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

•Low-level Cd promoted while high-level Cd inhibited broccoli seedlings' growth.•Transcriptome of broccoli grown under low- and high-level Cd was compared.•Ethylene and auxin signaling pathways were related with Cd-triggered hormesis.•Glucosinolate and GSH mediated signaling may be involved in Cd triggered hormesis.