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

本研究的目的是利用两个胡萝卜细胞系，不耐（CW）和耐Cd （CCd1）细胞来表征胡萝卜细胞对Cd的耐受机制。2个细胞用20-80 μM Cd预处理1 d后转移至无Cd培养基。与CW细胞不同，所有预处理的CCd1细胞在4天的时间内连续排除Cd。因此，在CCd1细胞中发现了更大的Cd消耗，这可能部分归因于Cd耐受性。然而，高的Cd排斥能力并不能完全解释CCd1细胞对Cd的耐受。因此，CCd1细胞中可能存在其他的Cd解毒机制。Cd胁迫下，CW细胞的植物螯合素（PCs）含量高于CCd1细胞。CW和CCd1细胞的抗坏血酸/谷胱甘肽循环对Cd的反应不同，抗坏血酸过氧化物酶活性分别降低80 ~ 160 μ Cd和160 μ Cd。Cd处理后，CW细胞中谷胱甘肽还原酶活性急剧下降；CCd1细胞的活性增加。Cd处理后，两种细胞的抗坏血酸含量均下降，但CCd1细胞的抗坏血酸含量高于CW细胞，且随时间延长而恢复。CCd1细胞的Cd耐受可能与活跃的抗坏血酸/谷胱甘肽循环有关。这些结果阐明了新的耐镉机制。CCd1细胞增强了CW细胞的自然Cd耐受机制。胡萝卜细胞对Cd的耐受与排除Cd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.