UC Davis

In flowering plants, double fertilization involves the fusion of one sperm cell with the egg cell to form the embryo and the other with the central cell to form the endosperm. Early embryogenesis is predominantly maternal in origin, but there are a few transcription factors that are expressed from the male genome in rice. The transcription factor BABY BOOM 1 (OsBBM1), which is initially expressed from the paternal genome, plays a pivotal role in initiating embryogenesis. Ectopic expression of OsBBM1 in the egg cell is sufficient to bypass fertilization and induce parthenogenesis in rice.This dissertation reveals that the WOX-family transcription factor WUSCHEL-related homeobox 9A (OsWOX9A), another paternally expressed transcription factor in rice zygotes, acts as a strong enhancer of OsBBM1-mediated parthenogenesis. Co-expression of OsWOX9A and OsBBM1 in egg cells results in 86-91% parthenogenesis, representing 4- to 15-fold increases over OsBBM1 alone. OsWOX9A and its paralog OsWOX9C together are required for normal embryo patterning and development. These findings suggest that embryo initiation is promoted by the synergistic action of paternal-genome-expressed transcription factors. A notable outcome of this process is the occurrence of polyembryony, particularly the formation of the twin progeny at high frequencies. While the phenomenon has been observed, its underlying mechanisms remain largely unexplored.We provide a brief description of the phenomenon and propose two possible models for the twinning events. The two proposed models are independent, but they can operate in combination. By evaluating the available evidence, we conclude that both models are required to account for the diversity of twins and triplets observed. These insights not only inform basic plant developmental biology but also provide practical guidance for reducing polyembryony when incorporating synthetic apomixis into crop plants such as maize, where uniform single-embryo seeds are desired.Parthenogenesis is an integral component of synthetic apomixis, a technology that enables clonal propagation of hybrids and the preservation of heterosis across generations. While previous studies have established parthenogenesis in diploid plants by ectopic expression of the BABY BOOM family of transcription factors, this dissertation extends synthetic apomixis to triploid rice. Triploid hybrids can achieve enhanced hybrid vigor compared to diploid hybrids. We demonstrated that triploid rice can be fertile through synthetic apomixis, thus it has the potential to be applied in triploid hybrids to enhance the agricultural traits.This dissertation deepens our understanding of rice early embryogenesis and uncovers OsWOX9A as a synergistic regulator of OsBBM1-induced parthenogenesis. It highlights key challenges such as polyembryony and provides a framework for future research to further investigate rice embryogenesis and optimize synthetic apomixis for crop improvement.