CE of phytosiderophores and related metal species in plants

Metal species involved in long distance metal transport in plants

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.

A sensitive LC-ESI-Q-TOF-MS method reveals novel phytosiderophores and phytosiderophore-iron complexes in barley

The direct analysis of phytosiderophores (PSs) and their metal complexes in plants is critical to understanding the biological functions of different PSs. Here we report on a rapid and highly sensitive liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry (LC-ESI-Q-TOF-MS) method for the direct and simultaneous determination of free PSs and their ferric complexes in plants. In addition to previously reported PSs--deoxymugineic acid (DMA), mugineic acid (MA) and epihydroxymugineic acid (epi-HMA)--two more PSs, avenic acid (AVA) and hydroxyavenic acid (HAVA), were identified by this method in roots of Hordeum vulgare cv Himalaya and in root exudates under iron (Fe) deficiency. The two identified PSs could be responsible for Fe acquisition under Fe deficiency because of their relative abundance and ability to form ferric complexes in secreted root exudates. This LC-ESI-Q-TOF-MS method greatly facilitates the identification of free PSs and PS-Fe complexes in one plant sample.

Copper in xylem and phloem saps from rice (Oryza sativa): the effect of moderate copper concentrations in the growth medium on the accumulation of five essential metals and a speciation analysis of copper-containing compounds

Copper (Cu) is an essential element for cereals, playing important roles as a cofactor of several enzymes. Copper and four other metals (Fe, Mn, Zn and Mo) taken up by roots are efficiently delivered to the shoots via xylem and phloem. Here we investigated the concentrations of Cu, Fe, Mn, Zn and Mo in the xylem and phloem saps as well as in tissues of rice (Oryza sativa L.) seedlings when they were grown under different Cu levels in culture solution. Although the Cu concentrations in the roots and the Mn concentrations in the mature shoot tissues were increased with the increase of the Cu level in the culture solution, the concentrations of Cu and the other four metals in the xylem and phloem saps and the Cu contents in the shoot tissues were only slightly affected by moderate increases in the Cu medium level. The results of our analyses using membrane filtration, size-exclusion chromatography and electrospray ionisation time-of-flight mass spectrometry indicate that Cu in the xylem sap is dominantly complexed by 2'-deoxymugineic acid, whereas Cu in the phloem sap is bound to several compounds, i.e. nicotianamine, histidine and other >3-kDa compounds.

Isoelectric focusing of small non-covalent metal species from plants

IEF is known as a powerful electrophoretic separation technique for amphoteric molecules, in particular for proteins. The objective of the present work is to prove the suitability of IEF also for the separation of small, non-covalent metal species. Investigations are performed with copper-glutathione complexes, with the synthetic ligand ethylenediamine-N,N'-bis(o-hydroxyphenyl)acetic acid (EDDHA) and respective metal complexes (Fe, Ga, Al, Ni, Zn), and with the phytosiderophore 2'-deoxymugineic acid (DMA) and its ferric complex. It is shown that ethylenediamine-N,N'-bis(o-hydroxyphenyl)acetic acid and DMA species are stable during preparative scale IEF, whereas copper-glutathione dissociates considerably. It is also shown that preparative scale IEF can be applied successfully to isolate ferric DMA from real plant samples, and that multidimensional separations are possible by combining preparative scale IEF with subsequent HPLC-MS analysis. Focusing of free ligands and respective metal complexes with di- and trivalent metals results in different pIs, but CIEF is usually needed for a reliable estimation of pI values. Limitations of the proposed methods (preparative IEF and CIEF) and consequences of the results with respect to metal speciation in plants are discussed.

A highly sensitive, quick and simple quantification method for nicotianamine and 2'-deoxymugineic acid from minimum samples using LC/ESI-TOF-MS achieves functional analysis of these components in plants

A highly sensitive quantitative method for assaying nicotianamine (NA) and 2'-deoxymugineic acid (DMA) using liquid chromatography/electrospray ionization time-of-flight mass spectrometry (LC/ESI-TOF-MS) was developed. The amino and hydroxyl groups of NA and DMA were derivatized using 9-fluorenylmethoxycarbonyl chloride. The amounts of NA and DMA in 10 mul of xylem sap from rice cultivated under iron (Fe)-sufficient and Fe-deficient conditions were quantified without concentration. In Fe-sufficient plants, the concentrations of NA and DMA were almost equal to that of Fe. In Fe-deficient plants, the concentration of NA did not change significantly, whereas that of DMA increased markedly.

Hydrophilic interaction chromatography of small metal species in plants using sulfobetaine- and phosphorylcholine-type zwitterionic stationary phases

Hydrophilic interaction chromatography (HILIC) was applied for the separation of small, noncovalent metal species, and free ligands in plants, using two different zwitterionic stationary phases at pH 7.3. Ligands ranged from amino acids and phytosiderophores to organic acids and synthetic chelators like EDTA. Our results confirmed the suitability of zwitterionic stationary phases for the separation of such ligands and their metal complexes. In particular, the chromatographic behavior of phytosiderophore complexes of copper, nickel, and zinc on a sulfobetaine-type material was compared to that on a phosphorylcholine-type material under otherwise identical conditions. A compression of the usable retention range for phytosiderophore species on the latter phase was found, which can be traced back to the reversed charge orientation of the zwitterionic functionalities. Differences were also found for the integrity of more labile metal chelates during separation on the two columns. In particular, Ni-malate could only be analyzed on the sulfobetaine phase, and Cu-glutathione, Ni-aspartate, and Ni-citrate complexes were only stable on the phosphorylcholine stationary phase at pH 7.3. Ni-histidine species were only found after lowering the pH to 4-5. The identification of separated species is possible by online ESI-MS in the negative ionization mode.