Toshihiko Komari

Transformation of rice mediated by Agrobacterium tumefaciens

Agrobacterium tumefaciens has been routinely utilized in gene transfer to dicotyledonous plants, but monocotyledonous plants including important cereals were thought to be recalcitrant to this technology as they were outside the host range of crown gall. Various challenges to infect monocotyledons including rice with Agrobacterium had been made in many laboratories, but the results were not conclusive until recently. Efficient transformation protocols mediated by Agrobacterium were reported for rice in 1994 and 1996. A key point in the protocols was the fact that tissues consisting of actively dividing, embryonic cells, such as immature embryos and calli induced from scutella, were co-cultivated with Agrobacterium in the presence of acetosyringonc, which is a potent inducer of the virulence genes. It is now clear that Agrobacterium is capable of transferring DNA to monocotyledons if tissues containing ‘competent’ cells are infected. The studies of transformation of rice suggested that numerous factors including genotype of plants, types and ages of tissues inoculated, kind of vectors, strains of Agrobacterium, selection marker genes and selective agents, and various conditions of tissue culture, are of critical importance. Advantages of the Agrobacterium-mediated transformation in rice, like on dicotyledons, include the transfer of pieces of DNA with defined ends with minimal rearrangements, the transfer of relatively large segments of DNA, the integration of small numbers of copies of genes into plant chromosomes, and high quality and fertility of transgenic plants. Delivery of foreign DNA to rice plants via A. tumefaciens is a routine technique in a growing number of laboratories. This technique will allow the genetic improvement of diverse varieties of rice, as well as studies of many aspects of the molecular biology of rice.

Agrobacterium -mediated transformation of rice using immature embryos or calli induced from mature seed

Here, we provide comprehensive, highly efficient protocols for Agrobacterium tumefaciens-mediated transformation of a wide range of rice genotypes. Methods that use either immature embryos (japonica and indica rice) or calli (japonica cultivars and the indica cultivar, Kasalath) as a starting material for inoculation with Agrobacterium are described. Immature embryos are pretreated with heat and centrifugal force, which significantly enhances the efficiency of gene transfer, and then infected with Agrobacterium. Callus is induced from mature seeds and infected. Transformed cells proliferated from these tissues are selected on the basis of hygromycin resistance, and transgenic plants are eventually regenerated. A single immature japonica or Kasalath embryo will produce between 10 and 18 independent transgenic plants; for other non-Kasalath indica varieties, the number of transgenic plants expected will be between 5 and 13. For japonica and Kasalath, transformants should be obtained from between 50 and 90% of calli. From inoculation with Agrobacterium to transplanting to soil will take 55 d for japonica and Kasalath, and 74 d for indica other than Kasalath using the immature embryo method, and 50 d for japonica and Kasalath using the callus method.

Agrobacterium-mediated transformation of maize

Maize may be transformed very efficiently using Agrobacterium tumefaciens-mediated methods. The most critical factor in the transformation protocol is the co-cultivation of healthy immature embryos of the correct developmental stage with A. tumefaciens; the embryos should be collected only from vigorous plants grown in well-conditioned glasshouses. With the protocol described here, approximately 50% of immature embryos from the inbred line A188 and 15% from inbred lines A634, H99 and W117 will produce transformants. About half of the transformed plants are expected to carry one or two copies of the transgenes, which are inherited by the progeny in a mendelian fashion. More than 90% of transformants are expected to be normal in morphology. The protocol takes about 3 months from the start of co-cultivation to the planting of transformants into pots.

Improved protocols for transformation of indica rice mediated by Agrobacterium tumefaciens

A highly efficient gene transfer method mediated by Agrobacterium tumefaciens was developed for Group I indica rice, which had been quite recalcitrant in tissue culture and transformation. Freshly isolated immature embryos from plants grown in a greenhouse were inoculated with A. tumefaciens LBA4404 that harbored super-binary vector pTOK233 or pSB134, which had a hygromycin-resistance gene and a GUS gene in the T-DNA. The efficiency of gene transfer varied with the kinds of gelling agents and the basic compositions of co-cultivation media. The highest activity of GUS after co-cultivation was observed when NB medium solidified with agarose was used. For the subsequent cultures, two types of media (modified NB and CC) were chosen to recover hygromycin-resistant cells efficiently. The transformation protocol thus developed worked very well in all of the varieties tested in this study, and the transformation frequency (number of independent hygromycin-resistant and GUS-positive plants per embryo) reached more than 30% in IR8, IR24, IR26, IR36, IR54, IR64, IR72, Xin Qing Ai 1, Nan Jin 11, and Suewon 258. Most of the transformants (T0) were normal in morphology and fertile. Stable integration, expression and inheritance of transgenes were demonstrated by molecular and genetic analysis of transformants in the T0 and T1 generations. For the recovery of multiple independent transgenic events from a single immature embryo, procedures were developed to section the embryo into as many as 30 pieces after non-selective cultures following co-cultivation. Transformants were then obtained from the pieces cultured on the selective media, and, in the highest case, more than seven independent transgenic plants per original embryo (transformation frequency of 738%) were produced. Thus, the efficiency of transformation was remarkably improved.

