2.5. RT-PCR
Total RNA was isolated with Trizol (Invitrogen, CA, USA), and single-stranded cDNA was synthesized from 2 μg of total RNA with BU-Script RT-Kit (Biunique, Jiangsu, China) according to the manufacturer's protocol. The cDNA was stored in −20°C. Reverse transcription was conducted with GoTaq Green Master Mix (Promega, WI, USA) according to the manufacturer's protocol.  Table 1 shows the primers and PCR parameters. PCR products were detected by agarose gel electrophoresis. The intensity of the bands was analyzed by ImageJ program. The level of β-actin was used as an internal standard.