Strong morphological defects in conditional Arabidopsis abp1 knock-down mutants generated in absence of functional ABP1 protein

The Auxin Binding Protein 1 (ABP1) is one of the most studied proteins in plants. Since decades ago, it has been the prime receptor candidate for the plant hormone auxin with a plethora of described functions in auxin signaling and development. The developmental importance of ABP1 has recently been questioned by identification of Arabidopsis thaliana abp1 knock-out alleles that show no obvious phenotypes under normal growth conditions. In this study, we examined the contradiction between the normal growth and development of the abp1 knock-outs and the strong morphological defects observed in three different ethanol-inducible abp1 knock-down mutants ( abp1-AS, SS12K, SS12S). By analyzing segregating populations of abp1 knock-out vs. abp1 knock-down crosses we show that the strong morphological defects that were believed to be the result of conditional down-regulation of ABP1 can be reproduced also in the absence of the functional ABP1 protein. This data suggests that the phenotypes in abp1 knock-down lines are due to the off-target effects and asks for further reflections on the biological function of ABP1 or alternative explanations for the missing phenotypic defects in the abp1 loss-of-function alleles

Cell Surface- and Rho GTPase-Based Auxin Signaling Controls Cellular Interdigitation in Arabidopsis

Auxin is a multi-functional hormone essential for plant development and pattern formation. A nuclear auxin signaling system controlling auxin-induced gene expression is well established, but cytoplasmic auxin signaling as in its coordination of cell polarization is unexplored. We found a cytoplasmic auxin signaling mechanism that modulates the interdigitated growth of Arabidopsis leaf epidermal pavement cells (PCs), which develop interdigitated lobes and indentations to form a puzzle-piece shape in a two-dimensional plane. PC interdigitation is compromised in leaves deficient in either auxin biosynthesis or its export mediated by PINFORMED 1 localized at the lobe tip. Auxin coordinately activates two Rho GTPases, ROP2 and ROP6, which promote the formation of complementary lobes and indentations, respectively. Activation of these ROPs by auxin occurs within 30 seconds and depends on AUXIN-BINDING PROTEIN 1. These findings reveal Rho GTPase-based novel auxin signaling mechanisms, which modulate the spatial coordination of cell expansion across a field of cells

The AUXIN BINDING PROTEIN 1 Is Required for Differential Auxin Responses Mediating Root Growth

Background In plants, the phytohormone auxin is a crucial regulator sustaining growth and development. At the cellular level, auxin is interpreted differentially in a tissue- and dose-dependent manner. Mechanisms of auxin signalling are partially unknown and the contribution of the AUXIN BINDING PROTEIN 1 (ABP1) as an auxin receptor is still a matter of debate. Methodology/Principal Findings Here we took advantage of the present knowledge of the root biological system to demonstrate that ABP1 is required for auxin response. The use of conditional ABP1 defective plants reveals that the protein is essential for maintenance of the root meristem and acts at least on the D-type CYCLIN/RETINOBLASTOMA pathway to control entry into the cell cycle. ABP1 affects PLETHORA gradients and confers auxin sensitivity to root cells thus defining the competence of the cells to be maintained within the meristem or to elongate. ABP1 is also implicated in the regulation of gene expression in response to auxin. Conclusions/Significance Our data support that ABP1 is a key regulator for root growth and is required for auxin-mediated responses. Differential effects of ABP1 on various auxin responses support a model in which ABP1 is the major regulator for auxin action on the cell cycle and regulates auxin-mediated gene expression and cell elongation in addition to the already well known TIR1-mediated ubiquitination pathway

Functional characterization of the auxin binding protein 1 (ABP) during arabidopsis development and in auxin signalling

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

ETUDE FONCTIONNELLE DE LA PROTEINE DE LIAISON D'AUXINE NT-ABP1 CHEZ NICOTIANA TABACUM

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Auxins

Auxin is a multifactorial phytohormone that is required for cell division. Fine gradients determine points of developmental change in time and space. It is associated intimately with the axiality of plant growth, and increasing doses lead to cell expansion or inhibition of cell expansion in different tissues. From embryonic patterning to fruit dehiscence every plant process has some involvement with auxin as a hormonal signal, including responses to wounding. Moreover, synthetic auxins have widespread uses as agrochemicals, particularly as selective herbicides. Despite the importance of auxin as a plant signal the pathways of its biosynthesis are still not clear. Much more is known about auxin perception and the mechanisms through which gene transcription is regulated. One receptor has been identified, and protein crystallography data has explained its auxin-binding capacity, but this is likely to control only a subset of auxin-mediated responses. Little is known of the signal transduction intermediates. A second receptor has been nominated and may be involved in controlling auxin-mediated gene transcription. A complex set of proteins comprising signalosome and proteasome contribute to the regulation of sets of transcription factors to confer regulation by derepression. A set of auxin transport proteins has been described with associated regulatory interactors, and these account for polar auxin flow and the control of auxin movements across cells, tissues, and around the plant. The gradients these transport systems build regulate the responses of growth and differentiation, including the plant's response to gravity. These areas are described and discussed by relating the physiology of the whole plant to the details of genetic and protein activities. (c) 2005 Elsevier Inc

Identification of a novel marker for auxin and ethylene cross-talk from tobacco seedlings

A novel early-response auxin-regulated gene, here termed Nt-AER (Auxin and Ethylene Regulated), was identified from a cDNA library prepared from auxin-treated tobacco (Nicotiana tabacum cv. Xanthi) seedlings. Comparison with sequence databases places Nt-AER in a family of transferase enzymes. Nt-AER was detected in a number of tobacco tissues with the exception of young leaves. In tobacco seedlings, Nt-AER is specifically up-regulated by auxins, but not their inactive analogues. A number of other plant growth regulators were tested of which the ethylene precursor ACC also induced Nt-AER accumulation. Steady-state levels of Nt-AER were relatively high in dark-grown tobacco seedlings but fell rapidly upon exposure to light and the possible role of ethylene in this response is discussed. Nt-AER was found to be rapidly induced following NAA or ACC treatment and maximum accumulation was detected for concentrations above 1 muM. Whilst ACC failed to induce Nt-AER in seedlings of transgenic ethylene insensitive tobacco, a response to NAA was retained. This suggests that Nt-AER is regulated by separable signalling pathways and not only by auxin-induced ethylene biosynthesis. As such, Nt-AER represents a novel marker for auxin and ethylene cross-talk