Monitoring the fate of paternal mitochondria and their elimination in rice zygotes

Mitochondria are preferentially transmitted from the maternal plant in most angiosperms, including rice, and paternal mitochondria are generally eliminated during microgametogenesis and/or in zygotes. The mechanism by which paternal mitochondria are eliminated progresses during plant reproductive processes. In the present study, we examined the distribution of paternal mitochondria in rice sperm cells and zygotes produced through the in vitro fertilization (IVF) of isolated rice gametes. Male gametes of rice possess mitochondria with nucleoids, suggesting the potential transfer of paternal mitochondria and their DNA into zygotes on fertilization and subsequent selective elimination of paternal mitochondria in the zygote. To intensively monitor the fate of rice paternal mitochondria in zygotes immediately after gamete fusion, time-lapse observations were conducted in paternal mitochondria labeled with GFP from rice zygotes produced using an IVF system. The results showed that the paternal mitochondria are progressively degraded during the early developmental stage at 1 to 3 h after fusion (HAF), leaving a small number of paternal mitochondria at 6 HAF. The remaining paternal mitochondria were considered to be degraded in later developmental-stage zygotes because paternal mitochondrial DNA-derived single-nucleotide polymorphisms were not detected in the sequencing reads of genomic DNA prepared from inter-subspecific hybrid rice. In addition, treatment with autophagy inhibitors stabilized the paternal mitochondria in zygotes. This suggests that the autophagy-dependent massive and selective elimination machinery for male mitochondria functions in rice zygotes immediately after gamete fusion and supports the strict maternal inheritance of mitochondria in rice.