Relationship between stresses of heavy metal and of active oxygen in plants

本研究旨在探讨胡萝卜(Daucus carota L.)对镉的耐性机制。细胞采用两株胡萝卜细胞系，不耐(CW)细胞和耐Cd(CCd1)细胞。两种细胞均用20~80μM Cd处理1d后转入无Cd培养液中。与CW细胞不同，所有处理的CCd1细胞在4天内持续排除Cd。因此，在CCd1细胞中发现了较大的Cd消耗，这可能部分归因于Cd耐受性。然而，高的Cd排除能力不能完全解释CCd1细胞对Cd的耐受性。因此，在CCd1细胞中可能存在其他的Cd解毒机制。在Cd胁迫下，CW细胞的植物螯合素(PC)含量高于CCd1细胞。CW细胞和CCd1细胞的抗坏血酸/谷胱甘肽循环对Cd的反应不同。CW细胞和CCd1细胞的抗坏血酸过氧化物酶活性分别下降80~160mCd和160mCd。经Cd处理后，CW细胞的谷胱甘肽还原酶活性显著降低，而CCd1细胞的谷胱甘肽还原酶活性升高。Cd处理后，两种细胞中抗坏血酸含量均下降，但CCd1细胞中抗坏血酸含量高于CW细胞，且随处理时间的延长而恢复。CCd1细胞对Cd的耐受性可能与活跃的抗坏血酸/谷胱甘肽循环有关。根据这些结果，阐明了植物对Cd耐性的新机制。CCd1细胞增强了CW细胞的自然耐镉机制。胡萝卜细胞对Cd的耐性与Cd的排斥和抗氧化防御系统，特别是抗坏血酸/谷胱甘肽循环有关。

The purpose of this study is to characterize the Cd tolerance mechanisms on carrot (Daucus carota L.) cells using a two carrot cell lines, non-tolerant (CW) and Cd tolerant (CCd1) cells. Both cells were pretreated with 20-80 μM Cd for 1 day and transferred to a Cd-free medium. The CCd1 cells in all the pretreatments continuously excluded Cd over a 4-day period, unlike the CW cells. Therefore, a larger depletion of Cd was found in the CCd1 cells, which may be partially attributed to Cd tolerance. However a high capacity for Cd exclusion cannot fully account for the Cd tolerance of the CCd1 cells. Hence, other mechanism of Cd detoxification should exist in the CCd1 cells. Under Cd stress, the contents of phytochelatins (PCs) of the CW cells were higher than those of the CCd1 cells. The ascorbate/glutathione cycle in the CW and CCd1 cells showed different responses against Cd. The activity of ascorbate peroxidase in the CW and CCd1 cells decreased by 80-160 and 160 μM Cd, respectively. After Cd treatment, the activity of glutathione reductase in the CW cells decreased drastically ; whereas, the activity in the CCd1 cells increased. The content of ascorbate in both cells decreased by Cd treatment, but that in the CCd1 cells was more than CW cells and recovered with increasing time. Cd tolerance in the CCd1 cells may be related to active ascorbate/glutathione cycle. From these results, new mechanism of Cd tolerance was clarified. The CCd1 cells strengthened the natural Cd tolerance-mechanism of the CW cells. Cd tolerance of carrot cells is related to exclusion of Cd and antioxidative defense system, particularly, ascorbate/glutathione cycle.