Gametic embryogenesis and haploid technology as valuable support to plant breeding

Efficient isolated microspore culture protocol for callus induction and plantlet regeneration in japonica rice (Oryza sativa L.)

BACKGROUND

Isolated microspore culture is a useful biotechnological technique applied in modern plant breeding programs as it can produce doubled haploid (DH) plants and accelerate the development of new varieties. Furthermore, as a single-cell culture technique, the isolated microspore culture provides an excellent platform for studying microspore embryogenesis. However, the reports on isolated microspore culture are rather limited in rice due to the low callus induction rate, poor regeneration capability, and high genotypic dependency. The present study developed an effective isolated microspore culture protocol for high-frequency androgenesis in four japonica rice genotypes. Several factors affecting the isolated microspore culture were studied to evaluate their effects on callus induction and plantlet regeneration.

RESULTS

Low-temperature pre-treatment at 4 ℃ for 10-15 days could effectively promote microspore embryogenesis in japonica rice. A simple and efficient method was proposed for identifying the microspore developmental stage. The anthers in yellow-green florets located on the second type of primary branch on the rice panicle were found to be the optimal stage for isolated microspore culture. The most effective induction media for callus induction were IM2 and IM3, depending on the genotype. The optimal concentration of 2, 4-D in the medium for callus induction was 1 mg/L. Callus induction was negatively affected by a high concentration of KT over 1.5 mg/L. The differentiation medium suitable for japonica rice microspore callus comprised 1/2 MS, 2 mg/L 6-BA, 0.5 mg/L NAA, 30 g/L sucrose, and 6 g/L agar. The regeneration frequency of the four genotypes ranged from 61-211 green plantlets per 100 mg calli, with Chongxiangjing showing the highest regeneration frequency.

CONCLUSIONS

This study presented an efficient protocol for improved callus induction and green plantlet regeneration in japonica rice via isolated microspore culture, which could provide valuable support for rice breeding and genetic research.

Unveiling the Genetic Basis Underlying Rice Anther Culturability via Segregation Distortion Analysis in Doubled Haploid Population

Anther culture (AC) is a valuable technique in rice breeding. However, the genetic mechanisms underlying anther culturability remain elusive, which has hindered its widespread adoption in rice breeding programs. During AC, microspores carrying favorable alleles for AC are selectively regenerated, leading to segregation distortion (SD) of chromosomal regions linked to these alleles in the doubled haploid (DH) population. Using the AC method, a DH population was generated from the japonica hybrid rice Shenyou 26. A genetic map consisting of 470 SNPs was constructed using this DH population, and SD analysis was performed at both the single- and two-locus levels to dissect the genetic basis underlying anther culturability. Five segregation distortion loci (SDLs) potentially linked to anther culturability were identified. Among these, SDL5 exhibited an overrepresentation of alleles from the female parent, while SDL1.1, SDL1.2, SDL2, and SDL7 displayed an overrepresentation of alleles from the male parent. Furthermore, six pairs of epistatic interactions (EPIs) that influenced two-locus SDs in the DH population were discovered. A cluster of genetic loci, associated with EPI-1, EPI-3, EPI-4, and EPI-5, overlapped with SDL1.1, indicating that the SDL1.1 locus may play a role in regulating anther culturability via both additive and epistatic mechanisms. These findings provide valuable insights into the genetic control of anther culturability in rice and lay the foundation for future research focused on identifying the causal genes associated with anther culturability.

Improvement of Rice Blast Resistance in TGMS Line HD9802S through Optimized Anther Culture and Molecular Marker-Assisted Selection

Rice blast caused by Magnaporthe oryzae is one of the most serious rice diseases worldwide. The early indica rice thermosensitive genic male sterile (TGMS) line HD9802S has the characteristics of stable fertility, reproducibility, a high outcrossing rate, excellent rice quality, and strong combining ability. However, this line exhibits poor blast resistance and is highly susceptible to leaf and neck blasts. In this study, backcross introduction, molecular marker-assisted selection, gene chipping, anther culture, and resistance identification in the field were used to introduce the broad-spectrum blast-resistance gene R6 into HD9802S to improve its rice blast resistance. Six induction media were prepared by varying the content of each component in the culture medium. Murashige and Skoog's medium with 3 mg/L 2,4-dichlorophenoxyacetic acid, 2 mg/L 1-naphthaleneacetic acid, and 1 mg/L kinetin and N6 medium with 800 mg/L casein hydrolysate, 600 mg/L proline, and 500 mg/L glutamine could improve the callus induction rate and have a higher green seedling rate and a lower white seedling rate. Compared to HD9802S, two doubled haploid lines containing R6 with stable fertility showed significantly enhanced resistance to rice blast and no significant difference in spikelet number per panicle, 1000-grain weight, or grain shape. Our findings highlight a rapid and effective method for improving rice blast resistance in TGMS lines.

Development of In Vitro Anther Culture for Doubled Haploid Plant Production in Indica Rice (Oryza sativa L.) Genotypes

Anther culture is an efficient biotechnological tool in modern plant breeding programs to produce new varieties and parental lines in hybrid seed productions. However, some bottlenecks-low induction rate, genotype dependency, albinism-restrict the widespread utilization of in vitro anther culture in rice breeding, especially in Oryza sativa ssp. indica (indica) genotypes, while an improved efficient protocol can shorten the process of breeding. Three different induction media (N₆NDK_, N₆NDZ, Ali-1) and four plant regeneration media (mMSNBK1, MSNBK3, MSNBKZ1, MSNBKZ2) were tested with five indica rice genotypes to increase the efficiency of in vitro androgenesis (number of calli and regenerated green plantlets). The production of calli was more efficient on the N₆NDK medium with an average 88.26 calli/100 anthers and N₆NDZ medium with an average of 103.88 calli/100 anthers as compared to Ali-1 with an average of 6.96 calli/100 anthers. The production of green plantlets was greater when calli was produced on N₆NDK medium (2.15 green plantlets/100 anthers) compared when produced on to N₆NDZ medium (1.18 green plantlets/100 anthers). Highest green plantlets production (4.7 green plantlets/100 anthers) was achieved when mMSNBK1 plant regeneration medium was used on calli produced utilizing N₆NDK induction medium. In the best overall treatment (N₆NDK induction medium and mMSNBK1 plant regeneration medium), four tested genotypes produced green plantlets. However, the genotype influenced the efficiency, and the green plantlets production ranged from 0.4 green plantlets/100 anthers to 8.4 green plantlets/100 anthers. The ploidy level of 106 acclimatized indica rice plantlets were characterized with flow cytometric analyses to calculate the percentage of spontaneous chromosome doubling. Altogether, 48 haploid-, 55 diploid-, 2 tetraploid- and 1 mixoploid plantlets were identified among the regenerant plantlets, and the spontaneous chromosome doubling percentage was 51.89%. Utilization of DH plants have been integrated as a routine method in the Hungarian rice breeding program. The tetraploid lines can be explored for their potential to offer new scopes for rice research and breeding directions in the future.

