Investigation of physiological and biochemical mechanisms of Zygophyllum fabago response to cadmium and zinc treatments

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 response to cadmium and zinc stress 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 and Zn accumulation, ii) to determine whether these modifications were related to a strategy of resistance or should be considered as symptoms of injuries. The potentiality of Z. fabago for heavy metal accumulation and tolerance was tested on nutrient solution in controlled environment. Seedlings of Z. fabago were exposed during 4 weeks to 10 µM CdCl2 or 50 µM ZnSO4. A strong heterogeneity in Cd and Zn accumulation and tolerance was observed between individuals within the considered population and some plants could exhibit high Cd or Zn 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, gaz exchange, the water and oxidative status in roots, stems and expanding leaves have been investigated in relation to the accumulation of Cd or Zn ions. The concentration of these two elements 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 two weeks in response to Zn treatment, and after four weeks in response to Cd treatment. Their stem and leaf water content and osmotic potential decreased already after 2 weeks of treatment. 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. Such a decrease did not occurred in response to Zn treatment and may be related to an increase in pigments concentration in order to compensate the lack of efficiency of the photosystem. In order to deeply investigate mechanisms involved in tolerance strategies, a cellular and sub-cellular localization of essential and non-essential elements in leaves of Z. fabago exposed to Cd and Zn treatments, as well as their pattern of accumulation, were investigated through micro-proton-induced X-ray emission (micro-PIXE). Moreover, in order to determine the metabolic changes in response to Cd or Zn 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. In relation to these modifications, a biochemical characterisation of Z. fabago was performed. All together, these results give some clues about tolerance mechanisms of Z. fabago to Cd and Zn toxicity that will be discussed