Somatic embryogenesis is an example of induced cellular totipotency, where embryos develop from vegetative cells rather than from gamete fusion. Somatic embryogenesis can be induced in vitro by exposing explants to growth regulators and/or stress treatments. The BABY BOOM (BBM) and LEAFY COTYLEDON1 (LEC1) and LEC2 transcription factors are key regulators of plant cell totipotency, as ectopic overexpression of either transcription factor induces somatic embryo formation from Arabidopsis (Arabidopsis thaliana) seedlings without exogenous growth regulators or stress treatments. Although LEC and BBM proteins regulate the same developmental process, it is not known whether they function in the same molecular pathway. We show that BBM transcriptionally regulates LEC1 and LEC2, as well as the two other LAFL genes, FUSCA3 (FUS3) and ABSCISIC ACID INSENSITIVE3 (ABI3). LEC2 and ABI3 quantitatively regulate BBM-mediated somatic embryogenesis, while FUS3 and LEC1 are essential for this process. BBM-mediated somatic embryogenesis is dose and context dependent, and the contextdependent phenotypes are associated with differential LAFL expression. We also uncover functional redundancy for somatic embryogenesis among other Arabidopsis BBM-like proteins and show that one of these proteins, PLETHORA2, also regulates LAFL gene expression. Our data place BBM upstream of other major regulators of plant embryo identity and totipotency.

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The QTL qhir8 affecting in vivo haploid induction in maize was mapped to a 789 kb region, embryo abortion rate and segregation ratios were analyzed, linkage markers for MAS were developed. The doubled-haploid (DH) technology has become an important tool for line development in modern maize breeding. However, the genetic basis of haploid induction remains elusive. In previous QTL mapping research, qhir8 besides qhir1 significantly affected haploid induction rate (HIR). Our objective was to fine map qhir8 and assess its effect on HIR, segregation distortion (SD) and embryo abortion (EmA). A total of 3989 F2 plants from the cross of inducers CAUHOI and UH400 were screened for recombinants in the qhir8 region. F2 plants and F3 plants from selfing progenies of 34 recombinant F2 plants were evaluated for HIR, SD and EmA. In parallel, we developed 31 new markers providing good coverage of the qhir8 region. We confirmed that qhir8 has an increasing effect on HIR and EmA, but not on SD. Moreover, we successfully narrowed down the qhir8 locus to a 789 kb region flanked by markers 4292232 and umc1867.

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The BABY BOOM (BBM) AINTEGUMENTA-LIKE (AIL) AP2/ERF domain transcription factor is a major regulator of plant cell totipotency, as it induces asexual embryo formation when ectopically expressed. Surprisingly, only limited information is available on the role of BBM during zygotic embryogenesis. Here we reexamined BBM expression and function in the model plant Arabidopsis thaliana ( Arabidopsis ) using reporter analysis and newly developed CRISPR mutants. BBM was expressed in the embryo from the zygote stage and also in the maternal (nucellus) and filial (endosperm) seed tissues. Analysis of CRISPR mutant alleles for BBM ( bbm-cr ) and the redundantly acting AIL gene PLETHORA2 ( PLT2 ) ( plt2-cr ) uncovered individual roles for these genes in the timing of embryo progression. We also identified redundant roles for BBM and PLT2 in endosperm proliferation and cellularization and the maintenance of zygotic embryo development. Finally, we show that ectopic BBM expression in the egg cell of Arabidopsis and the dicot crops Brassica napus and Solanum lycopersicon is sufficient to bypass the fertilization requirement for embryo development. Together these results highlight roles for BBM and PLT2 in seed development and demonstrate the utility of BBM genes for engineering asexual embryo development in dicot species.

Hetero-fertilization and failed egg–sperm cell fusion indicate that single fertilization plays a key role in in vivo haploid induction in maize.