Improvement of Anther Culture to integrate Doubled Haploid Technology in Temperate Rice (Oryza sativa L.) Breeding

Doubled haploid (DH) plant production, such as anther culture (AC), is an effective tool used in modern rice breeding programs. The improved efficient protocols applied can shorten the process of breeding. The effect of combinations of plant growth regulators (2.5 mg/L NAA, 1 mg/L 2,4-D and 0.5 mg/L kinetin; 2 mg/L 2,4-D and 0.5 mg/L BAP) in the induction medium were compared in AC for five rice breeding materials and combinations. Induction of calli ranged from 264.6 ± 67.07 to 468.8 ± 123.2 calli/100 anthers in AC of rice genotypes. Two basal media (MS and N₆) and two combinations of growth regulators (1 mg/L NAA, 1 mg/L BAP and 1 mg/L kinetin; 1.5 mg/L BAP, 0.5 mg/L NAA and 0.5 mg/L kinetin) were used as regeneration media. The in vitro green plant production was the highest with the application of the N₆NDK induction medium (NAA, 2,4-D and kinetin) and the MS-based regeneration medium (1 mg/L NAA, 1 mg/BAP and 1 mg/L kinetin) in anther culture of the '1009' genotype (95.2 green plantlets/100 anthers). The mean of five genotypes was 24.48 green plantlets/100 anthers for the best treatment. Flow cytometric analyses conducted identified the microspore origin of the haploid calli produced in AC, while the uniformity of spontaneous DH plants was checked in the DH₁ and DH₂ generations. Spontaneous chromosome doubling ranged from 38.1% to 57.9% (mean 42.1%), depending on the breeding source. The generated and selected DH lines were tested in micro- and small-plot field experiments to identify promising lines for a pedigree breeding program. The improved AC method was integrated in a Hungarian temperate rice pedigree breeding program.

The Production of Helianthus Haploids: A Review of Its Current Status and Future Prospects

The genus Helianthus comprises 52 species and 19 subspecies, with the cultivated sunflower (Helianthus annuus L.) representing one of the most important oilseed crops in the world, which is also of value for fodder and technical purposes. Currently, the leading direction in sunflower breeding is to produce highly effective heterosis F₁ hybrids with increased resistance to biotic and abiotic stresses. The production of inbred parental lines via repeated self-pollination takes 4-8 years, and the creation of a commercial hybrid can take as long as 10 years. However, the use of doubled haploid technology allows for the obtainment of inbred lines in one generation, shortening the time needed for hybrid production. Moreover, it allows for the introgression of the valuable genes present in the wild Helianthus species into cultivated sunflowers. Additionally, this technology makes it possible to manipulate the ploidy level, thereby restoring fertility in interspecific hybridization. This review systematizes and analyzes the knowledge available thus far about the production of haploid and dihaploid Helianthus plants using male (isolated anther and microspore cultures) and female (unpollinated ovaries and ovules culture) gametophytes, as well as by induced parthenogenesis using γ-irradiated pollen and interspecific hybridization. The genetic, physiological, and physical factors influencing the efficiency of haploid plant production are considered. A special section focuses on the approaches used to double a haploid chromosome set and the direct and indirect methods for determining the ploidy level. The current analyzed data on the successful application of haploid sunflower plants in breeding are summarized.

Advancing designer crops for climate resilience through an integrated genomics approach

Climate change and exponential population growth are exposing an immediate need for developing future crops that are highly resilient and adaptable to changing environments to maintain global food security in the next decade. Rigorous selection from long domestication history has rendered cultivated crops genetically disadvantaged, raising concerns in their ability to adapt to these new challenges and limiting their usefulness in breeding programmes. As a result, future crop improvement efforts must rely on integrating various genomic strategies ranging from high-throughput sequencing to machine learning, in order to exploit germplasm diversity and overcome bottlenecks created by domestication, expansive multi-dimensional phenotypes, arduous breeding processes, complex traits and big data.

Breeding an underutilized fruit crop: a long-term program for Hylocereus

This review describes three decades of introduction, agro-technology development, breeding and selection of Hylocereus species, known as pitaya or dragon fruit, as an example of a holistic program aimed to develop the horticultural potential of a perennial underutilized fruit crop. Interspecific homoploid and interploid crosses and embryo rescue procedures produced improved hybrids, some of which have been released to farmers. Molecular tools and morphological and phenological comparisons between the parental species and the resulting hybrids provided valuable information on dominant/recessive traits and on genetic relationships that could be exploited for further hybridizations. In addition, Hylocereus were crossed with species of the closely related genus Selenicereus, producing valuable intergeneric hybrids. In situ chromosome doubling resulted in the production of autopolyploid lines, from which an understanding of the effect of increased ploidy on fruit traits and metabolomic profiles was obtained. Gamete-derived lines were produced, adding to our biobank homozygote lines that were subsequently used for further hybridization. Spontaneous chromosome doubling occurred in haploid gamete-derived Hylocereus monacanthus lines and in interspecific interploid Hylocereus megalanthus × H. undatus hybrids obtained from an embryo rescue procedure, resulting in plants with double the expected ploidy. Challenging technical problems were addressed by the development of protocols for DNA isolation, flow cytometry, in situ chromosome doubling, androgenesis, gynogenesis and embryo rescue following interspecific and interploidy crosses. Current research leading to the development of genomics and molecular tools, including a draft genome of H. undatus, is also presented. Perspectives for further development of Hylocereus species and hybrids are discussed.

Tissue Culture Response of Ornamental and Medicinal Aesculus Species—A Review

Species of the genus Aesculus are very attractive woody ornamentals. Their organs contain numerous health-promoting phytochemicals. The most valuable of them—aescin—is used in commercial preparations for the treatment of venous insufficiency. The industrial source of aescin is horse chestnut seeds because the zygotic embryos are the main site of its accumulation. Horse chestnut somatic and zygotic embryos contain similar amount of aescin, hence somatic embryos could be exploited as an alternative source of aescin. Somatic embryogenesis, androgenesis and de novo shoot organogenesis were successfully achieved in several Aesculus species, as well as secondary somatic embryogenesis and shoot organogenesis, which enables mass production of embryos and shoots. In addition, an efficient method for cryopreservation of embryogenic tissue was established, assuring constant availability of the plant material. The developed methods are suitable for clonal propagation of elite specimens selected as the best aescin producers, the most attractive ornamentals or plants resistant to pests and diseases. These methods are also useful for molecular breeding purposes. Thus, in this review, the medicinal uses and a comprehensive survey of in vitro propagation methods established for Aesculus species, as well as the feasibility of in vitro production of aescin, are presented and discussed.

