Identification of high levels of phytochelatins, glutathione and cadmium in the phloem sap of Brassica napus. A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

• Published in: The Plant journal : for cell and molecular biology Vol. 54; no. 2; pp. 249 - 259
• Main Authors: Mendoza-Cózatl, David G, Butko, Emerald, Springer, Franziska, Torpey, Justin W, Komives, Elizabeth A, Kehr, Julia, Schroeder, Julian I
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.04.2008; Blackwell Publishing Ltd; Blackwell Science
• Subjects: Arabidopsis; Biochemistry; Biological and medical sciences; Biological Transport; Botany; Brassica napus; Brassica napus - drug effects; Brassica napus - metabolism; Cadmium - metabolism; Cadmium - pharmacology; cadmium translocation; Cell physiology; Fundamental and applied biological sciences. Psychology; Glutathione - metabolism; heavy metals; Iron - metabolism; iron content in xylem; long-distance transport; mass spectrometry; Metals; Peptides; Phloem - metabolism; Phytochelatins - metabolism; Plant physiology and development; Plasma membrane and permeation; Spectrum analysis; Sulfhydryl Compounds - metabolism; thiol-peptides; Xylem - metabolism; Translocation; Xylem; Root; Arabidopsis; heavy metals; Detoxification; cadmium translocation; long-distance transport; Heavy metal; iron content in xylem; thiol-peptides; Phloem; Cruciferae; Dicotyledones; Angiospermae; Brassica napus; Above ground plant part; mass spectrometry; Spermatophyta
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313X.2008.03410.x

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC-Cd and glutathione-Cd complexes.