Differential manifestation of seed mortality induced by seed-specific expression of the gene for diphtheria toxin A chain in Arabidopsis and tobacco

A dynamic WUSCHEL/Layer 1 interplay directs shoot apical meristem formation during regeneration in tobacco

De novo shoot apical meristem (SAM) organogenesis during regeneration in tissue culture has been investigated for several decades, but the precise mechanisms governing early-stage cell fate specification remain elusive. In contrast to SAM establishment during embryogenesis, in vitro SAM formation occurs without positional cues and is characterized by autonomous initiation of cellular patterning. Here, we report on the initial stages of SAM organogenesis and on the molecular mechanisms that orchestrate gene patterning to establish SAM homeostasis. We found that SAM organogenesis in tobacco calli starts with protuberance formation followed by the formation of an intact L1 layer covering the nascent protuberance. We also exposed a complex interdependent relationship between L1 and WUS expression and revealed that any disruption in this interplay compromises shoot formation. Silencing WUS in nascent protuberances prevented L1 formation and caused the disorganization of the outer cell layers exhibiting both anticlinal and periclinal divisions, suggesting WUS plays a critical role in the proper establishment and organization of L1 during SAM organogenesis. We further discovered that silencing TONNEAU1 prevents the exclusive occurrence of anticlinal divisions in the outermost layer of the protuberances and suppresses the acquisition of L1 cellular identity and L1 formation, ultimately impeding SAM formation and regeneration. This study provides a novel molecular framework for the characterization of a WUS/L1 interplay that mediates SAM formation during regeneration.

Induction of Male Sterility in Tobacco by Anther-Specific Expression of the Gene for Ricin Enzymatic Subunit A Chain RTA

Targeted gene expression in plants allows us to further study biological traits of interest, such as reproductive and developmental processes. Here, the tobacco TA29 anther-specific promoter was used to direct the expression of the ricin enzymatic subunit A (RTA) in transgenic tobacco plants, phenotypic analysis of the resulting positive transgenic tobacco (Nicotiana tabacum L.) plants demonstrated that RTA expression led to a reduction in pistil length and shriveling of anthers, as well as the grayish-brown color of anthers, the reduced pollen viability and male sterility. For the first time, a plant-derived ricin gene enzymatic subunit A (RTA) expression system under the tissue-specific promoter was demonstrated to be sensitive and efficient in controlling plant sterility and creating male-sterile materials. Consequently, it could be used to control other agronomic traits and produce hybrid seeds in plants in the future.

A mutated cytosine deaminase gene, codA (D314A), as an efficient negative selection marker for gene targeting in rice

Gene targeting (GT) is a powerful tool manipulating a gene of interest in a given genome specifically and precisely. To achieve efficient GT in higher plants, both positive and negative selection markers are required. In particular, a strong negative selection system is needed for enrichment of cells to eliminate those cells in which random integration of the introduced DNA has occurred in GT experiments. Currently, non-conditional negative selection marker genes are used for GT experiments in rice plants, and no conditional negative selection system is available. In this study, we describe the development of an efficient conditional negative selection system in rice plants using Escherichia coli cytosine deaminase (codA). We found that a mutant codA gene, codA(D314A), acts more efficiently than the wild-type codA for negative selection in rice plants. The codA(D314A) marker was further used as a negative selection marker for GT experiments in rice. Our conditional negative selection system effectively eliminated the cells in which random integration event(s) occurred; the enrichment factor was approximately 100-fold. This enrichment factor was similar to that found when Corynebacterium diphtheriae toxin fragment A was used. Our results suggest the codA(D314A) marker gene as a promising negative selection marker for GT of rice.

Organ boundary1 defines a gene expressed at the junction between the shoot apical meristem and lateral organs

We identify a gene, organ boundary1 (OBO1), by its unique pattern of enhancer- driven GFP expression at the boundaries between the apical meristems and lateral organs in Arabidopsis embryos, seedlings, and mature plants. OBO1 also is expressed at the root apical meristem and in distinct cell files surrounding this area. OBO1 is one of a 10-member plant-specific gene family encoding a single small domain (133 amino acids) with unknown function. One member of this gene family, OBO2, is identical to a previously studied gene, light-sensitive hypocotyl1. Overexpression of OBO1 causes an abnormal number and size of petals and petal-stamen fusions. The patterns of OBO1 gene expression are distinct but overlap with other genes involved in boundary formation in the Arabidopsis shoot apical meristem, including cup-shaped cotyledon, lateral organ boundaries, blade-on-petiole, asymmetric leaves, and lateral organ fusion. Nuclear localization of OBO1 suggests that it might act with one or more of the transcription factors encoded by the foregoing genes. Ablation of the specific cells expressing OBO1 leads to loss of the shoot apical meristem and lateral organs. Thus, the cells expressing OBO1 are important for meristem maintenance and organogenesis in Arabidopsis.

