An auxin research odyssey: 1989-2023

Abstract

The phytohormone auxin is at times called the master regulator of plant processes and has been shown to be a central player in embryo development, the establishment of the polar axis, early aspects of seedling growth, as well as growth and organ formation during later stages of plant development. The Plant Cell has been key, since the inception of the journal, to developing an understanding of auxin biology. Auxin-regulated plant growth control is accomplished by both changes in the levels of active hormones and the sensitivity of plant tissues to these concentration changes. In this historical review, we chart auxin research as it has progressed in key areas and highlight the role The Plant Cell played in these scientific developments. We focus on understanding auxin-responsive genes, transcription factors, reporter constructs, perception, and signal transduction processes. Auxin metabolism is discussed from the development of tryptophan auxotrophic mutants, the molecular biology of conjugate formation and hydrolysis, indole-3-butyric acid metabolism and transport, and key steps in indole-3-acetic acid biosynthesis, catabolism, and transport. This progress leads to an expectation of a more comprehensive understanding of the systems biology of auxin and the spatial and temporal regulation of cellular growth and development.

Figure 1.

Auxin signaling. A) Models proposed in the late 1980s for how the regulation of the auxin-inducible genes might be envisioned (Redrawn from Theologis 1986). “R” indicates a repressor and “A” an activator protein. B) A current diagram of auxin signaling, illustrating the remarkable progress in the last 35 years of research. A historically interesting progression of models for the regulation of auxin-inducible genes by derepression, as envisioned in 1985, 1995, and 2005, is diagrammed in Abel and Theologis (2010).

Figure 2.

The major proposed routes of IAA biosynthesis showing the IPyA 2 step pathway consisting of an amino transferase step (TAA1/TARs or VAS1) and a monooxygenase reaction (YUC), the indole (tryptophan-independent) option, and possible alternative steps from tryptophan. Figure modified from an original drawn by Dr. Molly Tillmann.

Figure 3.

A simplified diagram of the first steps in IAA inactivation pathways as currently envisioned. Dashed arrows represent metabolic steps for which a potential gene function has not been reported. IAA-AA and oxIAA-AA represent amide-linked conjugates with amino acids. The enzymes responsible for IAA metabolism shown in black boxes are as follows: DAO1, Dioxygenase for Auxin Oxidation 1; GH3, Gretchen Hagen 3; UGT84B1/UGT74D1, UDP-glucosyltransferase 84B1/74D1; ILR1, IAA-Leu-Resistance 1; ICR1/2, IAA-conjugate resistance 1 and 2; IAR3, IAA-Ala-resistant; ILL, ILR1-like. Updated and redrawn from Hladík et al. (2023).