Physiologia Plantarum 2014 © 2014 Scandinavian Plant Physiology Society, ISSN 0031-9317 MINIREVIEW Auxin is a central player in the hormone cross-talks that control adventitious rooting Daniel Ioan Pacurar a,† , Irene Perrone a,b,†,∗ and Catherine Bellini a,c a Department of Plant Physiology, Umeå Plant Science Centre, Umeå University, Umeå, SE-90187, Sweden b Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, Swedish University of Agricultural Sciences, Umeå, SE-90183, Sweden c Institut Jean-Pierre Bourgin, UMR1318 INRA-AgroParisTech, Versailles Cedex, 78026, France Correspondence *Corresponding author, e-mail: Irene.Perrone@slu.se Received 29 October 2013; revised 11 February 2014 doi:10.1111/ppl.12171 Vegetative propagation of economically important woody, horticultural and agricultural species rely on an efficient adventitious root (AR) formation. The formation of ARs is a complex genetic trait regulated by the interaction of environmental and endogenous factors among which the phytohormone auxin plays an essential role. This article summarizes the current knowledge related to the intricate network through which auxin controls adventitious rooting. How auxin and recently identified auxin-related compounds affect AR formation in different plant species is discussed. Particular attention is addressed to illustrate how auxin has a central role in the hormone cross-talk leading to AR development. In parallel, we describe the molecular players involved in the control of auxin homeostasis, transport and signaling, for a better understanding of the auxin action during adventitious rooting. Introduction Unlike the vast majority of species in the animal king- dom, who reproduce exclusively sexually, asexual repro- duction, also known as vegetative propagation, vegeta- tive multiplication or vegetative cloning, is a common form of reproduction in plants. Although most plants normally reproduce sexually, for many species vegeta- tive propagation can occur, either naturally or artificially when induced by different methods. This capacity of plants to reproduce vegetatively lays in the extraordi- nary plasticity of the plant cells to dedifferentiate and then redifferentiate, forming new organs (e.g. roots or shoots) that will eventually regenerate a new plant. For a Abbreviations – 2,4-D, 2,4-dichlorophenoxyacetic acid; ABA, abscisic acid; ABCB1, ATP BINDING CASSETTE TYPE B 1; AR, adventitious root; ARF, AUXIN RESPONSE FACTOR; ASA1, ANTHRANILATE SYNTHASE ALPHA 1; ASB1, ANTHRANILATE SYNTHASE BETA 1; BR, brassinosteroids; cGMP, cyclic guanosine monophosphate; CK, cytokinin; CsSCL1, Castanea sativa SCARECROW-like transcript; ET, ethylene; GBs, gibberellins; IBA, indole-3-butyric acid; IGs, indole glucosinolates; LR, lateral root; NAA, 1-naphthalene acetic acid; NO, nitric oxide; NPA, naphthylphthalamic acid; PAT, polar auxin transport; QC, quiescent center; RCE1, RUB-CONJUGATING ENZYME1; rtcs, rootless concerning crown and seminal roots; SA, salicylic acid; SL, strigolactone; TCL, thin cell layer; TIR1, TRANSPORT INHIBITOR RESISTANT1; WEI2, WEAK ETHYLENE INSENSITIVE 2; † These authors have equally contributed to the work. plant segment, usually a stem or a leaf cutting, to be able to live independently once detached from the mother plant, the regeneration and development of new roots is indispensable. These roots initiated post-embryonically from organs other than roots, are called adventitious roots (ARs). AR formation is part of the normal development of the plant and occurs naturally like in most monocotyle- donous for which they constitute the main root system, but can also be induced by stresses such as wound- ing, flooding, etiolation, or through horticultural prac- tices used for vegetative propagation of many dicotyle- donous species (reviewed in Geiss et al. 2009, Oinam et al. 2011). Artificial vegetative or clonal propagation is Physiol. Plant. 2014