Transcriptome reprogramming in the Arabidopsis male germline during the progamic phase

A pollen tube’s journey through the transmitting tissues of the pistil leads to changes in its gene expression profile. To which extent these changes occur in the vegetative cell (pollen tube) or in the sperm cells it transports has not been addressed so far. This question is of particular importance since sperm cells are believed to acquire competence for fertilization during pollen-pistil interactions. Here, we have compared the transcriptomes of Arabidopsis thaliana sperm cells and vegetative nuclei isolated from mature pollen grains and from pollen tubes grown semi in-vivo. We used an optimized semi in-vivo system and a fluorescent marker line to isolate GFP labelled sperm cells and RFP labelled vegetative nuclei by FACS, followed by RNA-seq. Moreover, to determine how many genes are elicited in the male gametes and the vegetative nuclei exclusively in the presence of female cues, we also analyzed sperm cells and vegetative nuclei from pollen tubes grown in vitro. Our data indicate that sperm cells undergo extensive transcriptomic changes during pollen tube growth, some of which are elicited exclusively when they are passively transported within the pistil, revealing hitherto unidentified transcripts that may be important for sperm maturation and gamete fusion. Similarly, vegetative nuclei undergo even more extensive transcriptome reprogramming during pollen tube growth, mainly through the upregulation of genes associated with pollen tube growth and vesicle mediated transport. Interestingly, comparison with published ATAC-seq data shows that chromatin at the promoters of genes up-regulated in sperm during pollen tube growth is already open in sperm of mature pollen grains, suggesting pre-configured promoter accessibility. Many of these genes were also found to be targets of H3K27Me3. Our study provides the most comprehensive overview of transcriptome dynamics of Arabidopsis sperm cells and vegetative nuclei during pollen tube growth to date