Manual Isolation of Living Early Embryos from Tobacco Seeds

Zygotic embryogenesis is one of key processes for fertile seed development and therefore has gained great attention for decades in the field of plant developmental biology. However, this process is deeply embedded in the maternal tissues. The inaccessibility of tiny early embryos has greatly hindered the study of early embryogenesis, especially limits direct observation and accurate omics investigations. In order to investigate the molecular mechanism regulating embryo development with modern technologies, it is necessary to develop a reliable method to isolate living embryos at different stages. For this purpose, plant scientists have been trying to develop different methods for isolating zygotes and early embryos in different plants such as maize, wheat, rice, and tobacco during past decades. Nicotiana tabacum has long been considered as an ideal model eudicot for the study of embryogenesis, which displays a traceable and predictable cell division pattern, spanning from the first zygotic division to the mature embryo formation. Here, we provide a detailed protocol for isolating living embryos from zygote to cotyledon embryo. Isolated living zygotes and early embryos could be used for several important studies such as cell type-specific transcriptome construction and clear GFP observation.

Expression of AtLEC2 and AtIPTs promotes embryogenic callus formation and shoot regeneration in tobacco

BACKGROUND

LEAFY COTYLEDON 2 (LEC2) acts throughout embryo morphogenesis and maturation phase to maintain embryogenic identity. Our previous study stated that Arabidopsis thaliana LEC2 (AtLEC2) driven by glucocorticoid receptor-dexamethasone (GR-DEX) inducible system (AtLEC2-GR) triggers embryogenic callus formation in tobacco (Nicotiana tabacum).

RESULTS

In this study, the adenosine phosphate isopentenyltransferase genes AtIPT3, AtIPT7 and the tRNA isopentenyltransferase gene AtIPT9 were overexpressed in the AtLEC2-GR transgenic background. In the AtIPT7-OE AtLEC2-GR and AtIPT9-OE AtLEC2-GR seedlings, high-quality embryogenic callus was obtained under the DEX condition, and the shoot regeneration efficiency was 2 to 3.5 folds higher than AtLEC2-GR alone on hormone free medium without DEX. Transcriptome analyses showed that up-regulated BBM, L1L, ABI3, and FUS3 might function during embryogenic callus formation. However, at the shoot regeneration stage, BBM, L1L, ABI3, and FUS3 were down-regulated and Type-B ARRs were up-regulated, which might contribute to the increased shoot regeneration rate.

CONCLUSIONS

A novel system for inducing shoot regeneration in tobacco has been developed using the GR-DEX system. Induced expression of AtLEC2 triggers embryogenic callus formation and overexpression of AtIPT7 or AtIPT9 improves shoot regeneration without exogenous cytokinin.

Retarded germination of Nicotiana tabacum seeds following insertion of exogenous DNA mimics the seed persistent behavior

Tobacco seeds show a coat-imposed dormancy in which the seed envelope tissues (testa and endosperm) impose a physical constraint on the radicle protrusion. The germination-limiting process is represented by the endosperm rupture which is induced by cell-wall weakening. Transgenic tobacco seeds, obtained by insertion of exogenous genes codifying for seed-based oral vaccines (F18 and VT2eB), showed retarded germination with respect to the wild type and modified the expression of endogenous proteins. Morphological and proteomic analyses of wild type and transgenic seeds revealed new insights into factors influencing seed germination. Our data showed that the interference of exogenous DNA influences the germination rather than the dormancy release, by modifying the maturation process. Dry seeds of F18 and VT2eB transgenic lines accumulated a higher amount of reserve and stress-related proteins with respect to the wild type. Moreover, the storage proteins accumulated in tobacco F18 and VT2eB dry seeds have structural properties that do not enable the early limited proteolysis observed in the wild type. Morphological observations by electron and light microscopy revealed a retarded mobilization of the storage material from protein and lipid bodies in transgenic seeds, thus impairing water imbibition and embryo elongation. In addition, both F18 and VT2eB dry seeds are more rounded than the wild type. Both the morphological and biochemical characteristics of transgenic seeds mimic the seed persistent profile, in which their roundness enables them to be buried in the soil, while the higher content of storage material enables the hypocotyl to elongate more and the cotyledons to emerge.

EMB2738, which encodes a putative plastid-targeted GTP-binding protein, is essential for embryogenesis and chloroplast development in higher plants

In higher plants, chloroplasts carry out many important functions, and normal chloroplast development is required for embryogenesis. Numerous chloroplast-targeted proteins involved in embryogenesis have been identified. Nevertheless, their functions remain unclear. In this study, a chloroplast-localized protein, EMB2738, was reported to be involved in Arabidopsis embryogenesis. EMB2738 knockout led to defective embryos, and the embryo development in emb2738 was interrupted after the globular stage. Complementation experiments identified the AT3G12080 locus as EMB2738. Cellular observation indicated that severely impaired chloroplast development was observed in these aborted embryos. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis showed that chloroplast-encoded photosynthetic genes, which are transcribed by plastid-encoded RNA polymerase (PEP), are predominantly decreased in defective embryogenesis, compared with those in the wild-type. In contrast, genes encoding PEP core subunits, which are transcribed by nucleus-encoded RNA polymerase (NEP), were increased. These results suggested that the knockout of EMB2738 strongly blocked chloroplast-encoded photosynthesis gene expression in embryos. Silencing of the EMB2738 orthologue in tobacco through a virus-induced genome silencing technique resulted in an albinism phenotype, vacuolated chloroplasts and decreased PEP-dependent plastid transcription. These results suggested that NtEMB2738 might be involved in plastid gene expression. Nevertheless, genetic analysis showed that the NtEMB2738 coding sequence could not fully rescue the defective embryogenesis of the emb2738 mutant, which suggested functional divergence between NtEMB2738 and EMB2738 in embryogenesis. Taken together, these results indicated that both EMB2738 and NtEMB2738 are involved in the expression of plastid genes in higher plants, and there is a functional divergence between NtEMB2738 and EMB2738 in embryogenesis.

The autonomous cell fate specification of basal cell lineage: the initial round of cell fate specification occurs at the two-celled proembryo stage

In angiosperms, the first zygotic division usually gives rise to two daughter cells with distinct morphologies and developmental fates, which is critical for embryo pattern formation; however, it is still unclear when and how these distinct cell fates are specified, and whether the cell specification is related to cytoplasmic localization or polarity. Here, we demonstrated that when isolated from both maternal tissues and the apical cell, a single basal cell could only develop into a typical suspensor, but never into an embryo in vitro. Morphological, cytological and gene expression analyses confirmed that the resulting suspensor in vitro is highly similar to its undisturbed in vivo counterpart. We also demonstrated that the isolated apical cell could develop into a small globular embryo, both in vivo and in vitro, after artificial dysfunction of the basal cell; however, these growing apical cell lineages could never generate a new suspensor. These findings suggest that the initial round of cell fate specification occurs at the two-celled proembryo stage, and that the basal cell lineage is autonomously specified towards the suspensor, implying a polar distribution of cytoplasmic contents in the zygote. The cell fate transition of the basal cell lineage to the embryo in vivo is actually a conditional cell specification process, depending on the developmental signals from both the apical cell lineage and maternal tissues connected to the basal cell lineage.

NtDRP is necessary for accurate zygotic division orientation and differentiation of basal cell lineage toward suspensor formation

Plant embryogenesis begins with an asymmetric division of the zygote, producing apical and basal cells with distinct cell fates. The asymmetric zygote division is thought to be critical for embryo pattern formation; however, the molecular mechanisms regulating this process, especially maintaining the accurate position and proper orientation of cell division plane, remain poorly understood. Here, we report that a dynamin-related protein in Nicotiana tabacum, NtDRP, plays a critical role in maintaining orientation of zygotic division plane. Down-regulation of NtDRP caused zygotic cell division to occur in different, incorrect orientations and resulted in disruption of suspensor formation, and even development of twin embryos. The basal cell lineage totally integrated with the apical cell lineage into an embryo-like structure, suggesting that NtDRP is essential to accurate zygotic division orientation and differentiation of basal cell lineage toward suspensor formation. We also reveal that NtDRP plays its role by modulating microtubule spatial organization and spindle orientation during early embryogenesis. Thus, we revealed that NtDRP is involved in orientation of the asymmetric zygotic division and differentiation of distinct suspensor and embryo domains, as well as subsequent embryo pattern formation.

