Advances in Plant Auxin Biology: Synthesis, Metabolism, Signaling, Interaction with Other Hormones, and Roles under Abiotic Stress

Abstract

Auxin is a key hormone that regulates plant growth and development, including plant shape and sensitivity to environmental changes. Auxin is biosynthesized and metabolized via many parallel pathways, and it is sensed and transduced by both normal and atypical pathways. The production, catabolism, and signal transduction pathways of auxin primarily govern its role in plant growth and development, and in the response to stress. Recent research has discovered that auxin not only responds to intrinsic developmental signals, but also mediates various environmental signals (e.g., drought, heavy metals, and temperature stresses) and interacts with hormones such as cytokinin, abscisic acid, gibberellin, and ethylene, all of which are involved in the regulation of plant growth and development, as well as the maintenance of homeostatic equilibrium in plant cells. In this review, we discuss the latest research on auxin types, biosynthesis and metabolism, polar transport, signaling pathways, and interactions with other hormones. We also summarize the important role of auxin in plants under abiotic stresses. These discussions provide new perspectives to understand the molecular mechanisms of auxin's functions in plant development.

Keywords: abiotic stress; auxin; auxin signal transduction; hormone.

Figure 1

Schematic presentation of auxin function in plants.

Figure 2

Examples of endogenous auxins (A) and some synthetic auxins (B) are presented. (A) IAA: indole-acetic acid; IBA: indole-3-butyric acid; 4-Cl-IAA: 4-chloroindole-3-acetic acid; and PAA: phenyl-acetic acid. (B) 1-NAA: 1-Naphthalene-acetic acid; 2,4-D: 2,4-dichlorophenoxyacetic acid; 2,4,5-T: 2,4,5-trichlorophenoxy-acetic acid; dicamba: 3,6-dichloro-2-methoxybenzoic acid, and picloram: 4-Amino-3,5,6-trichloropicolinic acid [48].

Figure 3

A model of the tryptophan (Trp)-dependent and Trp-independent indole acetic acid (IAA) biosynthetic pathways. IGP, indole-3-glycerol phosphate; INS, indole synthase gene; TAA1, tryptophan aminotransferase; TARs, TAA1-associated proteins; IPyA, indole-3-pyruvate; YUC, YUCCA; IAOx, indole-3-acetaldoxime; IAM, indole-3-acetamide; CYP79B2 and CYP79B3 in the cellular phosphorus P450 (CYP) mono-oxygenase family.

Figure 4

The auxin signaling transduction pathway in plants. Under low auxin concentration conditions, the auxin transduction repressor auxin/indole-acetic acid protein (Aux/IAA) forms a heterodimer with the auxin response factor (ARF), which inhibits the transcriptional activity of ARF, resulting in the suppression of auxin response gene expression. Under high auxin concentration, the auxin receptor transport inhibitor response 1 (TIR1) binds to Aux/IAA, ubiquitinates and degrades AUX/IAA by the action of the 26S proteasome, and ARF is released, activating the expression of auxin-responsive genes.