design: Total RNA was extracted and purified for each sample using the TRNzol reagent (Invitrogen) following the manufacturers protocols. The high-quality RNA was sheared into 500 bp fragments and reversely transcribed into cDNA. The cDNAs were then end-repaired and ligated with the Illumina sequencing adapter and sequenced on the Illumina HiSeq 2500 platform with paired-end 150 bp.