The molecular and physiological characterization of the <i>axr4-3</i> enhancer mutant lines WM20.2 and WM22.4 of <i>Arabidopsis thaliana</i>
Auxin is a hormone essential for plant growth and development that affects many areas of development including root development, gravitropic and phototropic responses. The auxin-resistant 4 (axr4) mutant of Arabidopsis is characterized by auxin-resistant roots with reduced lateral root growth and gravitropic response. The auxin importer AUX1 protein is mislocalized in axr4 mutants indicating that the AXR4 protein is necessary for proper AUX1 localization and regulates its function in membrane trafficking. To study the function of AXR4 protein, we focused on its enhancers. In previous characterization of some axr4-3 enhancer lines, they showed more defective phenotypes compared to axr4-3, such as more resistance to 2,4-D and IAA, less lateral root formation and reduced auxin uptake. Thus, the products of the enhancer genes and the AXR4 protein may be both involved in a pathway which regulates AUX1's function. In this project, two new axr4-3 enhancer lines were characterized: WM20.2 and WM22.4. They both showed higher relative root elongations compared to axr4-3 and wild type under the treatments of ethylene (ACC: 1-aminocyclopropane-1-carboxylic-acid) and cytokinin (kinetin) which indicated that they were resistant to these two hormones under the concentrations of 0.3 µM and 3.0 µM. They displayed less lateral root formations compared to axr4-3 and wild types under the treatments of ACC and kinetin. In the morphology test, both double mutant plants were shorter and had fewer flowers, fewer siliques (on their primary inflorescences) than axr4-3 and wild type. The enhancer lines also behaved similarly to previous double mutants during the auxin uptake assay, showing reduced auxin uptake compared to axr4-3. The result of the WM20.2, axr4-3 enhancer gene mapping showed this gene's position is located within a 0.97 Mbp region on chromosome V, and 277 genes were included in this range. Further characterization of these enhancer lines will provide a better understanding of axr4.