The dandelion PARTHENOGENESIS gene dominantly modifies Arabidopsis fertilization and embryogenesis

Parthenogenesis of totipotent egg cells is rare, yet widespread, across the tree of life but mechanistic insights into factors that control parthenogenesis remain sparse. The Taraxacum officinale PARTHENOGENESIS (ToPAR) gene encodes a C2H2-zinc finger and EAR domain containing protein which is required for parthenogenesis and clonal seed production in apomictic dandelions. Ectopic expression of ToPAR can trigger egg cell division in lettuce and maternal haploid induction in foxtail millet, and ToPAR has been employed in a high-penetrance synthetic apomixis system in hybrid rice. To date a convenient model system to study ToPAR function has yet to be established nor has the capacity for ToPAR to trigger cell division in non-gametic cells been tested. Here, we demonstrate that expression of ToPAR in egg cells of Arabidopsis thaliana using the EGG-CELL 1.1 promoter (pAtEC1.1) causes a reduction in seed set and can trigger egg cell division without fertilization. We found that the pAtEC1.1:ToPAR transgene is rarely transmitted through the female lineage where it causes aberrant cell divisions. Expression of ToPAR in sexual embryos under the WUSCHEL RELATED HOMEOBOX 8 (AtWOX8) promoter alters cell patterning disrupting morphogenesis. Our results demonstrate that A. thaliana can be a powerful system to dissect the mode of action of ToPAR, and that gamete-specific co-factors are not essential for its function.