Correspondents: Jonathan Gent, PhD and Shujun Ou, PhD
Funding: Innovative Genomics Institute & National Science Foundation
How does reproduction begin? In both plants and animals, the next generation is initiated by the fusion of differentiated gametes – sperm and egg - to form the zygote. The zygote is a unique totipotent cell that will give rise to a new organism. We have used rice as a model to study the molecular mechanisms that underlie the establishment of the zygote in plants. We found that unlike animals, rapid and extensive transcriptional activation of the zygotic genome occurs shortly after fertilization in plants. Early zygotic transcription is primarily from the female genome, but the male genome provides key transcription factors active during the initiation of embryogenesis. One such transcription factor gene BBM1is expressed in the sperm cell and also in early zygotes. We showed that this one factor is sufficient to convert an unfertilized egg cell into a zygote, that then goes on to make an asexually derived plant. Another transcription factor WOX9A, also expressed from the male genome, works synergistically with BBM1 to enhance this conversion of the pre-fertilization egg cell into a totipotent zygote. More generally, we hypothesize that these two transcription factors might function to promote pluripotency in plants, similar to the Yamanaka factors in mammals.
An important feature of this mechanism of embryo initiation is that differences in gene expression between the male and female genomes provide complementary functions to establish the zygote. These differences likely arise from different epigenetic modifications during plant gamete formation. We have observed extensive epigenetic reprogramming of the genome marked by genomic redistribution of 24nt siRNA loci in male and female gametes. These siRNAs are reset to a somatic pattern in the zygote, indicative of dynamic epigenome reprogramming during reproduction. The changes in the epigenome during this vital stage of plant reproduction are currently under investigation.
Understanding the mechanisms by which gametes give rise to embryos has important agricultural applications. We used this knowledge for the development of synthetic apomixis, a process by which seed formation by sexual reproduction is replaced by seeds with clonal asexual embryos. This is another major area of research in the laboratory, that has led to new methods for production of high-yielding hybrid seeds.
This research on ZGA includes a collaborative project with Dr. Jonathan Gent (U. Georgia) and Dr. Shujun Ou (Ohio State U.) funded by the NSF Plant Genome Research Program. The project website includes an integrated genome browser showing data from small RNA and mRNA transcriptomes, that can be accessed here.