Role of paternal Oryza sativa Baby Booms (OsBBMs) in initiating de novo gene expression and regulating early zygotic development in rice

Oryza sativa BABY BOOM 1 (OsBBM1), a member of the AP2/ERF family of transcription factors, is expressed from paternal allele in rice zygote and plays a crucial role in initiating zygotic development. However, the mechanism how the paternal OsBBM1 drives this development remains unclear. Rice zygotes with four different gamete combinations with or without functional paternal OsBBMs were produced by electrofusion, using gametes isolated from bbms triple mutants and wild-type rice plants. Developmental and gene expression profiles of these types of zygotes were intensively analyzed and compared. Mutations in OsBBM1, OsBBM2, and OsBBM3 on the paternal alleles caused developmental arrest or delay in the zygotes, while defects in OsBBMs on the maternal allele had minimal effects on zygotic development. Paternal allele of OsBBMs significantly influenced gene expression profiles related to regulation of basic cellular processes, such as chromosome/chromatin organization/assembly and cell cycle/division compared to the maternal allele of OsBBMs. Majority of these genes were upregulated in zygotes from paternal/parental alleles via paternal OsBBMs. Paternal OsBBMs initiate early development of rice zygotes through the regulation of expression profiles of genes controlling status of chromosome/chromatin and cell cycle/division.