The axhs1/dwf4 auxin-hypersensitive mutant of Arabidopsis thaliana defines a link for integration of auxin and brassinosteroid mediated root elongation
To understand the molecular mechanism of auxin-mediated root elongation, we carried out screening to isolate auxin-hypersensitive (axhs) mutants. The T-DNA insertional mutant axhs1 was selected on the basis of root phenotypes associated with auxin sensitivity. Results from brassinosteroid (BR) feeding experiments, Tail-PCR and genetic analyses indicate that AXHS1 encodes DWF4, which catalyzes the rate limiting step of BR biosynthesis. The axhs1 mutant shows increased sensitivity to
... ity to indole-3-acetic acid (IAA), the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), auxin transport inhibitors such as 2,3,5-triiodobenzoic acid (TIBA) and N-1-naphthylphthalamic acid (NPA), and the antiauxin p-chlorophenoxyisobutyric acid (PCIB) for the root elongation response. Analysis of the expression of the DR5:GUS and HS:AXR3NT-GUS reporter genes in wild type and axhs1 genotypes, and characterization of double mutants between axhs1 and mutants affected in auxin biosynthesis (wei2-1), auxin transport (aux1-7, eir1-1) and auxin signal transduction (tir1-1, axr1-3, axr2-1) indicate that auxin hypersensitivity in axhs1 is mediated by the auxin-signaling pathway and an AUX1, EIR1/ PIN2 dependent auxin uptake.