Molecular improvement of cereals

The changes in the reproductive barrier between hexaploid wheat (Triticum aestivum L.) and rye (Secale cereale L.): different states lead to different fates

The changes in the reproductive barrier between hexaploid wheat ( Triticum aestivum L.) and rye ( Secale cereale L.) can be induced using in situ embryo rescue of abnormal embryos, yielding stable fertile amphidiploid plants. In intergeneric crosses between hexaploid wheat (Triticum aestivum L.) and rye (Secale cereale L.), postzygotic barriers may occur at different stages of hybrid development. One such mechanism is embryo lethality, which is genetically determined by the interaction and expression of two incompatible genes in wheat (Eml-A1) and rye (Eml-R1). Using in vitro culture methods as stressors, we overcame this hybrid lethality. Normal and abnormal embryos were observed to build embryogenic calli and produce regenerated plantlets in a similar manner. The high regenerative capacity of the abnormal embryos led us to conclude that the reproductive barrier in these intergeneric hybrids may have an epigenetic origin that can be easily overcome by culturing immature embryos via callus induction. After colchicine treatment during callus culture, amphidiploid plants were obtained. However, most of these plants did not produce seeds, due mainly to sterility of the pollen but also of the embryo sacs. These findings demonstrate that hybrid sterility affects both male and female gametophytes in plants obtained from abnormal embryos. The key roles of double fertilization and stress factors in the implementation of the apical meristem formation program in embryos from incompatible intergeneric crosses between hexaploid wheat and rye during in vitro culture are discussed. We also propose a hypothetical model for a wheat-rye lethality system involving differential expression of incompatible wheat Eml-A1 and rye Eml-R1b alleles in an identical genetic background.

Edible Rabies Vaccines

Rabies has been one of the most feared diseases throughout history. Human rabies remains an important public health problem in many developing countries. The WHO reports that more than 55,000 people die of this disease every year. Most of these cases occur in developing countries. In most Latin American countries, the major reservoirs of rabies are the dog and the hematophagous bat (Desmodus rotundus), which is present in the tropical and subtropical areas from Northern Mexico to Northern Argentina and Chile and transmits the disease to cattle. One of the better options for controlling rabies is vaccination. The expression of rabies virus G protein in different plant systems for developing an oral rabies vaccine could reduce costs of production and distribution and would be convenient for developing countries where the disease is endemic.

Reliable transient transformation of intact maize leaf cells for functional genomics and experimental study

Maize (Zea mays) transformation routinely produces stable transgenic lines essential for functional genomics; however, transient expression of target proteins in maize cells is not yet routine. Such techniques are critical for rapid testing of transgene constructs and for experimental studies. Here, we report bombardment methods that depend on leaf developmental stage and result in successful expression with broad applications. Fluorescent marker genes were constructed and bombarded into five developmental regions in a growing maize leaf. Expression efficiency was highest in the basal-most 3 cm above the ligule of an approximately 50-cm growing adult leaf. Straightforward dissection procedures provide access to the receptive leaf regions, increasing efficiency from less than one transformant per cm(2) to over 21 transformants per cm(2). Successful expression was routine for proteins from full genomic sequences driven by native regulatory regions and from complementary DNA sequences driven by the constitutive maize polyubiquitin promoter and a heterologous terminator. Four tested fusion proteins, maize PROTEIN DISULFIDE ISOMERASE-Yellow Fluorescent Protein, GLOSSY8a-monomeric Red Fluorescent Protein and maize XYLOSYLTRANSFERASE, and maize Rho-of-Plants7-monomeric Teal Fluorescent Protein, localized as predicted in the endoplasmic reticulum, Golgi, and plasma membrane, respectively. Localization patterns were similar between transient and stable modes of expression, and cotransformation was equally successful. Coexpression was also demonstrated by transiently transforming cells in a stable line expressing a second marker protein, thus increasing the utility of a single stable transformant. Given the ease of dissection procedures, this method replaces heterologous expression assays with a more direct, native, and informative system, and the techniques will be useful for localization, colocalization, and functional studies.

High frequency plant regeneration from embryogenic callus of a popular indica rice (Oryza sativa L.)