NtKRP, a kinesin-12 protein, regulates embryo/seed size and seed germination via involving in cell cycle progression at the G2/M transition

Kinesins comprise a superfamily of microtubule-based motor proteins involved in essential processes in plant development, but few kinesins have been functionally identified during seed development. Especially, few kinesins that regulate cell division during embryogenesis have been identified. Here we report the functional characterization of NtKRP, a motor protein of the kinesin-12 family. NtKRP is predominantly expressed in embryos and embryonic roots. NtKRP RNAi lines displayed reductions in cell numbers in the meristematic zone, in embryonic root length, and in mature embryo and seed sizes. Furthermore, we also show that CDKA;1 binds to NtKRP at the consensus phosphorylation sites and that the decreased cell numbers in NtKRP-silenced embryos are due to a delay in cell division cycle at the G2/M transition. In addition, binding between the cargo-binding tail domain of NtKRP and CDKA; 1 was also determined. Our results reveal a novel molecular pathway that regulates embryo/seed development and critical role of kinesin in temporal and spatial regulation of a specific issue of embryo developmental.

Fertilization Independent Endosperm genes repress NbGH3.6 and regulate the auxin level during shoot development in Nicotiana benthamiana

The Fertilization Independent Endosperm (FIE) gene is required to restrict endosperm development without fertilization, and it represses flowering during embryo and seedling development in Arabidopsis thaliana However, the regulatory mechanism of the FIE gene in postembryonic shoot development is not well understood. Silencing of Nicotiana benthamiana homologues of the FIE gene, NbFIE1 and NbFIE2, resulted in the enhanced outgrowth of axillary buds and the impairment of secondary xylem differentiation. RNA sequencing analysis found that one of the auxin-responsive GRETCHEN HAGEN 3(GH3) family genes, NbGH3.6, was upregulated and maintained a high expression during the time course of silencing NbFIE genes. Chromatin immunoprecipiation (ChIP)-PCR results showed a lack of H3K27me3 marks on NbGH3.6 chromatin in NbFIE-silenced plants compared with negative control plants, indicating that NbGH3.6 was a direct target of NbFIE genes during postembryonic shoot development. Moreover, the free IAA content was reduced significantly in NbFIE-silenced plants, which might cause the enhanced outgrowth of axillary buds as well as impaired secondary xylem differentiation. These results clearly indicated that NbGH3.6 was a primary target of NbFIE genes during postembryonic shoot development, and NbFIE genes regulated axillary bud growth and secondary xylem formation through tuning endogenous auxin homeostasis, possibly by regulating the expression of the NbGH3.6 gene.

Exogenous auxin regulates multi-metabolic network and embryo development, controlling seed secondary dormancy and germination in Nicotiana tabacum L

BACKGROUND

Auxin was recognized as a secondary dormancy phytohormone, controlling seed dormancy and germination. However, the exogenous auxin-controlled seed dormancy and germination remain unclear in physiological process and gene network.

RESULTS

Tobacco seeds soaked in 1000 mg/l auxin solution showed markedly decreased germination compared with that in low concentration of auxin solutions and ddH2O. Using an electron microscope, observations were made on the seeds which did not unfold properly in comparison to those submerged in ddH2O. The radicle traits measured by WinRHIZO, were found to be also weaker than the other treatment groups. Quantified by ELISA, there was no significant difference found in β-1,3glucanase activity and abscisic acid (ABA) content between the seeds imbibed in gradient concentration of auxin solution and those soaked in ddH2O. However, gibberellic acid (GA) and auxin contents were significantly higher at the time of exogenous auxin imbibition and were gradually reduced at germination. RNA sequencing (RNA-seq), revealed that the transcriptome of auxin-responsive dormancy seeds were more similar to that of the imbibed seeds when compared with primary dormancy seeds by principal component analysis. The results of gene differential expression analysis revealed that auxin-controlled seed secondary dormancy was associated with flavonol biosynthetic process, gibberellin metabolic process, adenylyl-sulfate reductase activity, thioredoxin activity, glutamate synthase (NADH) activity and chromatin regulation. In addition, auxin-responsive germination responded to ABA, auxin, jasmonic acid (JA) and salicylic acid (SA) mediated signaling pathway (red, far red and blue light), glutathione and methionine (Met) metabolism.

CONCLUSIONS

In this study, exogenous auxin-mediated seed secondary dormancy is an environmental model that prevents seed germination in an unfavorable condition. Seeds of which could not imbibe normally, and radicles of which also could not develop normally and emerge. To complete the germination, seeds of which would stimulate more GA synthesis to antagonize the stimulation of exogenous auxin. Exogenous auxin regulates multi-metabolic networks controlling seed secondary dormancy and germination, of which the most important thing was that we found the auxin-responsive seed secondary dormancy refers to epigenetic regulation and germination to enhance Met pathway. Therefore, this study uncovers a previously unrecognized transcriptional regulatory networks and physiological development process of seed dormancy and germination with superfluous auxin signal activate.

Tobacco seeds as efficient production platform for a biologically active anti-HBsAg monoclonal antibody

The use of plants as heterologous hosts is one of the most promising technologies for manufacturing valuable recombinant proteins. Plant seeds, in particular, constitute ideal production platforms for long-term applications requiring a steady supply of starting material, as they combine the general advantages of plants as bioreactors with the possibility of biomass storage for long periods in a relatively small volume, thus allowing manufacturers to decouple upstream and downstream processing. In the present work we have used transgenic tobacco seeds to produce large amounts of a functionally active mouse monoclonal antibody against the Hepatitis B Virus surface antigen, fused to a KDEL endoplasmic reticulum retrieval motif, under control of regulatory sequences from common bean (Phaseolus vulgaris) seed storage proteins. The antibody accumulated to levels of 6.5 mg/g of seed in the T3 generation, and was purified by Protein A affinity chromatography combined with SEC-HPLC. N-glycan analysis indicated that, despite the KDEL signal, the seed-derived plantibody bore both high-mannose and complex-type sugars that indicate partial passage through the Golgi compartment, although its performance in the immunoaffinity purification of HBsAg was unaffected. An analysis discussing the industrial feasibility of replacing the currently used tobacco leaf-derived plantibody with this seed-derived variant is also presented.

Timing of the G1/S transition in tobacco pollen vegetative cells as a primary step towards androgenesis in vitro

KEY MESSAGE

Mid-bicellular pollen vegetative cells in tobacco escape from G1 arrest and proceed to the G1/S transition towards androgenesis within 1 day under glutamine starvation conditions in vitro. In the Nicotiana tabacum pollen culture system, immature pollen grains at the mid-bicellular stage can mature in the presence of glutamine; however, if glutamine is absent, they deviate from their native cell fate in a few days. The glutamine-starved pollen grains cannot undergo maturation, even when supplied with glutamine later. Instead, they undergo cell division towards androgenesis slowly within 10 days in a medium containing appropriate nutrients. During the culture period, they ought to escape from G1 arrest to proceed into S phase as the primary step towards androgenesis. However, this event has not been experimentally confirmed. Here, we demonstrated that the pollen vegetative cells proceeded to the G1/S transition within approximately 15-36 h after the start of culture. These results were obtained by analyzing transgenic pollen possessing a fusion gene encoding nuclear-localizing GFP under the control of an E2F motif-containing promoter isolated from a gene encoding one of DNA replication licensing factors. Observations using a 5-ethynyl-2'-deoxyuridine DNA labeling and detection technique uncovered that the G1/S transition was soon followed by S phase. These hallmarks of vegetative cells undergoing dedifferentiation give us new insights into upstream events causing the G1/S transition and also provide a novel strategy to increase the frequency of the androgenic response in tobacco and other species, including recalcitrants.

