Microtubule configurations and nuclear DNA synthesis during initiation of suspensor-bearing embryos from Brassica napus cv. Topas microspores

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.

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.

Methods for Chromosome Doubling

The completely homozygous genetic background of doubled haploids (DHs) has many applications in breeding programs and research studies. Haploid induction and chromosome doubling of induced haploids are the two main steps of doubled haploid creation. Both steps have their own complexities. Chromosome doubling of induced haploids may happen spontaneously, although usually at a low rate. Therefore, artificial/induced chromosome doubling of haploid cells/plantlets is necessary to produce DHs at an acceptable level. The most common method is using some mitotic spindle poisons that target the organization of the microtubule system. Colchicine is a well-known and widely used antimitotic. However, there are substances alternative to colchicine in terms of efficiency, toxicity, safety, and genetic stability, which can be applied in in vitro and in vivo pathways. Both pathways have their own advantages and disadvantages. However, in vitro-induced chromosome doubling has been much preferred in recent years, maybe because of the dual effect of antimitotic agents (haploid induction and chromosome doubling) in just one step, and the reduced generation of chimeras. Plant genotype, the developmental stage of initial haploids, and type-concentration-duration of application of antimitotic agents, are top influential parameters on chromosome doubling efficiency. In this review, we highlight different aspects related to antimitotic agents and to plant parameters for successful chromosome doubling and high DH yield.

In vitro androgenesis: spontaneous vs. artificial genome doubling and characterization of regenerants

Androgenesis has become the most frequently chosen method of doubled haploid (DH) production in major crops. Theoretically, plantlets derived from in vitro cultured microspore encompass half of the normal chromosome number of donor plants and thus, considered to be haploid. However, depending on species/genotype and the method of haploid production, either via anther or isolated microspore culture, different ratios of spontaneous DHs and diploid (2n) or even polyploid plants originating from somatic tissues or unreduced gametes may also arise in the cultures. Adopting the method of haploid identification, anti-microtubular agent for restoring fertility, and discriminating spontaneous DHs from undesired heterozygote plants will substantially affect the success of androgenesis in breeding programs. The recent advances in the last 2 decades have made it possible to characterize the in vitro regenerants efficiently either prior to genome duplication or using in breeding programs. The herein described approaches and antimicotubular agents are, therefore, expected to improve the efficiency of DH-based breeding pipeline through the in vitro androgenesis.

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.

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.

Failure of androgenesis in Miscanthus × giganteus in vitro culture of cytologically unbalanced microspores

Miscanthus × giganteus is a popular energy crop, which due to its hybrid origin is only vegetatively reproduced. Asexual embryogenesis in anther and microspore culture leading to double haploids production could allow to regain the ability for sexual reproduction and to increase the biodiversity of the species. Therefore, the goal of this paper was to investigate the requirements of androgenesis in Miscanthus. The standard protocols used for monocotyledonous plants were applied with many modifications regarding the developmental stage of the explants at the time of culture initiation, stress treatment applied to panicles and isolated anthers as well as various chemical and physical parameters of in vitro culture conditions. Our results indicated that the induction of androgenesis in M. × giganteus is possible. However, the very low efficiency of the process and the lack of regeneration ability of the androgenic structures presently prevent the use of this technique.

Endogenous ABA concentration and cytoplasmic membrane fluidity in microspores of oilseed rape (Brassica napus L.) genotypes differing in responsiveness to androgenesis induction

KEY MESSAGE

A better understanding of androgenesis with a focus on the changes in plasma membrane fluidity and endogenous ABA content affecting embryogenesis induction in microspore suspension of B. napus. Changes in plasma membrane fluidity (MF) and ABA content associated with androgenesis induction were under the study. Both parameters were monitored in microspores of two Brassica napus L. genotypes differing in their response to androgenic induction under heat (1 day at 32 °C). MF was assessed by DPH method. ABA content was evaluated by ELISA. Heat caused microspores' plasma membrane to become more rigid. Lower MF in microspores of 'DH 4079' (of high androgenic potential) seems to maintain proper cell protection and leads to efficient embryogenesis induction. Plasma membrane remodelling coincided with changes of ABA content in microspores and in the culture medium in both genotypes. ABA concentration (μM) and ABA content (fmol per 10(4) microspores or pmol g(-1) FW) were for the first time measured in microspores. ABA concentration (μM) in microspores and in the culture medium (nM) differed significantly for the genotype and the treatment. The interaction between both variables was also significant. In general, ABA content ranged from <3.5 to 87.1 fmol per 10(4) microspores. The highest content of ABA was detected in 'DH 4079' microspores at 32 °C. Assuming a mean microspores' radius of 10 μm, it corresponds to ABA concentration of 2.1 μM. Heat shock resulted in quantum of medium pH reduction (0.1-0.2) and increased levels of ABA in microspores and in the medium of both tested genotypes. However, heat induced increase of ABA content in microspores of non-responsive 'Campino' had no clear-cut impact, on androgenesis induction efficiency, which suggests a more complex mechanism of process initiation.

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

The roles of cell polarity and the first asymmetric cell division during early embryogenesis in apical-basal cell fate determination remain unclear. Previously, a novel Brassica napus microspore embryogenesis system was established, by which rape exine-dehisced microspores were induced by physical stress. Unlike traditional microspore culture, cell polarity and subsequent asymmetric division appeared in the exine-dehisced microspore, which finally developed into a typical embryo with a suspensor. Further studies indicated that polarity is critical for apical-basal cell fate determination and suspensor formation. However, the pattern of the first division was not only determined by cell polarity but was also regulated by the position of the ruptured exine. The first division could be equal or unequal, with its orientation essentially perpendicular to the polar axis. In both types of cell division, the two daughter cells could have different cell fates and give rise to an embryo with a suspensor, similar to zygotic apical-basal cell differentiation. The alignment of the two daughter cells is consistent with the orientation of the apical-basal axis of future embryonic development. Thus, the results revealed that exine dehiscing induces rape microspore polarization, and this polarity results in a different cell fate and fixes the apical-basal axis of embryogenesis, but is uncoupled from cell asymmetric division. The present study demonstrated the relationships among cell polarity, asymmetric cell division, and cell fate determination in early embryogenesis.