Enhancement of androgenesis by abiotic stress and other pretreatments in major crop species

Induction of Triticale (×Triticosecale Wittmack) In Vitro Androgenesis in Anther Cultures of F1 Hybrid Combinations, Varieties and Homogeneity Testing of Offspring Generation

In cereal breeding, in vitro androgenesis methods are frequently applied to achieve doubled haploid (DH) plants. The aim of this study was to determine the effects of genotype (three registered varieties and eight F1 crossing combinations) and induction medium (W14mf and P4mf) on anther cultures (ACs) of triticale (×Triticosecale Wittmack). Androgenesis was induced in the treatment of each tested genotype, and the genotype significantly influenced the efficiency of AC, including in embryo-like structures (ELSs), albinos, green plantlets, and transplanted plantlets. The utilized medium also had a significant effect on the number of ELSs, albinos, and transplanted plantlets. Both media were suitable for AC in triticale DH plant production. The efficiency of AC was higher when using the P4mf medium (103.7 ELS/100 anthers, 19.7 green plantlets/100 anthers) than when using the W14mf medium (90.0 ELS/100 anthers, 17.0 green plantlets/100 anthers). However, the green plantlet regeneration efficiency of microspore-derived structures was 18.0% when using the W14mf medium, while this value was 15.9% in the case of ELSs induced with the P4mf medium. After nursery seed evaluation and propagation (DH1), the genetic homogeneity of the offspring generation (DH2) was tested using a molecular genetic method. Most of the tested DH lines showed homogeneity and were progressed into a breeding program after agronomic selection. Some DH lines showed inhomogeneity, which could be explained by the outcross inclination of triticale. We would like to call breeders' attention to the outcross character of triticale and emphasize the vigilant propagation and maintenance of the triticale DH lines in breeding programs. Due to the outcross nature of triticale, even in self-pollinated genotypes, breeders should focus on careful maintenance, along with isolation in the case of line propagations, in triticale breeding programs.

Phospholipase C is a novel regulator at the early stages of microspore embryogenesis in Nicotiana tabacum

Microspore transfers the developmental fate into embryogenesis in vitro regulated by determinant factors of stress-induced. However, the key regulators of microspore embryogenesis (ME) are still largely undiscovered to reveal the mechanism of cell fate transition. Here, we report that Phospholipase C (PLC) is involved at the early stages of ME in Nicotiana tabacum. NtPLC2/3/4 are expressed at the initial stages of ME. The expression levels of NtPLC2/3 are transient activated after 3 days in culture, while the expression level of NtPLC4 maintains relatively stable. Inhibition of PLCs induces the decrease in NtPLC2/3/4 expression level and decline of ME yield. We confirm that lipids in microspore are degraded and then re-accumulate at first embryonic division stage. Inhibition of PLCs suppresses the lipids metabolism at the early stages of ME. Thus, we propose that PLCs-mediated lipid metabolism is a novel regulator at the early stages of ME.

MYB-6 and LDOX-1 regulated accretion of anthocyanin response to cold stress in purple black carrot (Daucus carota L.)

AIM

Anthocyanin, an essential ingredient of functional foods, is present in a wide range of plants, including black carrots. The current investigation was carried out to analyse the effect of cold stress on the expression of major anthocyanins and anthocyanin biosynthetic pathway genes, MYB6 and LDOX-1.

METHODS AND RESULTS

Five cultivated carrot genotypes belonging to the eastern group, having anthocyanin pigment, were used in the current study. The qRT-PCR analysis revealed that relative gene expression of transcription factor MYB-6 and LDOX1gene was highly expressed upon cold stress compared to non-stress samples. High-performance liquid chromatography-based quantification of Cyanidin 3-O-glucoside (Kuromanin chloride), Ferulic acid, 3,5-Dimethoxy-4-hydroxycinnamic acid (Sinapic acid), and Rutin revealed a significant increase in these major anthocyanins in response to cold stress when compared to control plants.

CONCLUSION

We conclude that MYB6 and LDOX1 gene expression increases upon cold stress, which induces accumulation of major anthocyanins in purple black carrot and suggests a possible cross-link between cold stress and anthocyanin biosynthesis in purple black carrot.

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.