The aim of the study is to establish a routine procedure for high frequency plant regeneration from in vitro raised embryogenic callus of abiotic salt sensitive indica rice (Oryza sativa L.) cultivar ADT 43. The effect of synthetic auxin 2,4-D on callus induction was optimized to achieve high frequency plant regeneration from fresh embryogenic callus without further subculture. Friable, nodular and creamish-white embryogenic callus cultures were raised from mature rice seeds on LS medium supplemented with 2.5 mg L(-1) 2,4-D and 1.0 mg L(-1) thiamine-HCL. Plant regeneration was achieved by the 24 days old embryogenic callus on MS medium supplemented with 1.0 mg L(-1) BAP and 1.5 mg L(-1)NAA. In vitro regenerated plants with multiple tillers and roots were transferred to sterile soil and maintained in the growth chamber. The regenerated plants exhibited normal growth and were phenotypically similar to plants maintained in the garden. Using the present protocol, 25-30 plantlets were regenerated from 50 individual mature seed derived callus within two to three months. This protocol has the potential for large-scale production of elite plants after genetic transformation.

The myth of plant transformation

Technology development is innovative to many aspects of basic and applied plant transgenic science. Plant genetic engineering has opened new avenues to modify crops, and provided new solutions to solve specific needs. Development of procedures in cell biology to regenerate plants from single cells or organized tissue, and the discovery of novel techniques to transfer genes to plant cells provided the prerequisite for the practical use of genetic engineering in crop modification and improvement. Plant transformation technology has become an adaptable platform for cultivar improvement as well as for studying gene function in plants. This success represents the climax of years of efforts in tissue culture improvement, in transformation techniques and in genetic engineering. Plant transformation vectors and methodologies have been improved to increase the efficiency of transformation and to achieve stable expression of transgenes in plants. This review provides a comprehensive discussion of important issues related to plant transformation as well as advances made in transformation techniques during three decades.

Whiskers-mediated maize transformation

There has been rapid progress in recent years in extending gene transfer capabilities to include plant species that fall outside the normal host range of Agrobacterium. Methods that allow direct DNA delivery into plant cells have contributed significantly to this expanded capability. Whiskers treatment is one means of delivering macromolecules, including DNA, to plant cells. Using relatively simple equipment and inexpensive materials, whiskers-mediated transformation of maize is possible. A critical prerequisite, however, is the establishment and maintenance of embryogenic tissue cultures as a source of totipotent, transformation-competent cells. Within hours of agitation in the presence of silicon carbide whiskers and DNA, embryogenic maize tissue cultures display transient gene expression, providing evidence for DNA uptake. Using appropriate selectable marker genes, following in vitro selection on inhibitory levels of a corresponding selection agent, stably transgenic tissue cultures can be generated from which fertile plants can be recovered. The timeline from whiskers treatment of embryogenic maize tissue cultures to fertile seed recovery is approximately 9 months, which is competitive with other methods of maize transformation.

Site-directed mutagenesis, heterologous expression of cyanamide hydratase gene and antimicrobial activity of cyanamide

Site-directed mutagenesis on a recombinant plasmid, pUC8, that contained the cah gene, was conducted and confirmed by sequence analysis. Single base substitution, G to A at nucleotide position 81 or T to C at nucleotide position 84 of cah gene does not change the amino acid sequence of cah enzyme but eliminates the HindIII site. The wild-type cah and its mutants were cloned and overexpressed in pQE-60 Escherichia coli expression system. Western blot analysis confirmed the production of 27.7-kDa cah enzyme by all the recombinants. The mutated cah gene devoid of HindIII site was used to generate a recombinant plant transformation vector (pCAMBIA-cah). Agrobacterium-mediated transformation was performed in Nicotiana tabaccum cv. Samsun plants by employing the leaf-disc method. The integration and expression of cah gene in transgenic plants were confirmed by polymerase chain reaction, Southern and Western blot analyses. Antimicrobial activity of cyanamide against phytopathogenic fungi and bacteria was determined. Cyanamide can be used as fertilizer as well as an antimicrobial salt against phytopathogenic fungi and bacteria. The present investigation reports the heterologous expression of the cah marker gene. Due to its innate ability to convert cyanamide to urea and the broad-spectrum antimicrobial activity of cyanamide, the cah gene can be used to facilitate plant growth promotion and biocontrol of phytopathogens.

