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

Analysis of auxin responses in the fern Ceratopteris richardii identifies the developmental phase as a major determinant for response properties

The auxin signaling molecule regulates a range of plant growth and developmental processes. The core transcriptional machinery responsible for auxin-mediated responses is conserved across all land plants. Genetic, physiological and molecular exploration in bryophyte and angiosperm model species have shown both qualitative and quantitative differences in auxin responses. Given the highly divergent ontogeny of the dominant gametophyte (bryophytes) and sporophyte (angiosperms) generations, however, it is unclear whether such differences derive from distinct phylogeny or ontogeny. Here, we address this question by comparing a range of physiological, developmental and molecular responses to auxin in both generations of the model fern Ceratopteris richardii. We find that auxin response in Ceratopteris gametophytes closely resembles that of a thalloid bryophyte, whereas the sporophyte mimics auxin response in flowering plants. This resemblance manifests both at the phenotypic and transcriptional levels. Furthermore, we show that disrupting auxin transport can lead to ectopic sporophyte induction on the gametophyte, suggesting a role for auxin in the alternation of generations. Our study thus identifies developmental phase, rather than phylogeny, as a major determinant of auxin response properties in land plants.

The maternal embrace: the protection of plant embryos

All land plants-the embryophytes-produce multicellular embryos, as do other multicellular organisms, such as brown algae and animals. A unique characteristic of plant embryos is their immobile and confined nature. Their embedding in maternal tissues may offer protection from the environment, but also physically constrains development. Across the different land plants, a huge discrepancy is present between their reproductive structures whilst leading to similarly complex embryos. Therefore, we review the roles that maternal tissues play in the control of embryogenesis across land plants. These nurturing, constraining, and protective roles include both direct and indirect effects. In this review, we explore how the maternal surroundings affect embryogenesis and which chemical and mechanical barriers are in place. We regard these questions through the lens of evolution, and identify key questions for future research.

The different response of Brassica napus genotypes to microspore embryogenesis induced by heat shock and trichostatin A is not determined by changes in cell wall structure and composition but by different stress tolerance

During microspore embryogenesis, microspores are induced to develop into haploid embryos. In Brassica napus, microspore embryogenesis is induced by a heat shock (HS), which initially produces embryogenic structures with different cell wall architectures and compositions, and with different potentials to develop into embryos. The B. napus DH4079 and DH12075 genotypes have high and very low embryo yields, respectively. In DH12075, embryo yield is greatly increased by combining HS and the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). However, we show that HS + TSA inhibits embryogenesis in the highly embryogenic DH4079 line. To ascertain why TSA has such different effects in these lines, we treated DH4079 and DH12075 microspore cultures with TSA and compared the cell wall structure and composition of the different embryogenic structures in both lines, specifically the in situ levels and distribution of callose, cellulose, arabinogalactan proteins and high and low methyl-esterified pectin. For both lines, HS + TSA led to the formation of cell walls unfavorable for embryogenesis progression, with reduced levels of arabinogalactan proteins, reduced cell adhesion of inner walls and altered pectin composition. Thus, TSA effects on cell walls cannot explain their different embryogenic response to TSA. We also applied TSA to DH4079 cultures at different times and concentrations before HS application, with no negative effects on embryogenic induction. These results indicate that DH4079 microspores are hypersensitive to combined TSA and HS treatments, and open up new hypotheses about the causes of such hypersensitivity.

New Epigenetic Modifier Inhibitors Enhance Microspore Embryogenesis in Bread Wheat

The use of doubled haploid (DH) technology enables the development of new varieties of plants in less time than traditional breeding methods. In microspore embryogenesis (ME), stress treatment triggers microspores towards an embryogenic pathway, resulting in the production of DH plants. Epigenetic modifiers have been successfully used to increase ME efficiency in a number of crops. In wheat, only the histone deacetylase inhibitor trichostatin A (TSA) has been shown to be effective. In this study, inhibitors of epigenetic modifiers acting on histone methylation (chaetocin and CARM1 inhibitor) and histone phosphorylation (aurora kinase inhibitor II (AUKI-II) and hesperadin) were screened to determine their potential in ME induction in high- and mid-low-responding cultivars. The use of chaetocin and AUKI-II resulted in a higher percentage of embryogenic structures than controls in both cultivars, but only AUKI-II was superior to TSA. In order to evaluate the potential of AUKI-II in terms of increasing the number of green DH plants, short and long application strategies were tested during the mannitol stress treatment. The application of 0.8 µM AUKI-II during a long stress treatment resulted in a higher percentage of chromosome doubling compared to control DMSO in both cultivars. This concentration produced 33% more green DH plants than the control in the mid-low-responding cultivar, but did not affect the final ME efficiency in a high-responding cultivar. This study has identified new epigenetic modifiers whose use could be promising for increasing the efficiency of other systems that require cellular reprogramming.