NtProRP1, a novel proline-rich protein, is an osmotic stress-responsive factor and specifically functions in pollen tube growth and early embryogenesis in Nicotiana tabacum

Proline-rich proteins (PRPs) are known to play important roles in sexual plant reproduction. Most of the known proteins in the family were found in styles or pollen and modulate pollen tube growth. Here, we identified a novel member of the gene family, NtProRP1, which is preferentially expressed in tobacco pollen grains, pollen tubes and zygotes. NtProRP1 could be secreted into the extracellular space including the cell wall, and the predicted N-terminal signal peptide is crucial for its secretion. In NtProRP1-RNAi plants, pollen germination and pollen tube growth were significantly slower and showed zigzag or swell morphology in vitro. Early embryogenesis also exhibited aberrant development, indicative of its critical role in both pollen tube growth and early embryogenesis. Further investigation revealed that NtProRP1 plays a crucial role in osmotic stress response during pollen tube growth and is likely regulated by Tsi, a stress-responsive gene, suggesting that the regulatory mechanism is also involved in the stress response during sexual plant reproduction. These data provide evidence that NtProRP1 functions as a downstream factor of Tsi1 in the stress response and converges the stress signal into the modulation of pollen tube growth and early embryogenesis.

A bipartite molecular module controls cell death activation in the Basal cell lineage of plant embryos

Plant zygote divides asymmetrically into an apical cell that develops into the embryo proper and a basal cell that generates the suspensor, a vital organ functioning as a conduit of nutrients and growth factors to the embryo proper. After the suspensor has fulfilled its function, it is removed by programmed cell death (PCD) at the late stages of embryogenesis. The molecular trigger of this PCD is unknown. Here we use tobacco (Nicotiana tabacum) embryogenesis as a model system to demonstrate that the mechanism triggering suspensor PCD is based on the antagonistic action of two proteins: a protease inhibitor, cystatin NtCYS, and its target, cathepsin H-like protease NtCP14. NtCYS is expressed in the basal cell of the proembryo, where encoded cystatin binds to and inhibits NtCP14, thereby preventing precocious onset of PCD. The anti-cell death effect of NtCYS is transcriptionally regulated and is repressed at the 32-celled embryo stage, leading to increased NtCP14 activity and initiation of PCD. Silencing of NtCYS or overexpression of NtCP14 induces precocious cell death in the basal cell lineage causing embryonic arrest and seed abortion. Conversely, overexpression of NtCYS or silencing of NtCP14 leads to profound delay of suspensor PCD. Our results demonstrate that NtCYS-mediated inhibition of NtCP14 protease acts as a bipartite molecular module to control initiation of PCD in the basal cell lineage of plant embryos.

Induced expression of AtLEC1 and AtLEC2 differentially promotes somatic embryogenesis in transgenic tobacco plants

Arabidopsis LEAFY COTYLEDON (LEC) genes, AtLEC1 and AtLEC2, are important embryonic regulators that play key roles in morphogenesis and maturation phases during embryo development. Ectopic expression of AtLEC1 and AtLEC2 in tobacco caused abnormality in transgenic seedling. When transgenic seeds germinated on medium containing 30 µM DEX, LEC1 transgenic seedlings were ivory and fleshy, with unexpanded cotyledons, stubby hypocotyls, short roots and no obvious callus formation at the shoot meristem position. While LEC2 transgenic seedlings formed embryonic callus on the shoot apical meristem and somatic embryo-like structures emerged from the surface of the callus. When callus were transferred to hormone free MS0 medium more shoots were regenerated from each callus. However, shoot formation was not observed in LEC1 overexpressors. To investigate the mechanisms of LEC2 in somatic embryogenesis, we studied global gene expression by digital gene expression profiling analysis. The results indicated that ectopic expression of LEC2 genes induced accumulation of embryo-specific proteins such as seed storage proteins, late embryogenesis abundant (LEA) proteins, fatty acid biosynthetic enzymes, products of steroid biosynthesis related genes and key regulatory genes of the embryo development. Genes of plant-specific transcription factors such as NAC domain protein, AP2 and GRAS family, resistance-related as well as salicylic acid signaling related genes were up-regulated in LEC2 transgenic seedlings. Ectopi c expression of LEC2 induced large number of somatic embryo formation and shoot regeneration but 20 d DEX induction of LEC1 is not sufficient to induce somatic embryogenesis and shoot formation. Our data provide new information to understand the mechanisms on LEC2 gene's induction of somatic embryogenesis.

The novel approach to enhance seed security: dual anti-counterfeiting methods applied on tobacco pelleted seeds

Seed security is of prime importance for agriculture. To protect true seeds from being faked, more secure dual anti-counterfeiting technologies for tobacco (Nicotiana tabacum L.) pelleted seed were developed in this paper. Fluorescein (FR), rhodamine B (RB), and magnetic powder (MP) were used as anti-counterfeiting labels. According to their different properties and the special seed pelleting process, four dual-labeling treatments were conducted for two tobacco varieties, MS Yunyan85 (MSYY85) and Honghua Dajinyuan (HHDJY). Then the seed germination and seedling growth status were investigated, and the fluorescence in cracked pellets and developing seedlings was observed under different excitation lights. The results showed that FR, RB, and MP had no negative effects on the germination, seedling growth, and MDA content of the pelleted seeds, and even some treatments significantly enhanced seedling dry weight, vigor index, and shoot height in MS YY85, and increased SOD activity and chlorophyll content in HHDJY as compared to the control. In addition, the cotyledon tip of seedlings treated with FR and MP together represented bright green fluorescence under illumination of blue light (478 nm). And the seedling cotyledon vein treated with RB and MP together showed red fluorescence under green light (546 nm). All seeds pelleted with magnetic powder of proper concentration could be attracted by a magnet. Thus, it indicated that those new dual-labeling methods that fluorescent compound and magnetic powder simultaneously applied in the same seed pellets definitely improved anti-counterfeiting technology and enhanced the seed security. This technology will ensure that high quality seed will be used in the crop production.

Polar protein transport between apical and basal cells during tobacco early embryogenesis

KEY MESSAGE : We found that protein trafficking between apical and basal cell can be unidirectional, which reveals the different roles of the two cells in the cell-to-cell communication between them during early embryogenesis. In most angiosperm species, asymmetric zygote division results in an apical cell and a basal cell that have distinct cell fates. Much has been speculated about possible communication between these cell types in relation to their cell fate determination. Here, we report on the use of photoactivatable green fluorescent protein (PA-GFP) in tobacco to trace intercellular communication between apical and basal cells during early embryogenesis. We found that PA-GFP was transported between apical and basal cells of a two-celled proembryo, and that protein trafficking was unidirectional toward the apical cell, highlighting different cell communication roles. Further ultrastructural analysis showed numerous plasmodesmata in the walls connecting the apical and basal cells, which may provide channels for protein trafficking. Our data show a possible unique method of cell-to-cell communication between apical and basal cells during early embryogenesis.

The cytological changes of tobacco zygote and proembryo cells induced by beta-glucosyl Yariv reagent suggest the involvement of arabinogalactan proteins in cell division and cell plate formation

BACKGROUND

In dicotyledonous plant, the first asymmetric zygotic division and subsequent several cell divisions are crucial for proembryo pattern formation and later embryo development. Arabinogalactan proteins (AGPs) are a family of extensively glycosylated cell surface proteins that are thought to have important roles in various aspects of plant growth and development, including embryogenesis. Previous results from our laboratory show that AGPs are concerned with tobacco egg cell fertilization and zygotic division. However, how AGPs interact with other factors involved in zygotic division and proembryo development remains unknown.

