Ethylene: Fine-tuning plant growth and development by stimulation and inhibition of elongation Jasper Dugardeyn, Dominique Van Der Straeten * Unit Plant Hormone Signaling and Bio-Imaging, Department of Molecular Genetics, Ghent University, Ledeganckstraat 35, B-9000 Gent, Belgium Received 20 December 2007; received in revised form 1 February 2008; accepted 4 February 2008 Available online 14 February 2008 Abstract Plant architecture is determined by a subtle balance between growth stimulation and inhibition conferring optimal plasticity in response to environmental changes. Ethylene is a gaseous hormone which plays an essential role in a myriad of plant developmental processes, both by stimulation and inhibition of growth. Ethylene can promote root hair formation, flowering, fruit ripening and abscission, as well as leaf and petal senescence and abscission. Its role is dependent on the environmental conditions, and the plant developmental stage, besides also being species dependent. Within a plant and even within a given organ, the effect of ethylene treatment can differ, depending on endogenous and environmental cues. Moreover, ethylene signalling and response are a part of an intricate network in cross-talk with other hormones. In this review, current insights in the role of ethylene on growth processes and the influence of these different factors are discussed. # 2008 Elsevier Ireland Ltd. All rights reserved. Keywords: Ethylene; Apical hook; Cross-talk; Growth; Hypocotyl; Root 1. The role of ethylene in root growth 1.1. Ethylene represses root growth The inhibitory effect of ethylene on root growth was described in 1901 by Neljubov [1]. Like other ethylene responses, the response of roots also differs from species to species [2]. Plants growing on well-aerated soils (Sinapsis alba L.) show a high ethylene production and a strong inhibition of root growth upon exogenous ethylene treatment. In contrast, wetland species as Oryza sativa L., have a lower rate of ethylene synthesis and are more resistant to exogenous ethylene treatment [2]. Three strategies may prevent the growth inhibitory effect of ethylene in wetland species [3]. Firstly, plants that grow in wetland conditions form aerenchyma, facilitating the diffusion of ethylene away from the root apex. Secondly, the root apices of these species also produce less ethylene compared to non-wetland species and thirdly, they are less susceptible to the inhibition of root growth by ethylene. In land plants, ethylene strongly inhibits root growth. Arabidopsis (a non-wetland species) plants that have been grown in the presence of 0.1 mM ACC (1-amino-cyclopropane- 1-carboxylic acid) for 7 days have a root that is half the length www.elsevier.com/locate/plantsci Available online at www.sciencedirect.com Plant Science 175 (2008) 59–70 Abbreviations: ACC, 1-amino-cyclopropane-1-carboxylic acid; ACO, ACC oxidase; ACS, ACC synthase; AS, anthranilate synthase; ASA, anthra- nilate synthase a; ASB, anthranilate synthase b; AUX, auxin permease; AVG, aminoethoxyvinylglycine; AXR, auxin resistant; BA, 6-benzylaminopurine; BR, brassinosteroids; BRZ, brassinazole; CBB, cabbage; CIN, cytokinin insen- sitive; COP, constitutive photomorphogenetic; CPD, constitutive photomorpho- genic dwarf; CTR, constitutive triple response; DET, de-etiolated; DEX, dexamethasone; EBF, EIN3 binding F-box protein; EBR, epi-brassinolide; EIL, ein3 like; EIN, ethylene insensitive; EIR, ethylene insensitive root; ETO, ethylene overproducer; ETR, ethylene resistant; GA, gibberellin; GAI, gibberellic acid insensitive; GFP, green fluorescent protein; GL, glabra; GUS, b-glucuronidase; HLS, hookless; HRGP, hydroxyproline-rich glycoproteins; IAA, indole-3-acetic acid; JA, jasmonic acid; JIN, jasmonate insensitive; LEH, length of the first epidermal cell with a visible root hair bulge; LNM, low nutrient medium; 1-MCP, 1-methylcyclopropene; MeJA, methyl jasmonate; NPA, 1-naphthylphthalamic acid; PAC, paclobutrazol; PIN, pin formed; QC, quiescent center; RGA, repressor of ga1-3; RHD, root hair defective; ROS, reactiveoxygen species; RTY, rooty; SAX, hypersensitive to abscisic acid and auxin; TIR, transport inhibitor response; Trp, tryptophan; TTG, transparent testa glabra; WEI, weak ethylene insensitive; WT, wild type. * Corresponding author. Fax: +32 9 264 53 33. E-mail address: dominique.vanderstraeten@ugent.be (D. Van Der Straeten). 0168-9452/$ – see front matter # 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2008.02.003