Comparative proteomic analysis provides new insights into regulation of microspore embryogenesis induction in winter triticale (× Triticosecale Wittm.) after 5-azacytidine treatment

Effective microspore embryogenesis (ME) requires substantial modifications in gene expression pattern, followed by changes in the cell proteome and its metabolism. Recent studies have awakened also interest in the role of epigenetic factors in microspore de-differentiation and reprogramming. Therefore, demethylating agent (2.5-10 μM 5-azacytidine, AC) together with low temperature (3 weeks at 4 °C) were used as ME-inducing tiller treatment in two doubled haploid (DH) lines of triticale and its effect was analyzed in respect of anther protein profiles, expression of selected genes (TAPETUM DETERMINANT1 (TaTPD1-like), SOMATIC EMBRYOGENESIS RECEPTOR KINASE 2 (SERK2) and GLUTATHIONE S-TRANSFERASE (GSTF2)) and ME efficiency. Tiller treatment with 5.0 µM AC was the most effective in ME induction; it was associated with (1) suppression of intensive anabolic processes-mainly photosynthesis and light-dependent reactions, (2) transition to effective catabolism and mobilization of carbohydrate reserve to meet the high energy demand of cells during microspore reprograming and (3) effective defense against stress-inducing treatment, i.e. protection of proper folding during protein biosynthesis and effective degradation of dysfunctional or damaged proteins. Additionally, 5.0 µM AC enhanced the expression of all genes previously identified as being associated with embryogenic potential of microspores (TaTPD1-like, SERK and GSTF2).

Effect of silver nitrate and growth regulators to enhance anther culture response in wheat (Triticum aestivum L.)

Application of chemical substances as stress pre-treatment factors may positively influence androgenetic responses in cereal and other crops. AgNO₃ is an anti-ethylene compounds that played a significant role in combination with other chemicals for anther culture responses in cereal and other crop plants. For this study two local wheat cultivars viz. Kheri and Akbar were considered to evaluate the effect of AgNO₃ and to optimize the suitable doses of plant growth regulators, amino acids and sucrose that supplemented in MS medium. Data were recorded on the basis of embryoids induction, regenerated green and albino plants. The results clearly stated that anther culture responses and its major outcomes on regeneration significantly increased with suitable dosages of chemicals. The most noteworthy increases embryo like structures and regenerated green plants accomplished by utilizing the combined effect of AgNO₃ (50 mg/l) and as plant growth regulators IAA (1.0 mg/l) + kinetin (0.5 mg/l). Best embryo like structures (79.17%) and green plants (33.33%) were recorded in Kheri. The results clearly stated that reducing albinism and increasing embryos induction and green plants 50–75 mg/l silver nitrate along with optimum doses of IAA and kinetin showed very effective results in wheat.

Maize transformation: history, progress, and perspectives

Maize functional genomics research and genetic improvement strategies have been greatly accelerated and refined through the development and utilization of genetic transformation systems. Maize transformation is a composite technology based on decades' efforts in optimizing multiple factors involving microbiology and physical/biochemical DNA delivery, as well as cellular and molecular biology. This review provides a historical reflection on the development of maize transformation technology including the early failures and successful milestones. It also provides a current perspective on the understanding of tissue culture responses and their impact on plant regeneration, the pros and cons of different DNA delivery methods, the identification of a palette of selectable/screenable markers, and most recently the development of growth-stimulating or morphogenic genes to improve efficiencies and extend the range of transformable genotypes. Steady research progress in these interdependent components has been punctuated by benchmark reports celebrating the progress in maize transformation, which invariably relied on a large volume of supporting research that contributed to each step and to the current state of the art. The recent explosive use of CRISPR/Cas9-mediated genome editing has heightened the demand for higher transformation efficiencies, especially for important inbreds, to support increasingly sophisticated and complicated genomic modifications, in a manner that is widely accessible. These trends place an urgent demand on taking maize transformation to the next level, presaging a new generation of improvements on the horizon. Once realized, we anticipate a near-future where readily accessible, genotype-independent maize transformation, together with advanced genomics, genome editing, and accelerated breeding, will contribute to world agriculture and global food security.

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.

β-1,3-Glucanases and chitinases participate in the stress-related defence mechanisms that are possibly connected with modulation of arabinogalactan proteins (AGP) required for the androgenesis initiation in rye (Secale cereale L.)

