Physiologia Plantarum 2014 © 2014 Scandinavian Plant Physiology Society, ISSN 0031-9317 Role of gibberellins during arbuscular mycorrhizal formation in tomato: new insights revealed by endogenous quantification and genetic analysis of their metabolism in mycorrhizal roots José Ángel Martín-Rodríguez a , Juan Antonio Ocampo a , Nuria Molinero-Rosales a , Danuše Tarkowská b , Omar Ruíz-Rivero c and José Manuel García-Garrido a,∗ a Departamento de Microbiología del suelo y sistemas simbióticos, Estación Experimental del Zaidín (EEZ), CSIC, Granada 18008, Spain b Laboratory of Growth Regulators, Faculty of Science, Palacký University and Institute of Experimental Botany AS CR, Olomouc CZ-783 71, Czech Republic c Instituto de Biología Molecular y Celular de Plantas (IBMCP), Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas (UPV-CSIC), Valencia 46022, Spain Correspondence *Corresponding author, e-mail: josemanuel.garcia@eez.csic.es Received 25 March 2014; revised 29 May 2014 doi:10.1111/ppl.12274 Gibberellins (GAs) are key regulators of plant growth and development and recent studies suggest also a role during arbuscular mycorrhizal (AM) forma- tion. Here, complementary approaches have been used to obtain a clearer pic- ture that correlates AM fungal development inside roots with GA metabolism. An extensive analysis of genes associated with GA metabolism as well as a quantification of GA content in roots was made. Application of GA 3 and its biosynthesis inhibitor prohexadione calcium (PrCa) combined with a GA-constitutive response mutant (procera) were used to determine whether fungal colonization is altered by the level of these hormones or by changes in the GA-signaling pathway. The increased levels of specific GAs from the 13-hydroxylation pathway in mycorrhizal roots correlate closely with the increased expression of genes coding enzymes from the GA biosynthetic trail. The imbalance of GAs in tomato roots caused by exogenous applica- tions of GA 3 or PrCa affects arbuscules in both negative and positive ways, respectively. In addition, procera plants were adversely affected by the myc- orrhization process. Our findings demonstrate that an imbalance in favor of an increased amount of GAs negatively affects the frequency of mycorrhiza- tion and particularly the arbuscular abundance in tomato mycorrhizal roots and the results point out that AM formation is associated with a change in the 13-hydroxylation pathway of GAs. Introduction The arbuscular mycorrhizal (AM) mutualistic association is the most widespread symbiosis in the plant kingdom. Fungal penetration and establishment in roots depend on a complex sequence of events and intracellular modifications that lead to an interchange of mineral Abbreviations – AM, arbuscular mycorrhiza; AON, autoregulation of nodulation; GAs, gibberellins; GinEF, R. irregularis elon- gation factor 1-alpha gene; PrCa, prohexadione calcium; qPCR, real-time polymerase chain reaction; SlDELLA, S. lycopersicum DELLA. nutrient uptakes by the fungus for plant carbohydrates (Bonfante and Requena 2011, Gutjahr and Parniske 2013). AM fungi penetration and growth in roots acti- vate symbiotic programs for functional mycorrhization in both symbionts, resulting in major alterations in the morphology and physiology of roots and fungal hyphae (Parniske 2008, Gutjahr and Parniske 2013). Physiol. Plant. 2014