Rapid and efficient production of transgenic bermudagrass and creeping bentgrass bypassing the callus formation phase

Callus culture has been an inevitable step in genetic transformation of monocotyledonous (monocot) species. The induction and maintenance of embryogenic calluses is time-consuming, laborious and also requires experience. A straightforward and callus-free transformation procedure was developed and demonstrated for two monocot species, bermudagrass (Cynodon spp.) and creeping bentgrass (Agrostis stolonifera). Stolon nodes were infected and co-cultivated with Agrobacterium tumefaciens harboring pCAMBIA or pTOK233 binary vectors. Green shoots were directly produced from infected stolon nodes 4-5 weeks after hygromycin selection. Without callus formation and with minimum tissue culture, this procedure allowed us to obtain well-rooted transgenic plantlets in only 7 weeks and greenhouse-grown plants in only 9 weeks. The established plants were screened by PCR; the transgenic nature of the plants was demonstrated by Southern hybridisation analysis. Expression of the transgenes was confirmed by northern hybridisation analysis and GUS staining. Based on the number of transgenic plants obtained and the number of stolon nodes inoculated, transformation frequencies of 4.8%-6.1% and 6.3%-11.3% were achieved for bermudagrass and creeping bentgrass, respectively. The rapid and efficient production of transgenic plants without callus induction is a significant improvement for genetic transformation of monocot species.

Mature embryo axis-based high frequency somatic embryogenesis and plant regeneration from multiple cultivars of barley (Hordeum vulgare L.)

A highly reproducible regeneration system through somatic embryogenesis from the excised mature embryos (MEs) of dry seeds of a range of European barley cultivars was developed. By minimizing the germination of plated MEs, primary callus could be obtained with high frequency which permitted efficient embryogenesis and regeneration of a large number of green plants. Different approaches were tested to reduce or prevent normal germination: (i) the use of a well defined balance of maltose and 2,4-D in the induction medium, (ii) soaking of seeds in water containing 2,4-D solution, (iii) direct culture of excised embryonic axes, (iv) longitudinally bisected MEs giving two halves, and (v) complete removal of the elongated main shoot including any roots within a week of culture initiation. Culturing of bisected MEs and whole embryonic axes gave the best responses with respect to large amounts of callus combined with minimal germination. The incorporation of BAP at low levels in the medium was found to be most effective for embryogenesis and the maintenance of long-term morphogenic capacity (more than 11 months up to now). This procedure allows the complete regeneration of plants in 16-20 weeks, from the initial isolation of MEs through all the steps to the development of plants ready to be transferred to the soil. The protocol was first developed for cv. Golden Promise and successfully applied to commercial cultivars. All cultivars tested formed embryogenic callus, with overall rates ranging from 22-55% and an average number of green plants per embryogenic callus from 1.5 to 7.5 across the genotypes.

Phytohormonal responses in enod40-overexpressing plants of Medicago truncatula and rice

Phytohormones are well-known regulators of the symbiotic Rhizobium-legume association in the plant host. The enod40 nodulin gene is associated with the earliest phases of the nodule organogenesis programme in the legume host and modifying its expression resulted in perturbations of nodule development in Medicago truncatula. Therefore in our pursuit to mimic the initial signal transduction steps of legume nodulation in the alien physiological set-up of a rice plant, we have expressed the Mtenod40 gene in rice. Molecular data confirm the stable integration, inheritance and transcription of the foreign gene in this non-legume. We have compared the phytohormonal responses of Mtenod40-overexpressing and control plants in a homologous legume background (M. truncatula) and in the non-legume rice. An enod40-mediated root growth response, induced by inhibition of ethylene biosynthesis, was observed in both plants. On the other hand, a significant differential effect of cytokinins was observed only in rice plants. This suggests that ethylene inhibits enod40 action both in legumes and non-legumes and reinforces that some of the early signal transduction steps of the nodule developmental programme may function in rice.

Promiscuity of hosting nitrogen fixation in rice: an overview from the legume perspective

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

Positive, negative and marker-free strategies for transgenic plant selection

In this review, the use of the most common selection marker genes on plant transformation and the effects of their respective selective agents are discussed. These genes could be divided in two categories according their mode of action: genes for positive and negative selection. The retention of the marker gene flow through chloroplast transformation is also discussed. Further, strategies to recover marker-free transgenic plants, involving multi-auto-transformation (MAT), co-transformation, site-specific recombination and intragenomic relocation of transgenes through transposable elements are reviewed.

