A female in vivo haploid-induction system via mutagenesis of egg cell-specific peptidases

AtRKD5 inhibits the parthenogenic potential mediated by AtBBM

Parthenogenesis, the development of unfertilized egg cells into embryos, is a key component of apomixis. AtBBM (BABY BOOM), a crucial regulator of embryogenesis in Arabidopsis, possesses the capacity to shift nutritional growth toward reproductive growth. However, the mechanisms underlying AtBBM-induced parthenogenesis remain largely unexplored in dicot plants. Our findings revealed that in order to uphold the order of sexual reproduction, the embryo-specific promoter activity of AtBBM as well as repressors that inhibit its expression in egg cells combine to limiting its ability to induce parthenogenesis. Notably, AtRKD5, a RWP-RK domain-containing (RKD) transcription factor, binds to the 3' end of AtBBM and is identified as one of the inhibitory factors for AtBBM expression in the egg cell. In the atrkd5 mutant, we successfully achieved enhanced ectopic expression of AtBBM in egg cells, resulting in the generation of haploid offspring via parthenogenesis at a rate of 0.28%. Furthermore, by introducing chimeric Arabidopsis and rice BBM genes into the egg cell, we achieved a significant 4.6-fold enhancement in haploid induction through the atdmp8/9 mutant. These findings lay a strong foundation for further exploration of the BBM-mediated parthenogenesis mechanism and the improvement of haploid breeding efficiency mediated by the dmp8/9 mutant.

One-step creation of CMS lines using a BoCENH3-based haploid induction system in Brassica crop

Heterosis utilization in a large proportion of crops depends on the use of cytoplasmic male sterility (CMS) tools, requiring the development of homozygous fertile lines and CMS lines1. Although doubled haploid (DH) technology has been developed for several crops to rapidly generate fertile lines2,3, CMS lines are generally created by multiple rounds of backcrossing, which is time consuming and expensive4. Here we describe a method for generating both homozygous fertile and CMS lines through in vivo paternal haploid induction (HI). We generated in-frame deletion and restored frameshift mutants of BoCENH3 in Brassica oleracea using the CRISPR/Cas9 system. The mutants induced paternal haploids by outcrossing. We subsequently generated HI lines with CMS cytoplasm, which enabled the generation of homozygous CMS lines in one step. The BoCENH3-based HI system provides a new DH technology to accelerate breeding in Brassica and other crops.

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

In planta haploid induction (HI), which reduces the chromosome number in the progeny after fertilization, has garnered increasing attention for its significant potential in crop breeding and genetic research. Despite the identification of several natural and synthetic HI systems in different plant species, the molecular and cellular mechanisms underlying these HI systems remain largely unknown. This review synthesizes the current understanding of HI systems in plants (with a focus on genes and molecular mechanisms involved), including the molecular and cellular interactions which orchestrate the HI process. As most HI systems can function across taxonomic boundaries, we particularly discuss the evidence for conserved mechanisms underlying the process. These include mechanisms involved in preserving chromosomal integrity, centromere function, gamete communication and/or fusion, and maintenance of karyogamy. While significant discoveries and advances on haploid inducer systems have arisen over the past decades, we underscore gaps in understanding and deliberate on directions for further research for a more comprehensive understanding of in vivo HI processes in plants.

EGG CELL 1 contributes to egg-cell-dependent preferential fertilization in Arabidopsis

During double fertilization in angiosperms, the pollen tube delivers two sperm cells into an embryo sac; one sperm cell fuses with an egg cell, and the other sperm cell fuses with the central cell. It has long been proposed that the preference for fusion with one or another female gamete cell depends on the sperm cells and occurs during gamete recognition. However, up to now, sperm-dependent preferential fertilization has not been demonstrated, and results on preferred fusion with either female gamete have remained conflicting. To investigate this topic, we generated Arabidopsis thaliana mutants that produce single sperm-like cells or whose egg cells are eliminated; we found that although the three different types of sperm-like cell are functionally equivalent in their ability to fertilize the egg and the central cell, each type of sperm-like cell fuses predominantly with the egg cell. This indicates that it is the egg cell that controls its preferential fertilization. We also found that sperm-activating small secreted EGG CELL 1 proteins are involved in the regulation of egg-cell-dependent preferential fertilization, revealing another important role for this protein family during double fertilization.

