Uptake, translocation and transformation of arsenate and arsenite in sunflower (Helianthus annuus): formation of arsenic-phytochelatin complexes during exposure to high arsenic concentrations

The aim of the study was to determine the time-dependent formation of arsenic-phytochelatin (As-PC) complexes in the roots, stems and leaves of an arsenic-nontolerant plant (Helianthus annuus) during exposure to 66 mol l(-1) arsenite (As-(III)) or arsenate (As-(V)).

We used our previously developed method of simultaneous element-specific (inductively coupled plasma mass spectrometry, ICP-MS) and molecular-specific (electrospray-ionization mass spectrometry, ES-MS) detection systems interfaced with a suitable chromatographic column and eluent conditions, which enabled us to identify and quantify As-PC complexes directly.

Roots of As-exposed H. annuus contained up to 14 different arsenic species, including the complex of arsenite with two (gamma-Glu-Cys)(2)-Gly molecules [As-(III)-(PC2)(2)], the newly identified monomethylarsonic phytochelatin-2 or (gamma-Glu-Cys)(2)-Gly CH3As (MA((III))-PC2) and at least eight not yet identified species. The complex of arsenite with (gamma-Glu-Cys)(3)-Gly (As-(III)-PC3) and the complex of arsenite with glutathione (GSH) and (gamma-Glu-Cys)(2)-Gly (GS-As-(III)-PC2) were present in all samples (roots, stems and leaves) taken from plants exposed to As. The GS-As-(III)-PC2 complex was the dominant complex after 1 h of exposure. As-(III)-PC3 became the predominant As-PC complex after 3 h, binding up to 40% of the As present in the exposed plants.

No As-PC complexes were found in sap (mainly xylem sap from the root system), in contrast to roots, stems and leaves, which is unequivocal evidence that As-PC complexes are not involved in the translocation of As from root to leaves of H. annuus.