The phytohormone auxin is involved in almost all developmental processes in land plants. Most, if not all, of these processes are mediated by changes in gene expression. Auxin acts on gene expression through a short nuclear pathway that converges upon the activation of a family of DNA-binding transcription factors. These AUXIN RESPONSE FACTORS (ARFs) are thus the effector of auxin response and translate the chemical signal into the regulation of a defined set of genes. Given the limited number of dedicated components in auxin signaling, distinct properties among the ARF family probably contribute to the establishment of multiple unique auxin responses in plant development. In the two decades following the identification of the first ARF in Arabidopsis, much has been learnt about how these transcription factors act, and how they generate unique auxin responses. Progress in genetics, biochemistry, genomics, and structural biology has helped to develop mechanistic models for ARF action. However, despite intensive efforts, many central questions are yet to be addressed. In this review, we highlight what has been learnt about ARF transcription factors, and identify outstanding questions and challenges for the near future.

The prominent and evolutionary ancient effect of the plant hormone auxin is the regulation of cell expansion1. Cell expansion requires ordered cytoskeleton arrangement2 but molecular mechanisms underlying its regulation by signaling molecules including auxin are unknown. Here we show in the model plant Arabidopsis thaliana that in elongating cells exogenous application of auxin or redistribution of endogenous auxin induces very rapid microtubule reorientation from transversal to longitudinal, coherent with the inhibition of cell expansion. This fast auxin effect requires Auxin Binding Protein1 (ABP1) and involves a contribution of downstream signaling components such as ROP6 GTPase, ROP-interactive protein RIC1 and microtubule severing protein Katanin. These components are required for rapid auxin and ABP1-mediated reorientation of microtubules to regulate cell elongation in roots and dark grown hypocotyls as well as asymmetric growth during gravitropic responses.

Auxin is an essential molecule that controls almost every aspect of plant development. Although the core signaling components that control auxin response are well characterized, the precise mechanisms enabling specific responses are not yet fully understood. Considering the significance of auxin in plant growth and its potential applications, deciphering further aspects of its biology is an important and exciting challenge.

Cell expansion is an increase in cell size and thus plays an essential role in plant growth and development. Phytohormones and the primary plant cell wall play major roles in the complex process of cell expansion. In shoot tissues, cell expansion requires the auxin receptor AUXIN BINDING PROTEIN1 (ABP1), but the mechanism by which ABP1 affects expansion remains unknown. We analyzed the effect of functional inactivation of ABP1 on transcriptomic changes in dark-grown hypocotyls and investigated the consequences of gene expression on cell wall composition and cell expansion. Molecular and genetic evidence indicates that ABP1 affects the expression of a broad range of cell wall-related genes, especially cell wall remodeling genes, mainly via an SCF TIR/AFB -dependent pathway. ABP1 also functions in the modulation of hemicellulose xyloglucan structure. Furthermore, fucosidase-mediated defucosylation of xyloglucan, but not biosynthesis of nonfucosylated xyloglucan, rescued dark-grown hypocotyl lengthening of ABP1 knockdown seedlings. In muro remodeling of xyloglucan side chains via an ABP1-dependent pathway appears to be of critical importance for temporal and spatial control of cell expansion. INTRODUCTIONThe essential protein AUXIN BINDING PROTEIN1 (ABP1) functions in the control of growth and development throughout plant life. Initially identified by its capacity to bind the phytohormone auxin, ABP1 was first shown to affect plasma membrane hyperpolarization via the modulation of ion fluxes across the membrane (Thiel et al., 1993;Barbier-Brygoo et al., 1996;Leblanc et al., 1999aLeblanc et al., , 1999b. These rapid ionic changes indicate a possible involvement of ABP1 in the control of cell expansion, at least in shoot tissues, thus providing preliminary molecular evidence supporting the acid growth theory. This theory states that auxin promotes the excretion of protons at the apoplast resulting in cell wall loosening and increased growth rate (Rayle and Cleland, 1992). Binding of auxin to ABP1 and increased amount of ABP1 at the plasma membrane promote protoplast swelling and enhance expansion of leaf cells (Jones et al., 1998;Steffens et al., 2001;Christian et al., 2006). Conversely, the functional inactivation of ABP1 severely impairs cell expansion in shoot tissues irrespective of their DNA content (Braun et al., 2008;Xu et al., 2010) but does not affect root cell elongation (Tromas et al., 2009). The effect of ABP1 on cell expansion varies in a cell-or tissue-dependent manner. Recent data indicate that ABP1 acts both constitutively and in response to auxin (Robert et al., 2010;Tromas et al., 2013). The mechanism by which ABP1 controls cell expansion remains poorly understood. In shoot tissues, it remains unclear whether the contribution of ABP1 to cell expansion relies solely on nongenomic responses or acts also via the regulation of gene expression. ABP1 was reported to affect expression of various genes in response to auxin, but little is known on the broader effects of ABP1 on gene expression (Braun et al.,...