SUMMARY
Tumbleweed (S. kali), a desert plant worldwide distributed, has been proposed as a potential Cdhyperaccumulator. X-ray studies showed that thiol and oxygen related compounds are involved in Cd sequestration within the plant. Thus, we have proposed that organic acids, cell wall, phytochelatins, and other glutathione related compounds might be involved in the mechanisms of Cd hyperaccumulation in tumbleweed. In this study, native plants were used to determine Cd content in phloem/xylem tissues and the related biochemical mechanisms of Cd uptake at the protein level. In addition, plant extracts were analyzed by high pressure liquid chromatography (HPLC) to identify and quantify organic acids. Plants were treated with 0, 20, 200, and 400 mg Cd L-1 for 48 h in hydroponic media. Cd incorporation was measured in roots, phloem/cortex, and xylem/pith, separately. It was found that in plants treated with Cd concentrations above 200 mg Cd L-1 , Cd content was higher in phloem than that in xylem. The protein profile in SDSPAGE showed that in Cd-treated plants, two peptides were enhanced while a new peptide was expressed. After G25 gel filtration and Cd codetermination it was found that two proteins (of 29 and 14 kDa) are probably associated to Cd. The use of degenerated primers of the Brassica family allowed the identification of a possible phytochelatin synthase gene. Citric and oxalic were the main acids identified in plant extracts. No significant differences were found in the concentration of citric acid in control and Cd-treated plants. On the other hand, less oxalic acid was quantified in Cd-treated plants as compared to controls. These data indicate that cadmium may have precipitated as oxalate crystals. The results reported herein will be helpful to better understand the mechanisms of Cd hyperaccumulation in S. kali.