Transcriptome analysis
The buds were collected and frozen in liquid nitrogen and preserved in -80℃ refrigerator. The RNA was extracted from the samples using the Trizol Reagent (Invitrogen Life Technologies, Carlsbad, USA). The quality of extracted RNA was tested by a NanoDrop spectrophotometer (Thermo Scientific, MA, USA). Sequencing libraries were constructed using the TruSeq mRNA Sample Prep Kit (Illumina, San Diego, CA, USA). The mRNA was isolated from total RNA and broken into fragments with the size of about 200-300 bp. The cDNA was synthesized from mRNA, and the cDNA with the fragments size of about 300-400 bp were chosen to construct the banks. DNA was assayed on a Bioanalyzer 2100 system (Agilent, Santa Clara, CA, USA). The sequencing library was then sequenced on a Hiseq platform (Illumina, San Diego, CA, USA). Taking HDEM genome sequence as a reference, clean and high-quality sequences were aligned to the reference genome. According to the alignment result, the expression level of each gene was calculated and the aligned reads were assembled into the transcript sequences.