APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) proteins comprise a large family of plant transcription factors with diverse functions in plant stress response and development (Feng et al., 2020). The AINTEGUMENTA-LIKE (AIL) clade of AP2 proteins comprises two subclades, one subclade with AINTEGUMENTA (ANT)/AIL1-like proteins and one subclade with BABY BOOM-like proteins, including the PLETHORA (PLT) proteins (Kerstens et al., 2020). AIL proteins have individual and redundant functions during development, including above and below ground meristem initiation, maintenance and differentiation (Horstman et al., 2014; Scheres and Krizek, 2018).

BBM is expressed in the embryo and root in Arabidopsis (Galinha et al., 2007) where it regulates development of these structures together with other PLT genes (Horstman et al., 2014). After fertilization, BBM is expressed in the zygotic embryo as early as the four-cell embryo stage (Horstman et al., 2015). BBM is initially expressed throughout the embryo proper and suspensor and then becomes restricted to the embryo root pole around the heart/torpedo stage (Galinha et al., 2007; Horstman et al., 2015). Mutant phenotypes were not reported in single bbm T-DNA insertion mutants, but double bbm plt2 mutant combinations were reported to be embryo lethal (Galinha et al., 2007), suggesting that BBM and PLT2 have shared roles in embryo development. Ectopic BBM expression is sufficient to induce asexual embryo development (Horstman et al., 2017a; Bui et al., 2017), and this trait has been widely exploited to improve plant transformation and clonal propagation (Gordon-Kamm et al., 2019). BBM expression must therefore be tightly regulated during sexual reproduction to prevent parthenogenesis, the development of the egg cell into an embryo in the absence of fertilization. However, egg cell parthenogenesis is a naturally occurring component of asexual seed reproduction pathways in apomictic plants (Vijverberg et al., 2019), and BBM-like genes map to the parthenogenesis locus in a number of apomictic grasses, including Pennisetum squalatum and Brachiaria spp.(Conner et al., 2015; Worthington et al., 2016, 2019). In apomictic P. squalatum ASGR-BBML is expressed in the egg cell (Ke et al., 2021) and expression of ASGR-BBML under its own promoter or an egg cell-expressed promoter can induce parthenogenesis in sexual grasses and tobacco (Conner et al., 2017; Zhang et al., 2020; Conner et al., 2015). This trait is not linked to the ‘apomictic’BBM allele per se, but rather to the timing of BBM expression, as heterochronic expression of a (sexual) rice (Oryza sativa) BBM allele in the egg cell is also sufficient to induce parthenogenesis in rice (Khanday et al., 2019). Thus, heterochronic differences in BBM expression can drive both sexual and asexual reproductive pathways. Here we show that the ArabidopsisBBMgene has a much broader expression pattern and function during seed development than previously reported. BBM is expressed transiently in the chalazal region of the ovule and seed, in the embryo starting from the zygote stage, and in the first few dividing endosperm cells. These expression patterns were reflected in novel BBM functions in endosperm and embryo development that were shared with PLT2. We also show that ectopic BBM expression in the egg cell induces parthenogenesis, a trait that can be enhanced by dmp maternal haploid inducers (Zhong et al., 2022, 2020,