Sperm cells are passive cargo of the pollen tube in plant fertilization

Sperm cells of seed plants have lost their motility and are transported by the vegetative pollen tube cell for fertilization, but the extent to which they regulate their own transportation is a long-standing debate. Here we show that Arabidopsis lacking two bHLH transcription factors produces pollen without sperm cells. This abnormal pollen mostly behaves like the wild type and demonstrates that sperm cells are dispensable for normal pollen tube development. A double mutant in bHLH transcription factors produces pollen without sperm cells. This pollen is able to germinate, grow, orient itself, penetrate the ovule and rupture, indicating that sperm cells are passive and do not participate in pollen tube development.