Exine dehiscing induces rape microspore polarity, which results in different daughter cell fate and fixes the apical-basal axis of the embryo

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.

Effects of genotype and culture conditions on microspore embryogenesis in radish (Raphanus sativus L.)

Radish (Raphanus sativus L.), an important annual or biennial root vegetable crop, is widely cultivated in the world for its high nutritive value. Isolated microspore culture (IMC) is one of the most effective methods for rapid development of homozygous lines. Due to imperfection of the IMC technology system, it is particularly important to establish an efficient IMC system in radish. In this study, the effects of different factors on radish microspore embryogenesis were investigated with 23 genotypes. Buds with the largest population of late-uninucleate-stage microspores were most suitable for embryogenesis, with a ratio of petal length to anther length (P/A) in buds of about 3/4 ~ 1. Cold pretreatment was found to be genotype specific, and the highest microspore-derived embryoid (MDE) yield occurred for treatment of the heat shock of 48 h. In addition, the supplement of 0.75 g/L activated charcoal (AC) could increase the yield of embryoids. It was found that genotypes, bud size, as well as temperature treatments had significant effects on microspore embryogenesis. Furthermore, somatic embryogenesis-related kinase (SERK) genes were profiled by reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis, which indicated that they are involved in the process of MDE formation and plantlet regeneration. The ploidy of microspore-derived plants was identified by chromosome counting and flow cytometry, and the microspore-derived plants were further proved as homozygous plants through expressed sequence tags-simple sequence repeats (EST-SSR) and genetic-SSR markers. The results would facilitate generating the large-scale double haploid (DH) from various genotypes, and promoting further highly efficient genetic improvement in radish.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01312-w.

Cytidinediphosphate diacylglycerol synthase—Mediated phosphatidic acid metabolism is crucial for early embryonic development of Arabidopsis

Embryonic development is a key developmental event in plant sexual reproduction; however, regulatory networks of plant early embryonic development, particularly the effects and functional mechanisms of phospholipid molecules are still unknown due to the limitation of sample collection and analysis. We innovatively applied the microspore-derived in vitro embryogenesis of Brassica napus and revealed the dynamics of phospholipid molecules, especially phosphatidic acid (PA, an important second messenger that plays an important role in plant growth, development, and stress responses), at different embryonic developmental stages by using a lipidomics approach. Further analysis of Arabidopsis mutants deficiency of CDS1 and CDS2 (cytidinediphosphate diacylglycerol synthase, key protein in PA metabolism) revealed the delayed embryonic development from the proembryo stage, indicating the crucial effect of CDS and PA metabolism in early embryonic development. Decreased auxin level and disturbed polar localization of auxin efflux carrier PIN1 implicate that CDS-mediated PA metabolism may regulate early embryogenesis through modulating auxin transport and distribution. These results demonstrate the dynamics and importance of phospholipid molecules during embryo development, and provide informative clues to elucidate the regulatory network of embryogenesis.

As an important stage of plant life cycle, embryogenesis initiates with fertilization and terminates with embryo maturation, and plays important roles for seed development. In Arabidopsis, embryogenesis can be divided into three general phases during which distinct developmental and physiological events occur: proembryo stage that starts with asymmetric division of zygote, the transition from globular to heart stage, and late embryogenesis which is marked by organ expansion and maturation. Genetics studies have identified the genes and network that play important regulatory roles in embryogenesis, however, effects and functional mechanisms of phospholipids molecules, the major building blocks of cell membranes and signal transduction cascades in mammals and plants, in embryogenesis remain unknown. To overcome the limitation of sample collection, we innovatively applied the microspore-derived in vitro embryogenesis of Brassica napus and revealed the dynamics of phospholipid molecules, especially phosphatidic acid (PA) at different embryonic developmental stages by using a lipidomics approach. Further genetics analysis revealed the crucial effect of CDS (cytidinediphosphate diacylglycerol synthase, key protein in PA metabolism)—mediated PA metabolism in early embryonic development through modulating auxin transport and distribution, demonstrating the dynamics and importance of phospholipid molecules during embryo development.

