 statistically over-represented 6-base-pair elements, and two of these were analyzed for their endogenous ABI3 expression. Together with a novel, genetic approach, this work was successful in identifying two regulators of ABI3 expression. A Dof transcription factor, Dof Affecting Germination2 (DAG2), is a negative regulator of ABI3 and ERA1 expression, and is genetically upstream of ERA1. The stripe 1-1 (stp1-1) mutation was isolated as a suppressor of the era1-2 mutation that negatively regulates ABI3 expression in the root. |