The effect of phytosulfokine alpha on haploid embryogenesis and gene expression of Brassica napus microspore cultures

Microspore embryogenesis (ME) is the most powerful tool for creating homozygous lines in plant breeding and molecular biology research. It is still based mainly on the reprogramming of microspores by temperature, osmotic and/or nutrient stress. New compounds are being sought that could increase the efficiency of microspore embryogenesis or even induce the formation of haploid embryos from recalcitrant genotypes. Among these, the mitogenic factor phytosulfokine alpha (PSK-α) is promising due to its broad spectrum of activity in vivo and in vitro. The aim of our study was to investigate the effect of PSK-α on haploid embryogenesis from microspores of oilseed rape (Brassica napus L., DH4079), one of the most important oil crops and a model plant for studying the molecular mechanisms controlling embryo formation. We tested different concentrations (0, 0.01, 0.1 and 1 µM) of the peptide and evaluated its effect on microspore viability and embryo regeneration after four weeks of culture. Our results showed a positive correlation between addition of PSK-α and cultured microspore viability and a positive effect also on the number of developed embryos. The analysis of transcriptomes across three time points (day 0, 2 and 4) with or without PSK-α supplementation (15 RNA libraries in total) unveiled differentially expressed genes pivotal in cell division, microspore embryogenesis, and subsequent regeneration. PCA grouped transcriptomes by RNA sampling time, with the first two principal components explaining 56.8% variability. On day 2 with PSK, 45 genes (15 up- and 30 down-regulated) were differentially expressed when PSK-α was added and their number increased to 304 by day 4 (30 up- and 274 down-regulated). PSK, PSKR, and PSI gene expression analysis revealed dynamic patterns, with PSK2 displaying the highest increase and overall expression during microspore culture at days 2 and 4. Despite some variations, only PSK1 showed significant differential expression upon PSK-α addition. Of 16 ME-related molecular markers, 3 and 15 exhibited significant differential expression in PSK-supplemented cultures at days 2 and 4, respectively. Embryo-specific markers predominantly expressed after 4 days of culture, with higher expression in medium without PSK, while on day 0, numerous sporophyte-specific markers were highly expressed.

Brassica napus Haploid and Double Haploid Production and Its Latest Applications

Rapeseed is one of the most important oil crops in the world. Increasing demand for oil and limited agronomic capabilities of present-day rapeseed result in the need for rapid development of new, superior cultivars. Double haploid (DH) technology is a fast and convenient approach in plant breeding as well as genetic research. Brassica napus is considered a model species for DH production based on microspore embryogenesis; however, the molecular mechanisms underlying microspore reprogramming are still vague. It is known that morphological changes are accompanied by gene and protein expression patterns, alongside carbohydrate and lipid metabolism. Novel, more efficient methods for DH rapeseed production have been reported. This review covers new findings and advances in Brassica napus DH production as well as the latest reports related to agronomically important traits in molecular studies employing the double haploid rapeseed lines.

The Highly Embryogenic Brassica napus DH4079 Line Is Recalcitrant to Agrobacterium-Mediated Genetic Transformation

Brassica napus is a species of high agronomic interest, used as a model to study different processes, including microspore embryogenesis. The DH4079 and DH12075 lines show high and low embryogenic response, respectively, which makes them ideal to study the basic mechanisms controlling embryogenesis induction. Therefore, the availability of protocols for genetic transformation of these two backgrounds would help to generate tools to better understand this process. There are some reports in the literature showing the stable transformation of DH12075. However, no equivalent studies in DH4079 have been reported to date. We explored the ability of DH4079 plants to be genetically transformed. As a reference to compare with, we used the same protocols to transform DH12075. We used three different protocols previously reported as successful for B. napus stable transformation with Agrobacterium tumefaciens and analyzed the response of plants. Whereas DH12075 plants responded to genetic transformation, DH4079 plants were completely recalcitrant, not producing any single regenerant out of the 1784 explants transformed and cultured. Additionally, an Agrobacterium rhizogenes transient transformation assay was performed on both lines, and only DH12075, but no DH4079 seedlings, responded to A. rhizogenes infection. Therefore, we propose that the DH4079 line is recalcitrant to Agrobacterium-mediated transformation.

Chromatin during plant regeneration: Opening towards root identity?