Embryogenesis of European Radish (Raphanus sativus L. subsp. sativus Convar. Radicula) in Culture of Isolated Microspores In Vitro

The European radish is one of the most unresponsive crops in the Brassicaceae family to embryogenesis in in vitro microspore culture. The aim of this work was to study the process of embryogenesis of European radish and its biological features. In this study, the embryogenesis of European radish is described in detail with illustrative data for the first time. For the first time for the entire family Brassicaceae, the following were found: microspores with intact exines with ordered-like divisions; microspores completely free of exines; and a new scheme of suspensors attachment to the apical parts of embryoids. The morphology of double and triple twin embryoids was described, and new patterns of their attachment to each other were discovered. Uneven maturation of European radish embryoids at all stages of embryogenesis was noted. The period of embryoid maturation to the globular stage of development corresponded, in terms of time, to the culture of B. napus, and into the cotyledonary stage of development, maturation was faster and amounted to 17-23 days. The rate of embryoid development with and without suspensors was the same.

Cell Wall Composition and Structure Define the Developmental Fate of Embryogenic Microspores in Brassica napus

Microspore cultures generate a heterogeneous population of embryogenic structures that can be grouped into highly embryogenic structures [exine-enclosed (EE) and loose bicellular structures (LBS)] and barely embryogenic structures [compact callus (CC) and loose callus (LC) structures]. Little is known about the factors behind these different responses. In this study we performed a comparative analysis of the composition and architecture of the cell walls of each structure by confocal and quantitative electron microscopy. Each structure presented specific cell wall characteristics that defined their developmental fate. EE and LBS structures, which are responsible for most of the viable embryos, showed a specific profile with thin walls rich in arabinogalactan proteins (AGPs), highly and low methyl-esterified pectin and callose, and a callose-rich subintinal layer not necessarily thick, but with a remarkably high callose concentration. The different profiles of EE and LBS walls support the development as suspensorless and suspensor-bearing embryos, respectively. Conversely, less viable embryogenic structures (LC) presented the thickest walls and the lowest values for almost all of the studied cell wall components. These cell wall properties would be the less favorable for cell proliferation and embryo progression. High levels of highly methyl-esterified pectin are necessary for wall flexibility and growth of highly embryogenic structures. AGPs seem to play a role in cell wall stiffness, possibly due to their putative role as calcium capacitors, explaining the positive relationship between embryogenic potential and calcium levels.

Live Imaging of embryogenic structures in Brassica napus microspore embryo cultures highlights the developmental plasticity of induced totipotent cells

In vitro embryo development is highly plastic; embryo cell fate can be re-established in tissue culture through different pathways. In most angiosperms, embryo development from the single-celled zygote follows a defined pattern of cell divisions in which apical (embryo proper) and basal (root and suspensor) cell fates are established within the first cell divisions. By contrast, embryos that are induced in vitro in the absence of fertilization show a less regular initial cell division pattern yet develop into histodifferentiated embryos that can be converted into seedlings. We used the Brassica napus microspore embryogenesis system, in which the male gametophyte is reprogrammed in vitro to form haploid embryos, to identify the developmental fates of the different types of embryogenic structures found in culture. Using time-lapse imaging of LEAFY COTYLEDON1-expressing cells, we show that embryogenic cell clusters with very different morphologies are able to form haploid embryos. The timing of surrounding pollen wall (exine) rupture is a major determinant of cell fate in these clusters, with early exine rupture leading to the formation of suspensor-bearing embryos and late rupture to suspensorless embryos. In addition, we show that embryogenic callus, which develops into suspensor-bearing embryos, initially expresses transcripts associated with both basal- and apical-embryo cell fates, suggesting that these two cell fates are fixed later in development. This study reveals the inherent plasticity of in vitro embryo development and identifies new pathways by which embryo cell fate can be established.