This work presents the biochemical, cytochemical and molecular studies on two groups of PR proteins, β-1,3-glucanases and chitinases, and the arabinogalactan proteins (AGP) during the early stages of androgenesis induction in two breeding lines of rye (Secale cereale L.) with different androgenic potential. The process of androgenesis was initiated by tillers pre-treatments with low temperature, mannitol and/or reduced glutathione and resulted in microspores reprogramming and formation of androgenic structures what was associated with high activity of β-1,3-glucanases and chitinases. Some isoforms of β-1,3-glucanases, namely several acidic isoforms of about 26 kDa; appeared to be anther specific. Chitinases were well represented but were less variable. RT-qPCR revealed that the cold-responsive chitinase genes Chit1 and Chit2 were expressed at a lower level in the microspores and whole anthers while the cold-responsive Glu2 and Glu3 were not active. The stress pre-treatments modifications promoted the AGP accumulation. An apparent dominance of some AGP epitopes (LM2, JIM4 and JIM14) was detected in the androgenesis-responsive rye line. An abundant JIM13 epitopes in the vesicles and inner cell walls of the microspores and in the cell walls of the anther cell layers appeared to be the most specific for embryogenesis.

Trichostatin A Affects Developmental Reprogramming of Bread Wheat Microspores towards an Embryogenic Route

Microspores can be developmentally reprogrammed by the application of different stress treatments to initiate an embryogenic pathway leading to the production of doubled haploid (DH) plants. Epigenetic modifications are involved in cell reprogramming and totipotency in response to stress. To increase microspore embryogenesis (ME) efficiency in bread wheat, the effect of the histone deacetylase inhibitor trichostatin A (TSA) has been examined in two cultivars of wheat with different microspore embryogenesis response. Diverse strategies were assayed using 0-0.4 µM TSA as a single induction treatment and after or simultaneously with cold or mannitol stresses. The highest efficiency was achieved when 0.4 µM TSA was applied to anthers for 5 days simultaneously with a 0.7 M mannitol treatment, producing a four times greater number of green DH plants than mannitol. Ultrastructural studies by transmission electron microscopy indicated that mannitol with TSA and mannitol treatments induced similar morphological changes in early stages of microspore reprogramming, although TSA increased the number of microspores with 'star-like' morphology and symmetric divisions. The effect of TSA on the transcript level of four ME marker genes indicated that the early signaling pathways in ME, involving the TaTDP1 and TAA1b genes, may be mediated by changes in acetylation patterns of histones and/or other proteins.

Early Development of Direct Embryos in the Cultured Anthers of Manihot esculenta Crantz

Cassava is one of the most important sources of energy. To meet the growing demand, genetic improvement is of utmost importance. Its cross-pollinating nature limits the opportunity of exploitation of hybrid vigor and demands the development of homozygous lines through doubled-haploid technologies. The problems in callus-mediated embryogenesis, such as longer processing time and genetically unstable nature, can be overcome by direct embryogenesis. Conditions to produce embryos directly from microspores in cultured anthers were optimized. The optimum stress pretreatment condition was 40 °C for 6 h after culturing the anthers into the induction medium. For proembryo formation, 2% sucrose and 5 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) or 1 mg/l 1-naphthaleneacetic acid were optimum. Globular embryos were formed by subculturing proembryos into the medium with 0.5 mg/l 2,4-D and 5 mg/l 6-benzylaminopurine after two weeks of culturing. Light microscopy of cultured anthers demonstrated the formation of multicellular structures and their further development into proembryos. Microscopic studies showed proembryos emerging through the damaged anther wall. Monoallelic banding in simple sequence repeat (SSR) analysis indicated homozygous or haploid states in some of the originated embryos. The conditions optimized in this study were effective in the early development of direct embryos after two weeks of culture initiation. This is the first report of the formation of direct embryos in cultured anthers of cassava.

Study of Androgenic Plant Families of Alloplasmic Introgression Lines (H. vulgare) -T. aestivum and the Use of Sister DH Lines in Breeding

One of the limitations in obtaining the genetic diversity of doubled haploid (DH) lines via anther culture is the development of families of regenerants, and each family represents a clone. This work examines the results of studying this phenomenon in anther culture of alloplasmic (H. vulgare)–T. aestivum and euplasmic lines with 1RS.1BL and 7DL-7Ai translocations and hybrids between them. Parameters of androgenesis such as the number of embryo-like structures, the total number of regenerants, and the number of green regenerants per 100 anthers varied depending on the genotype. In all genotypes from embryo-like structures, predominant development of families of plantlets rather than single plantlets was found. The source of family plantlets was polyembryos. About 75% of families consisted of regenerants at the same fertility level. On average, 37.74%4% of the R0 plants were fertile. The sister DH lines of three hybrid combinations were formed from seeds of R1 plants (2n = 42) with high fertility and in the presence of wheat–alien translocations. After four years of breeding trials, the sister DH lines of three families with fungal disease resistance increased yield, and some parameters of grain quality exceeding the controls were identified as promising for breeding.

