Multi-elemental speciation analysis of barley genotypes differing in tolerance to cadmium toxicity using SEC-ICP-MS and ESI-TOF-MS

Plants respond to Cd exposure by synthesizing heavy-metal-binding oligopeptides, called phytochelatins (PCs). These peptides reduce the activity of Cd ions in the plant tissues by forming Cd chelates. The main objective of the present work was to develop an analytical technique, which allowed identification of the most prominent Cd species in plant tissue by SEC-ICP-MS and ESI-TOF-MS. An integrated part of the method development was to test the hypothesis that differential Cd tolerance between two barley genotypes was linked to differences in Cd speciation. Only one fraction of Cd species, ranging from 700–1800 Da, was detected in the shoots of both genotypes. In the roots, two additional fractions ranging from 2900–4600 and 6700–15 000 Da were found. The Cd-rich SEC fractions were heart-cut, de-salted and demetallized using reversed-phase chromatography (RPC), followed by ESI-MS-TOF to identify the ligands. Three different families of PCs, viz. (gGlu-Cys)n-Gly (PCn), (gGlu-Cys)n-Ser (iso-PCn) and Cys-(gGlu-Cys)n-Gly (des-gGlu-PCn), the last lacking the N-terminal amino acid, were identified. The PCs induced by Cd toxicity also bound several essential trace elements in plants, including Zn, Cu, and Ni, whereas no Mn species were detected. Zn, Cu and Ni-species were distributed between the 700–1800 Da and 6700–15 000 Da fractions, whereas only Cd species were found in the 2900–4600 Da fraction dominated by PC3 ligands. Although the total tissue concentration of Cd was similar for the two species, the tolerant barley genotype synthesized significantly more CdPC3 species with a high Cd specificity than the intolerant genotype, clearly indicating a correlation between Cd tolerance and the Cd–PC speciation.