RESULTS

In this study, we used the tobacco in vitro zygote culture system and series of meticulous cell biology techniques to investigate the roles of AGPs in zygote and proembryo cell division. For the first time, we examined tobacco proembryo division patterns detailed to every cell division. The bright-field images and statistical results both revealed that with the addition of an exogenous AGPs inhibitor, beta-glucosyl Yariv (beta-GlcY) reagent, the frequency of aberrant division increased remarkably in cultured tobacco zygotes and proembryos, and the cell plate specific locations of AGPs were greatly reduced after beta-GlcY treatment. In addition, the accumulations of new cell wall materials were also significantly affected by treating with beta-GlcY. Detection of cellulose components by Calcofluor white stain showed that strong fluorescence was located in the newly formed wall of daughter cells after the zygotic division of in vivo samples and the control samples from in vitro culture without beta-GlcY treatment; while there was only weak fluorescence in the newly formed cell walls with beta-GlcY treatment. Immunocytochemistry examination with JIM5 and JIM7 respectively against the low- and high-esterified pectins displayed that these two pectins located in opposite positions of zygotes and proembryos in vivo and the polarity was not affected by beta-GlcY. Furthermore, FM4-64 staining revealed that endosomes were distributed in the cell plates of proembryos, and the localization pattern was also affected by beta-GlcY treatment. These results were further confirmed by subsequent observation with transmission electron microscopy. Moreover, the changes to proembryo cell-organelles induced by beta-GlcY reagent were also observed using fluorescent dye staining technique.

CONCLUSIONS

These results imply that AGPs may not only relate to cell plate position decision, but also to the location of new cell wall components. Correlated with other factors, AGPs further influence the zygotic division and proembryo pattern establishment in tobacco.

Multivesicular bodies in developing tobacco seed and mung bean are functionally equivalent

Protein storage vacuoles (PSVs) are the primarily storage organelles in cotyledon cells for protein preservation in seeds. Storage proteins are transported from the endoplasmic reticulum (ER) to the Golgi apparatus for subsequent delivery to PSVs via presumably Golgi-derived dense vesicles (DVs). However, recent studies demonstrated that storage proteins in early stage of developing cotyledon of mung beans reached the multivesicular bodies (MVBs) prior to the detection of DVs, indicating the possible involvement of MVBs in mediating transport of storage proteins during the early stage of seed development. Here, we further show that the MVBs in developing tobacco seeds are functionally and biochemically equivalent to those in developing mung beans. Thus, MVBs in developing tobacco seeds are structurally distinct from DVs, contain both vacuolar sorting receptors (VSRs) and storage proteins, and they are insensitive to treatments of wortmannin and brefeldin A (BFA).

Silencing MPK4 in Nicotiana attenuata enhances photosynthesis and seed production but compromises abscisic acid-induced stomatal closure and guard cell-mediated resistance to Pseudomonas syringae pv tomato DC3000

Mitogen-activated protein kinases (MAPKs) play pivotal roles in development and environmental interactions in eukaryotes. Here, we studied the function of a MAPK, NaMPK4, in the wild tobacco species Nicotiana attenuata. The NaMPK4-silenced N. attenuata (irNaMPK4) attained somewhat smaller stature, delayed senescence, and greatly enhanced stomatal conductance and photosynthetic rate, especially during late developmental stages. All these changes were associated with highly increased seed production. Using leaf epidermal peels, we demonstrate that guard cell closure in irNaMPK4 was strongly impaired in response to abscisic acid and hydrogen peroxide, and consistently, irNaMPK4 plants transpired more water and wilted sooner than did wild-type plants when they were deprived of water. We show that NaMPK4 plays an important role in the guard cell-mediated defense against a surface-deposited bacterial pathogen, Pseudomonas syringae pv tomato (Pst) DC3000; in contrast, when bacteria directly entered leaves by pressure infiltration, NaMPK4 was found to be less important in the resistance to apoplast-located Pst DC3000. Moreover, we show that salicylic acid was not involved in the defense against PstDC3000 in wild-type and irNaMPK4 plants once it had entered leaf tissue. Finally, we provide evidence that NaMPK4 functions differently from AtMPK4 and AtMPK11 in Arabidopsis (Arabidopsis thaliana), despite their sequence similarities, suggesting a complex functional divergence of MAPKs in different plant lineages. This work highlights the multifaceted functions of NaMPK4 in guard cells and underscores its role in mediating various ecologically important traits.

Comparative transcriptional analysis reveals differential gene expression between asymmetric and symmetric zygotic divisions in tobacco

Asymmetric cell divisions occur widely during many developmental processes in plants. In most angiosperms, the first zygotic cell division is asymmetric resulting in two daughter cells of unequal size and with distinct fates. However, the critical molecular mechanisms regulating this division remain unknown. Previously we showed that treatment of tobacco zygotes with beta-glucosyl Yariv (βGlcY) could dramatically alter the first zygotic asymmetric division to produce symmetric two-celled proembryos. In the present study, we isolated zygotes and two-celled asymmetric proembryos in vivo by micromanipulation, and obtained symmetric, two-celled proembryos by in vitro cell cultures. Using suppression-subtractive hybridization (SSH) and macroarray analysis differential gene expression between the zygote and the asymmetric and symmetric two-celled proembryos was investigated. After sequencing of the differentially expressed clones, a total of 1610 EST clones representing 685 non-redundant transcripts were obtained. Gene ontology (GO) term analysis revealed that these transcripts include those involved in physiological processes such as response to stimulus, regulation of gene expression, and localization and formation of anatomical structures. A homology search against known genes from Arabidopsis indicated that some of the above transcripts are involved in asymmetric cell division and embryogenesis. Quantitative real-time PCR confirmed the up- or down-regulation of the selected candidate transcripts during zygotic division. A few of these transcripts were expressed exclusively in the zygote, or in either type of the two-celled proembryos. Expression analyses of select genes in different tissues and organs also revealed potential roles of these transcripts in fertilization, seed maturation and organ development. The putative roles of few of the identified transcripts in the regulation of zygotic division are discussed. Further functional work on these candidate transcripts will provide important information for understanding asymmetric zygotic division, generation of apical-basal polarity and cell fate decisions during early embryogenesis.

Genes of both parental origins are differentially involved in early embryogenesis of a tobacco interspecies hybrid

BACKGROUND

In animals, early embryonic development is largely dependent on maternal transcripts synthesized during gametogenesis. However, in higher plants, the extent of maternal control over zygote development and early embryogenesis is not fully understood yet. Nothing is known about the activity of the parental genomes during seed formation of interspecies hybrids.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we report that an interspecies hybridization system between SR1 (Nicotiana tabacum) and Hamayan (N. rustica) has been successfully established. Based on the system we selected 58 genes that have polymorphic sites between SR1 and Hamayan, and analyzed the allele-specific expression of 28 genes in their hybrid zygotes (Hamayan x SR1). Finally the allele-specific expressions of 8 genes in hybrid zygotes were repeatedly confirmed. Among them, 4 genes were of paternal origin, 1 gene was of maternal origin and 3 genes were of biparental origin. These results revealed obvious biparental involvement and differentially contribution of parental-origin genes to zygote development in the interspecies hybrid. We further detected the expression pattern of the genes at 8-celled embryo stage found that the involvement of the parental-origin genes may change at different stages of embryogenesis.

CONCLUSIONS/SIGNIFICANCE

We reveal that genes of both parental origins are differentially involved in early embryogenesis of a tobacco interspecies hybrid and functions in a developmental stage-dependent manner. This finding may open a window to seek for the possible molecular mechanism of hybrid vigor.

Regulatory dephosphorylation of CDK at G₂/M in plants: yeast mitotic phosphatase cdc25 induces cytokinin-like effects in transgenic tobacco morphogenesis

BACKGROUND

During the last three decades, the cell cycle and its control by cyclin-dependent kinases (CDKs) have been extensively studied in eukaryotes. This endeavour has produced an overall picture that basic mechanisms seem to be largely conserved among all eukaryotes. The intricate regulation of CDK activities includes, among others, CDK activation by CDC25 phosphatase at G₂/M. In plants, however, studies of this regulation have lagged behind as a plant Cdc25 homologue or other unrelated phosphatase active at G₂/M have not yet been identified.