Small molecule inhibitors of mammalian GSK-3β promote in vitro plant cell reprogramming and somatic embryogenesis in crop and forest species

Plant in vitro regeneration systems, such as somatic embryogenesis, are essential in breeding; they permit propagation of elite genotypes, production of doubled-haploids, and regeneration of whole plants from gene editing or transformation events. However, in many crop and forest species, somatic embryogenesis is highly inefficient. We report a new strategy to improve in vitro embryogenesis using synthetic small molecule inhibitors of mammalian glycogen synthase kinase 3β (GSK-3β), never used in plants. These inhibitors increased in vitro embryo production in three different systems and species, microspore embryogenesis of Brassica napus and Hordeum vulgare, and somatic embryogenesis of Quercus suber. TDZD-8, a representative compound of the molecules tested, inhibited GSK-3 activity in microspore cultures, and increased expression of embryogenesis genes FUS3, LEC2, and AGL15. Plant GSK-3 kinase BIN2 is a master regulator of brassinosteroid (BR) signalling. During microspore embryogenesis, BR biosynthesis and signalling genes CPD, GSK-3-BIN2, BES1, and BZR1 were up-regulated and the BAS1 catabolic gene was repressed, indicating activation of the BR pathway. TDZD-8 increased expression of BR signalling elements, mimicking BR effects. The findings support that the small molecule inhibitors promoted somatic embryogenesis by activating the BR pathway, opening up the way for new strategies using GSK-3β inhibitors that could be extended to other species.

Non-homologous chromosome pairing during meiosis in haploid Brassica rapa

Cytological observations of chromosome pairing showed that evolutionarily genome duplication might reshape non-homologous pairing during meiosis in haploid B. rapa. A vast number of flowering plants have evolutionarily undergone whole genome duplication (WGD) event. Typically, Brassica rapa is currently considered as an evolutionary mesohexaploid, which has more complicated genomic constitution among flowering plants. In this study, we demonstrated chromosome behaviors in haploid B. rapa to understand how meiosis proceeds in presence of a single homolog. The findings showed that a diploid-like chromosome pairing was generally adapted during meiosis in haploid B. rapa. Non-homologous chromosomes in haploid cells paired at a high-frequency at metaphase I, over 50% of examined meiocytes showed at least three pairs of bivalents then equally segregated at anaphase I during meiosis. The fluorescence immunostaining showed that the cytoskeletal configurations were mostly well-organized during meiosis. Moreover, the expressed genes identified at meiosis in floral development was rather similar between haploid and diploid B. rapa, especially the expression of known hallmark genes pivotal to chromosome synapsis and homologous recombination were mostly in haploid B. rapa. Whole-genome duplication evolutionarily homology of genomic segments might be an important reason for this phenomenon, which would reshape the first division course of meiosis and influence pollen development in plants.

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.

Applications of Doubled Haploids in Plant Breeding and Applied Research

Manifold and diverse applications of doubled haploid (DH) plants have emerged in academy and in the plant breeding industry since the first discovery of a haploid mutant in the Jimson Weed (Datura stramonium), followed by the first reports about anther culture in the same species, maternal haploids by wide crosses in tobacco (Nicotiana tabacum L.) and barley (Hordeum vulgare L.), interspecific hybridization, ovary culture (gynogenesis), isolated microspore culture, and more recently the CENH3 approach in thale cress (Arabidopsis thaliana L.) and other species. Research and development efforts were and are still significant in both user groups. Luckily, often academic and industrial partners cooperate in challenging and sometimes voluminous projects worldwide. Not only to develop innovative DH protocols and technologies per se, but also to exploit the advantages of DH plants in a huge variety of research and development experiments. This review concentrates not on the DH technologies per se, but on the application of DHs in plant-related research and development projects.

In Vitro Anther Culture for Doubled Haploid Plant Production in Spelt Wheat

Doubled haploid (DH) plant production belongs to modern biotechnology methods of plant breeding. The main advantage of DH plant production methods is the development of genetically homozygous lines in one generation, whilst in conventional breeding programmes, the development of homozygous lines requires more generations. The present chapter describes an efficient protocol for DH plant production in spelt wheat genotypes using in vitro anther culture.

Doubled Haploids in Eggplant

Eggplant is a solanaceous crop cultivated worldwide for its edible fruit. Eggplant breeding programs are mainly aimed to the generation of F1 hybrids by crossing two highly homozygous, pure lines, which are traditionally obtained upon several self crossing generations, which is an expensive and time consuming process. Alternatively, fully homozygous, doubled haploid (DH) individuals can be induced from haploid cells of the germ line in a single generation. Several attempts have been made to develop protocols to produce eggplant DHs principally using anther culture and isolated microspore culture. Eggplant could be considered a moderately recalcitrant species in terms of ability for DH production. Anther culture stands nowadays as the most valuable technology to obtain eggplant DHs. However, the theoretical possibility of having plants regenerated from somatic tissues of the anther walls cannot be ruled out. For this reason, the use of isolated microspores is recommended when possible. This approach still has room for improvement, but it is largely genotype-dependent. In this review, we compile the most relevant advances made in DH production in eggplant, their application to breeding programs, and the future perspectives for the development of other, less genotype-dependent, DH technologies.

Identification and candidate gene screening of qCIR9.1, a novel QTL associated with anther culturability in rice (Oryza sativa L.)

A novel QTL, qCIR9.1, that controls callus induction rate in anther culture was identified on chromosome 9 in rice, and based on RNA-seq data, Os09g0551600 was the most promising candidate gene. Anther culture, a doubled haploid (DH) technique, has become an important technology in many plant-breeding programmes. Although anther culturability is the key factor in this technique, its genetic mechanisms in rice remain poorly understood. In this study, we mapped quantitative trait loci (QTLs) responsible for anther culturability by using 192 recombinant inbred lines (RILs) derived from YZX (Oryza sativa ssp. indica) × 02428 (Oryza sativa ssp. japonica) and a high-density bin map. A total of eight QTLs for anther culturability were detected in three environments. Among these QTLs, a novel major QTL for callus induction rate (CIR) named qCIR9.1 was repeatedly mapped to a ~ 100 kb genomic interval on chromosome 9 and explained 8.39-14.14% of the phenotypic variation. Additionally, RNA sequencing (RNA-seq) was performed for the parents (YZX and 02428), low- (L-Pool) and high-CIR RILs (H-Pool) after 16 and 26 days of culture. By using the RNA of the bulked RILs for background normalization, the number of differentially expressed genes (DEGs) both between the parents and between the bulked RILs after 26 days of culture was drastically reduced to only 78. Among these DEGs, only one gene, Os09g0551600, encoding a high-mobility group (HMG) protein, was located in the candidate region of qCIR9.1. qRT-PCR analysis of Os09g0551600 showed the same results as RNA-seq, and the expression of this gene was decreased in the low-callus-induction parent (YZX) and L-Pool. Our results provide a foundational step for further cloning of qCIR9.1 and will be very useful for improving anther culturability in rice.

