Physiologia Plantarum 2012 Copyright © Physiologia Plantarum 2012, ISSN 0031-9317 Metal response of transgenic tomato plants expressing P 1B -ATPase Anna Barabasz a , Anna Wilkowska a , Anna Ruszczy ´ nska b , Ewa Bulska b , Marc Hanikenne c , Magdalena Czarny d , Ute Kr ¨ amer e and Danuta Maria Antosiewicz a,∗ a Institute of Experimental Plant Biology and Biotechnology, Faculty of Biology, University of Warsaw, Miecznikowa 1, PL-02-096 Warszawa, Poland b Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland c Department of Life Sciences, University of Li ` ege, (B22), BE-4000 Li ` ege, Belgium d Department of Plant Genetics Breeding and Biotechnology, Faculty of Horticulture and Landscape Architecture, Warsaw University of Life Sciences, Nowoursynowska 166, 02-787 Warsaw, Poland e Lehrstuhl f ¨ ur Pflanzenphysiologie, Ruhr-Universit ¨ at Bochum, Universit ¨ atsstraße 150, DE-44801 Bochum, Germany Correspondence *Corresponding author, e-mail: dma@biol.uw.edu.pl Received 9 September 2011; revised 14 October 2011 doi:10.1111/j.1399-3054.2012.01584.x Heterologous expression of HMA4 (P 1B -ATPase) in plants is a useful strategy to engineer altered metal distribution in tissues for biofortification or phytoremediation purposes. This study contributes to understanding mechanisms underlying complex Zn-dependent phenotypes observed in transgenic plants and to better predict the consequences of transgene expression. Tomato was transformed with AhHMA4 p1 ::AhHMA4 from Arabidopsis halleri encoding the Zn export protein involved in xylem loading of Zn. Homozygous lines were tested for Zn tolerance, Zn and Fe concentrations in organs and in the apoplastic fluid, and for the expression of the transgene and tomato metal homeostasis endogenes. Expression of AhHMA4 facilitates root-to-shoot Zn translocation and induces Zn uptake in a Zn supply-dependent manner. Unexpectedly, it increases Zn excess- triggered Fe deficiency in leaves and transcriptional activation of Fe-uptake systems in roots. Moreover, AhHMA4 expression causes Zn overload of the apoplast, which may contribute to enhanced Zn sensitivity of transgenics and may lead to cell-wall remodeling. This study highlights that alteration of the apoplast/symplast Zn status through introduction of cellular Zn export activity via AhHMA4 may alter tomato metal homeostasis network, thus seems to be crucial in the generation of the phenotype of transgenic tomato. Introduction Enhancement of high Zn content in the above- ground plant parts through engineering of its facilitated translocation from the roots to the shoots is one of the strategies in plant biotechnology, both for biofortification and for phytoremediation purposes (Mayer et al. 2008, Palmgren et al. 2008, Verkleij et al. 2009). The amount of Zn which is allowed to enter the shoot is under Abbreviations – FRO, ferric chelate reductase; NA, nicotianamine; NAS, nicotianamine synthase; PME, pectin methylesterase; WT, wild-type. strict homeostatic control. Thus, more effective uptake from the soil does not always lead to substantial increase of Zn concentration in above-ground plant tissues. The molecular mechanisms determining the rate of long-distance Zn transport have been explored intensively, but our understanding of these processes remains fragmentary. This rate depends on the control of several key nodes of the metal homeostasis network: (1) metal retention in roots which regulates metal Physiol. Plant. 2012