| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 89 === When exposed to heavy metals, plant is induced to produce phytochelatin (PC) for the sequestration of toxic metals. PC has the general structure of (g-Glu-Cys)n-Gly (n =2-11), which contains high percentage of cysteine in order to bind heavy metals via its thiol groups. It is established that the heavy metal in the cell is sequestered by forming complex with several PC molecules. In the cytosol, a low-molecular-weight complex (LMW) is formed firstly to remove the invading toxic metals as soon as possible. Then LMW is transported into the vacuole by a transporter HMT1 (a heavy metal tolerance gene translated protein), and its molecular weight increases by incorporating more PC molecules forming the high-molecular-weight complex (HMW). The acid-labile sulfide (S2-) is added to the HMW complex, the component PC molecules might rearrange and produced a stable form for storage in the vacuole.
In this study, water hyacinth was treated with 0.01 mM CdCl2 for several days, then the Cd in various tissues (root, thick stalk and leaf) was analyzed. In root, both HMW and LMW complexes were detected in 5-day exposure, but only HMW complex remained for a longer period of Cd treatment. However, thick stalk accumulated only HMW complex after a short period of lag. The HMW complex was composed of mainly PC3 and PC4 as revealed by amino acid composition analysis and electron-spray ionization mass spectrum (ESI-MS) analysis. Not like its counterpart in the fission yeast, the LMW complex in water hyacinth was hard to isolate, and so its composition is still unclear. Only trace amount of the phytochelatin synthase (PCS) activity was detected in water hyacinth. Further studies are needed to explicit their roles in heavy metal sequestration mechanism.
To study the possible transportation route of Cd in the plant, the Cd treated water hyacinth plant was transferred to Cd-free medium and cultivated for days, and then the Cd content in different tissues was analyzed. It was found that the Cd content in the root and thick stalk decreased after the change. On the contrary, the Cd content increased in the leaf. This observation indicated that the Cd absorbed from the root might be transported to the stalk and then to the leaf.
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