Eugenol Confers Cadmium Tolerance via Intensifying Endogenous Hydrogen Sulfide Signaling in Brassica rapa

Eugenol, a plant-derived small compound, shows great medicinal potential. However, whether and how eugenol regulates crop physiology remains elusive. Here we reported that eugenol induced Cd (cadmium) tolerance in the root of Brassica rapa. Roots were treated with eugenol and CdCl2 simultaneously (eugenol + Cd) or pretreated with eugenol followed by CdCl2 treatment (eugenol -> Cd). Eugenol significantly attenuated Cd-induced growth inhibition, ROS accumulation, oxidative injury, and cell death, which were confirmed by in vivo histochemical analysis. Eugenol remarkably decreased free Cd2+ accumulation in root. Eugenol intensified GSH (glutathione) accumulation in roots upon CdCl2 exposure, which explained the decrease in free Cd2+ and attenuation of oxidative injury. Eugenol stimulated endogenous H2S (hydrogen sulfide) generation by upregulating the expression of BrLCD (L-cysteine desulfhydrase) and BrDCD (D-cysteine desulfhydrase) as well as their enzymatic activities in CdCl2-treated root. Application of H2S biosynthesis inhibitor or H2S scavenger led to the decrease in endogenous H2S level in Cd-treated root, which further compromised all the above effects of eugenol. These findings suggested that eugenol triggered H2S -> GSH signaling cassette in plants to combat Cd stress, which shed new light on eugenol-modulated plant physiology and the interaction between eugenol and H2S.