Developmental steps in acquiring competence for shoot development in Arabidopsis tissue culture

Arabidopsis shoots regenerate from root explants in tissue culture through a two-step process requiring preincubation on an auxin-rich callus induction medium (CIM) followed by incubation on a cytokinin-rich shoot induction medium (SIM). During CIM preincubation, root explants acquire competence to respond to shoot induction signals. During CIM preincubation, pericycle cells in root explants undergo cell divisions and dedifferentiate, losing the expression of a pericycle cell-specific marker. These cells acquire competence to form green callus only after one day CIM preincubation and to form shoots after 2-3 days CIM preincubation. Reversible DNA synthesis inhibitors interfered with the acquisition of competence to form shoots. Genes requiring CIM preincubation for upregulation on SIM were identified by microarray analysis and included RESPONSE REGULATOR 15 (ARR15), POLYGALACTURONASE INHIBITING PROTEIN 2 (PGIP2) and WUSCHEL (WUS). These genes served as developmental markers for the acquisition of competence because the CIM preincubation requirements for ARR15 and PGIP2 upregulation correlated well with the acquisition of competence to form green callus, and the CIM preincubation requirements for WUS upregulation matched those for shoot formation. Unlike ARR15, another cytokinin inducible, A-type ARR gene, ARR5, was upregulated on SIM, but the induction did not require CIM preincubation. These findings indicate that competencies for various events associated with shoot regeneration are acquired progressively during CIM preincubation, and that a set of genes, normally upregulated on SIM, are repressed by a process that can be relieved by CIM preincubation.

GAL4-GFP enhancer trap lines for genetic manipulation of lateral root development in Arabidopsis thaliana

Lateral root development occurs throughout the life of the plant and is responsible for the plasticity of the root system. In Arabidopsis thaliana, lateral root founder cells originate from pericycle cells adjacent to xylem poles. In order to study the mechanisms of lateral root development, a population of Arabidopsis GAL4-GFP enhancer trap lines were screened and two lines were isolated with GAL4 expression in root xylem-pole pericycle cells (J0121), i.e. in cells competent to become lateral root founder cells, and in young lateral root primordia (J0192). These two enhancer trap lines are very useful tools with which to study the molecular and cellular bases of lateral root development using targeted gene expression. These lines were used for genetic ablation experiments by targeting the expression of a toxin-encoding gene. Moreover, the molecular bases of the enhancer trap expression pattern were characterized. These results suggest that the lateral-root-specific GAL4 expression pattern in J0192 is due to a strong enhancer in the promoter of the LOB-domain protein gene LBD16.

beta-1,3-Glucanase gene expression in low-hydrated seeds as a mechanism for dormancy release during tobacco after-ripening

An air-dry developmental state with low-hydrated tissues is a characteristic of most plant seeds. Seed dormancy is an intrinsic block of germination and can be released during after-ripening, that is air-dry storage of mature seeds. Both seed-covering layers, testa and endosperm, cause the coat-imposed dormancy of tobacco (Nicotiana tabacum). After-ripening and over-expression of class I beta-1,3-glucanase (betaGlu I) confer maternal effects on testa rupture and dormancy release. Very little is known about the molecular mechanisms of after-ripening and whether gene expression is possible in low-hydrated seeds. Transient, low-level betaGlu I transcription and translation was detected during tobacco seed after-ripening. (1)H NMR 2D micro-imaging showed uneven distribution of proton mobility in seeds. betaGlu I gene expression is associated spatially with the inner testa and temporally with the promotion of testa rupture. Local elevation in moisture content seems to permit local, low-level betaGlu I gene transcription and translation in the maternal tissues of air-dry, low-hydrated seeds. De novo gene expression is therefore proposed to be a novel molecular mechanism for the release of coat-imposed dormancy during oilseed after-ripening.

