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

Abstract

BackgroundIn 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 FindingsHere 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/SignificanceOur 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.

Rapid Auxin-Induced Cell Expansion and Gene Expression: A Four-Decade-Old Question Revisited

“…It is well established that 1-aminocyclopropane-1-carboxylic acid synthase is coded by a gene inducible through the auxin-TIR1/AFB pathway (Abel et al, 1995;Wang et al, 2005), and there is evidence harking back to the 1960s that auxin transport is regulated by auxin (Apfelbaum and Burg, 1972). Cross talk between the TIR1 pathway and independent pathways has recently been shown for the control of root elongation growth (Tromas et al, 2009). …”

Auxin-Binding Protein 1 is a negative regulator of the SCFTIR1/AFB pathway

“…6a-c); conversely it is similar to the phenotype of double wei8-1tar mutants that contain reduced auxin 31 . Second, no alteration of free IAA was measured in roots, within up to 2 days following ABP1 inactivation in the immune-modulated SS12K9 line 16 , whereas alterations of gene expression are observed within few hours in similar conditions. Third, as shown above (Fig.…”

“…On the basis of the current model of auxindependent transcriptional regulation, an increased degradation of AUX/IAA repressors might result either from an increase in auxin content promoting interaction of TIR1/AFBs and AUX/ IAAs coreceptors, or from a modulation of AUX/IAA repressor half-life through differential interaction with TIR1/AFBs independently of the presence of auxin 28 . Several lines of evidence support that the inactivation of ABP1 does not result in an increase in auxin content that would be responsible for enhanced AUX/IAA degradation: first, the root phenotype of ABP1-inactivated plants with elongated cells 16 , reduced root hairs and few or no lateral roots is opposite to the root phenotype of plants with elevated auxin content or roots treated with exogenous auxin 29,30 ( Fig. 6a-c); conversely it is similar to the phenotype of double wei8-1tar mutants that contain reduced auxin 31 .…”

“…Loss of function of ABP1 and postembryonic functional inactivation of the protein by inducible cellular immunization or antisense were demonstrated to provoke lethality and severe developmental defects at all stages of development [14][15][16] . At the cellular level, ABP1 was shown to be required for cell division 14,16,17 and also for cell expansion in shoot tissues [14][15][16]18 . Interestingly, ABP1 was also reported to be required for a tight regulation of a number of auxin response genes 16 , thus suggesting that ABP1 and TIR1/AFB-AUX/IAA collectively contribute to mediate auxin responses.…”

“…At the cellular level, ABP1 was shown to be required for cell division 14,16,17 and also for cell expansion in shoot tissues [14][15][16]18 . Interestingly, ABP1 was also reported to be required for a tight regulation of a number of auxin response genes 16 , thus suggesting that ABP1 and TIR1/AFB-AUX/IAA collectively contribute to mediate auxin responses.…”