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Summary: Arabinogalactan proteins (AGPs) are extensively glycosylated hydroxyproline‐rich glycoproteins ubiquitous in all plant tissues and cells. AtAGP6 and AtAGP11, the only two functionally known pollen‐specific classical AGP encoding genes in Arabidopsis, are reported to have redundant functions in microspore development. BcMF18 and BcMF8 isolated from Brassica campestris are the orthologues of AtAGP6 and AtAGP11, respectively. In contrast to the functional redundancy of AtAGP6 and AtAGP11, single‐gene disruption of BcMF8 led to deformed pollen grains with abnormal intine development and ectopic aperture formation in B. campestris. Here, we further explored the action of BcMF18 and its relationship with BcMF8. BcMF18 was specifically expressed in pollen during the late stages of microspore development. Antisense RNA transgenic lines with BcMF18 reduction resulted in aberrant pollen grains with abnormal cellulose distribution, lacking intine, cytoplasm and nuclei. Transgenic plants with repressive expression of both BcMF8 and BcMF18 showed a hybrid phenotype, expressing a mixture of the phenotypes of the single gene knockdown plant lines. In addition, we identified functional diversity between BcMF18/BcMF8 and AtAGP6/AtAGP11, mainly reflected by the specific contribution of BcMF18 and BcMF8 to pollen wall formation. These results suggest that, unlike the orthologous genes AtAGP6 and AtAGP11 in Arabidopsis, BcMF18 and BcMF8 are both integral to pollen biogenesis in B. campestris, acting through independent pathways during microspore development. [ABSTRACT FROM AUTHOR]

Summary In Arabidopsis thaliana, the asymmetric cell division (ACD) of the zygote gives rise to the embryo proper and an extraembryonic suspensor, respectively. This process is controlled by the ERECTA‐YODA‐MPK3/6 receptor kinase‐MAP kinase‐signaling pathway, which also orchestrates ACDs in the epidermis. In this context, the bHLH transcription factor ICE1/SCRM is negatively controlled by MPK3/6‐directed phosphorylation. However, it is unknown whether this regulatory module is similarly involved in the zygotic ACD. We investigated the function of SCRM in zygote polarization by analyzing the effect of loss‐of‐function alleles and variants that cannot be phosphorylated by MPK3/6, protein accumulation, and target gene expression. Our results show that SCRM has a critical function in zygote polarization and acts in parallel with the known MPK3/6 target WRKY2 in activating WOX8. Our work further demonstrates that SCRM activity in the early embryo is positively controlled by MPK3/6‐mediated phosphorylation. Therefore, the effect of MAP kinase‐directed phosphorylation of the same target protein fundamentally differs between the embryo and the epidermis, shedding light on cell type‐specific, differential gene regulation by common signaling pathways. [ABSTRACT FROM AUTHOR]

In the context of plant regeneration, in vitro systems to produce embryos are frequently used. In many of these protocols, nonzygotic embryos are initiated that will produce shoot-like structures but may lack a primary root. By increasing the auxin-to-cytokinin ratio in the growth medium, roots are then regenerated in a second step. Therefore, in vitro systems might not or only partially execute a similar developmental program as employed during zygotic embryogenesis. There are, however, in vitro systems that can remarkably mimic zygotic embryogenesis such as Brassica microspore-derived embryos. In this case, the patterning process of these haploid embryos closely follows zygotic embryogenesis and all fundamental tissue types are generated in a rather similar manner. In this review, we discuss the most fundamental molecular events during early zygotic embryogenesis and hope that this brief summary can serve as a reference for studying and developing in vitro embryogenesis systems in the context of doubled haploid production., (© 2021. The Author(s), under exclusive license to Springer Science+Business Media, LLC, part of Springer Nature.)