Temperature sensitive diphtheria toxin confers conditional male-sterility in Arabidopsis thaliana

A gene encoding a temperature-sensitive diphtheria toxin A chain (DTA) polypeptide was fused to the Arabidopsis thaliana tapetum-specific A9 promoter. Expression of the chimaeric gene in transgenic A. thaliana lines resulted in plants that were male-sterile, but female-fertile, when grown at 18 degrees C, and fully self fertile at 26 degrees C. No pollen grains were found on the anthers of transgenic plants grown at 18 degrees C, although aggregated pollen grains were found inside the anthers. Electron microscopy revealed discrete alterations in the tapetal cells of the male-sterile transgenic plants. The strength of the phenotype observed in segregants correlated with the level of expression of the gene and the copy number. The low frequency at which fully male-sterile plants were generated suggests that the temperature-sensitive DTA protein is disabled as a cytotoxin, relative to the wild-type protein activity.

Transactivation of BARNASE under the AtLTP1 promoter affects the basal pole of the embryo and shoot development of the adult plant in Arabidopsis

Genetically controlled expression of a toxin provides a tool to remove a specific structure and consequently study its role during a developmental process. The availability of many tissue-specific promoters is a good argument for the development of such a strategy in plants. We have developed a conditional system for targeted toxin expression and demonstrated its use for generating embryo phenotypes that can bring valuable information about signalling during embryogenesis. The BARNASE gene was expressed in the Arabidopsis embryo under the control of two promoters, one from the cyclin AtCYCB1 gene and one from the AtLTP1 gene (Lipid Transfer Protein 1). One-hundred percent seed abortion was obtained with the cyclin promoter. Surprisingly however, the embryos displayed a range of lethal phenotypes instead of a single arrested stage as expected from this promoter. We also show that BARNASE expression under the control of the AtLTP1 promoter affects the basal pole of the globular embryo. Together with reporter expression studies, this result suggests a role of the epidermis in controlling the development of the lower tier of the embryo. This defect was not embryo-lethal and we show that the seedlings displayed a severe shoot phenotype correlated to epidermal defects. Therefore, the epidermis does not play an active role during organogenesis in seedlings but is important for the postgermination development of a viable plant.

Differential mercury volatilization by tobacco organs expressing a modified bacterial merA gene

Mercury pollution is a major environmental problem accompanying industrial activities. Most of the mercury released ends up and retained in the soil as complexes of the toxic ionic mercury (Hg2+), which then can be converted by microbes into the even more toxic methylmercury which tends to bioaccumulate. Mercury detoxification of the soil can also occur by microbes converting the ionic mercury into the least toxic metallic mercury (Hg0) form, which then evaporates. The remediation potential of transgenic plants carrying the MerA gene from E. coli encoding mercuric ion reductase could be evaluated. A modified version of the gene, optimized for plant codon preferences (merApe9, Rugh et al. 1996), was introduced into tobacco by Agrobacterium-mediated leaf disk transformation. Transgenic seeds were resistant to HgCl2 at 50 microM, and some of them (10-20% ) could germinate on media containing as much as 350 microM HgCl2, while the control plants were fully inhibited or died on 50 microM HgCl2. The rate of elemental mercury evolution from Hg2+ (added as HgCl2) was 5-8 times higher for transgenic plants than the control. Mercury volatilization by isolated organs standardized for fresh weight was higher (up to 5 times) in the roots than in shoots or the leaves. The data suggest that it is the root system of the transgenic plants that volatilizes most of the reduced mercury (Hg0). It also suggests that much of the mercury need not enter the vascular system to be transported to the leaves for volatilization. Transgenic plants with the merApe9 gene may be used to mercury detoxification for environmental improvement in mercury-contaminated regions more efficiently than it had been predicted based on data on volatilization of whole plants via the upper parts only (Rugh et al. 1996).

Genetic ablation of root cap cells in Arabidopsis

The root cap is increasingly appreciated as a complex and dynamic plant organ. Root caps sense and transmit environmental signals, synthesize and secrete small molecules and macromolecules, and in some species shed metabolically active cells. However, it is not known whether root caps are essential for normal shoot and root development. We report the identification of a root cap-specific promoter and describe its use to genetically ablate root caps by directing root cap-specific expression of a diphtheria toxin A-chain gene. Transgenic toxin-expressing plants are viable and have normal aerial parts but agravitropic roots, implying loss of root cap function. Several cell layers are missing from the transgenic root caps, and the remaining cells are abnormal. Although the radial organization of the roots is normal in toxin-expressing plants, the root tips have fewer cytoplasmically dense cells than do wild-type root tips, suggesting that root meristematic activity is lower in transgenic than in wild-type plants. The roots of transgenic plants have more lateral roots and these are, in turn, more highly branched than those of wild-type plants. Thus, root cap ablation alters root architecture both by inhibiting root meristematic activity and by stimulating lateral root initiation. These observations imply that the root caps contain essential components of the signaling system that determines root architecture.

