| Oilseed rape (Brassica napus L.) is one of the most important sources of edible oil in the world. Due to its prolific growth, B. napus could be grown for phytoremediation of cadmium (Cd) contaminated soils. However, growth of B. napus is negatively affected by several abiotic stresses like cold, drought, salinity and heavy metal toxcity. Heavy metal toxicity is one of the most serious threats to crop growth and yield of B. napus. Oilseed rape is also considered as a heavy metals tolerant crop, thus, it is utmost important to exploit its potential against heavy metal stress. Nowadays, use of plant growth regulators against heavy metals stress is one of the major objectives of researchers. The present study was carried out to determine the toxic effects of Cd on the plant growth of B. napus and to evaluate the ameliorative effects of5-aminolevulinic acid (ALA) regarding plant growth, molecular and ultrastructure changes in oilseed rape under Cd stress. Several experiments were performed and the following useful findings were achieved:(1) The toxic effects of Cd were evaluated in6days-old seedlings of two B. napus cultivars viz. Zheda619and ZS758. Results showed that Cd concentration was found significantly higher in ZS758as compared to Zheda619in different parts of seedlings. Higher concentration of Cd (500μM) decreased the photosynthetic pigments, and activities of antioxidant enzymes, as well as increased the level of malondialdehyde and reactive oxygen species (ROS). Electron micrographs illustrated that Cd-toxicity severely damaged the leaf and root tip cells in both cultivars. It was found that both varieties responded negatively against Cd stress but ZS758proved to be more sensitive as compared to Zheda619.(2) The plant responses to Cd-toxicity in combination with the growth regulator ALA, were investigated in oilseed rape hydroponically in greenhouse conditions under three levels (0,100, and500μM) of Cd with three levels (0,12.5and25mgl-1) of foliar application of ALA. Cd decreased the plant growth, root morphology as well as reduced the photosythetic parameters in B. napus plants. Cd-toxicity alone significantly reduced the nuterients concentartion in the leaves and roots. The higher concentration of Cd (500μM) enhanced the ROS in the leaves and roots of B. napus. At this level, antioxidants enzymes like superoxide dismutase (SOD), peroxidase (POD) and glutathione reductase (GR) reduced their activities in the leaves and roots; however, ascorbate peroxidase (APX) activity was increased in the leaves as compared to control. Moreover, higher Cd concentration totally damaged the leaf and root ultrastructures in B. napus. Meanwhile, foliar application of ALA improved the plant growth, root morphology, photosynthetic parameters and nutrients uptake in the leaves and roots under Cd-toxicity. It was also observed that application of ALA had synergetic effect on antioxidants system and it enhanced the antioxidant enzyme activities by lowering the production of ROS in the leaves and roots of B. napus under Cd-toxicity. Moreover, foliar applied ALA ameliorated the Cd-toxicity, and improved the leaf mesophyll and root tip ultrastructures in B. napus.(3) In order to investigate, how ALA alleviates the Cd-induced metabolic and proteomic changes, a study was carried out and plants were treated hydroponically at three Cd levels (0,100,500μM) with or without foliar spray of ALA (25mg/1). Results showed that application of ALA significantly improved the osmotic potential and different metabolites in the leaves. Moreover, data revealed that foliar applied ALA enhanced the photosynthetic fluorescence parameters such as F0. Fm, Fm’and Fv/Fm in the leaves of B. napus under Cd-toxicity. For proteomic analysis, total crude proteins were extracted for2-D gel electrophoresis and a total of34protein spots showed significant relative intensities between Cd and/or ALA-treated leaves proteomes of B. napus. Among34spots,17spots (50%) showed no matches, while, other17spots (50%) could be identified by proteomics analysis. Out of17identified reproducible protein spots.7proteins (41%) were down-regulated and10proteins (59%) were up-regulated. Out of34detected proteins,17Cd and/or ALA-responsive proteins were found to be involved in a nine different biological functions and pathways during the cellular adaptation to Cd and/or ALA. It was found that under Cd stress alone conditions, proteins belong to two categories i.e. CO2assimilation/photosynthesis and protein synthesis/regulation were significantly down-regulated as compared to control conditions. However, proteins belong to carbohydrate metabolism, CO2assimilation/photosynthesis, defense related, stress related proteins were up-regulated under Cd stress conditions as compared to control conditions. Under the combined treatment of ALA and Cd, there was only one protein of stress related category, which was significantly down-regulated as compared to Cd alone stress conditions. However, proteins belong to two categories i.e. CO2assimilation/photosynthesis and protein synthesis/regulation were found as up-regulated under the combined treatment of ALA and Cd as compared to Cd stress alone conditions. These findings provide insights into post-transcriptional regulatory mechanisms induced by ALA in plants under abiotic stress conditions. |
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