In recent years, there has been increasing awareness that multidisciplinary and integrative research is necessary to clarify the complexity of plant interactions with diverse biotic and abiotic cues. Since they are members of complex communities, plants interact with both antagonistic and beneficial organisms. Sulfur-containing compounds (including sulphate, glutathione [GSH], phytochelatins, metallothion-eins, low-molecular-weight thiols, various secondary metabolites, and sulfur-rich proteins) are crucial to the survival of plants under biotic and abiotic stress (Ernst et al. 2008; Hernandez-Allica et al. 2006; Rausch and Wachter 2005). As well as pre-existing morphological and biochemical adaptations, defence strategies comprise direct and/or indirect induced synthesis of chemical compounds (mainly plant secondary metabolites) (Bezemer and van Dam 2005). Glucosinolates (GS) are natural sulfur-containing products of the Brassicaceae and other related plant families (Fahey et al. 2001), and they represent one of the best known examples of preformed defence compounds. Their biosynthesis and the expression of their related enzymes and proteins vary during development and among organs, and they can be differentially regulated upon phytohormone treatment and insect herbivory, and in response to abiotic factors (Brown et al. 2003; Kliebenstein et al. 2002; Petersen et al. 2002; Pongrac et al. 2008; Rask et al. 2000). Regulation of plant responses to such cues is complex, and acts mainly via the salicylic acid

Department of Biology, University of Ljubljana, VeCna pot 111, 1000, Ljubljana, Slovenia e-mail: [email protected] e-mail: [email protected] e-mail: [email protected]

Laboratorio de Fisiología Vegetal, Facultad de Ciencias, Universidad Autónoma de Barcelona,

08193, Bellaterra, Spain e-mail: [email protected]

e-mail: [email protected]

e-mail: [email protected]

I. Sherameti and A. Varma (eds.), Soil Heavy Metals, Soil Biology, Vol 19, 139

DOI 10.1007/978-3-642-02436-8_7, © Springer-Verlag Berlin Heidelberg 2010

and jasmonic acid signalling networks (Freeman et al. 2004; Mikkelsen et al. 2003). Jasmonates contribute to signal transduction during insect herbivory, and they increase the total GS concentrations, primarily because of changes in indolyl GS concentrations (Cipollini et al. 2004; Reymond et al. 2004). Instead, salicylate, a pathogen attack signal, stimulates GS accumulation, either alone or in combination with other phytohormones, and it may counteract jasmonate-mediated GS induction (Kliebenstein et al. 2002). A model of a two-step regulation system of systemic resistance by airborne and vascular long-distance signals has been proposed, and it provides a carefully balanced systemic defence reaction to local attack by herbivores and pathogens (Heil and Ton 2008). Recent results indicate that the signals that are activated by plants in response to its "friends" and "foes" overlap. This indicates that the regulation of the adaptive response in the plant is finely balanced between protection against aggressors and acquisition of benefits (Pieterse and Dicke 2007).

Hyperaccumulation is a phenomenon that evolved in plants exposed to extreme metal concentrations (Baker 1981). Several Brassicaceae plant species are hyperac-cumulators that take up exceptionally high metal concentrations in their aboveg-round biomass without visible toxicity symptoms (Brooks et al. 1977). Extremely high concentrations of accumulated elements have also been proposed to act in plant biotic interactions in so-called elemental allelopathy (Boyd and Martens 1998). In addition, arbuscular mycorrhizal (AM) fungi are ubiquitous soil microbes that are considered essential for the survival and growth of plants in nutrient-deficient soils (Smith and Read 1997), and have been connected to metal tolerance in plants (Hall 2002; Regvar et al. 2006; Hildebrandt et al. 2007; Vogel-Mikus and Regvar 2006), mainly through fungal tolerance mechanisms (Joner et al. 2000). The presence of AM symbiosis in metal hyperaccumulating plants has been reviewed in detail recently (Regvar and Vogel-Mikus 2008). The relationships between the metal and organic defences that mediate ecological interactions in plants (including symbiosis, herbivory and pathogenesis) are, however, extremely complex and have rarely been studied simultaneously under field conditions (Hartl and Baldwin 2006; Noret et al. 2007; Poschenrieder et al. 2006a).

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