Agrobacterium-mediated transformation of Asparagus officinalis L. long-term embryogenic callus and regeneration of transgenic plants

Aspects of In Vitro Plant Tissue Culture and Breeding of Asparagus: A Review

In vitro plant tissue culture and biotechnology used to assist and support the development of plant breeding when classical methods of propagation must be accelerated or it was necessary to overcome barriers inaccessible by classical approaches. In asparagus, to improve multiple breeding tasks, a high number of in vitro methods have been used, such as plant regeneration methods through organogenesis, embryogenesis, manipulation of ploidy, protoplast isolation, genetic manipulation (protoplast fusion, genetic transformation), embryo rescue and germplasm preservation (in vitro, in vitro slow growth, cryopreservation). Plant tissue culture methods can overcome multiple problems in asparagus breeding such as, barriers of self and cross-incompatibility between asparagus species through embryo rescue of interspecific hybrids and protoplast fusion or genetic transformation, introgression of new genes, clonal propagation of elite genotypes of asparagus, mass screening, and the generation of haploid and polyploid genotypes, among others, becoming the tool of choice for asparagus breeding programs. Some of these in vitro methods are still under development.

Agrobacterium tumefaciens-mediated transformation of a hevein-like gene into asparagus leads to stem wilt resistance

Asparagus stem wilt, is a significant and devastating disease, typically leading to extensive economic losses in the asparagus industry. To obtain transgenic plants resistant to stem wilt, the hevein-like gene, providing broad spectrum bacterial resistance was inserted into the asparagus genome through Agrobacterium tumefaciens-mediated transformation. The optimal genetic transformation system for asparagus was as follows: pre-culture of embryos for 2 days, inoculation using a bacterial titre of OD600 = 0.6, infection time 10 min and co-culturing for 4 days using an Acetosyringone concentration of 200 μmol/L. Highest transformation frequencies reached 21% and ten transgenic asparagus seedlings carrying the hevein-like gene were identified by polymerase chain reaction. Moreover, integration of the hevein-like gene in the T1 generation of transgenic plants was confirmed by southern blot hybridization. Analysis showed that resistance to stem wilt was enhanced significantly in the transgenic plants, in comparison to non- transgenic plants. The results provide additional data for genetic improvement and are of importance for the development of new disease-resistant asparagus varieties.

Highly efficient Agrobacterium-mediated transformation of banana cv. Rasthali (AAB) via sonication and vacuum infiltration

A reproducible and efficient transformation method was developed for the banana cv. Rasthali (AAB) via Agrobacterium-mediated genetic transformation of suckers. Three-month-old banana suckers were used as explant and three Agrobacterium tumefaciens strains (EHA105, EHA101, and LBA4404) harboring the binary vector pCAMBIA1301 were used in the co-cultivation. The banana suckers were sonicated and vacuum infiltered with each of the three A. tumefaciens strains and co-cultivated in the medium containing different concentrations of acetosyringone for 3 days. The transformed shoots were selected in 30 mg/l hygromycin-containing selection medium and rooted in rooting medium containing 1 mg/l IBA and 30 mg/l hygromycin. The presence and integration of the hpt II and gus genes into the banana genome were confirmed by GUS histochemical assay, polymerase chain reaction, and southern hybridization. Among the different combinations tested, high transformation efficiency (39.4 ± 0.5% GUS positive shoots) was obtained when suckers were sonicated and vacuum infiltered for 6 min with A. tumefaciens EHA105 in presence of 50 μM acetosyringone followed by co-cultivation in 50 μM acetosyringone-containing medium for 3 days. These results suggest that an efficient Agrobacterium-mediated transformation protocol for stable integration of foreign genes into banana has been developed and that this transformation system could be useful for future studies on transferring economically important genes into banana.

Vegetables

The conscious promotion of health by an appropriate, balanced diet has become an important social request. Vegetable thereby possesses a special importance due to its high vitamin, mineral and dietary fibre content. Major progress has been made over the past few years in the transformation of vegetables. The expression of several genes has been inhibited by sense gene suppression, and new traits caused by new gene constructs are stably inherited. This chapter reviews advances in various traits such as disease resistance, abiotic stress tolerance, quality improvement, pharmaceutical and industrial application. Results are presented from most important vegetable families, like Solanaceae, Brassicaceae, Fabaceae, Cucurbitaceae, Asteraceae, Apiaceae, Chenopodiaceae and Liliaceae. Although many research trends in this report are positive, only a few transgenic vegetables have been released from confined into precommercial testing or into use.

Foreign gene delivery into monocotyledonous species

Monocotyledonous plants are generally more recalcitrant to genetic transformation than dicotyledonous species. The absence of reliable Agrobacterium-mediated transformation methods and the difficulties associated with the culture of monocotyledonous tissues in vitro are mainly responsible for this situation. Until recently, the genetic transformation of monocotyledons was essentially performed by direct transfer of DNA into regenerable protoplasts or intact cells cultured in vitro, via polyethylene glycol treatment, electroporation or particle bombardment. Since 1990, the use of particle gun technology has revolutionized the genetic engineering of monocotyledonous species, allowing transformation to be more independent of the in vitro culture requirements. Today, at least one genotype of each major monocotyledonous crop species, including cereals, can be genetically transformed.