The Resistance of Oilseed Rape Microspore-Derived Embryos to Osmotic Stress Is Associated With the Accumulation of Energy Metabolism Proteins, Redox Homeostasis, Higher Abscisic Acid, and Cytokinin Contents

The present study aims to investigate the response of rapeseed microspore-derived embryos (MDE) to osmotic stress at the proteome level. The PEG-induced osmotic stress was studied in the cotyledonary stage of MDE of two genotypes: Cadeli (D) and Viking (V), previously reported to exhibit contrasting leaf proteome responses under drought. Two-dimensional difference gel electrophoresis (2D-DIGE) revealed 156 representative protein spots that have been selected for MALDI-TOF/TOF analysis. Sixty-three proteins have been successfully identified and divided into eight functional groups. Data are available via ProteomeXchange with identifier PXD024552. Eight selected protein accumulation trends were compared with real-time quantitative PCR (RT-qPCR). Biomass accumulation in treated D was significantly higher (3-fold) than in V, which indicates D is resistant to osmotic stress. Cultivar D displayed resistance strategy by the accumulation of proteins in energy metabolism, redox homeostasis, protein destination, and signaling functional groups, high ABA, and active cytokinins (CKs) contents. In contrast, the V protein profile displayed high requirements of energy and nutrients with a significant number of stress-related proteins and cell structure changes accompanied by quick downregulation of active CKs, as well as salicylic and jasmonic acids. Genes that were suitable for gene-targeting showed significantly higher expression in treated samples and were identified as phospholipase D alpha, peroxiredoxin antioxidant, and lactoylglutathione lyase. The MDE proteome profile has been compared with the leaf proteome evaluated in our previous study. Different mechanisms to cope with osmotic stress were revealed between the genotypes studied. This proteomic study is the first step to validate MDE as a suitable model for follow-up research on the characterization of new crossings and can be used for preselection of resistant genotypes.

Improved Anther Culture Media for Enhanced Callus Formation and Plant Regeneration in Rice (Oryza sativa L.)

Anther culture technique is the most viable and efficient method of producing homozygous doubled haploid plants within a short period. However, the practical application of this technology in rice improvement is still limited by various factors that influence culture efficiency. The present study was conducted to determine the effects of two improved anther culture media, Ali-1 (A1) and Ali-2 (A2), a modified N6 medium, to enhance the callus formation and plant regeneration of japonica, indica, and hybrids of indica and japonica cross. The current study demonstrated that genotype and media had a significant impact (p < 0.001) on both callus induction frequency and green plantlet regeneration efficiency. The use of the A1 and A2 medium significantly enhanced callus induction frequency of japonica rice type, Nipponbare, and the hybrids of indica × japonica cross (CXY6, CXY24, and Y2) but not the indica rice type, NSIC Rc480. However, the A1 medium is found superior to the N6 medium as it significantly improved the green plantlet regeneration efficiency of CXY6, CXY24, and Y2 by almost 36%, 118%, and 277%, respectively. Furthermore, it substantially reduced the albino plantlet regeneration of the induced callus in two hybrids (CXY6 and Y2). Therefore, the improved anther culture medium A1 can produce doubled haploid rice plants for indica × japonica, which can be useful in different breeding programs that will enable the speedy development of rice varieties for resource-poor farmers.

Changes in plastid biogenesis leading to the formation of albino regenerants in barley microspore culture

BACKGROUND

Microspore embryogenesis is potentially the most effective method of obtaining doubled haploids (DH) which are utilized in breeding programs to accelerate production of new cultivars. However, the regeneration of albino plants significantly limits the exploitation of androgenesis for DH production in cereals. Despite many efforts, the precise mechanisms leading to development of albino regenerants have not yet been elucidated. The objective of this study was to reveal the genotype-dependent molecular differences in chloroplast differentiation that lead to the formation of green and albino regenerants in microspore culture of barley.

RESULTS

We performed a detailed analysis of plastid differentiation at successive stages of androgenesis in two barley cultivars, 'Jersey' and 'Mercada' that differed in their ability to produce green regenerants. We demonstrated the lack of transition from the NEP-dependent to PEP-dependent transcription in plastids of cv. 'Mercada' that produced mostly albino regenerants in microspore culture. The failed NEP-to-PEP transition was associated with the lack of activity of Sig2 gene encoding a sigma factor necessary for transcription of plastid rRNA genes. A very low level of 16S and 23S rRNA transcripts and impaired plastid translation machinery resulted in the inhibition of photomorphogenesis in regenerating embryos and albino regenerants. Furthermore, the plastids present in differentiating 'Mercada' embryos contained a low number of plastome copies whose replication was not always completed. Contrary to 'Mercada', cv. 'Jersey' that produced 90% green regenerants, showed the high activity of PEP polymerase, the highly increased expression of Sig2, plastid rRNAs and tRNAGlu, which indicated the NEP inhibition. The increased expression of GLKs genes encoding transcription factors required for induction of photomorphogenesis was also observed in 'Jersey' regenerants.

CONCLUSIONS

Proplastids present in microspore-derived embryos of albino-producing genotypes did not pass the early checkpoints of their development that are required for induction of further light-dependent differentiation of chloroplasts. The failed activation of plastid-encoded RNA polymerase during differentiation of embryos was associated with the genotype-dependent inability to regenerate green plants in barley microspore culture. The better understanding of molecular mechanisms underlying formation of albino regenerants may be helpful in overcoming the problem of albinism in cereal androgenesis.

Doubled Haploid Production in Watermelon

Doubled haploid (DH) technology is very advantageous in plant breeding. This technique is beneficial for reducing the time required to obtain pure lines and contributes to the selection efficiency. Using this technique, 100% homozygosity can be achieved in a single generation, while the development of stable lines using the traditional self-pollination method takes from 6 to 8 years. It has long been used in diverse crops including cucurbits. DHs can be obtained via parthenogenesis (pollination mostly with irradiated pollen), gynogenesis (in vitro culture of ovules and ovaries), and androgenesis (in vitro culture of microspores and anthers). All these methods have been used for over 30 years to develop haploid and DH lines in cucurbit crops. Nowadays, many researchers benefit from these techniques routinely. However, there are still many limits for using DH technology in watermelon breeding programmes. The number of developed DH lines is still very low.In this chapter, we present a protocol based on the different studies on haploids and DHs induced in watermelon through irradiated pollen technique, unfertilized ovule/ovary culture and anther/microspore culture. According to the results of all these studies, it is crucial to develop an efficient protocol for haploid embryo induction to enhance the frequency of obtaining haploid embryos in watermelon.

Doubled Haploid Parthenogenetic Production of Melon 'Piel de Sapo'

Parthenogenesis is the main technique to produce doubled haploid lines in melon. Although parthenogenesis is a genotype-dependent process and melon has a huge genotypic diversity, we developed a successful protocol for haploid embryo production via pollination with irradiated pollen and a protocol for chromosome doubling of haploid plants of 'Piel de Sapo' genotypes. 'Piel de sapo' genotype has lower efficiencies during the process in comparison with other genotypes, for instance, of the agrestis subspecies. Nevertheless, the doubled haploid lines produced have a great potential as pure parentals for hybrid seed production.