Impedance flow cytometry allows the early prediction of embryo yields in wheat (Triticum aestivum L.) microspore cultures

Haplomethods are key biotechnological tools that make it possible to rapidly produce perfectly homozygous lines, speeding up plant breeding programs. Under specific stress conditions, microspores are reprogrammed toward sporophytic pathways, leading to embryo formation. Various endogenous and exogenous factors affect embryo yield in androgenesis, so the improvement of androgenesis efficiency requires the development of early, reliable and robust reactivity markers. During the last decade, numerous cytological, cellular and biochemical approaches were carried out to finely characterize microspore development and fate during androgenesis. However, the different available markers are often species-dependent, and their development and application are time-consuming and cumbersome. In this study, we show the suitable use of impedance flow cytometry (IFC) to develop new robust, reliable and strong markers of androgenesis reactivity in wheat, leading to: (i) routine monitoring of the viability of heterogeneous cell cultures; (ii) quick and simple evaluation of stress treatment efficiency; and (iii) early prediction of embryo yields from microspore suspensions. IFC can therefore provide the fine characterization of all of the microspore developmental pathways that occur in a cell suspension, for embryogenic microspores as well as pollen-like microspores. IFC technology has become a very useful tool to track and characterize wheat microspores in androgenesis, but can also be adapted to other species and other in vitro cell culture systems.

Microspore embryogenesis in Brassica: calcium signaling, epigenetic modification, and programmed cell death

Stress induction followed by excessive calcium influx causes multiple changes in microspores resulting in chromatin remodeling, epigenetic modifications, and removal of unwanted gametophytic components via autophagy, switching microspores towards ME. In Brassica, isolated microspores that are placed under specific external stresses can switch their default developmental pathway towards an embryogenic state. Microspore embryogenesis is a unique system that speeds up breeding programs and, in the context of developmental biology, provides an excellent tool for embryogenesis to be investigated in greater detail. The last few years have provided ample evidence that has allowed Brassica researchers to markedly increase their understanding of the molecular and sub-cellular changes underlying this process. We review recent advances in this field, focusing mainly on the perception to inductive stresses, signal transduction, molecular and structural alterations, and the involvement of programmed cell death at the onset of embryogenic induction.

Intracellular delivery of fluorescent protein into viable wheat microspores using cationic peptides

Microspores are specialized generative cells with haploid genome that demonstrate the amenability toward embryogenesis under certain conditions. The induced microspore culture technique is largely exploited by the breeding programs of wheat and other crops due to its high efficiency for generation of the large number of haploid plants in the relatively short period of time. The ability to produce mature double haploid plant from a single cell has also attracted attention of the plant biotechnologists in the past few years. More importantly, the possibility to deliver proteins for improvement of embryogenesis and the genome modification purposes holds great potential for transgene-free wheat biotechnology. In the present study, we examined the ability of cationic and amphipathic cell penetrating peptides (CPPs) to convey a covalently-linked mCherry protein inside the viable microspores. We demonstrate that the affinity of CPPs to the microspore cells dependents on their charge with the highest efficiency of CPP-mCherry binding to the cells achieved by cationic CPPs (penetratin and R9). Additionally, due to overall negative charge of the microspore cell wall, the successful uptake of the protein cargo by live microspore cells is attained by utilization of a reversible disulfide bond between the R9 CPP and mCherry protein. Overall, the approach proposed herein can be applied by the other biotechnology groups for the fast and efficient screening of the different CPP candidates for their ability to deliver proteins inside the viable plant cells.

The isolation of early nuclear endosperm of Oryza sativa to facilitate gene expression analysis and screening imprinted genes

BACKGROUND

Since the quality and yield of rice production depends on endosperm development, previous studies have focused on the molecular mechanism that regulates this developmental process. Recently, how this process is epigenetically regulated has become an important topic. However, the gene expression analysis and screening imprinted genes during early endosperm development remain challenging since the isolation of early endosperm has not been possible. Here, we report a procedure for the isolation of endosperm at 24 or 48 HAP (hours after pollination) during the free nuclear stage of endosperm development.

RESULTS

This technique allows for rapid and convenient collection of pure free nuclear endosperm. Early endosperm RNA can then be extracted from the isolated endosperm cells using dynabeads. Our results showed that the quality of RNA is satisfactory for gene expression analysis and screening the parental-of-origin specific genes in early endosperm.

CONCLUSIONS

Thus, we offer a reliable method to overcome one of the major obstacles in the investigation of the molecular mechanisms of early endosperm development. Our approach can be used for accurate gene expression analysis and screening of imprinted genes, and facilitates the confirmation of endosperm-specific gene expression at the very early stages of endosperm development. This method could also be used in other species to collect early free nuclear endosperm.

