Total RNA was isolated from 100 mg fresh-weight samples by using Trizol reagent (Invitrogen, Gaithersburg, MD, USA) according to the manufacturer's instruction. DNA-free total RNA (2 μg) from different treatments was used for cDNA synthesis in a 20-μL reaction volume containing 2.5 U of AMV reverse transcriptase XL (TaKaRa, Dalian, China) and 1 μM oligo(dT) primer.