SCOPE

Failure to identify a plant mitotic CDK activatory phosphatase led to characterization of the effects of alien cdc25 gene expression in plants. Tobacco, expressing the Schizosaccharomyces pombe mitotic activator gene, Spcdc25, exhibited morphological, developmental and biochemical changes when compared with wild type (WT) and, importantly, increased CDK dephosphorylation at G₂/M. Besides changes in leaf shape, internode length and root development, in day-neutral tobacco there was dramatically earlier onset of flowering with a disturbed acropetal floral capacity gradient typical of WT. In vitro, de novo organ formation revealed substantially earlier and more abundant formation of shoot primordia on Spcdc25 tobacco stem segments grown on shoot-inducing media when compared with WT. Moreover, in contrast to WT, stem segments from transgenic plants formed shoots even without application of exogenous growth regulator. Spcdc25-expressing BY-2 cells exhibited a reduced mitotic cell size due to a shortening of the G₂ phase together with high activity of cyclin-dependent kinase, NtCDKB1, in early S-phase, S/G₂ and early M-phase. Spcdc25-expressing tobacco ('Samsun') cell suspension cultures showed a clustered, more circular, cell phenotype compared with chains of elongated WT cells, and increased content of starch and soluble sugars. Taken together, Spcdc25 expression had cytokinin-like effects on the characteristics studied, although determination of endogenous cytokinin levels revealed a dramatic decrease in Spcdc25 transgenics.

CONCLUSIONS

The data gained using the plants expressing yeast mitotic activator, Spcdc25, clearly argue for the existence and importance of activatory dephosphorylation at G₂/M transition and its interaction with cytokinin signalling in plants. The observed cytokinin-like effects of Spcdc25 expression are consistent with the concept of interaction between cell cycle regulators and phytohormones during plant development. The G₂/M control of the plant cell cycle, however, remains an elusive issue as doubts persist about the mode of activatory dephosphorylation, which in other eukaryotes is provided by Cdc25 phosphatase serving as a final all-or-nothing mitosis regulator.

Transcription profile analysis reveals that zygotic division results in uneven distribution of specific transcripts in apical/basal cells of tobacco

BACKGROUND

Asymmetric zygotic division in higher plants results in the formation of an apical cell and a basal cell. These two embryonic cells possess distinct morphologies and cell developmental fates. It has been proposed that unevenly distributed cell fate determinants and/or distinct cell transcript profiles may be the underlying reason for their distinct fates. However, neither of these hypotheses has convincing support due to technical limitations.

METHODOLOGY/PRINCIPAL FINDINGS

Using laser-controlled microdissection, we isolated apical and basal cells and constructed cell type-specific cDNA libraries. Transcript profile analysis revealed difference in transcript composition. PCR and qPCR analysis confirmed that transcripts of selected embryogenesis-related genes were cell-type preferentially distributed. Some of the transcripts that existed in zygotes were found distinctly existed in apical or basal cells. The cell type specific de novo transcription was also found after zygotic cell division.

CONCLUSIONS/SIGNIFICANCE

Thus, we found that the transcript diversity occurs between apical and basal cells. Asymmetric zygotic division results in the uneven distribution of some embryogenesis related transcripts in the two-celled proembryos, suggesting that a differential distribution of some specific transcripts in the apical or basal cells may involve in guiding the two cell types to different developmental destinies.

Dynamic changes of transcript profiles after fertilization are associated with de novo transcription and maternal elimination in tobacco zygote, and mark the onset of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is characterized by the turnover of zygote development from maternal to zygotic control, and has been extensively studied in animals. A majority of studies have suggested that early embryogenesis is maternally controlled and that the zygotic genome remains transcriptionally inactive prior to the MZT. However, little is known about the MZT in higher plants, and its timing and impact remain uncharacterized. Here, we constructed cDNA libraries from tobacco (Nicotiana tabacum) egg cells, zygotes and two-celled embryos for gene expression profiling analysis, followed by RT-PCR confirmation. These analyses, together with experiments using zygote microculture coupled with transcription inhibition, revealed that a marked change in transcript profiles occurs approximately 50 h after fertilization, and that the MZT is initiated prior to zygotic division in tobacco. Although maternal transcripts deposited in egg cells support several early developmental processes, they appear to be insufficient for zygotic polar growth and subsequent cell divisions. Thus, we propose that de novo transcripts are probably required to trigger embryogenesis in later zygotes in tobacco.

Loss of function of the HSFA9 seed longevity program

Gain of function approaches that have been published by our laboratory determined that HSFA9 (Heat Shock Factor A9) activates a genetic program contributing to seed longevity and to desiccation tolerance in plant embryos. We now evaluate the role(s) of HSFA9 by loss of function using different modified forms of HaHSFA9 (sunflower HSFA9), which were specifically overexpressed in seeds of transgenic tobacco. We used two inactive forms (M1, M2) with deletion or mutation of the transcription activation domain of HaHSFA9, and a third form (M3) with HaHSFA9 converted to a potent active repressor by fusion of the SRDX motif. The three forms showed similar protein accumulation in transgenic seeds; however, only HaHSFA9-SRDX showed a highly significant reduction of seed longevity, as determined by controlled deterioration tests, a rapid seed ageing procedure. HaHSFA9-SRDX impaired the genetic program controlled by the tobacco HSFA9, with a drastic reduction in the accumulation of seed heat shock proteins (HSPs) including seed-specific small HSP (sHSP) belonging to cytosolic (CI, CII) classes. Despite such effects, the HaHSFA9-SRDX seeds could survive developmental desiccation during embryogenesis and their subsequent germination was not reduced. We infer that the HSFA9 genetic program contributes only partially to seed-desiccation tolerance and longevity.

Auxin polar transport is essential for the development of zygote and embryo in Nicotiana tabacum L. and correlated with ABP1 and PM H+-ATPase activities

Auxin is an important plant growth regulator, and plays a key role in apical-basal axis formation and embryo differentiation, but the mechanism remains unclear. The level of indole-3-acetic acid (IAA) during zygote and embryo development of Nicotiana tabacum L. is investigated here using the techniques of GC-SIM-MS analysis, immunolocalization, and the GUS activity assay of DR5::GUS transgenic plants. The distribution of ABP1 and PM H(+)-ATPase was also detected by immunolocalization, and this is the first time that integral information has been obtained about their distribution in the zygote and in embryo development. The results showed an increase in IAA content in ovules and the polar distribution of IAA, ABP1, and PM H(+)-ATPase in the zygote and embryo, specifically in the top and basal parts of the embryo proper (EP) during proembryo development. For information about the regulation mechanism of auxin, an auxin transport inhibitor TIBA (2,3,5-triiodobenzoic acid) and exogenous IAA were, respectively, added to the medium for the culture of ovules at the zygote and early proembryo stages. Treatment with a suitable IAA concentration promoted zygote division and embryo differentiation, while TIBA treatment obviously suppressed these processes and caused the formation of abnormal embryos. The distribution patterns of IAA, ABP1, and PM H(+)-ATPase were also disturbed in the abnormal embryos. These results indicate that the polar distribution and transport of IAA begins at the zygote stage, and affects zygote division and embryo differentiation in tobacco. Moreover, ABP1 and PM H(+)-ATPase may play roles in zygote and embryo development and may also be involved in IAA signalling transduction.

The RY/Sph element mediates transcriptional repression of maturation genes from late maturation to early seedling growth

In orthodox seeds, the transcriptional activator ABI3 regulates two major stages in embryo maturation: a mid-maturation (MAT) stage leading to accumulation of storage compounds, and a late maturation (LEA) stage leading to quiescence and desiccation tolerance. Our aim was to elucidate mechanisms for transcriptional shutdown of MAT genes during late maturation, to better understand phase transition between MAT and LEA stages. Using transgenic and transient approaches in Nicotiana, we examined activities of two ABI3-dependent reporter genes driven by multimeric RY and abscisic acid response elements (ABREs) from a Brassica napus napin gene, termed RY and ABRE, where the RY reporter requires ABI3 DNA binding. Expression of RY peaks during mid-maturation and drops during late maturation, mimicking the MAT gene program, and in Arabidopsis thaliana RY elements are over-represented in MAT, but not in LEA, genes. The ABI3 transactivation of RY is inhibited by staurosporine, by a PP2C phosphatase, and by a repressor of maturation genes, VAL1/HSI2. The RY element mediates repression of MAT genes, and we propose that transcriptional shutdown of the MAT program during late maturation involves inhibition of ABI3 DNA binding by dephosphorylation. Later, during seedling growth, VAL1/HSI2 family repressors silence MAT genes by binding RY elements.

