Comparative transcriptome analysis of broccoli seedlings under different Cd exposure levels revealed possible pathways involved in hormesis

As a non-essential heavy metal, Cd has long been considered to cause plant damage and reduce crop productivity. Recently, accumulating studies have shown that low-level Cd shows beneficial effects on plants, called hormesis. However, the mechanism of low-level Cd triggered hormesis is largely unknown. In this study, we found that Cd showed a hormesis effect on broccoli seedlings. The comparative transcriptome analysis of broccoli seedlings under low-level and high-level Cd showed that the plants responded distinctively to different concentrations of Cd exposure. KEGG analysis suggested that plant hormone signaling and secondary metabolism, especially glucosinolate homeostasis, were important under both low and high-level Cd treatment. Interestingly, ethylene signaling pathway, auxin signaling pathway, glucosinolate degradation, and glutathione redox showed opposite responses to low and high-level Cd. Low-level Cd inhibited while high-level Cd promoted ethylene signaling. Low-level Cd enhanced auxin biosynthesis while high-level repressed auxin biosynthesis. Glucosinolate degradation was enhanced by low-level Cd and inhibited by high-level Cd. As an important indicator of cell redox state, the ratio of reduced glutathione to oxidized glutathione (GSH/GSSG) increased under low-level and decreased under high-level Cd. Since the above biological processes play important roles in both growth and Cd detoxification, their opposite response to different levels of Cd indicated their possible involvement in hormesis. The potential functions of these processes in hormesis are discussed. Our study shed a light on the underlying mechanisms of the Cd-mediated hormesis effect.