Increased soil cadmium concentrations enhanced poplar defence against a leaf pathogenic fungus infection

Abstract Since heavy metals are toxic to herbivores and pathogens, the Elemental defence hypothesis suggested that metals accumulated in plants might serve as defence against herbivores and pathogen attacks. However, several studies have tested this hypothesis with regard to pathogen resistance are limited. Most existing studies used herbaceous species as study model while studies on tree species are still lacking. In this study, we used poplar as a study model, to investigate whether soil treatment with five different cadmium (Cd) concentrations can protect plants from artificial infection of a leaf pathogenic fungus. The results showed that the Cd concentrations accumulated in plant leaves were positively correlated with increasing Cd concentrations amended in the soil while negatively correlated with the disease severity index (DSI) of plants after pathogen infection. Additional partial correlation analysis indicated that the plant DSI was only significantly correlated with Cd concentration accumulated in plant leaves while plant growth parameters and other leaf substances showed no clear correlations. The in vitro toxicity test of fungal mycelium grown on a series of Cd concentrations amended in the artificial medium further showed that the mycelium growth decreased with increasing Cd concentrations contained in the medium, which confirmed the biological toxicity of Cd on this fungus independently of other organic defensive substances present in poplar leaves. In conclusion, our study supported the Elemental defence hypothesis and proved that even a lower accumulation of Cd in poplar plants can provide sufficient protection against leaf pathogen infection.