High frequency of cytogenetic aberration in transgenic oat (Avena sativa L.) plants

Cytological abnormalities were observed in transgenic oat (Avena sativa L. cv. GAF/Park-1) produced by microprojectile bombardment of mature seed-derived highly regenerative tissues. Of the plants from 48 independent transgenic lines examined, plants from only 20 lines (42%) were karyotypically normal (2n=6x=42) without detectable chromosomal aberrations; plants from 28 lines (58%) had chromosomal variation, i.e. aneuploids and structural changes. No significant difference in cytological aberration was observed between the two different culturing systems used for transformation: 57% chromosomal abnormalities in plants derived from D'BC2 medium (2.0 mg/l 2,4-D, 0.1 mg/l BAP and 5.0 &mgr;M cupric sulfate) used for tissue initiation and maintenance and 60% in plants from tissue initiated on D'BC2 and maintained on DBC3 (1.0 mg/l 2,4-D, 0.5 mg/l BAP and 5.0 &mgr;M cupric sulfate). Comparative differences in chromosomal status frequently occurred among plants regenerated from the same T(0) line. The most common cytological aberration in transgenic plants was aneuploidy, followed by deletion of chromosomal segments; no change in ploidy level was observed. In contrast, nontransgenic plants, regenerated from tissues comparable in age and culture media to that used for transgenic tissues, had a much lower percentage of karyotypic abnormality (0-14%). Our data indicate that some stress(es) imposed by the transformation process, e.g. osmotic treatment, bombardment and selection, leads to cytological variation in transgenic oat plants, an observation similar to that observed in our recent studies with transgenic barley plants.

Field performance of transgenic elite commercial hybrid rice expressing bacillus thuringiensis delta-endotoxin

Here we describe development of transgenic elite rice lines expressing a Bt fusion gene derived from cryIA(b) and cryIA(c) under the control of rice actinI promoter. The lines used in the study were indica CMS restorer line of Minghui 63 and its derived hybrid rice Shanyou 63. The level of Bt fusion protein CryIA(b)/CryIA(c) detected in Minghui 63 (T51-1) plants was 20 ng/mg soluble protein. The Bt Shanyou 63 was field-tested in natural and repeated heavy manual infestation of two lepidopteran insects, leaffolder and yellow stem borer. The transgenic hybrid plants showed high protection against both insect pests without reduced yield.

High frequency of cytogenetic aberration in transgenic oat (Avena sativa L.) plants

Cytological abnormalities were observed in transgenic oat (Avena sativa L. cv. GAF/Park-1) produced by microprojectile bombardment of mature seed-derived highly regenerative tissues. Of the plants from 48 independent transgenic lines examined, plants from only 20 lines (42%) were karyotypically normal (2n=6x=42) without detectable chromosomal aberrations; plants from 28 lines (58%) had chromosomal variation, i.e. aneuploids and structural changes. No significant difference in cytological aberration was observed between the two different culturing systems used for transformation: 57% chromosomal abnormalities in plants derived from D'BC2 medium (2.0 mg/l 2,4-D, 0. 1 mg/l BAP and 5.0 µM cupric sulfate) used for tissue initiation and maintenance and 60% in plants from tissue initiated on D'BC2 and maintained on DBC3 (1.0 mg/l 2,4-D, 0.5 mg/l BAP and 5.0 µM cupric sulfate). Comparative differences in chromosomal status frequently occurred among plants regenerated from the same T(0) line. The most common cytological aberration in transgenic plants was aneuploidy, followed by deletion of chromosomal segments; no change in ploidy level was observed. In contrast, nontransgenic plants, regenerated from tissues comparable in age and culture media to that used for transgenic tissues, had a much lower percentage of karyotypic abnormality (0-14%). Our data indicate that some stress(es) imposed by the transformation process, e.g. osmotic treatment, bombardment and selection, leads to cytological variation in transgenic oat plants, an observation similar to that observed in our recent studies with transgenic barley plants.

Relaxation, linearization and fragmentation of supercoiled circular DNA by tungsten microprojectiles

The aim of the study was to characterize DNA lesions caused by microprojectile bombardment and by the post-bombardment presence of tungsten particles in transformed cells. For the sake of simplicity, plasmid DNA was used as a target for bombardment with naked tungsten particles. Unexpectedly extensive DNA degradation was observed under standard bombardment conditions. However, no further DNA fragmentation occurred under post-bombardment conditions, simulated by incubation of plasmid DNA with a suspension of tungsten particles. Instead, relaxation and linearization of supercoiled circular plasmids (pAHC25 and others) took place. It is concluded that the observed linearization (a single site double-strand break in DNA circle) results from the ability of tungsten to catalyse the hydrolysis of phosphodiester bonds in torsionally strained sites of native DNA selectively.