Production of Haploid and Doubled Haploid Lines in Nut Crops: Persian Walnut, Almond, and Hazelnut

This chapter deals with induction of haploidy via parthenogenesis in Persian walnut and via microspore embryogenesis in almond and hazelnut. Haploid induction through in situ parthenogenesis using pollination with irradiated pollen to stimulate the embryogenic development of the egg cell, followed by in vitro culture of the immature haploid embryos. Microspore embryogenesis allows the induction of immature pollen grains (microspores), to move away from the normal gametophytic developmental route in the direction of the sporophytic one, yielding homozygous organisms (embryos in this case). Unlike other fruit crops (such as Citrus), regeneration of entire plants has not yet been obtained in our studied nut crops; however, it gives the methodology should be used to continue the roadmap.

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.

The commitment of barley microspores into embryogenesis correlates with miRNA-directed regulation of members of the SPL, GRF and HD-ZIPIII transcription factor families

Microspore embryogenesis is a model for developmental plasticity and cell fate decisions. To investigate the role of miRNAs in this development, we sequenced sRNAs and the degradome of barley microspores collected prior to (day 0) and after (days 2 and 5) the application of a stress treatment known to induce embryogenesis. Microspores isolated at these timepoints were uniform in both appearance and in their complements of sRNAs. We detected 68 miRNAs in microspores. The abundance of 51 of these miRNAs differed significantly during microspore development. One group of miRNAs was induced when the stress treatment was applied, prior to being repressed when microspores transitioned to embryogenesis. Another group of miRNAs were up-regulated in day-2 microspores and their abundance remained stable or increased in day-5 microspores, a timepoint at which the first clear indications of the transition toward embryogenesis were visible. Collectively, these miRNAs might play a role in the modulation of the stress response, the repression of gametic development, and/or the gain of embryogenic potential. A degradome analysis allowed us to validate the role of miRNAs in regulating 41 specific transcripts. We showed that the transition of microspores toward the embryogenesis pathway involves miRNA-directed regulation of members of the ARF, SPL, GRF, and HD-ZIPIII transcription factor families. We noted that 41.5% of these targets were shared between day-2 and day-5 microspores while 26.8% were unique to day-5 microspores. The former set may act to disrupt transcripts involved in pollen development while the latter set may drive the commitment to embryogenesis.

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.

Quantitative Trait Loci (QTLs) Associated with Microspore Culture in Raphanus sativus L. (Radish)

The radish is a highly self-incompatible plant, and consequently it is difficult to produce homozygous lines. Bud pollination in cross-fertilization plants should be done by opening immature pollen and attaching pollen to mature flowers. It accordingly takes a lot of time and effort to develop lines with fixed alleles. In the current study, a haploid breeding method has been applied to obtain homozygous plants in a short period of time by doubling chromosomes through the induction of a plant body in the haploid cells, in order to shorten the time to breed inbred lines. We constructed genetic maps with an F1 population derived by crossing parents that show a superior and inferior ability to regenerate microspores, respectively. Genetic maps were constructed from the maternal and parental maps, separately, using the two-way pseudo-testcross model. The phenotype of the regeneration rate was examined by microspore cultures and a quantitative trait loci (QTL) analysis was performed based on the regeneration rate. From the results of the culture of microspores in the F1 population, more than half of the group did not regenerate, and only a few showed a high regeneration rate. A total of five significant QTLs were detected in the F1 population, and five candidate genes were found based on the results. These candidate genes are divided into two classes, and appear to be related to either PRC2 subunits or auxin synthesis.

Plastid differentiation during microgametogenesis determines green plant regeneration in barley microspore culture

Developing plants from in vitro culture of microspores or immature pollen grains (androgenesis) is a highly genotype-dependent process whose effectiveness in cereals is significantly reduced by occurrence of albino regenerants. Here, we examined a hypothesis that the molecular differentiation of plastids in barley microspores prior to in vitro culture affects the genotype ability to regenerate green plants in culture. At the mid-to-late uninucleate (ML) stage, routinely used to initiate microspore culture, the expression of most genes involved in plastid transcription, translation and starch synthesis was significantly higher in microspores of barley cv. 'Mercada' producing 90% albino regenerants, than in cv. 'Jersey' that developed 90% green regenerants. The ML microspores of cv. 'Mercada' contained a large proportion of amyloplasts filled with starch, while in cv. 'Jersey' there were only proplastids. Using additional spring barley genotypes that differed in their ability to regenerate green plants we confirmed the correlation between plastid differentiation prior to culture and albino regeneration in culture. The expression of GBSSI gene (Granule-bound starch synthaseI) in early-mid (EM) microspores was a good marker of a genotype potential to produce green regenerants during androgenesis. Initiating culture from EM microspores that significantly improved regeneration of green plants may overcome the problem of albinism.

Clonal Reproduction through Seeds in Sight for Crops

Apomixis or asexual reproduction through seeds, enables the preservation of hybrid vigor. Hybrids are heterozygous and segregate for genotype and phenotype upon sexual reproduction. While apomixis, that is, clonal reproduction, is intuitively antithetical to diversity, it is rarely obligate and actually provides a mechanism to recover and maintain superior hybrid gene combinations for which sexual reproduction would reveal deleterious alleles in less fit genotypes. Apomixis, widespread across flowering plant orders, does not occur in major crop species, yet its introduction could add a valuable tool to the breeder's toolbox. In the past decade, discovery of genetic mechanisms regulating meiosis, embryo and endosperm development have facilitated proof-of-concept for the synthesis of apomixis, bringing apomictic crops closer to reality.

In situ Parthenogenetic Doubled Haploid Production in Melon "Piel de Sapo" for Breeding Purposes

Doubled haploids in cucurbit species are produced through in situ parthenogenesis via pollination with irradiated pollen for further use as parental lines for hybrid F1 production. In this study, seven genotypes of melon "Piel de Sapo" were appraised for agronomic traits and pathogen resistances to evaluate its commercial value and used as donor plant material for the parthenogenetic process. Then, in situ parthenogenetic capacity of melon "Piel de Sapo" germplasm was evaluated and optimized. Several steps of the parthenogenetic process were assessed in this study such as melon fruit set after pollination with irradiated pollen, haploid embryo obtention, in vitro germination and growth of parthenogenetic embryos and plantlets, in vitro and in vivo chromosome doubling with colchicine or oryzalin and fruit set of doubled haploid lines. Parthenogenetic efficiencies of "Piel de Sapo" genotypes showed a high genotypic dependency during the whole process. Three different methods were assayed for parthenogenetic embryo detection: one-by-one, X-ray and liquid medium. X-ray radiography of seeds was four times faster than one-by-one method and jeopardized eight times less parthenogenetic embryo obtention than liquid medium. One third of melon fruits set after pollination with irradiated pollen contained at least one parthenogenetic embryo. The 50.94% of the embryos rescued did not develop into plantlets because failed to germinate or plantlet died at the first stages of development because of deleterious gene combination in haploid homozygosity. The distribution of the ploidy-level of the 26 parthenogenetic plantlets obtained was: 73.08% haploid, 23.08% spontaneous doubled haploid and 3.84% mixoploid. Two in vitro chromosome doubling methods, with colchicine or oryzalin, were compared with a third in vivo colchicine method. In vivo immersion of apical meristems in a colchicine solution for 2 h showed the highest results of plant survival, 57.33%, and chromosome doubling, 9.30% mixoploids and 20.93% doubled haploids. Fruit set and seed recovery of doubled haploids lines was achieved. In this study, doubled haploid lines were produced from seven donor genotypes of melon "Piel de Sapo," however, further improvements are need in order to increase the parthenogenetic efficiency.

