Odyssey of auxin

Abstract

The history of plant biology is inexorably intertwined with the conception and discovery of auxin, followed by the many decades of research to comprehend its action during growth and development. Growth responses to auxin are complex and require the coordination of auxin production, transport, and perception. In this overview of past auxin research, we limit our discourse to the mechanism of auxin action. We attempt to trace the almost epic voyage from the birth of the hormonal concept in plants to the recent crystallographic studies that resolved the TIR1-auxin receptor complex, the first structural model of a plant hormone receptor. The century-long endeavor is a beautiful illustration of the power of scientific reasoning and human intuition, but it also brings to light the fact that decisive progress is made when new technologies emerge and disciplines unite.

Figure 1.

The progression of models for the regulation of auxin-inducible genes by derepression. The model on the left was proposed by Theologis (1986) based on published data (Theologis et al. 1985) and suggests control of primary genes by a short-lived protein repressor (R) that inhibits a transcriptional activator (A). Ten years later, this model was refined (center panel) after analyzing the auxin-responsive region of the PS-IAA4/5 promoter by linker scanning mutations, which identified two domains (A and B). Domain A contains typical auxin-responsive DNA elements (AuxRE) and both domains interact with positive transcription factors (Ballas et al. 1995), presumably with auxin-response factors (ARF) (Ulmasov et al. 1997). The current simplified model (right panel) reflects the progress made during the past 15 years by combining molecular, biochemical, and foremost genetic approaches taken by several laboratories (see text), which culminated in the identification of TIR1 as an auxin receptor (Dharmasiri et al. 2005; Kepinski and Leyser 2005) and in the structural model of a TIR1-auxin complex in association with a degron peptide of Aux/IAA domain II (Tan et al. 2007).

Figure 2.

Advances in auxin research. A reductionist approach to the understanding of auxin action (indicated by the vertical trapeze) was facilitated (1) by the development of new technologies, such as adopting recombinant DNA technology, or engineering GFP (green fluorescent protein) to a noninvasive intracellular reporter system (Chalfie et al. 1994), (2) by the introduction of Arabidopsis thaliana as a model and reference organism for research in plant biology and genomics (Meyerowitz and Pruitt 1985; Arabidopsis Genome Initiative 2000), and (3) by the synergism of merging different disciplines. The inset shows two classic biological systems for studying auxin response, dark-grown pea and Arabidopsis seedlings.