Characterization of Arabidopsis activation-tagged fumonisin B1-resistant (fbr) mutants in programmed cell death (PCD) and plant development

A forward molecular genetics approach was used to isolate gh3.9-1 and D-fbr7 from populations of Arabidopsis activation-tagged T-DNA mutants in a germination/development-based selection scheme to identify fumonisin B1-resistant (fbr) mutants. The characterization of the functions of affected genes in gh3.9-1 and D-fbr7 is the primary focus of the studies presented in this thesis. The GH3.9 gene, belonging to the Arabidopsis Group II primary auxin-responsive GH3 gene family, was found to be affected by the T-DNA insertion in the gh3.9-1 mutant. Phenotypic characterization studies performed with gh3.9-1 mutant along with GH3.9 RNAi lines and additional T-DNA insertions identified in the same region, revealed that GH3.9 might be involved in root development-related processes, as the mutants displayed a long-root length phenotype. GH3.9 expression analysis revealed that GH3.9 transcript accumulation was repressed in seedlings treated with exogenous IAA treatment. Furthermore, gh3.9-1 T-DNA insertions and RNAi lines showed sensitivity to IAA-mediated root growth inhibition and resistance towards Methyl Jasmonate-mediated root growth inhibition. These results have provided new functions for GH3.9 in roots, where it likely controls auxin activity through its amino acid conjugation activity. D-fbr7 was identified as a novel FB1-resistant ( fbr) mutant that was subsequently also found to be defective in embryo development. Different physiological assays conducted to determine resistance of D-fbr7 to FB1-induced cell death at different stages of plant development showed that at both seedling and mature development stages, D-fbr7 displayed resistance to FB1 and FB1-induced senescence. Expression analysis revealed that CLE19, member of CLAVATA3-LIKE (CLE) gene family is involved in causing the fbr phenotype and small RNAs identified in the intergenic region adjacent to D-fbr7 might be involved in conferring embryo lethality. These small RNAs identified by web-based computational approaches were validated by northern expression analysis. These studies have provided evidence of novel roles for CLE19 and small RNAs in affecting plant development