Metal species involved in long distance metal transport in plants

Abstract

The mechanisms plants use to transport metals from roots to shoots are not completely understood. It has long been proposed that organic molecules participate in metal translocation within the plant. However, until recently the identity of the complexes involved in the long-distance transport of metals could only be inferred by using indirect methods, such as analyzing separately the concentrations of metals and putative ligands and then using in silico chemical speciation software to predict metal species. Molecular biology approaches also have provided a breadth of information about putative metal ligands and metal complexes occurring in plant fluids. The new advances in analytical techniques based on mass spectrometry and the increased use of synchrotron X-ray spectroscopy have allowed for the identification of some metal-ligand species in plant fluids such as the xylem and phloem saps. Also, some proteins present in plant fluids can bind metals and a few studies have explored this possibility. This study reviews the analytical challenges researchers have to face to understand long-distance metal transport in plants as well as the recent advances in the identification of the ligand and metal-ligand complexes in plant fluids.

Keywords: metal complexes; metals; phloem; transport; xylem.

Figure 1

Schematic representation showing the metal species found in the xylem and phloem saps of non-grass (upper panels) and grass plant species (lower panels) grown at low and adequate metal supply (left panels) or at high metal supply (right panels). Metals occurred in free ionic forms and in complexes with different ligands such as nicotianamine (NA), 2′-deoxymugineic acid (DMA), carboxylates (e.g., citrate), histidine (His), phytochelatins (PCs) and proteins. Putative metal species proposed to occur in plant fluids as supported by strong evidence from indirect approaches (e.g., molecular biology and others) are shown with faded symbols. ^*Most of the data with high metal supply have been described in metal hyper-accumulator plant species.