Auxin metabolism and homeostasis during plant development

Abstract

SummaryAuxin plays important roles during the entire life span of a plant. This small organic acid influences cell division, cell elongation and cell differentiation, and has great impact on the final shape and function of cells and tissues in all higher plants. Auxin metabolism is not well understood but recent discoveries, reviewed here, have started to shed light on the processes that regulate the synthesis and degradation of this important plant hormone. Key words: Auxin metabolism, Biosynthesis, Conjugation and degradation, Plant hormone, Arabidopsis thaliana, Root and shoot development IntroductionThe plant hormone auxin or indole-3-acetic acid (IAA) (Fig. 1) was discovered ~70 years ago, although the concept of plant hormones and speculation over their roles in plant development were conceived much earlier (reviewed by Abel and Theologis, 2010). Auxin research took off in the 1980s following the discovery of a battery of genes involved in auxin responses, and more recently with the discovery of the TIR1 auxin receptor family (reviewed by Calderón-Villalobos et al., 2010) and various auxin transporters (reviewed by Zažímalová et al., 2010). These discoveries boosted a wave of research into auxin signalling and the roles of these newly discovered genes during plant development, mostly using the model plant species Arabidopsis thaliana as an experimental system.Although our understanding of auxin signalling has flourished during the last few decades, progress in understanding auxin metabolism (which includes auxin biosynthesis, conjugation and degradation) has been hampered by the lack of suitable tools for the quantification and visualisation of auxin metabolites on a tissue and cellular level. Until recently, key components in these metabolic pathways were missing and the regulation of auxin metabolism was poorly understood, but genetic and biochemical evidence in combination with sensitive methods for auxin metabolite identification and quantification have greatly improved our knowledge in this respect.This Primer describes the recent progress that has been made in our understanding of auxin metabolism, with the aim of putting these discoveries into the context of plant growth and development. I will focus on auxin metabolism in A. thaliana, as these metabolic processes are much better understood in Arabidopsis than in other plant species. Still, it is important to remember that there are differences between plant species, and also that IAA metabolism in some species (including Arabidopsis) is intertwined with, and affected by, the secondary metabolism of plant defence compounds (Normanly, 2010). An overview of auxin transport and signalling pathwaysAuxin is a weak organic acid consisting of a planar indole ring structure coupled to a side chain harbouring a terminal carboxyl group (Fig. 1A,B). The carboxyl group is protonated at low pH, making the molecule less polar (IAA -+ H + ⇔ IAA-H). In this form it can diffuse across cell membranes, whereas the molecule in its unprotonated, negatively charged form ...

Regulation of Auxin Homeostasis and Gradients in Arabidopsis Roots through the Formation of the Indole-3-Acetic Acid Catabolite 2-Oxindole-3-Acetic Acid

“…Although growth of the root has the potential to result in the continual dilution of the auxin concentration within the root apex, and some auxin is actively transported away from the tip, it is likely that IAA conjugation and degradation processes are important for maintaining the auxin gradient and avoiding overaccumulation of the hormone in the root tip. To date, only genes involved in IAA conjugation have been identified in plants (reviewed in Normanly, 2010;Ludwig-Müller, 2011;Korasick et al, 2013;Ljung, 2013). To be able to answer whether general, constitutive catabolic activity is present in most cells of the root apex, or if specific catabolic sites exist in certain cell types, we performed cell type-specific analysis of IAA and oxIAA levels in four different % Figure 11.…”

“…In order to ascertain the major contributors to the conjugation and degradation of IAA, we analyzed the concentration of the hormone and the major catabolites and conjugates in root tissue from the three IAA-overproducing mutants YUCCA1 (35S:YUC1; Zhao et al, 2001), superoot1-3 (sur1-3;Boerjan et al, 1995;Mikkelsen et al, 2004), and sur2-1 (Barlier et al, 2000) at 10 d after germination (DAG; Figure 2). The YUC1 gene is believed to be directly involved in Trp-dependent IAA biosynthesis, while the sur1-3 and sur2-1 mutations block indole glucosinolate biosynthesis, thereby redirecting metabolism into IAA biosynthesis, resulting in IAA overproduction (reviewed in Ljung, 2013). Both sur1-3 and sur2-1 show increased IAA levels in root tissues from 10-DAG seedlings compared with the wild-type ecotype Columbia (Col-0), while the difference between the wild type and 35S:YUC1 is less pronounced (Figure 2A).…”

“…The most comprehensively studied IAA biosynthetic pathways rely on Trp as a precursor (reviewed in Normanly, 2010;Korasick et al, 2013;Ljung, 2013). Recent data have shown the importance of Trp-dependent IAA biosynthesis in numerous plant processes.…”

Modelling Plant Hormone Gradients

“…In the root tip, the biosynthesis of auxin is activated by ethylene (Stepanova et al, 2007;Swarup et al, 2007;Ljung 2013), and can be inhibited by cytokinin (Nordstrom et al, 2004, Jones et al, 2011Ljung, 2013). It is known that both auxin and cytokinin can synergistically activate the biosynthesis of ethylene (Vogel et al, 1998;Stepanova et al, 2007).…”

“…A regulatory network showing these relationships can be constructed and is termed the "hormonal crosstalk network" (Liu et al, 2010;2013;2014;Moore et al, 2015a). An example of a hormonal crosstalk network is shown in Figure 1, which describes the regulatory relationships between auxin, ethylene, cytokinin, PIN1, PIN2, AUX1 and the activity of other associated genes (Moore et al, 2015a).…”