Inhibition of cell expansion by rapid ABP1-mediated auxin effect on microtubules.

The prominent and evolutionarily ancient role of the plant hormone auxin is the regulation of cell expansion. Cell expansion requires ordered arrangement of the cytoskeleton but molecular mechanisms underlying its regulation by signalling 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 re-orientation from transverse to longitudinal, coherent with the inhibition of cell expansion. This fast auxin effect requires auxin binding protein 1 (ABP1) and involves a contribution of downstream signalling components such as ROP6 GTPase, ROP-interactive protein RIC1 and the microtubule-severing protein katanin. These components are required for rapid auxin- and ABP1-mediated re-orientation of microtubules to regulate cell elongation in roots and dark-grown hypocotyls as well as asymmetric growth during gravitropic responses.

Auxin Response Factors: output control in auxin biology.

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.

AUXIN BINDING PROTEIN1 links cell wall remodeling, auxin signaling, and cell expansion in arabidopsis.

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.

Q&A: Auxin: the plant molecule that influences almost anything.

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.

Auxin-binding protein 1 is a negative regulator of the SCF(TIR1/AFB) pathway.

Auxin is a major plant hormone that controls most aspects of plant growth and development. Auxin is perceived by two distinct classes of receptors: transport inhibitor response 1 (TIR1, or auxin-related F-box (AFB)) and auxin/indole-3-acetic acid (AUX/IAA) coreceptors, that control transcriptional responses to auxin, and the auxin-binding protein 1 (ABP1), that controls a wide variety of growth and developmental processes. To date, the mode of action of ABP1 is still poorly understood and its functional interaction with TIR1/AFB-AUX/IAA coreceptors remains elusive. Here we combine genetic and biochemical approaches to gain insight into the integration of these two pathways. We find that ABP1 is genetically upstream of TIR1/AFBs; ABP1 knockdown leads to an enhanced degradation of AUX/IAA repressors, independently of its effects on endocytosis, through the SCF(TIR1/AFB) E3 ubiquitin ligase pathway. Combining positive and negative regulation of SCF ubiquitin-dependent pathways might be a common mechanism conferring tight control of hormone-mediated responses.