Doubled haploid (DH) technology substantially accelerates crop breeding process. Wheat haploid production through interspecific hybridization requires embryo rescue and is dependent on genetic background. In vivo haploid induction (HI) in maize has been widely used and demonstrated to be independent of genetic background and to produce haploids efficiently. Recent studies revealed that loss of function of the gene MTL/ZmPLA1/NLD triggers HI (Gilles et al., 2017; Kelliher et al., 2017; Liu et al., 2017). In addition to producing homozygous DH lines, HI system has also been used for gene editing in different genetic backgrounds without introducing the genome of the male parent (Kelliher et al., 2019; Wang et al., 2019).

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.

Summary: In vivo haploid induction has been extended from maize to monocotyledonous plants like rice, wheat, millet and dicotyledonous plants such as tomato, rapeseed, tobacco and cabbage. Accurate identification of haploids is a crucial step of doubled haploid technology, where a useful identification marker is very pivotal. R1‐nj is an extensively used visual marker for haploid identification in maize. RFP and eGFP have been shown to be feasible in identifying haploid. However, these methods are either limited to specific species, or require specific equipment. It still lacks an efficient visual marker that is practical across different crop species. In this study, we introduced the RUBY reporter, a betalain biosynthesis system, into maize and tomato haploid inducers as a new marker for haploid identification. Results showed that expression of RUBY could result in deep betalain pigmentation in maize embryos as early as 10 days after pollination, and enabled 100% accuracy of immature haploid embryo identification. Further investigation in tomato revealed that the new marker led to deep red pigmentation in radicles and haploids can be identified easily and accurately. The results demonstrated that the RUBY reporter is a background‐independent and efficient marker for haploid identification and would be promising in doubled haploid breeding across different crop species.

Mutation of the MATRILINEAL gene in sugarcane induces in vivo maternal haploids and suggests prospects for sugarcane breeding.