Drought tolerance through overexpression of monoubiquitin in transgenic tobacco

Ubiquitin (Ub) is present in all eukaryotic species examined. It is a multifunctional protein and one of its main known functions is to tag proteins for selective degradation by the 26S proteasome. In this study, Ta-Ub2, a cDNA sequence containing a single Ub repeat and a 3' non-coding region of a polyubiquitin gene, was isolated from wheat (Triticum aestivum) by reverse transcription-polymerase chain reaction (RT-PCR). A PBI sense vector with Ta-Ub2 was constructed and transformed into tobacco plants. Ub expression in wheat leaves, monitored by semi-quantitative RT-PCR, responded to drought stress. In transgenic tobacco, determined by protein gel blot analysis, we found higher amounts of Ub-protein conjugates than in control (tobacco carrying a PBI GUS vector without Ta-Ub2) and wild-type (WT) lines. However, free Ub levels did not significantly differ in the 3 genotypes. Seeds from transgenic, Ub-overexpressing tobacco germinated faster and seedlings grew more vigorously than control and WT samples, both under drought and non-drought conditions. Furthermore, CO(2) assimilation of transgenic plants was significantly higher under drought stress. Our results indicate that Ub may be involved in the response of plants to drought stress and that overexpression of monoubiquitin might be an effective strategy for enhancing drought tolerance.

Combination of reversible male sterility and doubled haploid production by targeted inactivation of cytoplasmic glutamine synthetase in developing anthers and pollen

Reversible male sterility and doubled haploid plant production are two valuable technologies in F(1)-hybrid breeding. F(1)-hybrids combine uniformity with high yield and improved agronomic traits, and provide self-acting intellectual property protection. We have developed an F(1)-hybrid seed technology based on the metabolic engineering of glutamine in developing tobacco anthers and pollen. Cytosolic glutamine synthetase (GS1) was inactivated in tobacco by introducing mutated tobacco GS genes fused to the tapetum-specific TA29 and microspore-specific NTM19 promoters. Pollen in primary transformants aborted close to the first pollen mitosis, resulting in male sterility. A non-segregating population of homozygous doubled haploid male-sterile plants was generated through microspore embryogenesis. Fertility restoration was achieved by spraying plants with glutamine, or by pollination with pollen matured in vitro in glutamine-containing medium. The combination of reversible male sterility with doubled haploid production results in an innovative environmentally friendly breeding technology. Tapetum-mediated sporophytic male sterility is of use in foliage crops, whereas microspore-specific gametophytic male sterility can be applied to any field crop. Both types of sterility preclude the release of transgenic pollen into the environment.

Tobacco zygotic embryogenesis in vitro: the original cell wall of the zygote is essential for maintenance of cell polarity, the apical-basal axis and typical suspensor formation

We have developed a reliable in vitro zygotic embryogenesis system in tobacco. A single zygote of a dicotyledonous plant was able to develop into a fertile plant via direct embryogenesis with the aid of a co-culture system in which fertilized ovules were employed as feeders. The results confirmed that a tobacco zygote could divide in vitro following the basic embryogenic pattern of the Solanad type. The zygote cell wall and directional expansion are two critical points in maintaining apical-basal polarity and determining the developmental fate of the zygote. Only those isolated zygotes with an almost intact original cell wall could continue limited directional expansion in vitro, and only these directionally expanded zygotes could divide into typical apical and basal cells and finally develop into a typical embryo with a suspensor. In contrast, isolated zygote protoplasts deprived of cell walls could enlarge but could not directionally elongate, as in vivo zygotes do before cell division, even when the cell wall was regenerated during in vitro culture. The zygote protoplasts could also undergo asymmetrical division to form one smaller and one larger daughter cell, which could develop into an embryonic callus or a globular embryo without a suspensor. Even cell walls that hung loosely around the protoplasts appeared to function, and were closely correlated with the orientation of the first zygotic division and the apical-basal axis, further indicating the essential role of the original zygotic cell wall in maintaining apical-basal polarity and cell-division orientation, as well as subsequent cell differentiation during early embryo development in vitro.

Transcriptional and metabolic profiles of stress-induced, embryogenic tobacco microspores

Higher plant microspores, when subjected to various stress treatments in vitro, are able to reprogram their regular gametophytic development towards the sporophytic pathway to form haploid embryos and plants. Suppression subtractive hybridization (SSH) and metabolic profiling were used to characterize this developmental switch. Following differential reverse Northern hybridizations 90 distinct up-regulated sequences were identified in stressed, embryogenic microspores (accessible at www.univie.ac.at/ntsm). Sequence analyses allowed the classification of these genes into functional clusters such as metabolism, chromosome remodeling, signaling, transcription and translation, while the putative functions of half of the sequences remained unknown. A comparison of metabolic profiles of non-stressed and stressed microspores using gas chromatography/mass spectrometry (GC/MS) identified 70 compounds, partly displaying significant changes in metabolite levels, e.g., highly elevated levels of isocitrate and isomaltose in stressed microspores compared to non-stressed microspores. The formation of embryogenic microspores is discussed on the basis of the identified transcriptional and metabolic profiles.

Two promoters conferring active gene expression in vegetative nuclei of tobacco immature pollen undergoing embryogenic dedifferentiation

In order to visualize the specific state of tobacco pollen undergoing dedifferentiation from immature pollen to embryogenic cells, we established tobacco marker lines transgenic for a vital reporter gene regulated under the transcriptional control of an 840 bp fragment, named A22pro. This fragment was obtained from the 5'-flanking region of a gene corresponding to a cDNA named A22 that was previously isolated through differential screening from a cDNA library prepared from tobacco pollen undergoing dedifferentiation. The reporter gene, named H3sGFP, consisting of synthetic green fluorescent protein gene (sGFP) and tobacco H3 histone gene for nuclear localization, was designed to distinguish the gene expression in the generative cell from that in the vegetative cell in a pollen grain. The marker line produced pollen showing a green fluorescent signal in the generative nuclei (GN) but the expression level of the transgene was low. Pollen after culture for dedifferentiation showed an intense signal transiently in the vegetative nuclei (VN), at a specific developmental stage of pollen, with a rapid increase of expression level of the transgene. Serial observations revealed that all androgenic embryos originated from the pollen grains that had shown the signal in their VN. Thus, A22pro is originally functional in gametogenesis but is activated in VN of pollen undergoing embryogenic dedifferentiation. Additionally, we observed a gene expression pattern identical to that described above, using another 5'-flanking region of a gene for a cDNA, named B27pro, homologous to A22 as a promoter of the reporter gene.

Regulation of carotenoid and ABA accumulation during the development and germination of Nicotiana plumbaginifolia seeds

Abscisic acid (ABA) is derived from epoxycarotenoid cleavage and regulates seed development and maturation. A detailed carotenoid analysis was undertaken to study the contribution of epoxycarotenoid synthesis to the regulation of ABA accumulation in Nicotiana plumbaginifolia developing seeds. Maximal accumulation of xanthophylls occurred at mid-development in wild type seeds, when total ABA levels also peaked. In contrast, in ABA-deficient mutants xanthophyll synthesis was delayed, in agreement with the retardation in seed maturation. Seed dormancy was restored in mutants impaired in the conversion of zeaxanthin into violaxanthin by zeaxanthin epoxidase (ZEP), by the introduction of the Arabidopsis AtZEP gene under the control of promoters inducing expression during later stages of seed development compared to wild type NpZEP, and in dry and imbibed seeds. Alterations in the timing and level of ZEP expression did not highly affect the temporal regulation of ABA accumulation in transgenic seeds, despite notable perturbations in xanthophyll accumulation. Therefore, major regulatory control of ABA accumulation might occur downstream of epoxycarotenoid synthesis.