Analysis of habituated embryogenic lines in Asparagus officinalis L.: growth characteristics, hormone content and ploidy level of calli and regenerated plants

Habituated asparagus embryogenic lines derived from eleven genotypes were maintained on hormone-free medium and grew actively through secondary embryogenesis. Secondary embryos were of single cell origin and emerged from the transversal division of some epidermal or subepidermal cotyledonary cells of primary embryos. The intensity of secondary embryogenesis was found to be variable between embryogenic lines. Plants regenerated from three of these lines have been previously demonstrated to carry a mutation whose phenotype was the direct appearance of somatic embryos on apices or nodes cultured on hormone-free medium. Habituated lines of embryogenic calli and various tissues of embryogenic mutant and wild type plants were analysed for their hormonal content in ABA, IAA, iP, Z and their metabolites ABA-GE, iPA, iMP, ZR. No significant difference was found between different embryogenic lines, except the level of iPA, or between cladophyll or apex cultures of mutant and wild type plants. Flow cytometry analyses indicated only 34% of the embryogenic lines were diploid, most of the others being tetraploid, but 62% of regenerated plants from these lines were diploid. This indicated the process of maturation and conversion selected diploid embryos in the embryogenic lines.

Decrease in the regeneration potential of long-term cell suspension cultures of Lilium formosanum Wallace and its restoration by the auxin transport inhibitor, 2,3,5-triiodobenzoic acid

Cell suspension cultures of Lilium formosanum Wallace were initiated from bulb scale-derived calli and subcultured every 2 weeks using a medium containing 5 µM 4-amino-3,5,6-trichloropicolinic acid (picloram). Almost all cell clumps from the suspension cultures developed numerous somatic embryos following their transfer onto a plant growth regulator-free medium, while they vigorously produced shoot buds on media containing 0.5 or 5 µM 6-benzyladenine (BA). The high regeneration potential on a plant growth regulator-free medium was maintained for up to 54 months, but it gradually decreased thereafter, and only a few adventitious shoots and embryos were obtained from 75-month-old cultures. For restoring the regeneration potential of these cultures, various treatments with plant growth regulators were applied, among which about 10-fold increases in the number of regenerated shoot buds were obtained with 0.5 or 5 µM 2,3,5-triiodobenzoic acid (TIBA) in combination with 0.5 or 5 µM BA or N-(1,2,3-thiadiazol-5-yl)-N'-phenylurea (thidiazuron). Only shoot buds were produced from the cell clumps cultured on TIBA-containing media, and these shoot buds developed into complete plantlets after they were excised from the calli and transferred to a plant growth regulator-free medium.

An efficient particle bombardment system for the genetic transformation of asparagus (Asparagus officinalis L.)

The microprojectile bombardment method was used to transfer DNA into embryogenic callus of asparagus (Asparagus officcinalis L.) and to produce stably transformed asparagus plants. Embryogenic callus, derived from UC 157 and UC72 asparagus cultivars, was bombarded with tungsten particles coated with plasmid DNA that contained genes encoding hygromycin phosphotransferase, phosphinothricin acetyl transferase and β-glucuronidase. Putatively transformed calli were identified from the bombarded tissue after 4 months selection on 25 mg/L hygromycin B plus 4 mg/L phosphinothricin (PPT). By selecting embryogenic callus on hygromycin plus PPT the overall transformation and selection efficiencies were substantially improved over selection with hygromycin or PPT alone, where no transgenic clones were recovered. The transgenic nature of the selected material was demonstrated by GUS histochemical assays and Southern blot hybridization analysis. Transgenic asparagus plants were found to withstand the prescribed levels of the PPT-based herbicide BASTATM for weed control.

Production of Agrobacterium-mediated transgenic fertile plants by direct somatic embryogenesis from immature zygotic embryos of Datura innoxia

This work describes a new method to obtain transgenic somatic embryos from Agrobacterium-infected immature zygotic embryos of Datura innoxia. It has several advantages over previous transformation methods such as the absence of a callus phase, an average transformation rate of 76% and a high regeneration frequency. Critical steps for optimal transformation were the embryo stage and a short preculture treatment. The marker gene beta-glucuronidase and light microscopy were used to identify the competent embryogenic cells which, after transformation, passed through the classical stages of embryo development. The transgenes were transmitted to the progeny in a Mendelian fashion. The plants regenerated via direct somatic embryogenesis were cytologically and morphologically uniform. We also observed that: (1) wounding or wound-induced divisions were not required for zygotic embryo transformation; (2) epidermal cells were competent for both transformation and regeneration; and (3) competency for Agrobacterium infection was developmental stage-specific. This new method should facilitate the development of new strategies to routinely transform recalcitrant plant species.