重金屬是環境中主要的有毒污染物之一，隨著快速的工業化，使環境中重金屬的含量增加而對所有的生物體造成危害。在植物中，過去研究資料顯示，當植物體內累積過量的重金屬，會使植物體的一些生理現象產生變化，如蒸散作用減少，光合作用速率降低，並改變(通常是抑制)一些酵素的活性，其對於植物生長與發育的影響亦已被證實。然而，其機制卻尚未清楚明瞭。
植物根部是植物體接觸環境土壤的主要器官，當環境中的重金屬過量，會對植物根部造成嚴重的傷害。本實驗以阿拉伯芥及水稻兩種植物為代表，探討重金屬是透過那些分子機制影響植物根部生長及發育。結果顯示，在影響植物根部發育方面，從阿拉伯芥的下胚軸癒傷組織(hypocotyl callus)誘導根部再分化時，會受到重金屬鎘及銅處理的抑制；在影響植物根部生長方面，重金屬銅會抑制水稻植物根部生長，並造成根部尖端細胞死亡及DNA斷裂(DNA fragmentation)。若前處理抗氧化劑—GSH, DTT, NAC、細胞外鈣離子螯合劑—EGTA、細胞膜上鈣離子通道抑制劑—La3+、攜鈣素(calmodulin)抑制劑—W7、蛋白質去磷酸酶抑制劑—cantharidin、sodium orthovanadate及MAPK kinase (mitogen-activated protein kinase kinase)活性抑制劑—PD098059、U0126，則皆可抑制銅所造成的根部尖端細胞死亡。推測活化氧族(Reactive oxygen species, ROS)的產生、細胞外鈣離子流入細胞內、蛋白質去磷酸酶以及MAPK pathway可能參與銅所誘導的水稻根部尖端細胞死亡之過程。
生物體常藉由磷酸化或去磷酸化蛋白質分子調控其活性，進而影響細胞生理反應。在哺乳動物中，重金屬會抑制細胞的生長及分化；且細胞內接受酪胺酸 (tyrosine) 磷酸化之蛋白質已被證實會受到重金屬處理而改變；然而，在植物中，蛋白質酪胺酸磷酸化的研究卻有限。利用抗體偵測酪胺酸磷酸化蛋白質，以探討其在重金屬抑制阿拉伯芥根部發育及重金屬誘導水稻根部細胞死亡過程中的角色，發現在鎘抑制阿拉伯芥根部發育中，有一分子量大約80 kDa的酪胺酸磷酸化蛋白質的表現量會上升；而在銅誘導水稻根部細胞死亡情形中，有一分子量大約是45 kDa的酪胺酸磷酸化蛋白質的表現量會下降。為進一步了解酪胺酸去磷酸化在銅誘導根部細胞死亡下所
扮演的角色，利用外加蛋白質去磷酸酶抑制劑sodium orthovanadate、抗氧化劑GSH及鈣離子螯合劑EGTA，發現皆可抑制銅所誘導的蛋白質酪胺酸去磷酸化。此外，以MAPK為主的訊息傳遞系統在生物體面臨生物性及非生物性壓力時，亦扮演重要角色。由於水稻中OsMAPK2參與某些逆境訊息的傳遞，因此，進一步探討水稻中OsMAPK2在銅處理下的表現及其調節機制，結果顯示：OsMAPK2基因表現量會受到重金屬銅及過氧化氫逆境刺激的誘導而增加。抗氧化劑GSH、鈣離子螯合劑EGTA、細胞膜上鈣離子通道抑制劑La3+及蛋白質去磷酸酶抑制劑cantharidin，皆可抑制重金屬銅所誘導的OsMAPK2基因表現。這些結果顯示，在重金屬抑制阿拉伯芥根部發育及重金屬誘導水稻根部細胞死亡之過程中，可能與酪胺酸磷酸化/去磷酸化及MAPK pathway的活化有密切關聯。然而，其間的相互關係及是否有其他因子參與，則有待進一步研究探討。

Heavy metal toxicity is one of the major environment health problems in modern society. Rapid industrialization and urbanization have enhanced the levels of toxic heavy metals in the environment posing a potential health hazard for all living organisms. In plants, these heavy metals when accumulated in excess amounts can function as stressors causing physiological constraints and alterations in various vital growth processes, such as transpiration, photosynthesis, enzyme activities. It has been known that heavy metals exert inhibitory effects in plant growth and development. However, the mechanism involved in its effect is not well understood.
Roots of plants are damaged if the dose of heavy metals exceeds threshold levels. In this study, we used two model plants, Arabidopsis and rice, to study the molecular mechanism(s) of heavy metals on root growth and development. In Arabidopsis, the redifferentiation of adventitious roots from hypocotyl callus was severely damaged by Cd and Cu treatments. In rice, copper treatment caused the inhibition of root growth as well as the cell death and DNA fragmentation of root cells. Pre-treatment of rice roots with antioxidants (GSH, NAC, DTT), calcium chelator (EGTA), plasma-membrane calcium channel blocker (La3+), calmodulin antagonist (W-7), protein phosphatase inhibitors (sodium orthovanadate, cantharidin) and MAPK kinase (MEK) inhibitors (PD098059, U0126) blocked copper-induced cell death. We suggest that ROS production, extracellular calcium ions, protein phosphatase and activation of MAPK pathway are probably involved in copper-induced cell death of root-tip cells in rice.
Protein phosphorylation and dephosphorylation are involved in the regulation of many eukaryotic intracellular processes. It has been known that heavy metals inhibit the proliferation and differentiation of cells in animals. The variations in protein tyrosine phosphorylation caused by heavy metals has been studied in mammalian cells. However, data related to the involvement of protein tyrosine phosphorylation in plant growth and development influenced by heavy metals was limited. In this study, the possible roles of
protein tyrosine phosphorylation in cadmium-inhibited root development in Arabidopsis and copper-induced cell death of root-tip cells in rice were investigated. In cadmium- inhibited root development in Arabidopsis, a 80-kDa protein was highly tyrosine phosphorylated. In copper-induced cell death of rice root-tip cells, the level of tyrosine phosphorylation of a 45-kDa protein was decreased. Pre-treatment of rice roots with tyrosine phosphatase inhibitor sodium orthovanadate, the antioxidant GSH and the calcium chelator EGTA significantly abolished copper-induced protein tyrosine dephosphorylation. In addition, MAPKs play important roles in signal transduction in responses to biotic and abiotic stresses. Since OsMAPK2 function in specific stress-signalling pathway, the MAP kinase gene (OsMAPK2) expression in rice under copper stress was also investigated. OsMAPK2 transcript accumulation was enhanced by copper and H2O2 in rice root-tip cells. Using Northern blot analysis, it was shown that antioxidant agent (GSH), calcium chelator (EGTA), plasma-membrane calcium channel blocker (La3+) and protein phosphatase inhibitor (cantharidin) inhibited copper-induced OsMAPK2 gene expression. These results indicate that tyrosine phosphorylation/ dephosphorylation and activation of MAPK pathway are probably involved in heavy metals-inhibited root development in Arabidopsis and heavy metals-induced cell death of rice root-tip cells. These results provide some information for further studies on the mechanism(s) of heavy metal toxicity in plants.

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