Nicotiana (Nicotiana tobaccum, Nicotiana benthamiana)

Agrobacterium-mediated transformation of Nicotiana species, namely, Nicotiana tobaccum and Nicotiana benthamiana, using leaf disks as the target explant has provided the plant community with a valuable tool for rapid evaluation of transgenes in higher plants. This protocol has a number of desirable attributes: readily available explant material, high efficiency, and a relatively quick turnaround time. The in vitro regeneration scheme of the leaf disks is prolific and follows an indirect organogenic differentiation. N. tobaccum and N. benthamiana are highly susceptible to a variety of disarmed A. tumefaciens strains, and the transformation system is amenable to a number of selection agents. This protocol has proved to be easily transferable to a point at which a single investigator with minimal training can generate thousand of events per year. Moreover, seed size and quantity per event permit monitoring of segregation in Petri plates, and sufficient biomass can be accrued from an individual plant, which can be either clonally propagated or allowed to self-pollinate or easily outcrossed.

Expression of the promoter of HyPRP, an embryo-specific gene from Zea mays in maize and tobacco transgenic plants

zmHyPRP is a gene specifically expressed in maize immature embryos where its transcripts are mainly observed in the scutellum. It has been shown that zmHyPRP expression in the embryo is arrested when ABA levels increase at the beginning of the maturation stage. Here we report the ability of 2 Kb zmHyPRP promoter to reproduce the zmHyPRP gene specific expression pattern in the maize embryo and its repression by ABA at the end of the morphogenetic process. Three different approaches have been used, transient particle bombardment of maize immature excised embryos and stable transformation of maize and tobacco plants with a construct containing 2 Kb of zmHyPRP promoter fused to the GUS gene. This construct has shown to confer specific expression to maize and tobacco embryos but in tobacco expression in the embryo was very low. The same construct was also negatively regulated by ABA in embryos of both species. This suggests that 2 Kb of the zmHyPRP promoter contain all regulatory elements sufficient to confer the developmental expression patterns of the gene characterized to date.

Cytokinin-induced abnormal shoot organogenesis is associated with elevated Knotted1-type homeobox gene expression in tobacco

The molecular mechanisms that regulate the transcription of key developmental genes involved in shoot organogenesis have yet to be fully elucidated. However, it is clear that plant growth regulators, such as cytokinin, play a critical role in the differentiation of adventitious shoots. In Nicotiana tabacum zz100 leaf discs, high frequency shoot formation could be induced with 5 microM of the cytokinin N6-benzyladenine (BA). Increasing the exogenous BA concentration to greater than 20 microM resulted in stunted explants with abnormal shoot morphology and altered mineral composition. Explants with abnormal shoots did not appear to be hyperhydric. Abnormalities were, however, associated with an increase in the expression of a knotted1-type homeobox gene (TobH1) isolated from normal shoot-forming cultures. The results suggest that the development of cytokinin-induced abnormal shoot morphology possibly involves changes in TobH1 gene expression.

Inhibition of plant protein synthesis by the cyanobacterial hepatotoxin, cylindrospermopsin

Cylindrospermopsin, a cyanobacterial guanidine alkaloid hepatotoxin and protein synthesis-inhibitor, was assayed for its effects on the germination of pollen from tobacco (Nicotiana tabacum cv Samsun NN). Pollen germination, measured by Alcian Blue dye-binding, was inhibited by cylindrospermopsin between 5 and 1000 microg ml(-1). As a protein synthesis-inhibitor, cylindrospermopsin did not inhibit pollen germination to the same extent as cycloheximide on a gravimetric basis, but significantly reduced the amount of (14)C-(U)-l-leucine labelling in pollen tubes. The inhibition of tobacco pollen germination may be amenable for development as a bioassay for cylindrospermopsin, although this would require a pre-concentration step for the monitoring of environmental samples. Implications of these observations for current spray-irrigation practices are discussed.

Maternal synthesis of abscisic acid controls seed development and yield in Nicotiana plumbaginifolia

The role of maternally derived abscisic acid (ABA) during seed development has been studied using ABA-deficient mutants of Nicotiana plumbaginifolia Viviani. ABA deficiency induced seed abortion, resulting in reduced seed yield, and delayed growth of the remaining embryos. Mutant grafting onto wild-type stocks and reciprocal crosses indicated that maternal ABA, synthesized in maternal vegetative tissues and translocated to the seed, promoted early seed development and growth. Moreover ABA deficiency delayed both seed coat pigmentation and capsule dehiscence. Mutant grafting did not restore these phenotypes, indicating that ABA synthesized in the seed coat and capsule envelope may have a positive effect on capsule and testa maturation. Together these results shed light on the positive role of maternal ABA during N. plumbaginifolia seed development.

The tobacco A-type cyclin, Nicta;CYCA3;2, at the nexus of cell division and differentiation

Although most of the components of the cell cycle machinery are conserved in all eukaryotes, plants differ strikingly from animals by the absence of a homolog of E-type cyclin, an important regulator involved in G1/S-checkpoint control in animals. By contrast, plants contain a complex range of A-type cyclins, with no fewer than 10 members in Arabidopsis. We previously identified the tobacco A-type cyclin Nicta;CYCA3;2 as an early G1/S-activated gene. Here, we show that antisense expression of Nicta;CYCA3;2 in tobacco plants induces defects in embryo formation and impairs callus formation from leaf explants. The green fluorescent protein (GFP)-Nicta;CYCA3;2 fusion protein was localized in the nucleoplasm. Transgenic tobacco plants overproducing GFP-Nicta;CYCA3;2 could not be regenerated from leaf disc transformation, whereas some transgenic Arabidopsis plants were obtained by the floral-dip transformation method. Arabidopsis plants that overproduce GFP-Nicta;CYCA3;2 showed reduced cell differentiation and endoreplication and a dramatically modified morphology. Calli regenerated from leaf explants of these transgenic Arabidopsis plants were defective in shoot and root regeneration. We propose that Nicta;CYCA3;2 has important functions, analogous to those of cyclin E in animals, in the control of plant cell division and differentiation.

Expansion of the cell plate in plant cytokinesis requires a kinesin-like protein/MAPKKK complex

The tobacco mitogen-activated protein kinase kinase kinase NPK1 regulates lateral expansion of the cell plate at cytokinesis. Here, we show that the kinesin-like proteins NACK1 and NACK2 act as activators of NPK1. Biochemical analysis suggests that direct binding of NACK1 to NPK1 stimulates kinase activity. NACK1 is accumulated specifically in M phase and colocalized with NPK1 at the phragmoplast equator. Overexpression of a truncated NACK1 protein that lacks the motor domain disrupts NPK1 concentration at the phragmoplast equator and cell plate formation. Incomplete cytokinesis is also observed when expression of NACK1 and NACK2 is repressed by virus-induced gene silencing and in embryonic cells from Arabidopsis mutants in which a NACK1 ortholog is disrupted. Thus, we conclude that expansion of the cell plate requires NACK1/2 to regulate the activity and localization of NPK1.

Ontogeny of plants under various gravity condition

The results of experiments performed under conditions of microgravity (MG) or under its simulation on the horizontal clinostat (HC) with the callus, seedlings of various species and embryogenic structures have revealed a definite role of gravity as an ecological factor in the processes of cytomorphogenesis, growth, and development. The transformation of differentiated somatic cells of arabidopsis seed into undifferentiated callus was not inhibited under MG, though modifications of the whole callus morphology and of mean cell and nucleus size were observed. The morphogenesis of polar structures such as root-hair bearing cells of Lactuca primary root has been shown to be modified in the course of differentiation under mass acceleration diminished below 0.1 g. Seed germination and seedling morphogenesis under MG follow their normal course, but a significant stimulation of shoot growth with no effect on primary root growth has been determined. A successful in vitro regeneration of Nicotiana tabacum plantlets from leaf cells and subsequent formation of shoots and roots on a continuously rotating HC as well as the formation of viable seeds during seed-to-seed growth of Arabidopsis plants under MG have indicated that gravity plays but a limited role in the processes of embryogenesis and organogenesis.

