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.
Plants have a potential for the uptake and accumulation of essential and non-essential trace elements. The ability to take up and tolerate metals varies between and within species as well as between metals. For most metals, the mechanisms involved in plant tolerance, uptake and accumulation are still not fully known and it is not known to what extent the plant response is metal-specific rather than a general stress response. In the present study, the growth response of soybean to Cd, As, Al and NaCl was compared and contrasted to simple sequence repeat (SSR) marker analysis results for Cda1, a dominant gene located in a major quantitative trait locus that regulates Cd accumulation in soybean, to evaluate the hypothesis that general effect patterns are induced by the individual metals. Principal component analysis revealed that the root growth response was most diverse for Al exposure and decreased in the order of Al > As > Cd > NaCl. NaCl did not exert a differentiating effect, indicating response mechanisms similar, at least partially, to metal exposure. The applied stressors yielded a distinguishable pattern of root responses, indicating the potential of such screens to identify agents acting similarly or differently. The SSR marker analysis also facilitated characterization of the Cd accumulation potential of the 22 soybean cultivars studied, and thereby identification of cultivars with potential health risk under cultivation in Cd-contaminated soils.