Root exudates released into soil have important functions in mobilizing metal micronutrients and for causing selective enrichment of plant beneficial soil micro-organisms that colonize the rhizosphere. Analysis of plant root exudates typically has involved chromatographic methods that rely on a priori knowledge of which compounds might be present. In the research reported here, the combination of multinuclear and 2-D NMR with GC-MS and high-resolution MS provided de novo identification of a number of components directly in crude root exudates of different plant types. This approach was applied to examine the role of exudate metal ion ligands (MIL) in the acquisition of Cd and transition metals by barley and wheat. The exudation of mugineic acids and malate was enhanced by Fe deficiency, which in turn led to an increase in the tissue content of Cu, Mn, and Zn. The presence of elevated Cd maintained at a free activity pCd of 8.8 (10-8.8 M), resulted in reduced phytosiderophore production by Fe deficient plants. The buffer morpholinoethane sulfonate (MES), which is commonly used in chelator-buffering nutrient solutions, was detected in the root exudate mixture, suggesting uptake and re-secretion of this compound by the roots. The ability to detect this compound in complex mixtures containing organic acids, amino acids, and other substances suggests that the analytical methods used here provide an unbiased method for simultaneous detection of all major components contained in root exudates.
Bibliographical noteFunding Information:
This work was supported by US Department of Energy grant numbers DE-FG07-96ER20255, US EPA grant No. R825960010, and US EPA funded (No. R819658) Center for Ecological Health Research at University of California at Davis. We thank Dr. Steven Howell of NIMR for recording the nanospray ES-MS spectra. NMR spectra were recorded at the MRC Biomedical NMR Centre, Mill Hill.
- Fe deficiency
- High-resolution MS
- Hordeum vulgare
- Metal ion ligands
- Metal sequestration
- Mugineic acids
- Multinuclear and 2-D NMR spectroscopy
- Triticum aestivum