Constitutive chemical defense mechanism of inner bark of Sorbus commixta against Trametes versicolor.

Tree bark is a crucial tissue that defends tree stems from invasions by microorganisms. However, our understanding of the constitutive chemical defense mechanisms of the tree barks remains limited. Our group recently discovered that the inner bark of Sorbus commixta Hedl. exhibited potent inhibitory effects on the growth of the white-rot fungus, Trametes versicolor (Linnaeus) Lloyd. It was hypothesized that this growth suppression was due to hydrogen cyanide (HCN) originating from cyanogenic glycosides such as amygdalin and prunasin, which are secondary metabolites in the inner bark of S. commixta. To test this hypothesis, we first quantified the amygdalin content in the inner bark of S. commixta and evaluated the antifungal activity (AFA) of HCN against T. versicolor by placing paper discs on potato dextrose agar with T. versicolor. Subsequently, we identified HCN in the inner bark of S. commixta using a picric acid paper. In the AFA tests, the amygdalin + β-glucosidase solution, the potassium cyanide (KCN) + benzaldehyde solution and the KCN solution exhibited elevated AFA against T. versicolor. Conversely, the amygdalin solution, the β-glucosidase solution and the benzaldehyde solution did not exhibit significant AFA. These findings demonstrate that HCN acts as an antifungal agent against T. versicolor. Furthermore, HCN was detected in the freeze-dried inner bark pieces and the trunks of living S. commixta. This suggests that the disruption of the cells in the inner bark of S. commixta caused the cyanogenic glycosides and β-glucosidase to interact, which resulted in the hydrolysis of cyanogenic glycosides and generation of HCN. From the perspective of constitutive chemical defense mechanisms, HCN plays a crucial role in the high AFA of the inner bark of S. commixta against T. versicolor, creating an unfavorable environment for the growth of white-rot fungus after injury breaches the periderm.