Plant Science 167 (2004) 607–620 The delay in hormonal treatment modulates the expression of LESK1,a gene encoding a putative serine-threonine kinase, marker of in vitro caulogenesis in tomato (Lycopersicon esculentum Mill.) Anna Torelli ∗ , Michela Borinato, Elisabetta Soragni 1 , Rossano Bolpagni 2 , Chiara Bottura 2 , Camillo Branca Department of Evolutive and Functional Biology, University of Parma, Parco delle Scienze 11/A, 43100 Parma, Italy Received 22 December 2003; received in revised form 26 April 2004; accepted 7 May 2004 Available online 2 June 2004 Abstract Growth regulators play a multiple role in somatic organogenesis, affecting the phases of competence acquisition, determination and organ differentiation. The establishment of these phases is conceivably determined by a differential gene expression. We have recently found that the expression of LESK1, a gene that encodes a putative serine/threonine kinase marks an elevated intrinsic caulogenetic attitude in tomato hypocotyl, that does not require the presence of exogenous growth regulators. In tomato cotyledon explants, caulogenesis is instead induced supplying the culture medium with growth regulators. In these explants, LESK1 expression peak precedes the induction phase and marks the acquisition of caulogenic competence. The removal of growth regulators just after the reaching of the peak prevents the achievement of an optimal shoot production. A delay in hormonal treatment greatly reduced caulogenesis, affecting both competence acquisition, as suggested by the reduced LESK1 expression, and consequently the proper induction, driving to a sharp decrease in shoot primordia per explant. We hypothesize that LESK1 kinase is activated during the transduction of the hormonal signal driving to caulogenesis and required for the fulfillment of the inductive phase. LESK1 expression is differently modulated in tomato horticultural varieties showing different caulogenic attitude. As all the examined cultivars have one copy of this gene, a different regulation of the hormonal signal transduction pathway involving LESK1, is conceivably responsible for the diverse morphogenetic outcomes in these varieties. © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Kinase; Auxin/cytokinin; Caulogenic competence; Somatic organogenesis 1. Introduction Organogenesis in higher plants is achieved by a patterned control of the number, places and planes of cell divisions occurring in the meristems, followed by a harmonic cellu- lar expansion. These finely regulated processes occurring during the normal development of the flowering plants, are re-established when a tissue explant is “in vitro” cultured and induced to produce new organs. During “in vitro” ∗ Corresponding author. Tel.: +39 0521 905567; fax: +39 0521 905403. E-mail address: torelli@biol.unipr.it (A. Torelli). 1 Present address: Center for Molecular Genetics, University of Cali- fornia San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0634, USA. 2 Present address: Department of Environmental Sciences, University of Parma, Parco delle Scienze 11/A, 43100 Parma, Italy. organogenesis, plant cells and tissues go through three dis- crete, experimentally distinguishable developmental states defined as “morphogenic competence”, “developmental determination” and “morphological differentiation” [1–5]. Competent cells are sensitive to induction, being able to recognize the environmental signals that can alter their de- velopmental fate. Following this stage, the cells become developmentally determined and proceed toward organ dif- ferentiation. It is conceivable that a correct development is the product of a strictly regulated gene expression achieved by the transduction of environmental and inner signals as light, nutrients, and mainly growth regulators sensed by the cells and diffused by an efficient cell to cell commu- nication. It is already well established that, auxin and/or cytokinin, play a key role on “in vitro” culture, and that their accurate balance determines the morphogenic outcome of the explants. The activation of specific patterns of gene 0168-9452/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2004.05.007