Reduced Naphthylphthalamic Acid Binding in the tir3 Mutant of Arabidopsis Is Associated with a Reduction in Polar Auxin Transport and Diverse Morphological Defects

Abstract

Polar auxin transport plays a key role in the regulation of plant growth and development. To identify genes involved in this process, we have developed a genetic procedure to screen for mutants of Arabidopsis that are altered in their response to auxin transport inhibitors. We recovered a total of 16 independent mutants that defined seven genes, called TRANSPORT INHIBITOR RESPONSE (TIR) genes. Recessive mutations in one of these genes, TIR3, result in altered responses to transport inhibitors, a reduction in polar auxin transport, and a variety of morphological defects that can be ascribed to changes in indole-3-acetic acid distribution. Most dramatically, tir3 seedlings are strongly deficient in lateral root production, a process that is known to depend on polar auxin transport from the shoot into the root. In addition, tir3 plants display a reduction in apical dominance as well as decreased elongation of siliques, pedicels, roots, and the inflorescence. Biochemical studies indicate that tir3 plants have a reduced number of N-1-naphthylphthalamic (NPA) binding sites, suggesting that the TIR3 gene is required for expression, localization, or stabilization of the NPA binding protein (NBP). Alternatively, the TIR3 gene may encode the NBP. Because the tir3 mutants have a substantial defect in NPA binding, their phenotype provides genetic evidence for a role for the NBP in plant growth and development.

TheTRANSPORT INHIBITOR RESPONSE2Gene Is Required for Auxin Synthesis and Diverse Aspects of Plant Development

“…The mutant displays an NPAresistant phenotype similar to other tir mutants, such as tir1 and tir3 (Ruegger et al, 1997). When grown on 5 mM NPA, wild-type seedlings exhibited approximately 60% root elongation after 3 d compared to untreated controls, while tir2-1 seedlings displayed 75% root elongation (Fig.…”

“…Thus, mutants that are resistant to NPA may have defects in synthesis, transport, or response because roots of these mutants are expected to have lower levels of IAA or reduced sensitivity to IAA. This screen succeeded in isolating mutations in seven genes with weak NPAresistant phenotypes, including genes related to auxin signaling (TIR1), auxin transport (TIR3), and auxin synthesis (TIR7; Ruegger et al, 1997Ruegger et al, , 1998Ljung et al, 2005).…”

“…To identify genes that are required for auxin synthesis, transport, and signaling, we previously screened for Arabidopsis (Arabidopsis thaliana) mutants that are resistant to auxin transport inhibitors, such as N-1-napthylpthalamic (NPA; Ruegger et al, 1997). The treatment of seedlings with NPA results in auxin accumulation in the root tip (Ljung et al, 2005).…”

Cytokinin-Deficient Transgenic Arabidopsis Plants Show Multiple Developmental Alterations Indicating Opposite Functions of Cytokinins in the Regulation of Shoot and Root Meristem Activity

“…A subsequent series of highly ordered transverse cell divisions leads to the emergence of the newly formed lateral root (Malamy and Benfey, 1997). Numerous studies have shown that auxin, and in particular its polar transport, is necessary for the initiation and the later development of lateral roots (Ruegger et al, 1997;Reed et al, 1998;Casimiro et al, 2001). The initiation phase of lateral root development is dependent on basipetal polar auxin transport and can be blocked by the auxin transport inhibitor N-1-naphthylphthalamic acid (Casimiro et al, 2001).…”

Auxin Transport Is Required for Hypocotyl Elongation in Light-Grown but Not Dark-Grown Arabidopsis1

“…The endogenous auxin IAA does not compete with NPA for this binding site (Lomax et al, 1995). In the tir3 mutant of Arabidopsis, reduced NPA binding is associated with defects in auxin-regulated growth processes, suggesting that the NPA-binding site is important for auxin transport (Ruegger et al, 1997).…”