Plants that are exposed to toxic concentrations of non-essential elements Cd, Hg, Pb and As synthesize phytochelatins (PCs) in order to survive (Rea 2012; Fischer et al. 2014). These peptides are composed of the general structure (γ-Glu-Cys)n-Gly and there formation is catalyzed non-ribosomally by phytochelatin synthases (PCS) from glutathione (Grill et al. 1985, 1987). PCS are constitutively expressed but the synthesis of PCs is activated in the presence of a range of heavy metal ions including Cd2+, Cu2+, Hg2+, Pb2+, Zn2+ (Grill et al. 1987; Ha et al. 1999; Vatamaniuk et al. 2000). Heavy metal ions are subsequently complexed by the free thiol groups of PCs (Grill et al. 1987, 1989). PCS are ubiquitously distributed in higher plants but exist also in all larger groups of eukaryotes and in many prokaryotes as well (Clemens & Persoh 2009). The wide distribution of PCS sequences and the constitutive expression of the respective proteins suggest an alternative physiological function besides heavy metal detoxification. A role of PCS in the essential heavy metal homeostasis as well as in nonhost resistance was recently demonstrated (Tennstedt et al. 2009; Clay et al. 2009). Experiments on artificially contaminated soil confirmed a role of PCs in the detoxification of the essential heavy metal Zn and the toxic metal Cd. Moreover, it was shown that PC-formation is indispensable for plants tolerance towards these metals inside and outside metal rich habitats and that PCs contribute to their accumulation. Furthermore, PCS participate in general Zn homeostasis, whereas no evidence was provided for a function of PCs in the response upon Zn deficiency. In addition, the hypothesis of the AtPCS1 C-terminal part being the responsible region for the activation of PC-synthesis by metal ions, could be underlined by growth experiments on contaminated soil comparing wild type, null allele cad1-3 and the T-DNA insertion line cad1-6, which carries a C-terminally truncated version of the protein. PC-analysis of soil grown and liquid culture derived plant material strengthened the idea of the existence of different binding sites for different metals in the C-terminus. Furthermore, the loss of C-terminal sequences appears to interfere with long-chain PC-synthesis. The development of a sensitive method for PC-detection by use of UPLC-ESI-QTOF-MS enabled the detection of small PC2 levels in the AtPCS1 mutant cad1-3, which result from the action of the second PCS in A. thaliana, AtPCS2. This was the first evidence for the functionality of AtPCS2 in planta. Furthermore AtPCS2 is able to complement the Cd hypersensitivity of cad1-3, which lacks a functional AtPCS1. Using different test systems for the assessment of heavy metal tolerance of AtPCS1 and AtPCS2 overexpression lines in the cad1-3 background it was demonstrated, that the activation of PC-formation of AtPCS2 is, in contrast to AtPCS1, less influenced by the presence of heavy metals. Furthermore, AtPCS2 seems to be impaired in the synthesis of long-chain PCs, which renders the quenching of the dysfunction of AtPCS1 in heavy metal tolerance impossible in planta. This suggests a biological function for this protein apart from heavy metal detoxification. Assays with cad1-3 plants expressing the phylogenetic diverse PCS variant from C. elegans in comparison with those expressing AtPCS1 demonstrated the independent action of transpeptidase and peptidase activity in planta. Overexpression lines of both genes could complement the Cd hypersensitive phenotype but not the penetration phenotype induced by P. infestans. The ABC-transporter PEN3 was shown, like AtPCS1, to take part in plants Cd and Zn tolerance. Although AtPCS1 is the key component of As detoxification, PEN3 plays only a minor role. Experiments with a crossing of single mutants of both genes revealed an additive effect in Zn hypersensitivity, implying the coordinated action of both proteins in Zn but not in Cd tolerance.
|School:||Universitaet Bayreuth (Germany)|
|Source:||DAI-C 81/4(E), Dissertation Abstracts International|
|Subjects:||Genetics, Plant sciences, Biogeochemistry, Soil sciences|
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