Utilization of in Vitro Anther Culture in Spelt Wheat Breeding

The efficiency of in vitro anther culture was screened in a full diallel population of four spelt wheat genotypes and ten F1 hybrids. Genotype dependency was observed based on the data of embryo-like structures (ELS), green-, albino plantlets. In the diallel population and ten F1 hybrids, the green plantlets production ranged from 13.75 to 85.00 and from 6.30 to 51.00, respectively. The anther culture-derived plants of F1 hybrids were grown up in the nursery. At the harvest, 436 spontaneous doubled haploid (DH) plants were identified among the 1535 anther culture-derived transplanted and grown up individual plants. The mean of spontaneous rediploidization was 28.4% which ranged from 9.76% to 54.24%. In two consecutive years, the agronomic values of 'Tonkoly.pop1' advanced line were compared with seven DH lines of 'Tonkoly.pop1' in the nursery. The DH lines achieved competitive values in comparison with 'Tonkoly.pop1' advanced line based on the 11 measured parameters (heading date, plant height, yield, hardness, width and length of seed, TKW, hulling yield, flour yield, protein and wet gluten content). These observations presage the efficient utilization of anther culture in spelt wheat breeding.

Application of genetics and biotechnology for improving medicinal plants

Plant tissue culture has been used for conservation, micropropagation, and in planta overproduction of some pharma molecules of medicinal plants. New biotechnology-based breeding methods such as targeted genome editing methods are able to create custom-designed medicinal plants with different secondary metabolite profiles. For a long time, humans have used medicinal plants for therapeutic purposes and in food and other industries. Classical biotechnology techniques have been exploited in breeding medicinal plants. Now, it is time to apply faster biotechnology-based breeding methods (BBBMs) to these valuable plants. Assessment of the genetic diversity, conservation, proliferation, and overproduction are the main ways by which genetics and biotechnology can help to improve medicinal plants faster. Plant tissue culture (PTC) plays an important role as a platform to apply other BBBMs in medicinal plants. Agrobacterium-mediated gene transformation and artificial polyploidy induction are the main BBBMs that are directly dependent on PTC. Manageable regulation of endogens and/or transferred genes via engineered zinc-finger proteins or transcription activator-like effectors can help targeted manipulation of secondary metabolite pathways in medicinal plants. The next-generation sequencing techniques have great potential to study the genetic diversity of medicinal plants through restriction-site-associated DNA sequencing (RAD-seq) technique and also to identify the genes and enzymes that are involved in the biosynthetic pathway of secondary metabolites through precise transcriptome profiling (RNA-seq). The sequence-specific nucleases of transcription activator-like effector nucleases (TALENs), zinc-finger nucleases, and clustered regularly interspaced short palindromic repeats-associated (Cas) are the genome editing methods that can produce user-designed medicinal plants. These current targeted genome editing methods are able to manage plant synthetic biology and open new gates to medicinal plants to be introduced into appropriate industries.

The evolving EU regulatory framework for precision breeding

Plant breeding has always relied on progress in various scientific disciplines to generate and enable access to genetic variation. Until the 1970s, available techniques generated mostly random genetic alterations that were subject to a selection procedure in the plant material. Recombinant nucleic acid technology, however, started a new era of targeted genetic alterations, or precision breeding, enabling a much more targeted approach to trait management. More recently, developments in genome editing are now providing yet more control by enabling alterations at exact locations in the genome. The potential of recombinant nucleic acid technology fueled discussions about potentially new associated risks and, starting in the late 1980s, biosafety legislation for genetically modified organisms (GMOs) has developed in the European Union. However, the last decade has witnessed a lot of discussions as to whether or not genome editing and other precision breeding techniques should be encompassed by the EU GMO legislation. A recent ruling from the Court of Justice of the European Union indicated that directed mutagenesis techniques should be subject to the provisions of the GMO Directive, essentially putting many precision breeding techniques in the same regulatory basket. This review outlines the evolving EU regulatory framework for GMOs and discusses some potential routes that the EU may take for the regulation of precision breeding.

Identifying and Engineering Genes for Parthenogenesis in Plants

Parthenogenesis is the spontaneous development of an embryo from an unfertilized egg cell. It naturally occurs in a variety of plant and animal species. In plants, parthenogenesis usually is found in combination with apomeiosis (the omission of meiosis) and pseudogamous or autonomous (with or without central cell fertilization) endosperm formation, together known as apomixis (clonal seed production). The initiation of embryogenesis in vivo and in vitro has high potential in plant breeding methods, particularly for the instant production of homozygous lines from haploid gametes [doubled haploids (DHs)], the maintenance of vigorous F1-hybrids through clonal seed production after combining it with apomeiosis, reverse breeding approaches, and for linking diploid and polyploid gene pools. Because of this large interest, efforts to identify gene(s) for parthenogenesis from natural apomicts have been undertaken by using map-based cloning strategies and comparative gene expression studies. In addition, engineering parthenogenesis in sexual model species has been investigated via mutagenesis and gain-of-function strategies. These efforts have started to pay off, particularly by the isolation of the PsASGR-BabyBoom-Like from apomictic Pennisetum, a gene proven to be transferable to and functional in sexual pearl millet, rice, and maize. This review aims to summarize the current knowledge on parthenogenesis, the possible gene candidates also outside the grasses, and the use of these genes in plant breeding protocols. It shows that parthenogenesis is able to inherit and function independently from apomeiosis and endosperm formation, is expressed and active in the egg cell, and can induce embryogenesis in polyploid, diploid as well as haploid egg cells in plants. It also shows the importance of genes involved in the suppression of transcription and modifications thereof at one hand, and in embryogenesis for which transcription is allowed or artificially overexpressed on the other, in parthenogenetic reproduction. Finally, it emphasizes the importance of functional endosperm to allow for successful embryo growth and viable seed production.

Regeneration of highland papaya (Vasconcellea pubescens) from anther culture

PREMISE OF THE STUDY

Vasconcellea pubescens is an important Caricaceae species cultivated in several countries of South America. The objective of this study was to investigate different media compositions and plant growth regulators to induce plant regeneration.

