Physiological interactions of antiauxins with auxin in roots

Mechanisms of the enantioselective effects of phenoxyalkanoic acid herbicides DCPP and MCPP

Phenoxyalkanoic acids (PAAs), synthetic indole-3-acetic acid (IAA) auxin mimics, are widely used as herbicides. Many PAAs are chiral molecules and show strong enantioselectivity in their herbicidal activity; however, there is a lack of understanding of mechanisms driving enantioselectivity. This study aimed to obtain a mechanistic understanding of PAA enantioselectivity using dichlorprop and mecoprop as model PAA compounds. Molecular docking, in vitro ³H-IAA binding assay, and surface plasmon resonance analysis showed that the R enantiomer was preferentially combined with TIR1-IAA7 (Transport Inhibitor Response1- Auxin-Responsive Protein IAA7) than the S enantiomer. In vivo tracking using ¹⁴C-PAAs showed a greater absorption of the R enantiomer by Arabidopsis thaliana, and further comparatively enhanced translocation of the R enantiomer to the nucleus where the auxin co-receptor is located. These observations imply that TIR1-IAA7 is a prior target for DCPP and MCPP, and that PAA enantioselectivity occurs because the R enantiomer has a stronger binding affinity for TIR1-IAA7 as well as a greater plant absorption and translocation capability than the S enantiomer. The improved understanding of PAA enantioselectivity is of great significance, as the knowledge may be used to design "green" molecules, such as R enantiomer enriched products, leading to improved plant management and environmental sustainability.

Mitogen-activated protein kinases concentrate in the vicinity of chromosomes and may regulate directly cellular patterning in Vicia faba embryos

Mitogen-activated protein kinases seem to mark genes which are set up to be activated in daughter cells and thus they may play a direct role in cellular patterning during embryogenesis. Embryonic patterning starts very early and after the first division of zygote different genes are expressed in apical and basal cells. However, there is an ongoing debate about the way these different transcription patterns are established during embryogenesis. The presented data indicate that mitogen-activated protein kinases (MAPKs) concentrate in the vicinity of chromosomes and form visible foci there. Cells in the apical and basal regions differ in number of foci observed during the metaphase which suggests that cellular patterning may be determined by activation of diverse MAPK-dependent genes. Different number of foci in each group of separating chromatids and the specified direction of these mitoses in apical-basal axis indicate that the unilateral auxin accumulation in a single cell may regulate the number of foci in each group of chromatids. Thus, we put forward a hypothesis that MAPKs localized in the vicinity of chromosomes during mitosis mark those genes which are set up to be activated in daughter cells after division. It implies that the chromosomal localization of MAPKs may be one of the mechanisms involved in establishment of cellular patterns in some plant species.

Investigating a Potential Auxin-Related Mode of Hormetic/Inhibitory Action of the Phytotoxin Parthenin

Parthenin is a metabolite of Parthenium hysterophorus and is believed to contribute to the weed's invasiveness via allelopathy. Despite the potential of parthenin to suppress competitors, low doses stimulate plant growth. This biphasic action was hypothesized to be auxin-like and, therefore, an auxin-related mode of parthenin action was investigated using two approaches: joint action experiments with Lactuca sativa, and dose-response experiments with auxin/antiauxin-resistant Arabidopsis thaliana genotypes. The joint action approach comprised binary mixtures of subinhibitory doses of the auxin 3-indoleacetic acid (IAA) mixed with parthenin or one of three reference compounds [indole-3-butyric acid (IBA), 2,3,5-triiodobenzoic acid (TIBA), 2-(p-chlorophenoxy)-2-methylpropionic acid (PCIB)]. The reference compounds significantly interacted with IAA at all doses, but parthenin interacted only at low doses indicating that parthenin hormesis may be auxin-related, in contrast to its inhibitory action. The genetic approach investigated the response of four auxin/antiauxin-resistant mutants and a wildtype to parthenin or two reference compounds (IAA, PCIB). The responses of mutant plants to the reference compounds confirmed previous reports, but differed from the responses observed for parthenin. Parthenin stimulated and inhibited all mutants independent of resistance. This provided no indication for an auxin-related action of parthenin. Therefore, the hypothesis of an auxin-related inhibitory action of parthenin was rejected in two independent experimental approaches, while the hypothesis of an auxin-related stimulatory effect could not be rejected.

The inter-kingdom volatile signal indole promotes root development by interfering with auxin signalling

Recently, emission of volatile organic compounds (VOCs) has emerged as a mode of communication between bacteria and plants. Although some bacterial VOCs that promote plant growth have been identified, their underlying mechanism of action is unknown. Here we demonstrate that indole, which was identified using a screen for Arabidopsis growth promotion by VOCs from soil-borne bacteria, is a potent plant-growth modulator. Its prominent role in increasing the plant secondary root network is mediated by interfering with the auxin-signalling machinery. Using auxin reporter lines and classic auxin physiological and transport assays we show that the indole signal invades the plant body, reaches zones of auxin activity and acts in a polar auxin transport-dependent bimodal mechanism to trigger differential cellular auxin responses. Our results suggest that indole, beyond its importance as a bacterial signal molecule, can serve as a remote messenger to manipulate plant growth and development.

A combinatorial TIR1/AFB-Aux/IAA co-receptor system for differential sensing of auxin

The plant hormone auxin regulates virtually every aspect of plant growth and development. Auxin acts by binding the F-box protein transport inhibitor response 1 (TIR1) and promotes the degradation of the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors. Here we show that efficient auxin binding requires assembly of an auxin co-receptor complex consisting of TIR1 and an Aux/IAA protein. Heterologous experiments in yeast and quantitative IAA binding assays using purified proteins showed that different combinations of TIR1 and Aux/IAA proteins form co-receptor complexes with a wide range of auxin-binding affinities. Auxin affinity seems to be largely determined by the Aux/IAA. As there are 6 TIR1/AUXIN SIGNALING F-BOX proteins (AFBs) and 29 Aux/IAA proteins in Arabidopsis thaliana, combinatorial interactions may result in many co-receptors with distinct auxin-sensing properties. We also demonstrate that the AFB5-Aux/IAA co-receptor selectively binds the auxinic herbicide picloram. This co-receptor system broadens the effective concentration range of the hormone and may contribute to the complexity of auxin response.

Auxin: a major player in the shoot-to-root regulation of root Fe-stress physiological responses to Fe deficiency in cucumber plants

The aim of this study was to investigate the effects of IAA and ABA in the shoot-to-root regulation of the expression of the main Fe-stress physiological root responses in cucumber plants subjected to shoot Fe functional deficiency. Changes in the expression of the genes CsFRO1, CsIRT1, CsHA1 and CsHA2 (coding for Fe(III)-chelate reductase (FCR), the Fe(II) transporter and H+-ATPase, respectively) and in the enzyme activity of FCR and the acidification capacity were measured. We studied first the ability of exogenous applications of IAA and ABA to induce these Fe-stress root responses in plants grown in Fe-sufficient conditions. The results showed that IAA was able to activate these responses at the transcriptional and functional levels, whereas the results with ABA were less conclusive. Thereafter, we explored the role of IAA in plants with or without shoot Fe functional deficiency in the presence of two types of IAA inhibitors, affecting either IAA polar transport (TIBA) or IAA functionality (PCIB). The results showed that IAA is involved in the regulation at the transcriptional and functional levels of both Fe root acquisition (FCR, Fe(II) transport) and rhizosphere acidification (H+-ATPase), although through different, and probably complementary, mechanisms. These results suggest that IAA is involved in the shoot-to-root regulation of the expression of Fe-stress physiological root responses.