A temperature-sensitive auxin auxotroph not deficient in indole-3-acetic acid

The diageotropica gene differentially affects auxin and cytokinin responses throughout development in tomato

The interactions between the plant hormones auxin and cytokinin throughout plant development are complex, and genetic investigations of the interdependency of auxin and cytokinin signaling have been limited. We have characterized the cytokinin sensitivity of the auxin-resistant diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) in a range of auxin- and cytokinin-regulated responses. Intact, etiolated dgt seedlings showed cross-resistance to cytokinin with respect to root elongation, but cytokinin effects on hypocotyl growth and ethylene synthesis in these seedlings were not impaired by the dgt mutation. Seven-week-old, green wild-type and dgt plants were also equally sensitive to cytokinin with respect to shoot growth and hypocotyl and internode elongation. The effects of cytokinin and the dgt mutation on these processes appeared additive. In tissue culture organ regeneration from dgt hypocotyl explants showed reduced sensitivity to auxin but normal sensitivity to cytokinin, and the effects of cytokinin and the mutation were again additive. However, although callus induction from dgt hypocotyl explants required auxin and cytokinin, dgt calli did not show the typical concentration-dependent stimulation of growth by either auxin or cytokinin observed in wild-type cells. Cross-resistance of the dgt mutant to cytokinin thus was found to be limited to a small subset of auxin- and cytokinin-regulated growth processes affected by the dgt mutation, indicating that auxin and cytokinin regulate plant growth through both shared and separate signaling pathways.

A novel root gravitropism mutant of Arabidopsis thaliana exhibiting altered auxin physiology

A root gravitropism mutant was isolated from the DuPont Arabidopsis thaliana T-DNA insertional mutagenesis collection. This mutant has reduced root gravitropism, hence the name rgr1. Roots of rgr1 are shorter than those of wild-type, and they have reduced lateral root formation. In addition, roots of rgr1 coil clockwise on inclined agar plates, unlike wild-type roots which grow in a wavy pattern. The rgr1 mutant has increased resistance, as measured by root elongation, to exogenously applied auxins (6-fold to indole-3-acetic acid, 3-fold to 2,4-dichlorophenoxyacetic acid, and 2-fold to napthyleneacetic acid). It is also resistant to polar auxin transport inhibitors (2-fold to triiodobenzoic acid and 3- to 5-fold to napthylphthalamic acid). The rgr1 mutant does not appear to be resistant to other plant hormone classes. When grown in the presence of 10(-7) M 2,4-dichlorophenoxyacetic acid, rgr1 roots have fewer root hairs than wild type. All these rgr1 phenotypes are Mendelian recessives. Complementation tests indicate that rgr1 is not allelic to previously characterized agravitropic or auxin-resistant mutants. The rgr1 locus was mapped using visible markers to 1.4 +/- 0.6 map units from the CH1 locus at 1-65.4. The rgr1 mutation and the T-DNA cosegregate, suggesting that rgr1 was caused by insertional gene inactivation.

Approaches to understanding auxin action

The effects of auxin on plant growth and development have been studied for decades, but the molecular mechanisms of auxin action remain unknown. These mechanisms have primarily been investigated by characterization of auxin physiology mutants and analysis of auxin-binding proteins and auxin-regulated genes. These efforts are now converging, since some mutants have recently been shown to have altered expression of specific auxin-binding proteins and auxin-regulated genes. The features of these proteins and genes are providing the first tantalizing clues to the organization of auxin signal transduction pathways.

Genetic approaches to auxin action

Answers to long-standing questions concerning the molecular mechanism of auxin action and auxin's exact functions in plant growth and development are beginning to be uncovered through studies using mutant and transgenic plants. We review recent work in this area in vascular plants. A number of conclusions can be drawn from these studies. First, auxin appears essential for cell division and viability, as auxin auxotrophs isolated in tissue culture are dependent on auxin for growth and cannot be regenerated into plants even when auxin is supplied exogenously. Secondly, plants with transgenes that alter auxin levels are able to regulate cellular auxin concentrations by synthesis and conjugation; wild-type plants are probably also capable of such regulation. Thirdly, the phenotypes of transgenic plants with altered auxin levels and of mutant plants with altered sensitivity to auxin confirm earlier physiological studies which indicated a role for auxin in regulation of apical dominance, in development of roots and vascular tissue, and in the gravitropic response. Finally, the cloning of a mutationally identified gene important for auxin action, along with accumulating biochemical evidence, hints at a major role for protein degradation in the auxin response pathway.

The plant hormone auxin: insight in sight

Physiological experiments conducted over the last 60 years indicate that the plant hormone auxin regulates a diverse set of developmental processes via changes in cell division, cell elongation and cell differentiation. Recent studies using transgenic plants with altered auxin levels support these conclusions and promise to provide more detailed information on the role of auxin during plant development. Although it is possible that all auxin responses are mediated by the same primary biochemical events, the studies described in this review are more consistent with multiple modes of auxin action. The development of molecular and genetic approaches to the study of hormone action should resolve this issue. The accelerated rate of progress in this field suggests that real insight into the mechanism of auxin action may be forthcoming.