Long-distance transport of Cd in plants
- 1. Understanding long-distance transportof heavy metals in plants David G. Mendoza-Cozatl 2nd Georgian Bay International Conference on Bioinorganic Chemistry May 26th-29th, 2009 Parry Sound, ON, Canada 1
- 2. Heavy metals - Canbe divided in essential (Fe, Zn, Cu) and non essential (Pb, Cd, Hg) heavy metals - Extremely reactive and widely used in the industry - Global contamination was fully recognized in the 80’s caused by an inadequate disposal of industrial residues 2 *using phytoremediation as a keyword
- 3. Superfund sites withinthe US EPA has identified > 1000 sites requiring urgent attention 1. Arsenic* 2. Lead 3. Mercury 4. Vinyl Chloride 5. PCBs 6. Benzene 7. Cadmium 9. Polycyclic hydrocarbons Superfund sites in the National Priority List (EPA, 2007) 3 * Agency for Toxic Substances and Disease Registry (www.atsdr.cdc.gov)
- 4. Two main phytoremediationapproaches ✗ Phytostabilization Currently used in mine tailings to avoid spreading by wind Phytoextraction ✔ Useful in leaky soils to avoid further contamination 4
- 5. Different processes areinvolved in the homeostasis of heavy metals 5. Sequestration Pb Zn Sn Mn 4. Unloading and V tissue distribution Zn 3. Vascular transport Ag Mn Cr Zn Mn Cd 1. Mobilization 2. Uptake Zn Cu Mn Fe Zn Co 5 Clemens et al., (2002) Trends Plant Sci.
- 6. Phytochelatins (PCs) inactivateheavy metals GSSG GR Cys Gly ATP-Mg ATP-Mg H2O2 GPx γ - ECS γ-glutamylcysteine GS GSH (γ-EC ) PCS* Glu Xe Cd2+ GSTs (γ -Glu-Cys)2-Gly γ -Glu-Cys-Gly Xe PCs PC2-Cd-PC2 cytosol vacuole 6 *PCS gene was first cloned independently by 3 labs (Clemens et al., 1999; Ha et al., 1999; Vatamaniuk et al., 1999)
- 7. PCs are essentialfor Cd2+ resistance Col cad1-3 Col cad1-3 Wildtype (PC-deficient) Wildtype (PC-deficient) Control + Cd 40 µM, 8 days PCs were once considered solely as an intracellular detoxification system 7
- 8. Arabidopsis hydroponic cultures 1 week 2 weeks 1 week 1 week 3 weeks Advantages: 1) Allows clean separation from leaves and roots 2) Better control on heavy metal concentration in solution 4 weeks
- 9. PCs can betransported between shoots and roots Shoot specific expression of PC-synthase Which vascular PCs system ? - Xylem - Phloem PCs Root specific expression of PC-synthase Independently of the site of expression, PCs were found in roots and shoots 9 Gong et al., 2003; Chen et al., 2006
- 10. PCs occur inphloem sap from Brassica napus Xylem sap and phloem sap were analyzed for their Cd and thiol content and No Cd Cd 1 week Cd 2 weeks PCs were found only in the phloem 10 Mendoza-Cózatl et al. (2008), Plant J. 54:249-259.
- 11. No PCs werefound in the xylem sap Xylem sap GSH Fluorescence m/z 920 Da Xylem + Cd 1d GSH m/z Cadmium in phloem (8-60 µΜ) is more concentrated overtime than in xylem (4-20 µM) 11
- 12. PC concentration issufficient to inactivate Cd in the phloem sap Affinity towards Cd (Kd) *PCs/Cd ratio PC2 7.9 x 10-17 M GSH 3.1 x 10-11 M 1 week 1.2 Histidine 2.2 x 10-6 M 2 weeks 5.1 Glutamate 1.2 x 10-4 M PC2-Cd-PC2 12 * Thiols from PCs (each PC2 has 2 thiols).
- 13. PC-Cd complexes determinedby mass spectrometry Isotopic distribution of cadmium PC2-Cd complex spectra in negative mode PC2 = 536 Da (540 - 4H+ @ pH 7.4) counts Spectra recorded @ pH 7.4 13
- 14. Cd transport throughthe phloem is mediated by thiol compounds Phloem-mediated shoot-to-root Xylem transport is transport unidirectional Root-to-shoot Cd transport is mediated by organic acids and amino Cd re-distribution acids as thiol-Cd complexes Transporters ? 14 Salt et al, 1995; Kerkeb and Kramer, 2003, Mendoza-Cozatl et al., 2008
- 15. Summary - Long distancetransport of phytochelatin-Cd complexes occurs through the phloem. - Phloem transport contributes to define the plant tissue where Cd is finally stored. - The next step: Identification of transporters. 15
- 16. UCSD Julian Schroeder Joan Chen GaroAkmakjian Tim Jobe Zhongchun Zhang Garrick Yuen Maik Böhmer Noriyuki Nishimura TSRI Paul Russell UCSD Biochemistry Elizabeth Komives 16