The influence of heat stress on auxin distribution in transgenic B. napus microspores and microspore-derived embryos

Plant embryogenesis is regulated by differential distribution of the plant hormone auxin. However, the cells establishing these gradients during microspore embryogenesis remain to be identified. For the first time, we describe, using the DR5 or DR5rev reporter gene systems, the GFP- and GUS-based auxin biosensors to monitor auxin during Brassica napus androgenesis at cellular resolution in the initial stages. Our study provides evidence that the distribution of auxin changes during embryo development and depends on the temperature-inducible in vitro culture conditions. For this, microspores (mcs) were induced to embryogenesis by heat treatment and then subjected to genetic modification via Agrobacterium tumefaciens. The duration of high temperature treatment had a significant influence on auxin distribution in isolated and in vitro-cultured microspores and on microspore-derived embryo development. In the "mild" heat-treated (1 day at 32 °C) mcs, auxin localized in a polar way already at the uni-nucleate microspore, which was critical for the initiation of embryos with suspensor-like structure. Assuming a mean mcs radius of 20 μm, endogenous auxin content in a single cell corresponded to concentration of 1.01 μM. In mcs subjected to a prolonged heat (5 days at 32 °C), although auxin concentration increased dozen times, auxin polarization was set up at a few-celled pro-embryos without suspensor. Those embryos were enclosed in the outer wall called the exine. The exine rupture was accompanied by the auxin gradient polarization. Relative quantitative estimation of auxin, using time-lapse imaging, revealed that primordia possess up to 1.3-fold higher amounts than those found in the root apices of transgenic MDEs in the presence of exogenous auxin. Our results show, for the first time, which concentration of endogenous auxin coincides with the first cell division and how the high temperature interplays with auxin, by what affects delay early establishing microspore polarity. Moreover, we present how the local auxin accumulation demonstrates the apical-basal axis formation of the androgenic embryo and directs the axiality of the adult haploid plant.

Plasticity in Cell Division Patterns and Auxin Transport Dependency during in Vitro Embryogenesis in Brassica napus

In Arabidopsis thaliana, zygotic embryo divisions are highly regular, but it is not clear how embryo patterning is established in species or culture systems with irregular cell divisions. We investigated this using the Brassica napus microspore embryogenesis system, where the male gametophyte is reprogrammed in vitro to form haploid embryos in the absence of exogenous growth regulators. Microspore embryos are formed via two pathways: a zygotic-like pathway, characterized by initial suspensor formation followed by embryo proper formation from the distal cell of the suspensor, and a pathway characterized by initially unorganized embryos lacking a suspensor. Using embryo fate and auxin markers, we show that the zygotic-like pathway requires polar auxin transport for embryo proper specification from the suspensor, while the suspensorless pathway is polar auxin transport independent and marked by an initial auxin maximum, suggesting early embryo proper establishment in the absence of a basal suspensor. Polarity establishment in this suspensorless pathway was triggered and guided by rupture of the pollen exine. Irregular division patterns did not affect cell fate establishment in either pathway. These results confirm the importance of the suspensor and suspensor-driven auxin transport in patterning, but also uncover a mechanism where cell patterning is less regular and independent of auxin transport.

Somatic embryogenesis: life and death processes during apical-basal patterning

Somatic embryogenesis (SE) is a process of differentiation of cells into a plant bypassing the fusion of gametes. As such, it represents a very powerful tool in biotechnology for propagation of species with a long reproductive cycle or low seed set and production of genetically modified plants with improved traits. SE is also a versatile model to study cellular and molecular mechanisms of plant embryo patterning. The morphology and molecular regulation of SE resemble those of zygotic embryogenesis and begin with establishment of apical-basal asymmetry. The apical domain, the embryo proper, proliferates and eventually gives rise to the plantlet, while the basal part, the embryo suspensor, is terminally differentiated and gradually removed via vacuolar programmed cell death (PCD). This PCD is essential for normal development of the apical domain. Emerging evidence demonstrates that signalling events in the apical and basal domains share homologous components. Here we provide an overview of the main pathways controlling the life and death events during SE.

