The Roads to Haploid Embryogenesis

Abstract

Although zygotic embryogenesis is usually studied in the field of seed biology, great attention has been paid to the methods used to generate haploid embryos due to their applications in crop breeding. These mainly include two methods for haploid embryogenesis: in vitro microspore embryogenesis and in vivo haploid embryogenesis. Although microspore culture systems and maize haploid induction systems were discovered in the 1960s, little is known about the molecular mechanisms underlying haploid formation. In recent years, major breakthroughs have been made in in vivo haploid induction systems, and several key factors, such as the matrilineal (MTL), baby boom (BBM), domain of unknown function 679 membrane protein (DMP), and egg cell-specific (ECS) that trigger in vivo haploid embryo production in both the crops and Arabidopsis models have been identified. The discovery of these haploid inducers indicates that haploid embryogenesis is highly related to gamete development, fertilization, and genome stability in ealry embryos. Here, based on recent efforts to identify key players in haploid embryogenesis and to understand its molecular mechanisms, we summarize the different paths to haploid embryogenesis, and we discuss the mechanisms of haploid generation and its potential applications in crop breeding. Although these haploid-inducing factors could assist egg cells in bypassing fertilization to initiate embryogenesis or trigger genome elimination in zygotes after fertilization to form haploid embryos, the fertilization of central cells to form endosperms is a prerequisite step for haploid formation. Deciphering the molecular and cellular mechanisms for haploid embryogenesis, increasing the haploid induction efficiency, and establishing haploid induction systems in other crops are critical for promoting the application of haploid technology in crop breeding, and these should be addressed in further studies.

Strategy for unlimited cycles of scarless oligonucleotide directed gene editing inChlamydomonas reinhardtii

“…While simplest to employ in haploid organisms it can also be used in diploids (e.g. Arabidopsis ) especially where selective mutation or artificial generation of haploids (Shen et al, 2023) can reduce the endogenous marker to a single copy.…”

Haploid rhapsody: the molecular and cellular orchestra of in vivo haploid induction in plants

“…This allows breeders to quickly stabilize favourable genotypes or evaluate agronomical traits in a fixed, inter‐generationally stable genetic background (Gilles et al ., 2017b; Jacquier et al ., 2020; Shen et al ., 2023). DH technologies require the induction of haploid embryos or plants through a haploid induction (HI) system (Gilles et al ., 2017b; Jacquier et al ., 2020; Shen et al ., 2023), coupled with a system to reduplicate the haploid genome (Jacquier et al ., 2020; Hooghvorst & Nogués, 2021). Multiple strategies to develop DHs have been employed using in vitro or in vivo techniques (Gilles et al ., 2017b).…”

“…Functional pollen‐specific orthologues of ZmMTL have only been found in other monocots and not in eudicots (Zhong et al ., 2020; Shen et al ., 2023), possibly indicating that some haploid inducer genes ( CENH3 , ECS , KLP and DMP ) evolved before the divergence of these lineages. It has also been recently observed that double mutants of ECS1/2 in Arabidopsis can generate semigametic production haploid plants, possibly by affecting the nuclear fusion in Arabidopsis as and in rice (X. Zhang et al ., 2023).…”

CRISPR/Cas Technology Revolutionizes Crop Breeding

“…Double haploid technology, including haploid induction and double haploid development, can greatly accelerate the breeding process by rapidly generating homozygous plants, and has been widely applied in various crops [5,128,129]. Using CRISPR/Cas genome editing technology, many advances have been made in the mechanisms and application of haploid induction in different crops [130]. In maize, key genes involved in haploid induction such as ZmPOD65, ZmPLD3, ZmDMP7, and ZmMTL have been characterized and show potential for breeding haploid inducers [131][132][133].…”