Evidence of oxidative attenuation of auxin signalling

Abstract

Indole-3-acetic acid (IAA) is the principle auxin in Arabidopsis and is synthesized primarily in meristems and nodes. Auxin is transported to distal parts of the plant in response to developmental programming or environmental stimuli to activate cell-specific responses. As with any signalling event, the signal must be attenuated to allow the system to reset. Local auxin accumulations are thus reduced by conjugation or catabolism when downstream responses have reached their optima. In most cell types, localized auxin accumulation increases both reactive oxygen species (ROS) and an irreversible catabolic product 2-oxindole-3-acid acid (oxIAA). oxIAA is inactive and does not induce expression of the auxin-responsive reporters DR5 or 2XD0. Here it is shown that oxIAA is not transported from cell to cell, although it appears to be a substrate for the ATP-binding cassette subfamily G (ABCG) transporters that are positioned primarily on the outer lateral surface of the root epidermis. However, oxIAA and oxIAA-Glc levels are higher in ABCB mutants that accumulate auxin due to defective cellular export. Auxin-induced ROS production appears to be at least partially mediated by the NAD(P)H oxidase RbohD. oxIAA levels are higher in mutants that lack ROS-scavenging flavonoids (tt4) and are lower in mutants that accumulate excess flavonols (tt3). These data suggest a model where IAA signalling is attenuated by IAA catabolism to oxIAA. Flavonoids appear to buffer ROS accumulations that occur with localized increases in IAA. This buffering of IAA oxidation would explain some growth responses observed in flavonoid-deficient mutants that cannot be explained by their established role in partially inhibiting auxin transport.

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

“…Additional IAA metabolites have been identified in Arabidopsis, such as IAA-Glc, IAA-Val, IAA-Phe, 6-OH-IAA-Val, and 6-OH-IAA-Phe (Kai et al, 2007), IAA-Trp (Staswick 2009), and IAA-Ala and IAA-Leu (Kowalczyk and Sandberg, 2001;Rampey et al, 2004). These and other studies (Tam et al, 2000;Kubeš et al, 2012;Novák et al, 2012;Peer et al, 2013) clearly show that oxIAA is a major IAA degradation product in Arabidopsis tissues and that IAA amino acid conjugates are normally present in much lower quantities. and oxIAA (open triangles) was monitored after feeding 7-DAG wild-type Arabidopsis Col-0 seedlings with liquid medium containing 10 mM of the labeled precursor for 3, 6, 12, and 24 h. Enrichment of the IAA and oxIAA pools is expressed as the isotope ratio, the ratio of labeled to unlabeled compound, after correction for natural isotope abundances.…”

“…It has recently been suggested that oxIAA could instead function as a substrate for the ABC SUBFAMILY G (ABCG) transporters ABCG36/ABCG37. ABCG transporters are known to transport a large variety of organic molecules (reviewed in Verrier et al, 2008), and the proposed function in Arabidopsis roots would be to transport oxIAA out of the root epidermis (Peer et al, 2013).…”

“…It is known that auxin can be conjugated to amino acids and sugars to either storage forms or to irreversible degradation products, and several genes involved in IAA conjugation and deconjugation have been identified (reviewed in Ludwig-Müller, 2011;Korasick et al, 2013). Data from our laboratory and others have demonstrated that oxidation of IAA to 2-oxindole-3-acetic acid (oxIAA) is another important pathway for IAA degradation in plant species such as Arabidopsis (Östin et al, 1998;Kowalczyk and Sandberg 2001;Kai et al, 2007;Novák et al, 2012;Peer et al, 2013), maize (Zea mays; Reinecke and Bandurski, 1983), and Pinus sylvestris (Ernstsen et al, 1987). These findings have mainly been based on the use of radioactive and labeled precursor feeding and/or mass spectrometry identification and quantification of the main IAA degradation products.…”

“…These findings have mainly been based on the use of radioactive and labeled precursor feeding and/or mass spectrometry identification and quantification of the main IAA degradation products. To our knowledge, no genes and/or enzymes have so far been identified in IAA catabolic/oxidation Oxidation of IAA to oxIAA, and subsequent conjugation with Glc to form oxIAA-Glc, is a major route for IAA catabolism in Arabidopsis, based on feeding experiments with labeled precursors and mass spectrometry-based quantification of IAA metabolites (Östin et al, 1998;Kowalczyk and Sandberg, 2001;Kai et al, 2007;Kubeš et al, 2012;Peer et al, 2013). IAA can also be conjugated to either Glc or amino acids (AA), and these compounds can be further metabolized to oxIAA-Glc, 6-OH-IAA-AA, and oxIAA-AA.…”

Glutathione – linking cell proliferation to oxidative stress

“…Auxin accumulates in stem cells and at sites of cell division and is important in the production of lateral roots, adventitious roots and root hairs, as well as in the control of apical dominance and stem elongation (52)(53)(54). IAA is removed by conjugation or catabolism when downstream responses have reached their optima (56) Oxidation of IAA to 2-oxindole-3-acid acid (oxIAA) attenuates the auxin signal.…”