Transcriptionally active regions are highly limited in the nucleus of the plant sperm cell

Flowering plants have immotile sperm cells that are delivered to the female gametes by the pollen tube cell. Although the sperm chromatin in flowering plants is highly condensed like animal sperm chromatin, it is generally accepted that the transcription status in the sperm chromatin of flowering plants is active because of the existence of active gene promoters and active histone modification status. However, the level of transcriptional activity in the sperm chromatin of flowering plants has remained unknown. In this study, we observed the distribution of C-terminal domain-phosphorylated RNA polymerase II (RNAPII), which represents a transcriptionally active status in living pollen, and fixed isolated vegetative and sperm nuclei of Arabidopsis thaliana. We found that the transcriptionally active regions were highly limited in sperm chromatin compared with those in vegetative chromatin. RNA sequencing (RNA-seq) analysis showed that the transcription of some RNAPII subunit-encoding genes was highly suppressed in sperm cells, suggesting that the amount of functional RNAPII would be small. In addition, the C-terminal domains of some RNAPII phosphatase-encoding genes, which act as a regulator of the transcription cycle, were actively transcribed in sperm cells. These findings indicate that transcriptional activity is highly suppressed in the sperm chromatin in A. thaliana due to the lack of completely assembled RNAPII and the inhibition of transcriptional elongation of RNAPII by the phosphatase activities.