Hygromycin resistance is an effective selectable marker for biolistic transformation of black spruce (Picea mariana)

Stable transformation of embryogenic tissues of Pinus nigra Arn. using a biolistic method

The stable transformation of embryogenic tissues of Pinus nigra Arn., cell line E104, has been achieved using a biolistic approach. The introduced DNA consisted of the uidA reporter gene under the control of the double CaMV 35S promoter and the nptII selection gene controlled by the single CaMV 35S promoter. Three days after bombardment, putative transformed tissues were selected for continued proliferation on a medium containing 20 mg geneticin l(-1). Resistant embryogenic tissue recovery required 10-12 weeks. The integration of the nptII and uidA genes was confirmed by both histochemical/fluorimetric GUS assays and PCR amplification of the inserts in the five geneticin resistant sub-lines of line E104. The activity of the uidA reporter gene in transgenic, embryogenic tissue lines was stable and could be detected after one year of culture. Somatic embryo maturation was, however, poor and no plantlet regeneration could be obtained.

Agrobacterium tumefaciens-mediated transformation of embryogenic tissue and transgenic plant regeneration in Chamaecyparis obtusa Sieb. et Zucc

A genetic transformation procedure for Chamaecyparis obtusa was developed after co-cultivation of embryogenic tissues with disarmed Agrobacterium tumefaciens strain C58/pMP90, which harbours the sgfp (synthetic green fluorescent protein) visual reporter and nptII (neomycin phoshotransferase II) selectable marker genes. The highest transformation frequency was 22.5 independent transformed lines per dish (250 mg embryogenic tissue) following selection on kanamycin medium. Transgenic plantlets were regenerated through the maturation and germination of somatic embryos. The intensity of GFP fluorescence, observed under a fluorescence microscope, varied from very faint to relatively strong, depending on the transgenic line or part of the transgenic plant. The integration of the genes into the genome of regenerated plantlets was confirmed by Southern blot analysis.

Selectable marker genes in transgenic plants: applications, alternatives and biosafety

Approximately fifty marker genes used for transgenic and transplastomic plant research or crop development have been assessed for efficiency, biosafety, scientific applications and commercialization. Selectable marker genes can be divided into several categories depending on whether they confer positive or negative selection and whether selection is conditional or non-conditional on the presence of external substrates. Positive selectable marker genes are defined as those that promote the growth of transformed tissue whereas negative selectable marker genes result in the death of the transformed tissue. The positive selectable marker genes that are conditional on the use of toxic agents, such as antibiotics, herbicides or drugs were the first to be developed and exploited. More recent developments include positive selectable marker genes that are conditional on non-toxic agents that may be substrates for growth or that induce growth and differentiation of the transformed tissues. Newer strategies include positive selectable marker genes which are not conditional on external substrates but which alter the physiological processes that govern plant development. A valuable companion to the selectable marker genes are the reporter genes, which do not provide a cell with a selective advantage, but which can be used to monitor transgenic events and manually separate transgenic material from non-transformed material. They fall into two categories depending on whether they are conditional or non-conditional on the presence of external substrates. Some reporter genes can be adapted to function as selectable marker genes through the development of novel substrates. Despite the large number of marker genes that exist for plants, only a few marker genes are used for most plant research and crop development. As the production of transgenic plants is labor intensive, expensive and difficult for most species, practical issues govern the choice of selectable marker genes that are used. Many of the genes have specific limitations or have not been sufficiently tested to merit their widespread use. For research, a variety of selection systems are essential as no single selectable marker gene was found to be sufficient for all circumstances. Although, no adverse biosafety effects have been reported for the marker genes that have been adopted for widespread use, biosafety concerns should help direct which markers will be chosen for future crop development. Common sense dictates that marker genes conferring resistance to significant therapeutic antibiotics should not be used. An area of research that is growing rapidly but is still in its infancy is the development of strategies for eliminating selectable marker genes to generate marker-free plants. Among the several technologies described, two have emerged with significant potential. The simplest is the co-transformation of genes of interest with selectable marker genes followed by the segregation of the separate genes through conventional genetics. The more complicated strategy is the use of site-specific recombinases, under the control of inducible promoters, to excise the marker genes and excision machinery from the transgenic plant after selection has been achieved. In this review each of the genes and processes will be examined to assess the alternatives that exist for producing transgenic plants.

Agrobacterium tumefaciens-mediated transformation of Indian mulberry, Morus indica cv. K2: a time-phased screening strategy

An efficient and reproducible protocol for the production of transgenic plants was developed for Morus indica cv. K2 by Agrobacterium tumefaciens-mediated transformation. The hypocotyls, cotyledon, leaf and leaf callus explants precultured for 5 days on regeneration medium were co-cultivated with a bacterial suspension at 10(9) cells/ml for 3 days in the dark. Infectivity of A. tumefaciens strain LBA4404 was more than that of strains GV2260 and A281, and among the various plasmids tried, pBI121 and pBI101:Act1 transformed nearly 100% of the explants followed closely by p35SGUSINT. About 90-100% of the explants tested positive in the beta-glucuronidase (GUS) histochemical assay performed after 3 days of co-cultivation. This high level of transient expression, however, decreased to 20-25% after 15 days. Gus activity was most stable in the callus explants, which emerged as the explant of choice for transformation. The transformed explants were selected on 50-75 mg/l kanamycin for 1 month, and 25-50% of the explants developed adventitious buds. On the basis of kanamycin-resistant shoots produced from the total number of explants inoculated, the transformation efficiency was 44%. After 1 month, 40% of these shoots displayed high gus activity as assessed by the GUS fluorometric assay. On a selection-free root induction medium, 80% of the shoots developed roots and 90% of the potted plantlets acclimatized to the growth room conditions. The 3-month-old regenerates showed gus and nptII(neomycin phosphotransferase II) gene activity as assayed by the GUS fluorometric assay and nptII enzyme assay, followed by PCR polymerase chain reaction (54.5%) analysis after 6-months. Transgene integration into the nuclear genome of 1-year-old regenerates was confirmed in 10 of the 18 transformants tested by Southern analysis. The transformation efficiency as defined by the number of transgenic plants produced from the total number of explants co-cultivated was 6%.