From microspore to haploid embryo : Dissecting the key molecular and cellular factors driving microspore embryogenesis

Abstract

The main aim of this thesis is to dissect the key molecular and cellular factors involved in Brassica napus in vitro microspore embryogenesis. Microspore embryogenesis is a form of in vitro totipotency where the male gametophyte is reprogrammed to form haploid embryos, usually after exposure to an abiotic stress treatment. Induced embryogenic structures can follow different developmental pathways that can be similar or very different to zygotic embryogenesis. In this Chapter, a comparison is made between zygotic embryogenesis and in vitro microspore embryogenesis. The similarities and differences between these two types of embryogenesis are discussed, focusing on their development and transcriptional regulation.The role of the hormone auxin and the role of histone acetylation are also discussed in detail.Totipotency is defined as the ability of a single cell to form an entire organism . The single-celled zygote is the only totipotent cell that can form an entire organism during normal sexual reproduction. Remarkably, many other plant cells are also totipotent and develop into embryos in the absence of fertilization, either via asexual reproduction or in vitro in response to an inducer treatment . Somatic embryogenesis (SE) is a form of plant cell totipotency where embryos are formed from vegetative cells, generating plants with the same genetic composition and ploidy level as the donor plant (i.e., clones). SE occurs naturally in the ovule tissues of some apomictic plants and along the leaf margins of other plants (Garcês et al., 2007;Ozias-Akins & Conner, 2020), but can also be induced in vitro in many different cells in a wide range of model and crop plants . In addition to SE, both male and female gametophytic cells can also form embryos without fertilization in a process called gametophytic embryogenesis (Reynolds, 1997; Soriano et al., 2013b). In some apomictic species, diploid embryos develop spontaneously from an egg cell formed without meiotic recombination or chromosome reduction (parthenogenesis) .Isolated haploid gametophytic cells can also be induced in vitro to form haploid embryos, but in this case the embryos are derived from meiotically produced cells and are genetically distinct from the mother plant. Haploid embryogenesis from the female gametophyte/egg cell, as in apomicts, is called gynogenesis or parthenogenesis , while haploid embryogenesis from the male gametophyte/pollen grain is termed androgenesis or microspore embryogenesis (ME) (Dong et al., 2021;Hale et al., 2022;. In both cases, the resulting haploid embryo can be converted into a Embryo proper development begins with a transverse division of the distal-most suspensor cell, which continues to divide like the typical ordered zygotic embryo .Suspensors can also develop as small protrusions or with multiple cell files , which are observed in the low response genotype DH12075 used in this study. However, it is not known if these embryos develop in a similar way to suspensor embryos in Topas DH4079 cultures, i.e., first the development of...