Accelerated production of transgenic wheat (Triticum aestivum L.) plants

We have developed a method for the accelerated production of fertile transgenic wheat (Triticum aestivum L.) that yields rooted plants ready for transfer to soil in 8-9 weeks (56-66 days) after the initiation of cultures. This was made possible by improvements in the procedures used for culture, bombardment, and selection. Cultured immature embryos were given a 4-6 h pre-and 16 h post-bombardment osmotic treatment. The most consistent and satisfactory results were obtained with 30 μg of gold particles/bombardment. No clear correlation was found between the frequencies of transient expression and stable transformation. The highest rates of regeneration and transformation were obtained when callus formation after bombardment was limited to two weeks in the dark, with or without selection, followed by selection during regeneration under light. Selection with bialaphos, and not phosphinothricin, yielded more vigorously growing transformed plantlets. The elongation of dark green plantlets in the presence of 4-5 mg/l bialaphos was found to be reliable for identifying transformed plants. Eighty independent transgenic wheat lines were produced in this study. Under optimum conditions, 32 transformed wheat plants were obtained from 2100 immature embryos in 56-66 days, making it possible to obtain R3 homozygous plants in less than a year.

The similarities of bar and pat gene products make them equally applicable for plant engineers

The bar and pat genes, isolated from different Streptomyces species, both encode a phosphinothricin acetyltransferase (PAT) and are widely applied in plant genetic engineering. The genes were expressed in Escherichia coli and the corresponding proteins were purified and used for functional and structural comparison. Both proteins are homodimers regardless of whether they are expressed in microorganisms or in plants. They have comparable molecular weights and show immuno-cross-reactivity to their respective polyclonal antisera. The enzymes have a similar substrate affinity towards L-phosphinothricin and do not acetylate any of the other L-amino acids tested. In model digestion experiments using simulated human gastric fluids, their enzymatic activity is decreased within seconds, accompanied by a rapid and complete breakdown of both proteins. These data demonstrate the structural and functional equivalence of the PAT proteins, which is also reflected in the comparable performance of transgenic plants carrying the bar or pat gene.

Integration and expression of the high-molecular-weight glutenin subunit 1Ax1 gene into wheat

The unique bread-making characteristic of wheat flour is closely related to the elasticity and extensibility of the gluten proteins stored in the starchy endosperm, particularly the high-molecular-weight glutenin subunits (HMW-GS), which are important in determining gluten and dough elasticity. The quality of wheat cultivars depends on the number and composition of the HMW-GS present. We have introduced the HMW-GS 1Ax1 gene, known to be associated with good bread-making quality, into the Bob White cultivar of wheat (Triticum aestivum L.), in which it is not present in nature, by the biolistic bombardment of cultured immature embryos. Of the 21 independent transformed lines selected, 20 expressed the selectable bar gene, and nine the 1Ax1 gene. The amount of HMW-GS 1Ax1 protein produced in the different transgenic lines varied from 0.6% to 2.3% of the total protein, resulting in an increase of up to 71% in total HMW-GS proteins. The transgenic plants were normal, fertile, and showed Mendelian segregation of the transgenes. The accumulation of HMW-GS 1Ax1 was consistent and stable up to the R3 seed generation. These results demonstrate that it is possible to manipulate both the quantity and quality of HMW-GS, which influence the bread-making quality of wheat.

Fertile transgenic barley generated by direct DNA transfer to protoplasts

We report the generation of transgenic barley plants via PEG-mediated direct DNA uptake to protoplasts. Protoplasts isolated from embryogenic cell suspensions of barley (Hordeum vulgare L. cv 'Igri') were PEG-treated in a solution containing a plasmid which contained the neomycin phosphotransferase (NPT II) gene under the control of the rice actin promoter and the nos terminator. Colonies developing from the treated protoplasts were incubated in liquid medium containing the selective antibiotic G418. Surviving calli were subsequently transferred to solid media containing G418, on which embryogenic calli developed. These calli gave rise to albino and green shoots on antibiotic-free regeneration medium. NPT II ELISA revealed that approximately half of the morphogenic calli expressed the foreign gene. In total, 12 plantlets derived from NPT-positive calli survived transfer to soil. Southern hybridization analysis confirmed the stable transformation of these plants. However, the foreign gene seemed to be inactivated in plants from one transgenic line. Most of the transgenic plants set seed, and the foreign gene was transmitted and expressed in their progenies, which was ascertained by Southern hybridization and NPT II ELISA.