The homeodomain-leucine zipper (HD-Zip) class I transcription factors ATHB7 and ATHB12 modulate abscisic acid signalling by regulating protein phosphatase 2C and abscisic acid receptor gene activities Ana Elisa Valde ´s • Elin O ¨ verna ¨s • Henrik Johansson • Alvaro Rada-Iglesias • Peter Engstro ¨m Received: 3 April 2012 / Accepted: 16 August 2012 / Published online: 12 September 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Plants perceiving drought activate multiple responses to improve survival, including large-scale alter- ations in gene expression. This article reports on the roles in the drought response of two Arabidopsis thaliana homeodomain-leucine zipper class I genes; ATHB7 and ATHB12, both strongly induced by water-deficit and abscisic acid (ABA). ABA-mediated transcriptional regu- lation of both genes is shown to depend on the activity of protein phosphatases type 2C (PP2C). ATHB7 and ATHB12 are, thus, targets of the ABA signalling mechanism defined by the PP2Cs and the PYR/PYL family of ABA receptors, with which the PP2C proteins interact. Our results from chromatin immunoprecipitation and gene expression anal- yses demonstrate that ATHB7 and ATHB12 act as positive transcriptional regulators of PP2C genes, and thereby as negative regulators of abscisic acid signalling. In support of this notion, our results also show that ATHB7 and ATHB12 act to repress the transcription of genes encoding the ABA receptors PYL5 and PYL8 in response to an ABA stimulus. In summary, we demonstrate that ATHB7 and ATHB12 have essential functions in the primary response to drought, as mediators of a negative feedback effect on ABA signalling in the plant response to water deficit. Keywords HD-Zip  PP2C  ABA  SnRK2  ABA receptors  Drought stress response Introduction Plants perceive changes in water availability and respond by specific adaptive alterations in physiology, growth and development, aimed at the efficient use of water resources. Limiting water conditions, or drought, elicit a signalling cascade, to a large extent dependent on abscisic acid (ABA) as a systemic mediator that eventually activates genes involved in the stress adaptive process (Xiong et al. 2002; Shinozaki et al. 2003). Several categories of tran- scriptional regulators participate in the signal transduction network from perception of drought stress signals to stress- responsive gene expression (Bray 2004). Among these, members of the plant-specific homeodomain-leucine zipper (HD-Zip) class I gene family have roles related to drought stress and ABA-signalling in different plant species (So ¨derman et al. 1996, Dezar et al. 2005; Deng et al. 2006; Agalou et al. 2008). In Arabidopsis thaliana the paralo- gous, and highly similar ARABIDOPSIS THALIANA HOMEOBOX7 (ATHB7) and ARABIDOPSIS THALIANA HOMEOBOX12 (ATHB12) are strongly and rapidly up-regulated after ABA treatment, and under drought and osmotic stress conditions (So ¨derman et al. 1996; Olsson et al. 2004; Henriksson et al. 2005). Constitutive expres- sion of ATHB7 causes a suppression of cell expansion and growth in the stem, leaves and roots (Hjellstro ¨m et al. 2003), mimicking one of the primary plant growth responses to water limitation. An additive phenotypic Electronic supplementary material The online version of this article (doi:10.1007/s11103-012-9956-4) contains supplementary material, which is available to authorized users. A. E. Valde ´s (&)  E. O ¨ verna ¨s  H. Johansson  P. Engstro ¨m Physiological Botany, Uppsala BioCenter, Uppsala University, Almas Alle ´ 5, 75651 Uppsala, Sweden e-mail: anaelisa.valdes@ebc.uu.se A. E. Valde ´s  P. Engstro ¨m Linnean Center for Plant Biology, Uppsala, Sweden A. Rada-Iglesias Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA, USA 123 Plant Mol Biol (2012) 80:405–418 DOI 10.1007/s11103-012-9956-4