Modeling of asymmetric division of somatic cell in protoplasts culture of higher plants

The key result of the work is the selection of factors for the cultivation of protoplasts of higher plants in vitro, which allowed induction of asymmetrical cell division during the first cell cycle phase. Gibberellin has been proved to be one of the main cofactors of asymmetric division of plant cells. The objects of research were plants of the following cultivars aseptically grown in hormone-free MS medium: tobacco (Nicotiana tabacum L.), SR-1 line; Arabidopsis thaliana var. columbia (L.) Heynh; potato (Solanum tuberosum L.), Zarevo cultivar; cultivated white head cabbage (Brassica oleraceae var. capitata L.) of the following varieties: Kharkivska zymnia, Ukrainska osin, Yaroslavna, Lika, Lesya, Bilosnizhka, Dithmarscher Früher, Iyunskarannya; rape (Brassica napus L.) of Shpat cultivar; winter radish (Raphanus sativus L.) of Odessa-5 cultivar. In experiments with mesophilic and hypocotyl protoplasts of the above-mentioned plant species it has been proved that short-term osmotic stress within 16–18 hours being combined with subsequent introduction of high doses of gibberellin GK3 (1 mg/L) into the modified liquid nutrient media TM and SW led to the occurrence of pronounced morphological traits of cytodifferentiation already at the initial stages of the development of mitotically active cells in a number of higher plants. Meanwhile, in all analyzed species, there was observed the division of the initial genetically homogeneous population of mitotically active cells into two types of asymmetric division: by the type of division of the mother cell into smaller daughter cells and by the type of the first asymmetric division of the zygotic embryo in planta. In this case, the first type of asymmetric division occurred during unusual cytomorphism of the mother cells: a pronounced heart-shaped form even before the first division, which is inherent in the morphology of somatic plant embryo in vitro at the heart-shaped stage. A particular study of the effect of osmotic stress influencing protoplasts of various cultivars of white cabbage, isolated from hypocotyls of 7–9 day etiolated seedlings, revealed quite a typical consistent pattern: the acquisition and maintenance of the axis of symmetry in growing microcolonies occurred without extra exogenous gibberellin (GK3), which was added to the nutrient medium earlier. While analyzing the effect of growth regulators on the formation of microcolonies with traits of structural organization, the conclusion was made regarding the commonality of the revealed morphogenetic reactions of cells within the culture of protoplasts of higher plants in vitro with similar reactions studied earlier on other plants, both in vitro and in planta. Modeling of asymmetric cell division in protoplast culture in vitro has become possible by carrying out a balanced selection of growth regulators as well as their coordinated application through time along with changes in osmotic pressure of a nutrient medium.

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.

Doubled haploid production in alfalfa (Medicago sativa L.) through isolated microspore culture

Rapid production of doubled haploids (DHs) through isolated microspore culture is an important and promising method for genetic study of alfalfa. To induce embryogenesis in alfalfa, isolated microspores were submitted to abiotic stresses during their initial culture, in order to stimulate them to form embryos and plantlets. ‘Baoding’ and ‘Zhongmu No 1’ alfalfa cultivars supported reproducible and reliable proliferation response irrespective of any stress treatment of microspores. The microspore developmental stage for isolated microspore culture was studied and we found that uninucleate microspores were best to initiate culture. Exposure of microspores to appropriate low temperature or heat shock stresses were able to increase the efficiency of embryogenesis. The most effective low-temperature treatment was 4 °C for 24 h and the frequency of plantlets induction was 20.0%. The most effective heat shock treatment was 32 °C for 2 d and the frequency of plantlets induction was 14.17%. The analysis of ploidy level performed by flow cytometer revealed that the majority of 278 regenerated plantlets were haploid (65.83%) or doubled haploid (33.81%). This is the first report of haploid production in alfalfa through isolated microspore culture.

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.

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

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

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).

