Abstract
Phytochelatins play a key role in the detoxification of metals in plants and many other eukaryotes. Their formation is catalysed by phytochelatin synthases (PCS) in the presence of metal excess. It appears to be common among higher plants to possess two PCS genes, even though in Arabidopsis thaliana only AtPCS1 has been demonstrated to confer metal tolerance. Employing a highly sensitive quantification method based on ultraperformance electrospray ionization quadrupole time-of-flight mass spectrometry, we detected AtPCS2-dependent phytochelatin formation. Overexpression of AtPCS2 resulted in constitutive phytochelatin accumulation, i.e. in the absence of metal excess, both in planta and in a heterologous system. This indicates distinct enzymatic differences between AtPCS1 and AtPCS2. Furthermore, AtPCS2 was able to partially rescue the Cd hypersensitivity of the AtPCS1-deficient cad1-3 mutant in a liquid seedling assay, and, more importantly, when plants were grown on soil spiked with Cd to a level that is close to what can be found in agricultural soils. No rescue was found in vertical-plate assays, the most commonly used method to assess metal tolerance. Constitutive AtPCS2-dependent phytochelatin synthesis suggests a physiological role of AtPCS2 other than metal detoxification. The differences observed between wild-type plants and cad1-3 on Cd soil demonstrated: (i) the essentiality of phytochelatin synthesis for tolerating levels of Cd contamination that can naturally be encountered by plants outside of metal-rich habitats, and (ii) a contribution to Cd accumulation under these conditions.
Highlights
Due to their sessile nature, plants have to cope with various biotic and abiotic stress conditions
The saturation that the response curves for PC2 and PC3 showed at concentrations >25 μM was negligible because concentrations above 13 μM were never observed, even in extracts derived from Cd2+-treated plants
The synthesis of PCs is well established as the major detoxification mechanism for Cd, As, and Hg (Cobbett and Goldsbrough, 2002)
Summary
Due to their sessile nature, plants have to cope with various biotic and abiotic stress conditions. Changing availabilities of heavy metals are among the latter. They pose a threefold challenge for plants. Essential micronutrients such as Fe, Zn, Cu, Mn, and Mo have to be acquired in sufficient amounts. Their toxic effects have to be limited when present in supraoptimal concentrations. The co-occurrence of non-essential heavy metals or metalloids such as Cd and As, which are toxic and taken up as a result of their chemical similarity to essential nutrients, i.e. through transport proteins with a limited substrate specificity, necessitates an array of rapid and flexible and specific detoxification mechanisms
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