Targeting antioxidant pathways for improved tolerance to chromium exposure

With advancement in functional genomics, it is now possible to engineer plants for heavy metal tolerance by altering the expression of proteins and genes involved in heavy metal stress resistance. The present study was carried out to evaluate the tomato transgenic plants overexpressing the genes associated with ascorbate-glutathione (AsA-GSH) pathway, namely, APX (ascorbate peroxidase), MDHAR (mono-dehydroascorbate reductase), DHAR (dehydroascorbate reductase), GR (glutathione reductase), and SOD (superoxide dismutase) for the response to chromium stress (Cr). Transgenic tomato plants with altered antioxidant pathway had higher levels of carotenoids (41.64%) and anthocyanins (34.60%) besides improved photosynthetic rate, transpiration, and stomatal conductance compared to untransformed wild type (WT) when supplied with potassium dichromate (K2Cr2O7) at a concentration of 100 µM. Moreover, transgenic tomato showed increased osmolytes and phenolic concentrations with a simultaneous reduction in electrolyte leakage, malondialdehyde (MDA) and reactive oxygen species (ROS). Transcript analysis indicated higher expression of all the transgenes, whereas scanning electron microscopy (SEM) revealed lesser deformities in transgenic plants in response to Cr stress. Transgenic lines accumulated higher Cr in leaves (68%) and roots (56.89%) compared to the WT under Cr stress. We conclude that overexpressing the AsA-GSH pathway in tomato makes them suitable for use as phytoremediator. We conclude that transgenic tomato plants with altered Ascorbate-Glutathione antioxidant pathway accumulate higher levels of pigments, besides developing improved photosynthetic rate, transpiration, and stomatal conductance compared to untransformed (WT) plants on exposure to chromium stress. Scanning electron microscopy (SEM) revealed lesser deformities in transgenic tomato plants in response to Cr stress. Overexpressing the Asc-GSH pathway in tomato makes them suitable for use as phytoremediator due to higher Cr accumulation in leaves and roots compared to the WT under Cr stress.随着功能基因组学的发展，如今可通过调控参与重金属胁迫抗性的蛋白与基因的表达，培育具备重金属耐受能力的工程植物。本研究旨在评估过表达抗坏血酸-谷胱甘肽（ascorbate-glutathione, AsA-GSH）通路相关基因的转基因番茄植株对铬（Cr）胁迫的响应，所涉及的基因包括抗坏血酸过氧化物酶（APX, ascorbate peroxidase）、单脱氢抗坏血酸还原酶（MDHAR, mono-dehydroascorbate reductase）、脱氢抗坏血酸还原酶（DHAR, dehydroascorbate reductase）、谷胱甘肽还原酶（GR, glutathione reductase）以及超氧化物歧化酶（SOD, superoxide dismutase）。在施加浓度为100 µM的重铬酸钾（K₂Cr₂O₇）处理下，与未转化的野生型（WT）植株相比，抗氧化通路发生改变的转基因番茄类胡萝卜素含量提升41.64%、花青素含量提升34.60%，同时光合速率、蒸腾速率及气孔导度均得到改善。此外，转基因番茄的渗透调节物质与酚类物质浓度升高，同时电解质渗漏率、丙二醛（MDA, malondialdehyde）与活性氧（ROS, reactive oxygen species）水平降低。转录分析显示所有外源转基因均呈现更高的表达水平；扫描电子显微镜（SEM, scanning electron microscopy）观察发现，转基因植株在铬胁迫下的形态畸形程度更低。在铬胁迫条件下，转基因株系的叶片铬积累量较野生型提升68%，根部铬积累量较野生型提升56.89%。本研究证实，在番茄中过表达AsA-GSH通路可使植株作为植物修复剂具备应用潜力。综上，经抗坏血酸-谷胱甘肽抗氧化通路修饰的转基因番茄，在遭受铬胁迫时，不仅色素含量更高，同时光合速率、蒸腾速率及气孔导度均得到改善；扫描电子显微镜观察显示其在铬胁迫下的形态畸形程度更低；且由于在铬胁迫条件下其叶片与根部的铬积累量均高于野生型，因此在番茄中过表达AsA-GSH通路可使其作为植物修复剂具备应用潜力。