Trichostatin A increases embryo and green plant regeneration in wheat

Chemical agents such as trichostatin A (TSA) can assist in optimization of doubled haploidy for rapid improvements in wheat germplasm and addressing recalcitrance issues in cell culture responses. In wheat, plant regeneration through microspore culture is an integral part of doubled haploid (DH) production. However, low response to tissue culture and genotype specificity are two major constraints in the broad deployment of this breeding tool. Recently, the structure of chromatin was shown to be linked with cell transitions during tissue culture. Specifically, repression of genes that are required for cell morphogenesis, through acetylation of histones, may play an important role in this process. Reduction of histone acetylation by chemical inhibition may increase tissue culture efficiency. Here, the role of trichostatin A (TSA) in inducing microspore-derived embryos was investigated in wheat. The optimal dose of TSA was determined for wheat cultivars and subsequently validated in F₁ hybrids. A significant increase in the efficiency of DH production was observed in both cultivated varieties and F₁ hybrids. Thus, the inclusion of TSA in DH protocols for wheat breeding programs is advocated.

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.

Transcriptome analysis reveals translational regulation in barley microspore-derived embryogenic callus under salt stress

Transcriptome analysis of barley embryogenic callus from isolated microspore culture under salt stress uncovered a role of translation inhibition and selective activation of stress-specific proteins in cellular defense. Soil salinity is one of the major abiotic stresses which constrains the plant growth and reduces the productivity of field crops. In this study, it was observed that the salt stress in barley isolated microspore culture impacted not only on the quantity of embryogenic callus but also on the quality for later differentiation. The barley microspore-derived embryogenic callus, a transient intermediate form linked cells and plants, was employed for a global transcriptome analysis by RNA sequencing to provide new insights into the cellular adaptation or acclimation to stress. A total of 596 differentially expressed genes (DEGs) were identified, in which 123 DEGs were up-regulated and 473 DEGs were down-regulated in the embryogenic callus produced from microspore culture under salt stress as compared to the control conditions. KEGG pathway analysis identified 'translation' (27 DEGs; 12.56 %) as the largest group and followed by 'folding, sorting and degradation' (25 DEGs; 11.63 %) in 215 mapped metabolic pathways. The results of RNA-Seq data and quantitative real-time polymerase chain reaction validation showed that the genes related to translation regulation (such as eIF1A, RPLP0, RPLP2, VARS) were down-regulated to control general protein synthesis, and the genes related to endoplasmic reticulum stress response (such as small heat shock protein genes) were selectively up-regulated against protein denaturing during microspore embryogenesis under continuous salt stress. These transcriptional remodeling might affect the essential protein synthesis for the cell development to fulfill totipotency under salt stress.

Molecular cloning, characterization and expression analysis of a heat shock protein 10 (Hsp10) from Pennisetum glaucum (L.), a C4 cereal plant from the semi-arid tropics

Heat shock proteins (Hsp10) belong to the ubiquitous family of heat-shock molecular chaperones found in the organelles of both prokaryotes and eukaryotes. Chaperonins assist the folding of nascent and stress-destabilized proteins. A cDNA clone encoding a 10 kDa Hsp was isolated from pearl millet, Pennisetum glaucum (L.) by screening a heat stress cDNA library. The fulllength PgHsp10 cDNA consisted of 297 bp open reading frame (ORF) encoding a 98 amino acid polypeptide with a predicted molecular mass of 10.61 kDa and an estimated isoelectric point (pI) of 7.95. PgHsp10 shares 70-98 % sequence identity with other plant homologs. Phylogenetic analysis revealed that PgHsp10 is evolutionarily close to the maize Hsp10 homolog. The predicted 3D model confirmed a conserved eight-stranded ß-barrel with active site between the ß-barrel comprising of eight-strands, with conserved domain VLLPEYGG sandwiched between two ß-sheets. The gene consisted of 3 exons and 2 introns, while the position and phasing of these introns were conserved similar to other plant Hsp10 family genes. In silico analysis of the promoter region of PgHsp10 presented several distinct set of cis-elements and transcription factor binding sites. Quantitative RT-PCR analysis showed that PgHsp10 gene was differentially expressed in response to abiotic stresses with the highest level of expression under heat stress conditions. Results of this study provide useful information regarding the role of chaperonins in stress regulation and generated leads for further elucidation of their function in plant stress tolerance.

Low temperatures are required to induce the development of fertile flowers in transgenic male and female early flowering poplar (Populus tremula L.)