Characterization of polarity development through 2- and 3-D imaging during the initial phase of microspore embryogenesis in Brassica napus L

Isolated microspores of B. napus in culture change their developmental pathway from gametophytic to sporophytic and form embryo-like structures (ELS) upon prolonged heat shock treatment (5 days at 32 °C). ELS express polarity during the initial days of endosporic development. In this study, we focussed on the analysis of polarity development of ELS without suspensor. Fluorescence microscopy and 3-D confocal laser scanning microscopy (CLSM) without tissue interfering enabled us to get a good insight in the distribution of nuclei, mitochondria and endoplasmic reticulum (ER), the architecture of microtubular (MT) cytoskeleton and the places of 5-bromo-2'-deoxy-uridine (BrdU) incorporation in successive stages of microspore embryogenesis. Scanning electron microscopy (SEM) analysis revealed, for the first time, the appearance of a fibrillar extracellular matrix-like structure (ECM-like structure) in androgenic embryos without suspensor. Two types of endosporic development were distinguished based upon the initial location of the microspore nucleus. The polarity of dividing and growing cells was recognized by the differential distributions of organelles, by the organization of the MT cytoskeleton and by the visualization of DNA synthesis in the cell cycle. The directional location of nuclei, ER, mitochondria and starch grains in relation to the MTs configurations were early polarity indicators. Both exine rupture and ECM-like structure on the outer surfaces of ELS are supposed to stabilize ELS's morphological polarity. As the role of cell polarity during early endosporic microspore embryogenesis in apical-basal cell fate determination remains unclear, microspore culture system provides a powerful in vitro tool for studying the developmental processes that take place during the earliest stages of plant embryogenesis.

The histone deacetylase inhibitor trichostatin a promotes totipotency in the male gametophyte

The haploid male gametophyte, the pollen grain, is a terminally differentiated structure whose function ends at fertilization. Plant breeding and propagation widely use haploid embryo production from in vitro-cultured male gametophytes, but this technique remains poorly understood at the mechanistic level. Here, we show that histone deacetylases (HDACs) regulate the switch to haploid embryogenesis. Blocking HDAC activity with trichostatin A (TSA) in cultured male gametophytes of Brassica napus leads to a large increase in the proportion of cells that switch from pollen to embryogenic growth. Embryogenic growth is enhanced by, but not dependent on, the high-temperature stress that is normally used to induce haploid embryogenesis in B. napus. The male gametophyte of Arabidopsis thaliana, which is recalcitrant to haploid embryo development in culture, also forms embryogenic cell clusters after TSA treatment. Genetic analysis suggests that the HDAC protein HDA17 plays a role in this process. TSA treatment of male gametophytes is associated with the hyperacetylation of histones H3 and H4. We propose that the totipotency of the male gametophyte is kept in check by an HDAC-dependent mechanism and that the stress treatments used to induce haploid embryo development in culture impinge on this HDAC-dependent pathway.

Microspore embryogenesis: establishment of embryo identity and pattern in culture

The developmental plasticity of plants is beautifully illustrated by the competence of the immature male gametophyte to change its developmental fate from pollen to embryo development when exposed to stress treatments in culture. This process, referred to as microspore embryogenesis, is widely exploited in plant breeding, but also provides a unique system to understand totipotency and early cell fate decisions. We summarize the major concepts that have arisen from decades of cell and molecular studies on microspore embryogenesis and put these in the context of recent experiments, as well as results obtained from the study of pollen and zygotic embryo development.

Microspore embryogenesis: establishment of embryo identity and pattern in culture

The developmental plasticity of plants is beautifully illustrated by the competence of the immature male gametophyte to change its developmental fate from pollen to embryo development when exposed to stress treatments in culture. This process, referred to as microspore embryogenesis, is widely exploited in plant breeding, but also provides a unique system to understand totipotency and early cell fate decisions. We summarize the major concepts that have arisen from decades of cell and molecular studies on microspore embryogenesis and put these in the context of recent experiments, as well as results obtained from the study of pollen and zygotic embryo development.