Plants show exceptional developmental plasticity and the ability to reprogram cell identities during regeneration. Although regeneration has been used in plant propagation for decades, we only recently gained detailed cellular and molecular insights into this process. Evidently, not all cell types have the same regeneration potential, and only a subset of regeneration-competent cells reach pluripotency. Pluripotent cells exhibit transcriptional similarity to root stem cells. In different plant regeneration systems, transcriptional reprogramming involves transient release of chromatin repression during pluripotency establishment and its restoration during organ or embryo differentiation. Incomplete resetting of the epigenome leads to somaclonal variation in regenerated plants. As single-cell technologies advance, we expect novel, exciting insights into epigenome dynamics during the establishment of pluripotency.

Microtubule organization changes severely after mannitol and n-butanol treatments inducing microspore embryogenesis in bread wheat

BACKGROUND

A mannitol stress treatment and a subsequent application of n-butanol, known as a microtubule-disrupting agent, enhance microspore embryogenesis (ME) induction and plant regeneration in bread wheat. To characterize changes in cortical (CMT) and endoplasmic (EMT) microtubules organization and dynamics, associated with ME induction treatments, immunocytochemistry studies complemented by confocal laser scanning microscopy (CLSM) were accomplished. This technique has allowed us to perform advanced 3- and 4D studies of MT architecture. The degree of MT fragmentation was examined by the relative fluorescence intensity quantification.

RESULTS

In uni-nucleated mannitol-treated microspores, severe CMT and EMT fragmentation occurs, although a complex network of short EMT bundles protected the nucleus. Additional treatment with n-butanol resulted in further depolymerization of both CMT and EMT, simultaneously with the formation of MT aggregates in the perinuclear region. Some aggregates resembled a preprophase band. In addition, a portion of the microspores progressed to the first mitotic division during the treatments. Bi-nucleate pollen-like structures showed a high MT depolymerization after mannitol treatment and numerous EMT bundles around the vegetative and generative nuclei after n-butanol. Interestingly, bi-nucleate symmetric structures showed prominent stabilization of EMT.

CONCLUSIONS

Fragmentation and stabilization of microtubules induced by mannitol- and n-butanol lead to new configurations essential for the induction of microspore embryogenesis in bread wheat. These results provide robust insight into MT dynamics during EM induction and open avenues to address newly targeted treatments to induce ME in recalcitrant species.

Proteins, Small Peptides and Other Signaling Molecules Identified as Inconspicuous but Possibly Important Players in Microspores Reprogramming Toward Embryogenesis

In this review, we describe and integrate the latest knowledge on the signaling role of proteins and peptides in the stress-induced microspore embryogenesis (ME) in some crop plants with agricultural importance (i.e., oilseed rape, tobacco, barley, wheat, rice, triticale, rye). Based on the results received from the most advanced omix analyses, we have selected some inconspicuous but possibly important players in microspores reprogramming toward embryogenic development. We provide an overview of the roles and downstream effect of stress-related proteins (e.g., β-1,3-glucanases, chitinases) and small signaling peptides, especially cysteine—(e.g., glutathione, γ-thionins, rapid alkalinization factor, lipid transfer, phytosulfokine) and glycine-rich peptides and other proteins (e.g., fasciclin-like arabinogalactan protein) on acclimation ability of microspores and the cell wall reconstruction in a context of ME induction and haploids/doubled haploids (DHs) production. Application of these molecules, stimulating the induction and proper development of embryo-like structures and green plant regeneration, brings significant improvement of the effectiveness of DHs procedures and could result in its wider incorporation on a commercial scale. Recent advances in the design and construction of synthetic peptides–mainly cysteine-rich peptides and their derivatives–have accelerated the development of new DNA-free genome-editing techniques. These new systems are evolving incredibly fast and soon will find application in many areas of plant science and breeding.

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.

Differential Expression Profiling Reveals Stress-Induced Cell Fate Divergence in Soybean Microspores

Stress-induced microspore embryogenesis is a widely employed method to achieve homozygosity in plant breeding programs. However, the molecular mechanisms that govern gametophyte de- and redifferentiation are understood poorly. In this study, RNA-Seq was used to evaluate global changes across the microspore transcriptome of soybean (Glycine max [L.] Merrill) as a consequence of pretreatment low-temperature stress. Expression analysis revealed more than 20,000 differentially expressed genes between treated and control microspore populations. Functional enrichment illustrated that many of these genes (e.g., those encoding heat shock proteins and cytochrome P450s) were upregulated to maintain cellular homeostasis through the mitigation of oxidative damage. Moreover, transcripts corresponding to saccharide metabolism, vacuolar transport, and other pollen-related developmental processes were drastically downregulated among treated microspores. Temperature stress also triggered cell wall modification and cell proliferation-characteristics that implied putative commitment to an embryonic pathway. These findings collectively demonstrate that pretreatment cold stress induces soybean microspore reprogramming through suppression of the gametophytic program while concomitantly driving sporophytic development.