Until now, artificial early flowering poplar systems have mostly led to the development of sterile flowers. In this study, several strategies aimed at inducting fertile flowers in pHSP::AtFT transgenic poplar were evaluated, in particular the influence of temperature and photoperiod. Our results provide evidence that temperature, and not photoperiod, is the key factor required for the development of fertile flowers in early flowering poplar. Fertile flowers were only obtained when a cold treatment phase of several weeks was used after the heat treatment phase. Heat treatments induced AtFT gene activity through activation of the heat-shock promoter (pHSP). Photoperiod did not show a similar influence on flower fertility as pollen grains were obtained under both long- and short-day conditions. Fertility was confirmed in flowers of both male and female plants. For the first time, crosses were successfully performed with transgenic female early flowering poplar. All mature flowers obtained after 8 weeks of inductive treatments were fertile. Gene expression studies also confirmed that cold temperatures influenced expression of poplar genes homologous to 'pollen development genes' from Arabidopsis thaliana (L.) Heynh. Homology and expression patterns suggested a role for PtTDF1, PtBAM1, PtSERK1/2 and PtMS1 on anther and pollen development in poplar flowers. The system developed in this study allows a fast and very reliable induction of fertile poplar flowers in a very short period of time. The non-reproductive phase, usually 7-10 years, can now be shortened to 6-10 months, and fertile flowers can be obtained independently of the season. This system is a reliable tool for breeding purposes (high-speed breeding technology), genomics and biosafety research.

Simple and robust determination of the activity signature of key carbohydrate metabolism enzymes for physiological phenotyping in model and crop plants

The analysis of physiological parameters is important to understand the link between plant phenotypes and their genetic bases, and therefore is needed as an important element in the analysis of model and crop plants. The activities of enzymes involved in primary carbohydrate metabolism have been shown to be strongly associated with growth performance, crop yield, and quality, as well as stress responses. A simple, fast, and cost-effective method to determine activities for 13 key enzymes involved in carbohydrate metabolism has been established, mainly based on coupled spectrophotometric kinetic assays. The comparison of extraction buffers and requirement for dialysis of crude protein extracts resulted in a universal protein extraction protocol, suitable for the preparation of protein extracts from different organs of various species. Individual published kinetic activity assays were optimized and adapted for a semi-high-throughput 96-well assay format. These assays proved to be robust and are thus suitable for physiological phenotyping, enabling the characterization and diagnosis of the physiological state. The potential of the determination of distinct enzyme activity signatures as part of a physiological fingerprint was shown for various organs and tissues from three monocot and five dicot model and crop species, including two case studies with external stimuli. Differential and specific enzyme activity signatures are apparent during inflorescence development and upon in vitro cold treatment of young inflorescences in the monocot ryegrass, related to conditions for doubled haploid formation. Likewise, treatment of dicot spring oilseed rape with elevated CO2 concentration resulted in distinct patterns of enzyme activity responses in leaves.

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

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

Effect of air desiccation and salt stress factors on in vitro regeneration of rice (Oryza sativa L.)

Enhancement of callus induction and its regeneration efficiency through in vitro techniques has been optimized for 2 abiotic stresses (salt and air desiccation) using 3 rice genotypes viz. BR10, BRRI dhan32 and BRRI dhan47. The highest frequency of callus induction was obtained for BRRI dhan32 (64.44%) in MS medium supplemented with 2, 4-D (2.5 mgL(-1)) and Kin (1.0 mgL(-1)). Different concentrations of NaCl (2.9, 5.9, 8.8 and 11.7 gL(-1)) were used and its effect was recorded on the basis of viability of calli (VC), relative growth rate (RGR), tolerance index (TI) and relative water content (RWC). It was observed that in all cases BRRI dhan47 showed highest performance on tolerance to VC (45.33%), RGR (1.03%), TI (0.20%) and RWC (10.23%) with 11.7 gL(-1) NaCl. Plant regeneration capability was recorded after partial air desiccation pretreatment to calli for 15, 30, 45 and 60 h. In this case BRRI dhan32 gave maximum number of regeneration (76.19%) when 4 weeks old calli were desiccated for 45 h. It was observed that air desiccation was 2-3 folds more effective for enhancing green plantlet regeneration compared to controls. Furthermore, desiccated calli also showed the better capability to survive in NaCl induced abiotic stress; and gave 1.9 fold (88.80%) increased regeneration in 11.7 gL(-1) salt level for BRRI dhan47. Analysis of variance (ANOVA) showed that the genotypes, air desiccation and NaCl had significant effect on plant regeneration at P < 0.01.

Microspore embryogenesis: establishment of embryo identity and pattern in culture

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

Microspore embryogenesis: establishment of embryo identity and pattern in culture

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