Study On The Response Mechanism Of Border Cells In The Root Tip Of Cucumber(Cucumber Sativus L.) To Copper Stress

Copper is one of the most common heavy metals in the contaminated soil in China, which seriously threatens the agricultural production and health safety. Root border cells (RBCs), a pioneer in response to stress in plant, are directly involved in the response to heavy metals and play an important role in regulating heavy metals accumulation. However, the response mechanism of border cells to copper stress needs for further research. In this paper, we studied the characters of cucumber’s (Cucumis sativus L.) border cells and their response to copper stress, a typical copper sensitive crop. Transmission electron microscopy, fluorescence microscopy and other advanced techniques are used in this study in order to provide some new pathways for copper-resistant mechanism. The main conclusions are as follows:The physiological characteristics of cucumber RBCs were closely related to root growth, and the development of RBCs was almost synchronized with the growth of roots. Pectolytic enzyme (PME) activity detection showed that there was a closing correlation between border cell development and PME activity in the root tips of cucumber, and PME activity played an important role in the release of RBCs. RBCs maintained high viability and the mucilage thickness was stable with6.5-7.5μm during root growing. According to these results, border cells of roots with20-25mm length were used in the following experiments in this paper.Copper stress can obviously inhibit the formation and development of RBCs by affecting the PME activity. The effect of copper on the viability and the mucilage thickness of RBCs was very obvious. In addition, under high copper concentrations or long time copper treatment, border cell and its mucilage may have a role in protecting cucumber roots. RBCs may relieve the copper toxicity to plant root by accelerating its own death and secreting mucilage.ROS-mediated programmed cell death (PCD) may be one of mechanisms of border cells response to copper stress. Hoechst33342-PI fluorescent staining experiment found the typical characters of PCD of border cells under1-20μM copper treatment. RBCs tended to transform from apoptotis towards death when stressed by higher copper concentration. The results of DCFH-DA staining showed that copper induced PCD possibly through a ROS-activated signal transduction pathway. The ROS induced by copper stress accumulated in the RBCs and caused the collapse of mitochondrial membrane potential, which triggered a series of apoptosis, while caspase may be the executor of PCD.