CRISPR/Cas Technology Revolutionizes Crop Breeding

Crop breeding is an important global strategy to meet sustainable food demand. CRISPR/Cas is a most promising gene-editing technology for rapid and precise generation of novel germplasm and promoting the development of a series of new breeding techniques, which will certainly lead to the transformation of agricultural innovation. In this review, we summarize recent advances of CRISPR/Cas technology in gene function analyses and the generation of new germplasms with increased yield, improved product quality, and enhanced resistance to biotic and abiotic stress. We highlight their applications and breakthroughs in agriculture, including crop de novo domestication, decoupling the gene pleiotropy tradeoff, crop hybrid seed conventional production, hybrid rice asexual reproduction, and double haploid breeding; the continuous development and application of these technologies will undoubtedly usher in a new era for crop breeding. Moreover, the challenges and development of CRISPR/Cas technology in crops are also discussed.

Embryo Rescue in Plant Breeding

Embryo rescue (ER) techniques are among the oldest and most successful in vitro tissue culture protocols used with plant species. ER refers to a series of methods that promote the development of an immature or lethal embryo into a viable plant. Intraspecific, interspecific, or intergeneric crosses allow the introgression of important alleles of agricultural interest from wild species, such as resistance or tolerance to abiotic and biotic stresses or morphological traits in crops. However, pre-zygotic and post-zygotic reproductive barriers often present challenges in achieving successful hybridization. Pre-zygotic barriers manifest as incompatibility reactions that hinder pollen germination, pollen tube growth, or penetration into the ovule occurring in various tissues, such as the stigma, style, or ovary. To overcome these barriers, several strategies are employed, including cut-style or graft-on-style techniques, the utilization of mixed pollen from distinct species, placenta pollination, and in vitro ovule pollination. On the other hand, post-zygotic barriers act at different tissues and stages ranging from early embryo development to the subsequent growth and reproduction of the offspring. Many crosses among different genera result in embryo abortion due to the failure of endosperm development. In such cases, ER techniques are needed to rescue these hybrids. ER holds great promise for not only facilitating successful crosses but also for obtaining haploids, doubled haploids, and manipulating the ploidy levels for chromosome engineering by monosomic and disomic addition as well substitution lines. Furthermore, ER can be used to shorten the reproductive cycle and for the propagation of rare plants. Additionally, it has been repeatedly used to study the stages of embryonic development, especially in embryo-lethal mutants. The most widely used ER procedure is the culture of immature embryos taken and placed directly on culture media. In certain cases, the in vitro culture of ovule, ovaries or placentas enables the successful development of young embryos from the zygote stage to maturity.

ECS1 and ECS2 suppress polyspermy and the formation of haploid plants by promoting double fertilization

The current pace of crop plant optimization is insufficient to meet future demands and there is an urgent need for novel breeding strategies. It was previously shown that plants tolerate the generation of triparental polyspermy-derived plants and that polyspermy can bypass hybridization barriers. Polyspermy thus has the potential to harness previously incompatible climate-adapted wild varieties for plant breeding. However, factors that influence polyspermy frequencies were not previously known. The endopeptidases ECS1 and ECS2 have been reported to prevent the attraction of supernumerary pollen tubes by cleaving the pollen tube attractant LURE1. Here, we show that these genes have an earlier function that is manifested by incomplete double fertilization in plants defective for both genes. In addition to supernumerary pollen tube attraction, ecs1 ecs2 mutants exhibit a delay in synergid disintegration, are susceptible to heterofertilization, and segregate haploid plants that lack a paternal genome contribution. Our results thus uncover ECS1 and ECS2 as the first female factors triggering the induction of maternal haploids. Capitalizing on a high-throughput polyspermy assay, we in addition show that the double mutant exhibits an increase in polyspermy frequencies. As both haploid induction and polyspermy are valuable breeding aims, our results open new avenues for accelerated generation of climate-adapted cultivars.

Loss-of-function of gynoecium-expressed phospholipase pPLAIIγ triggers maternal haploid induction in Arabidopsis

Production of in planta haploid embryos that inherit chromosomes from only one parent can greatly increase breeding efficiency via quickly generating homozygous plants, called doubled haploid. One of the main players of in planta haploid induction is a pollen-specific phospholipase A, which is able, when mutated, to induce in vivo haploid induction in numerous monocots. However, no functional orthologous gene has been identified in dicots plants. Here, we show that loss-of-function of gynoecium-expressed phospholipase AII (pPLAIIγ) triggers maternal haploid plants in Arabidopsis, at an average rate of 1.07%. Reciprocal crosses demonstrate that haploid plants are triggered from the female side and not from the pollen, and the haploid plants carry the maternal genome. Promoter activity of pPLAIIγ shows enriched expression in the funiculus of flower development stages 13 and 18, and pPLAIIγ fused to yellow fluorescent protein reveals a plasma-membrane localization Interestingly, the polar localized PIN1 at the basal plasma membrane of the funiculus was all internalized in pplaIIγ mutants, suggesting that altered PIN1 localization in female organ could play a role in maternal haploid induction.

The Roads to Haploid Embryogenesis

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.