Genetic analysis of auxin homeostasis: Conjugate sensitivity and auxin supersensitivity
Doctor of Philosophy
Through their regulation of gene expression and cell growth, auxins affect many aspects of plant growth and development. One mechanism through which auxin homeostasis is achieved is by conjugating indole-3-acetic acid (IAA, the most common natural form of auxin) to amino acids or sugars. The Arabidopsis mutant ilr2-1 was isolated as an I&barbelow;AA-L&barbelow;eucine r&barbelow;esistant mutant that retains wild-type sensitivity to free IAA. ilr2-1 is resistant to IAA-Leu and IAA-Phe, but not to other conjugated forms of IAA. It is also resistant to cobalt and manganese. ilr2-1 has fewer lateral roots than wild type, which can be rescued by exogenous IAA conjugates; and it has a shorter root than wild type, which can be rescued by exogenous metals, including cobalt. Using a map-based approach, I cloned the ILR2 gene, which encodes a novel protein that is polymorphic between Arabidopsis ecotypes. A T-DNA insertion in the second form of ILR2, ilr2-2, results in cobalt supersensitivity. The ILR2 transcript appears to be expressed at very low levels, and accumulates in aerial tissues and in response to exogenous cobalt. Auxin resistant mutant screens have provided valuable information about how auxin is sensed and regulated; they also suggest that a considerable part of auxin homeostasis is dependent on negative regulation. To identify these negative regulators, we developed a genetic screen that allows the identification of three a&barbelow;ux&barbelow;in s&barbelow;upersensitive (axs) mutants. These mutants exhibit abnormal responses in conditions leading to auxin accumulation, such as auxin transport inhibitors and growth at high temperatures. We used recombination mapping to identify the genes defective in these mutants. The axs1 mutant that is resistant to low concentrations of auxin transport inhibitors, and the axs2 mutant that has a higher number of lateral roots than wild type. Both mutations were mapped, based on a defective root-curling phenotype, to the region in chromosome 1 contained between the markers nga280 and nga111; this mapping location may represent a QTL involved in root-curling. The axs3 mutant, supersensitive to high temperature, was mapped to the region in chromosome I contained between the markers nga62 and nga280. The analysis of mutants such as axs3 is expected to contribute to our understanding of how auxin levels are spatially and temporally regulated in the plant.