library_construction_protocol: Sperm cells were isolated from pollen by FACS. RNA was extracted with Direct-zol RNA Microprep (ZYMO RESEARCH) and converted to cDNA by RevertAid First Strand cDNA Synthesis Kit (Thermo Fisher Scientific). Libraries were prepared with the Nugen Universal RNA-Seq library preparation kit (0364) and sequenced on the NextSeq 500 (Illumina) with 76 bp single end reads at the John Innes Centre, Norwich, UK.