Molecular characterization of cDNAs encoding G protein alpha and beta subunits and study of their temporal and spatial expression patterns in Nicotiana plumbaginifolia Viv

We have isolated cDNA sequences encoding alpha and beta subunits of potential G proteins from a cDNA library prepared from somatic embryos of Nicotiana plumbaginifolia Viv. at early developmental stages. The predicted NPGPA1 and NPGPB1 gene products are 75-98% identical to the known respective plant alpha and beta subunits. Southern hybridizations indicate that NPGPA1 is probably a single-copy gene, whereas at least two copies of NPGPB1 exist in the N. plumbaginifolia genome. Northern analyses reveal that both NPGPA1 and NPGPB1 mRNA are expressed in all embryogenic stages and plant tissues examined and their expression is obviously regulated by the plant hormone auxin. Immunohistological localization of NPGPalpha1 and NPGPbeta1 preferentially on plasma and endoplasmic reticulum membranes and their immunochemical detection exclusively in microsomal cell fractions implicate membrane association of both proteins. The temporal and spatial expression patterns of NPGPA1 and NPGPB1 show conformity as well as differences. This could account for not only cooperative, but also individual activities of both subunits during embryogenesis and plant development.

Cloning of cDNA encoding NtEPc, a marker protein for the embryogenic dedifferentiation of immature tobacco pollen grains cultured in vitro

We partially purified three Nicotiana tabacum L. embryogenic pollen-abundant phosphoproteins (NtEPa to c) which appeared in the cells undergoing a dedifferentiation process from immature pollen grains to embryogenic cells, caused by glutamine-deficiency in vitro. All the NtEPs had a highly conserved N-terminal amino acid sequence. Using degenerate oligonucleotide probes designed from the amino acid sequences, the cDNA for NtEPc was isolated from a cDNA library of pollen cultured in glutamine-free medium The cDNA sequence showed moderate homology with several type-1 copper-binding glycoproteins and with a kind of early nodulin though its function could not be predicted. Expression analysis revealed that the level of mRNA for NtEPc was high during the dedifferentiation and also in the very early period of pollen embryogenesis but it was low in the developmental process of microspores/pollen in anthers, in the in vitro maturation process and both in the stational and logarithmic growth phases of tobacco BY-2 cells. Furthermore, an acidic medium pH, which promoted the induction of dedifferentiation increased the level of mRNA for NtEPc, whereas the presence of 6-benzylaminopurine, which inhibited it, decreased the level. These results suggest that the expression of NtEPc gene is correlated with the dedifferentiation but not with pollen development or cell division.

Dual regulation of a heat shock promoter during embryogenesis: stage-dependent role of heat shock elements

Transgenic tobacco expression was analysed of chimeric genes with point mutations in the heat shock element (HSE) arrays of a small heat shock protein (sHSP) gene from sunflower: Ha hsp17.7 G4. The promoter was developmentally regulated during zygotic embryogenesis and responded to heat stress in vegetative tissues. Mutations in the HSE affected nucleotides crucial for human heat shock transcription factor 1 (HSF1) binding. They abolished the heat shock response of Ha hsp17.7 G4 and produced expression changes that demonstrated dual regulation of this promoter during embryogenesis. Thus, whereas activation of the chimeric genes during early maturation stages did not require intact HSE, expression at later desiccation stages was reduced by mutations in both the proximal (-57 to -89) and distal (-99 to -121) HSE. In contrast, two point mutations in the proximal HSE that did not severely affect gene expression during zygotic embryogenesis, eliminated the heat shock response of the same chimeric gene in vegetative organs. Therefore, by site-directed mutagenesis, it was possible to separate the heat shock response of Ha hsp17.7 G4 from its developmental regulation. The results indicate the co-existence, in a single promoter, of HSF-dependent and -independent regulation mechanisms that would control sHSP gene expression at different stages during plant embryogenesis.

Heat shock elements are involved in heat shock promoter activation during tobacco seed maturation

The soybean Gmhsp 17.3-B heat shock promoter is developmentally regulated in transgenic tobacco, as indicated by the constitutive expression of a beta-glucuronidase reporter in seeds [16]. In this paper, we show that both the heat shock promoter-driven beta-glucuronidase activity and the mRNA of the endogenous Nthsp18P gene accumulate coincident with the onset of seed desiccation. Deletions of the soybean Gmhsp17.3-B promoter, encompassing the heat shock element (HSE)-containing regions, revealed a co-localization of sequences responsible for heat induction and developmental expression. Moreover, synthetic HSEs fused to a TATA box sequence had the potential to stimulate the developmental expression of a GUS reporter gene in seeds of transgenic plants.

Different sets of cis-elements contribute to the expression of a catalase gene from castor bean during seed formation and postembryonic development in transgenic tobacco

Deletion analysis of the promoter region of a gene for catalase, cat2, from castor bean (Ricinus communis) was performed to identify the cis-regulatory elements responsible for the expression of a beta-glucuronidase (GUS) fusion gene during seed formation and postembryonic development in transgenic tobacco. The analysis showed that multiple cis-elements contribute to the activity of the cat2 promoter during seed formation and postembryonic development. The 5'-upstream regions from -1,241 to -816 bp, from -720 to -682 bp, and from -632 to -535 bp, relative to the site of initiation of translation of cat2, contributed positively to the activity of the cat2 promoter during both stages. By contrast, the region from -816 to -720 bp had a negative effect at both stages. The region from -682 to -632 bp contributed positively to the activity during seed formation but negatively during postembyonic development. Histochemical analysis revealed that the multiple cis-elements determined not only the level of expression of the chimeric gene but also the tissue-specificity of such expression. For example, the region from -1,241 to -816 bp allowed expression of the chimeric gene in the axis of the embryo of the dry seed, as well as in the cortex of the middle part of the hypocotyl and at the base of epicotyl in the young seedling.

Isolation of a cDNA encoding a cytochrome b5 specifically expressed in developing tobacco seeds

Polymerase chain reaction (PCR) was used to amplify transcripts encoding cytochrome b5 from cDNA synthesised from RNA isolated from developing seeds of tobacco (Nicotiana tabacum L.). The sequence of the amplified products indicated that the clones encoded a second form of tobacco cytochrome b5, different from that previously characterised (Smith et al. 1994, Plant Mol Biol 25:527-537). Rapid amplification of cDNA ends (RACE)-PCR was used to amplify the 5' and 3' ends of the transcript. Northern blotting and RNAse protection assays of RNA samples isolated from different tobacco tissues indicated that this second cytochrome b5 form was expressed only in developing seeds. Therefore, it seems likely that this message is the product of a tobacco cytochrome b5 gene specifically expressed in seeds.

Differences in cell type-specific expression of the gene Opaque 2 in maize and transgenic tobacco

The Opaque 2 (O2) gene encodes a transcriptional activator of the basic region/leucine zipper family, which controls the synthesis of a major storage protein class in maize endosperm, the 22 kDa alpha-zeins, and of several other non-zein polypeptides including b32. We demonstrate, by analysing O2 mRNAs in different organs of maize plants, that the O2 gene is only active in the endosperm. Its transcription is precisely controlled during seed development: O2 mRNAs are first detected 10 days after pollination and accumulate in the endosperm over a period of 20 days. When introduced into tobacco plants, the O2 promoter directs the expression of the beta-glucuronidase (GUS) reporter gene in endosperm, but also in the embryo, cotyledons and pollen. The first 185 bp of the O2 promoter is sufficient for developmentally regulated expression in tobacco seeds. A distinct cis-acting element, located between positions -185 and -520, directs expression in the cotyledons of tobacco seedlings. The possible origins of this breakdown in promoter specificity in the heterologous host are discussed.