METHODS

Anthers were cultured in Murashige and Skoog medium with varying concentrations of naphthalene acetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) plus a cytokinin (N-(2-chloro-4-pyridyl)-N'-phenylurea). The effect of the basal medium supplemented with auxins and cytokinins on shoot regeneration from the induced calli was also evaluated. Addition of maltose to the basal medium was also tested.

RESULTS

The combination of 0.54 μM NAA and 22.66 μM 2,4-D induced the highest rate of calli formation. Regeneration via organogenesis was obtained in Murashige and Skoog and Woody Plant Medium supplemented with maltose and containing 8.88 μM 6-benzylaminopurine, 5.71 μM indoleacetic acid, and 2.28 μM zeatin.

DISCUSSION

The plant regeneration protocol reported here permits the development of haploid and double haploid plants that can be useful for propagation purposes, allow a better molecular understanding of the species, and facilitate the production of new cultivars.

Differential Expression Profiling of Microspores During the Early Stages of Isolated Microspore Culture Using the Responsive Barley Cultivar Gobernadora

In barley, it is possible to induce embryogenesis in the haploid and uninucleate microspore to obtain a diploid plant that is perfectly homozygous. To change developmental fates in this fashion, microspores need to engage in cellular de-differentiation, interrupting the pollen formation, and restore totipotency prior to engaging in embryogenesis. In this work, we used the barley cultivar Gobernadora to characterize the transcriptome of microspores prior to (day 0) and immediately after (days 2 and 5) the application of a stress pretreatment. A deep RNA-seq analysis revealed that microspores at these three time points exhibit a transcriptome of ∼14k genes, ∼90% of which were shared. An expression analysis identified a total of 3,382 differentially expressed genes (DEGs); of these, 2,155 and 2,281 DEGs were respectively identified when contrasting expression at days 0 and 2 and at days 2 and 5. These define 8 expression profiles in which DEGs share a common up- or down-regulation at these time points. Up-regulation of numerous glutathione S-transferase and heat shock protein genes as well as down-regulation of ribosomal subunit protein genes was observed between days 0 and 2. The transition from microspores to developing embryos (days 2 vs. 5) was marked by the induction of transcription factor genes known to play important roles in early embryogenesis, numerous genes involved in hormone biosynthesis and plant hormonal signal transduction in addition to genes involved in secondary metabolism. This work sheds light on transcriptional changes accompanying an important developmental shift and provides candidate biomarkers for embryogenesis in barley.

Rapid Generation of Barley Mutant Lines With High Nitrogen Uptake Efficiency by Microspore Mutagenesis and Field Screening

In vitro mutagenesis via isolated microspore culture provides an efficient way to produce numerous double haploid (DH) lines with mutation introduction and homozygosity stabilization, which can be used for screening directly. In this study, 356 DH lines were produced from the malt barley (Hordeum vulgare L.) cultivar Hua-30 via microspore mutagenic treatment with ethyl methane sulfonate or pingyangmycin during in vitro culture. The lines were subjected to field screening under high nitrogen (HN) and low nitrogen (LN) conditions, and the number of productive tillers was used as the main screening index. Five mutant lines (A1-28, A1-84, A1-226, A16-11, and A9-29) with high numbers of productive tillers were obtained over three consecutive years of screening. In the fifth year, components related to N-use efficiency (NUE), including N accumulation, utilization, and translocation, were characterized for these lines based on N uptake efficiency (NUpE), N utilization efficiency (NUtE), and N translocation efficiency (NTE). The results show that the NUpE of four mutant lines (A1-84, A1-226, A9-29, and A16-11) improved significantly under HN, whereas that of two lines (A1-84 and A9-29) improved under LN. As a result, their NUE improved greatly. No improvement in NUtE was observed in any of the five mutant lines. A1-84 and A9-29 were selected as an enhanced genotype in N uptake, and A1-28 showed improved NTE at the grain-filling stage. Our results imply that high-NUpE mutants can be produced through microspore mutagenesis and field screening, and that improvement of NUE in barley depends on enhancement of N uptake.

Micropropagation in the Twenty-First Century

Despite more than a century of research on effective biotechnological methods, micropropagation continues to be an important tool for the large-scale production of clonal plantlets of several important plant species that retain genetic fidelity and are pest-free. In some cases, micropropagation is the only technique that supports the maintenance and promotes the economic value of specific agricultural species. The micropropagation of plants solved many phytosanitary problems and allowed both the expansion and access to high-quality plants for growers from different countries and economic backgrounds, thereby effectively contributing to an agricultural expansion in this and the last century. The challenges for micropropagation in the twenty-first century include cost reduction, enhanced efficiency, developing new technologies, and combining micropropagation with other systems/propagation techniques such as microcuttings, hydroponics, and aeroponics. In this chapter, we discuss the actual uses of micropropagation in this century, its importance and limitations, and some possible techniques that can effectively increase its wider application by replacing certain conventional techniques and technologies.

Genetic variation of salt-stressed durum wheat (Triticum turgidum subsp. durum Desf.) genotypes under field conditions and gynogenetic capacity

Agriculture has new challenges against the climate change: the preservation of genetic resources and the rapid creation of new varieties better adapted to abiotic stress, specially salinity. In this context, the agronomic performance of 25 durum wheat (Triticum turgidum subsp. durum Desf.) genotypes (nineteen landraces and six improved varieties), cultivated in two semi-arid regions in the center area of Tunisia, were assessed. These sites (Echbika, 2.2 g l^-1; Barrouta, 4.2 g l^-1) differ by their degree of salinity of the water irrigation. The results showed that most of the agronomic traits (e.g. spike per meter square, thousand kernels weight and grain yield) were reduced by salinity. Durum wheat landraces, Mahmoudi and Hmira, and improved varieties, Maali and Om Rabia showed the widest adaptability to different quality of irrigation water. Genotypes including Jneh Kotifa and Arbi were estimated as stable genotypes under adverse conditions. Thereafter, salt-tolerant (Hmira and Jneh Khotifa) and the most cultivated high-yielding (Karim, Razzak and Khiar) genotypes were tested for their gynogenetic ability to obtain haploids and doubled haploid lines. Genotypes with good induction capacity had not necessarily a good capacity of regeneration of haploid plantlets. In our conditions, Hmira and Khiar exhibited the best gynogenetic ability (3.1% and 2.9% of haploid plantlets, respectively).

Novel technologies in doubled haploid line development

haploid inducer line can be transferred (DH) technology can not only shorten the breeding process but also increase genetic gain. Haploid induction and subsequent genome doubling are the two main steps required for DH technology. Haploids have been generated through the culture of immature male and female gametophytes, and through inter- and intraspecific via chromosome elimination. Here, we focus on haploidization via chromosome elimination, especially the recent advances in centromere-mediated haploidization. Once haploids have been induced, genome doubling is needed to produce DH lines. This study has proposed a new strategy to improve haploid genome doubling by combing haploids and minichromosome technology. With the progress in haploid induction and genome doubling methods, DH technology can facilitate reverse breeding, cytoplasmic male sterile (CMS) line production, gene stacking and a variety of other genetic analysis.

