Investigation of proteomic and physiological response of Zygophyllum fabago to cadmium treatment

The aim of our research was to identify some new interesting accumulators which may associate an important biomass production with an effective absorption and translocation of heavy metals. The present study focused on cadmium stress response in a metallicolous population of the Syrian bean caper (Zygophyllum fabago), a succulent perennial shrub. Its goal was i) to determine the metabolic and physiological parameters modified in relation to Cd accumulation, ii) to identify if these modifications were related to a strategy of resistance or should be considered as symptoms of injury. The potentiality of Z. fabago heavy metal accumulation and tolerance was tested on nutritive solution in controlled environment. Seedlings of Z. fabago were exposed during 4 weeks to 10 µM CdCl2. In order to determine the metabolic changes in response to Cd accumulation, a quantitative proteomic analysis of Z. fabago leaves was performed by two-dimensional gels combined with fluorescent labeling. Differentially expressed proteins, determined by statistical analysis, were identified by MALDI-TOF-TOF analysis. Among proteins which abundance was lowered in stressed plants, some proteins related to photosynthetic apparatus and energy metabolism were found, while the abundance of some proteins related to carbohydrate metabolism and plant defence (such as antioxidants and pathogenesis-related proteins) was increased in Cd-treated plants. In relation to these modifications, a physiological characterization of Z. fabago has been performed, indicating that water stress, modification of some photosynthesis-related parameters and oxidative constraints occurred in response to 10 µM Cd. A strong heterogeneity in Cd accumulation and tolerance was observed between individuals within the considered population and some plants could exhibit high Cd accumulation without exhibiting any biomass reduction. A comparison was performed to determine the main physiological differences between these promising individuals and more sensitive ones. Thus, the photosynthetic activity, gas exchange, the water and oxidative status in roots, stems and expanding leaves have been investigated in relation to the accumulation of Cd ions. The Cd concentration surprisingly increased more rapidly in plants showing the highest tolerance. Those plants were able to maintain a water status similar to untreated ones for a longer duration than sensitive individuals, thus leading to a faster leaf expansion. The less tolerant plants strongly decreased their stomatal conductivity and transpiration rate after 4 weeks in response to Cd treatment. Their stem and leaf water content and osmotic potential decreased already after 2 weeks of treatment, concomitantly with an increase in soluble sugars and proline concentration. A limited diffusion of CO2 indicated by a decrease in C isotope discrimination, as well as a decrease in efficiency of the photosystem II, lead to a reduction of the photosynthetic rate in response to Cd