Advances in cereal gene transfer

Over the past five years, transgenic strains of various major cereals have been produced, with transformation of rice and maize being most common. A majority of the cereal transformants obtained to date has been generated by the particle bombardment technique, but Agrobacterium-mediated transformation is rapidly becoming the method of choice. Rice, the plant in which transformation-related technology is most advanced, appears to be the model monocotyledon for basic and applied studies.

Transformation of cultured cells of Chenopodium quinoa by binary vectors that carry a fragment of DNA from the virulence region of pTiBo542

SummaryA 15.2-kb KpnI fragment from the virulence region of pTiBo542, the Ti plasmid harbored by Agrobacterium tumefaciens strain A281, was introduced into binary vectors. The fragment contained the virB, virC and virG genes, and it is known to have the ability to increase the virulence of strains of A. tumefaciens. The strains of A. tumefaciens that carried the resulting plasmids were able to transform cells in a suspension culture of Chenopodium quinoa Willd cells which were not transformable by common vectors. Although the sizes of the plasmids was very large, a foreign segment of DNA was introduced into one of the plasmids by homologous recombination in A. tumefaciens cells, and the segment was subsequently transferred to plant cells.

Improved frequency of transformation in rice and maize by treatment of immature embryos with centrifugation and heat prior to infection with Agrobacterium tumefaciens

The efficiency of transformation was improved by treating immature embryos with heat and centrifugation before infection with Agrobacterium tumefaciens in rice and maize. Because the effects were detected both in the levels of transgene expression after co-cultivation and in the number of independent transgenic plants obtained per embryo, conditions were first optimized based on the transgene expression, and then transformants were produced. The optimal conditions varied considerably depending on species and genotypes, but reasonably good parameters were identified for Japonica rice, Indica rice or maize. As a general tendency, the effect of centrifugation was greater than that of heat in Japonica rice, whereas that of heat was greater than that of centrifugation in Indica rice and maize A188, and the combination of the treatments was the most effective in all of the genotypes tested. The frequency of transformation was improved several fold in rice and maize. In addition, transformation of certain genotypes of maize, which were not transformable before, and transformation of maize with a less efficient vector, which could not transform maize before, became possible by these pre-treatments. In the highest case, 18 independent transgenic plants were obtained from a single immature embryo of Japonica rice. Although nothing is known about the mechanism, these pre-treatments seemed to render cells of rice and maize more competent for transformation mediated by A. tumefaciens.

Transformation of callus cultures of nine plant species mediated by agrobacterium

Abstract A simple method for the Agrobacterium-mediated transformation of callus cultures of nine plant species, Lycopersicum esculentum Mill, Petunia hybrida Vilm, Pimpinella anisum L., Solanum melongena L., S. tuberosum L., Nicotiana glauca Graham, N. glutinosa L., N. plumbaginifolia Viviani and N. tabacum L., is described. Plant calli were resuspended in liquid media, co-cultivated with A. tumefaciens, and plated on restrictive media. The combination of a gene for kanamycin resistance and a gene for firefly luciferase was convenient in the selection and confirmation of hundreds of transformants. Four strains of A. tumefaciens, A208, A348, A281, and PC2760, were employed. All of the callus cultures were successfully transformed with at least one strain of A. tumefaciens, and A281 was the most effective of the four strains. N. glutinosa, N. plumbaginifolia, N. tabacum, P. hybrida and L. esculentum were transformed more efficiently than the other species tested.

Current Status of Binary Vectors and Superbinary Vectors

A binary vector was invented soon after it had been elucidated that crown gall tumorigenesis was caused by genetic transformation of plant cells with a piece of DNA, T-DNA for transferred DNA, from a Ti plasmid (tumor-inducing plasmid) harbored by the soil bacterium Agrobacterium tumefaciens ([

The promoter of rbcS in a C3 plant (rice) directs organ-specific, light-dependent expression in a C4 plant (maize), but does not confer bundle sheath cell-specific expression

The small subunit of ribulose-bisphosphate carboxylase (Rubisco), encoded by rbcS, is essential for photosynthesis in both C3 and C4 plants, even though the cell specificity of rbcS expression is different between C3 and C4 plants. The C3 rbcS is specifically expressed in mesophyll cells, while the C4 rbcS is expressed in bundle sheath cells, and not mesophyll cells. Two chimeric genes were constructed consisting of the structural gene encoding β-glucuronidase (GUS) controlled by the two promoters from maize (C4) and rice (C3) rbcS genes. These constructs were introduced into a C4 plant, maize. Both chimeric genes were specifically expressed in photosynthetic organs, such as leaf blade, but not in non-photosynthetic organs. The expressions of the genes were also regulated by light. However, the rice promoter drove the GUS activity mainly in mesophyll cells and relatively low in bundle sheath cells, while the maize rbcS promoter induced the activity specifically in bundle sheath cells. These results suggest that the rice promoter contains some cis-acting elements responding in an organ-pecific and light-inducible regulation manner in maize but does not contain element(s) for bundle sheath cell-specific expression, while the maize promoter does contain such element(s). Based on this result, we discuss the similarities and differences between the rice (C3) and maize (C4) rbcS promoter in terms of the evolution of the C4 photosynthetic gene.