Sequencing of Single Pollen Nuclei Reveals Meiotic Recombination Events at Megabase Resolution and Circumvents Segregation Distortion Caused by Postmeiotic Processes

Meiotic recombination is a fundamental mechanism to generate novel allelic combinations which can be harnessed by breeders to achieve crop improvement. The recombination landscape of many crop species, including the major crop barley, is characterized by a dearth of recombination in 65% of the genome. In addition, segregation distortion caused by selection on genetically linked loci is a frequent and undesirable phenomenon in double haploid populations which hampers genetic mapping and breeding. Here, we present an approach to directly investigate recombination at the DNA sequence level by combining flow-sorting of haploid pollen nuclei of barley with single-cell genome sequencing. We confirm the skewed distribution of recombination events toward distal chromosomal regions at megabase resolution and show that segregation distortion is almost absent if directly measured in pollen. Furthermore, we show a bimodal distribution of inter-crossover distances, which supports the existence of two classes of crossovers which are sensitive or less sensitive to physical interference. We conclude that single pollen nuclei sequencing is an approach capable of revealing recombination patterns in the absence of segregation distortion.

Androgenesis, gynogenesis, and parthenogenesis haploids in cucurbit species

Haploids and doubled haploids are critical components of plant breeding. This review is focused on studies on haploids and double haploids inducted in cucurbits through in vitro pollination with irradiated pollen, unfertilized ovule/ovary culture, and anther/microspore culture during the last 30 years, as well as comprehensive analysis of the main factors of each process and comparison between chromosome doubling and ploidy identification methods, with special focus on the application of double haploids in plant breeding and genetics. This review identifies existing problems affecting the efficiency of androgenesis, gynogenesis, and parthenogenesis in cucurbit species. Donor plant genotypes and surrounding environments, developmental stages of explants, culture media, stress factors, and chromosome doubling and ploidy identification are compared at length and discussed as methodologies and protocols for androgenesis, gynogenesis, and parthenogenesis in haploid and double haploid production technologies.

A Comparison of In Vitro and In Vivo Asexual Embryogenesis

In plants, embryogenesis generally occurs through the sexual process of double fertilization, which involves a haploid sperm cell fusing with a haploid egg cell to ultimately give rise to a diploid embryo. Embryogenesis can also occur asexually in the absence of fertilization, both in vitro and in vivo. Somatic or gametic cells are able to differentiate into embryos in vitro following the application of plant growth regulators or stress treatments. Asexual embryogenesis also occurs naturally in some plant species in vivo, from either ovule cells as part of a process defined as apomixis, or from somatic leaf tissue in other species. In both in vitro and in vivo asexual embryogenesis, the embryo precursor cells must attain an embryogenic fate without the act of fertilization. This review compares the processes of in vitro and in vivo asexual embryogenesis including what is known regarding the genetic and epigenetic regulation of each process, and considers how the precursor cells are able to change fate and adopt an embryogenic pathway.

Induced parthenogenesis by gamma-irradiated pollen in loquat for haploid production

Successful haploid induction in loquat (Eriobotrya japonica (Thunb.) Lindl.) through in situ-induced parthenogenesis with gamma-ray irradiated pollen has been achieved. Female flowers of cultivar 'Algerie' were pollinated using pollen of cultivars 'Changhong-3', 'Cox' and 'Saval Brasil' irradiated with two doses of gamma rays, 150 and 300 Gy. The fruits were harvested 90, 105 and 120 days after pollination (dap). Four haploid plants were obtained from 'Algerie' pollinated with 300-Gy-treated pollen of 'Saval Brasil' from fruits harvested 105 dap. Haploidy was confirmed by flow cytometry and chromosome count. The haploids showed a very weak development compared to the diploid plants. This result suggests that irradiated pollen can be used to obtain parthenogenetic haploids.

Extent and overlap of segregation distortion regions in 12 barley crosses determined via a Pool-GBS approach

Extent and overlap of segregation distortion regions in 12 barley crosses determined via a Pool-GBS approach. Segregation distortion is undesirable as it alters the frequency of alleles and can reduce the chances of obtaining a particular combination of alleles. In this work, we have used a pooled genotyping-by-sequencing (Pool-GBS) approach to estimate allelic frequencies and used it to examine segregation distortion in 12 segregating populations of barley derived from androgenesis. Thanks to the extensive genome-wide SNP coverage achieved (between 674 and 1744 markers), we determined that the proportion of distorted markers averaged 28.9 % while 25.3 % of the genetic map fell within segregation distortion regions (SDRs). These SDRs were characterized and identified based on the position of the marker showing the largest distortion and the span of each SDR. Summed across all 12 crosses, 36 different SDR peaks could be distinguished from a total of 50 SDRs and a majority of these SDRs (27 of 36) were observed in only one population. While most shared SDRs were common to only two crosses, two SDRs (SDR3.1 and SDR4.2) were exceptionally recurrent (seen in five and four crosses, respectively). Because of the broad span of most SDRs, an average of 30 % of crosses showed segregation distortion in any given chromosomal segment. In reciprocal crosses, although some SDRs were clearly shared, others were unique to a single direction. In summary, segregation distortion is highly variable in its extent and the number of loci underpinning these distortions seems to be quite large even in a narrow germplasm such as six-row spring barley.

Analysis of wheat microspore embryogenesis induction by transcriptome and small RNA sequencing using the highly responsive cultivar "Svilena"

BACKGROUND

Microspore embryogenesis describes a stress-induced reprogramming of immature male plant gametophytes to develop into embryo-like structures, which can be regenerated into doubled haploid plants after whole genome reduplication. This mechanism is of high interest for both research as well as plant breeding. The objective of this study was to characterize transcriptional changes and regulatory relationships in early stages of cold stress-induced wheat microspore embryogenesis by transcriptome and small RNA sequencing using a highly responsive cultivar.

RESULTS

Transcriptome and small RNA sequencing was performed in a staged time-course to analyze wheat microspore embryogenesis induction. The analyzed stages were freshly harvested, untreated uninucleate microspores and the two following stages from in vitro anther culture: directly after induction by cold-stress treatment and microspores undergoing the first nuclear divisions. A de novo transcriptome assembly resulted in 29,388 contigs distributing to 20,224 putative transcripts of which 9,305 are not covered by public wheat cDNAs. Differentially expressed transcripts and small RNAs were identified for the stage transitions highlighting various processes as well as specific genes to be involved in microspore embryogenesis induction.

CONCLUSION

This study establishes a comprehensive functional genomics resource for wheat microspore embryogenesis induction and initial understanding of molecular mechanisms involved. A large set of putative transcripts presumably specific for microspore embryogenesis induction as well as contributing processes and specific genes were identified. The results allow for a first insight in regulatory roles of small RNAs in the reprogramming of microspores towards an embryogenic cell fate.