Complexation of Hg with phytochelatins is important for plant Hg tolerancepce

Abstract

Three-week-old alfalfa (Medicago sativa), barley

(Hordeum vulgare) and maize (Zea mays) were exposed

for 7 d to 30 mM of mercury (HgCl2) to characterize the Hg

speciation in root, with no symptoms of being poisoned.

The largest pool (99%) was associated with the particulate

fraction, whereas the soluble fraction (SF) accounted for a

minor proportion (<1%). Liquid chromatography coupled

with electro-spray/time of flight mass spectrometry showed

that Hg was bound to an array of phytochelatins (PCs)

in root SF, which was particularly varied in alfalfa (eight

ligands and five stoichiometries), a species that also accu mulated homophytochelatins. Spatial localization of Hg in

alfalfa roots by microprobe synchrotron X-ray fluorescence

spectroscopy showed that most of the Hg co-localized with

sulphur in the vascular cylinder. Extended X-ray Absorp tion Fine Structure (EXAFS) fingerprint fitting revealed

that Hg was bound in vivo to organic-S compounds, i.e.

biomolecules containing cysteine. Albeit a minor propor tion of total Hg, Hg–PCs complexes in the SF might be

important for tolerance to Hg, as was found with Arabi dopsis thaliana mutants cad2-1 (with low glutathione

content) and cad1-3 (unable to synthesize PCs) in compari son with wild type plants. Interestingly, high-performance

liquid chromatography-electrospray ionization-time of

flight analysis showed that none of these mutants accumu lated Hg–biothiol complexes.