Soybean roots defence against cadmium and its dependence on dose in a non-linear manner

Metal pollution represents a serious issue for sustainable agronomy and food safety. Activation of plants' protective mechanisms has been shown to depend on the extent of soil contamination with metal, but reports on dose-dependent responses (especially to extreme concentrations) are rather rare. In this research we exposed soybean (Glycine max L.) roots at a very early stage to a scale of cadmium concentrations, including doses far exceeding the amounts found in nature. Spectrophotometrical and enzyme in-gel detection assays were used to examine ongoing defence responses. The results confirmed commonly reported findings on inhibited growth and the activation of several superoxide dismutase isoforms in a dose-dependent manner. Contrary to expectations, the hydrogen peroxide levels, proline accumulation and the rate of lipid peroxidation were suppressed with increasing metal doses. More importantly, the linearity of Cd accumulation in soybean roots was interrupted at 200 mg.l^-1 of cadmium, which coincides with peculiar responses of several chitinase family members. No such nonlinear response was observable for proline accumulation or any of the SOD isoforms. Possible explanations are provided and the importance of considering the metal stress dose is stressed to avoid false generalizations on plant defence responses.