Negative selection systems for transgenic barley (Hordeum vulgare L.): comparison of bacterial codA- and cytochrome P450 gene-mediated selection

Efficient negative selection systems are increasingly needed for numerous applications in plant biology. In recent years various counter-selectable genes have been tested in six dicotyledonous species, whereas there are no data available for the use of negative selection markers in monocotyledonous species. In this study, we compared the applicability and reliability of two different conditional negative selection systems in transgenic barley. The bacterial codA gene encoding cytosine deaminase, which converts the non-toxic 5-fluorocytosine (5-FC) into the toxic 5-fluorouracil (5-FU), was used for in vitro selection of germinating seedlings. Development of codA-expressing seedlings was strongly inhibited by germinating the seeds in the presence of 5-FC. For selecting plants in the greenhouse, a bacterial cytochrome P450 mono-oxygenase gene, the product of which catalyses the dealkylation of a sulfonylurea compound, R7402, into its cytotoxic metabolite, was used. T1 plants expressing the selectable marker gene showed striking morphological differences from the non-transgenic plants. In experiments with both negative selectable markers, the presence or absence of the transgene, as predicted from the physiological appearance of the plants under selection, was confirmed by PCR analysis. We demonstrate that both marker genes provide tight negative selection; however, the use of the P450 gene is more amenable to large-scale screening under greenhouse or field conditions.

Variable expression of the herpes simplex virus thymidine kinase gene in Nicotiana tabacum affects negative selection

The potentials and limitations of negative-selection systems based on the human herpes simplex virus thymidine kinase type-1 (HSVtk) gene, which causes sensitivity to the nucleoside analog ganciclovir, were examined in tobacco as a model system. There were great differences between individual HSVtk(+) transgenic plants in ganciclovir sensitivity. Inhibition of growth while under selection correlated with HSVtk-tianscnpt levels. Negative selection against HSVtk(+) transformants at the level of Agrobacterium-mediated transformation using a ganciclo-vir/kanamycin double-selection medium (the positive selection marker neomycin phosphotransferase-II gene was in the transformation vector) resulted in a three- to six-fold reduction in the frequency of kanamycin-resistant shoots. The efficiency of negative selection in this case was limited due to the great variation in HSVtk expression, i.e., the frequently occurring transformants with low, or no, ganciclovir sensitivity escaping negative selection. Two independently constructed HSVtk genes showed the same variability of the phenotype in Nicotiana tabacum transformants. Distinct phenotypes, ranging from no regeneration through abnormal or delayed regeneration, were observed when leaf segments were placed on shoot-inducing medium supplemented with 10(-6)-10(-3) M ganciclovir. The highest HSVtk mRNA and ganciclovir sensitivity levels were observed in plants which were transformed with the pSLJ882 chimeric construct. The pSLJ882 plant expression vector carried the coding sequence of HSVtk, whereas plasmid pCX305.1 carried an HSVtk construct retaining the untranslated 5 leader and viral 3 regions. The pCX305.1 transformants showed, at most, a delayed formation of shoots with thin stems and very narrow leaves. Ganciclovir sensitivity showed typical Mendelian segregation. A gene-dosage effect was also seen at the seedling level in the progeny of two transgenic lines.

Use of the four-and-a-half clearing technique to study gymnosperm embryology: Cunninghamia lanceolata

Since its introduction in 1971, the four-and-a-half clearing technique has been widely applied to the study of ovule and female gametophyte development in flowering plants as an alternative to the more arduous paraffin section methods. The technique has undergone several modifications that have broadened its application in studies of Angiosperm embryology. To date, however, the technique has not been successfully applied to embryological features of Gymnosperms. Dark coloration caused by naturally occurring substances and by-products of fixation render the clearing fluid ineffective, and special pretreatment methods used to remove dark substances in Angiosperm ovules have little or no effect on Gymnosperm material. In the technique reported here, paraffin sections of ovules and young seeds of Cunninghamia lanceolata 80-120 microns thick are cleared in benzyl benzoate-4 1/2 clearing fluid and examined with phase contrast optics. Observations of the mature female gametophyte in these cleared preparations are compared with those obtained from 10 microns sections, stained with safranin and fast green, and examined with bright-field optics. Although contrast and definition are more pronounced in stained sections than in cleared ones, the differences would not alter one's interpretation of characteristic structural features. The thick, cleared section offers an advantage over the thin, stained one in that many structural entities are contained within a single section rather than spread through several serial sections. The time required for clearing thick sections is much shorter than that required for making permanent stained preparations.