Aspects of plant genetic manipulation

Gene transfer in cereals

Until recently, gene transfer in plants was achieved only by sexual hybridization. Now, in addition, plant genetic manipulation, with the use of both recombinant DNA and protoplast fusion technology, is being applied to an increasing range of plants. The soil bacterium Agrobacterium tumefaciens, with its associated plasmid, is used as a vector for introducing DNA into the genomes of dicotyledonous plants, but it has not proved suitable for cereals. Instead, the direct uptake of plasmid DNA into cereal protoplasts is being used for the transformation of cells in rice, wheat, and maize. Transformation efficiencies, in some cases, are becoming comparable to those obtained in dicotyledons with Agrobacterium. In rice it is now possible to regenerate efficiently whole plants from protoplasts, and this capability may soon be extended to the other cereals. By means of direct interaction of cereal protoplasts with plasmids, coupled with improved procedures for the regeneration of plants from their protoplasts, gene transfer in the cereals is becoming established at the frontiers of recombinant DNA technology.

Intergeneric somatic hybrid production through protoplast fusion between Brassica juncea and Diplotaxis muralis

The need to transfer genetic traits from Diplotaxis muralis (2n=42) to Brassica juncea (2n=36), a major oil seed crop of the tropical world, was realised. Since the two plant types are sexually incompatible, attempts were made to evolve parasexual hybrids as the result of protoplast fusion. Protoplasts of hypocotyl-derived calli of two cultivars of B. juncea were fused with normal and γ-irradiated mesophyll protoplasts of Diplotaxis muralis. Regeneration of 110 plants from the fused products was successfully achieved. Upon analysis of some of them, we realised that true somatic hybrids and partial somatic hybrids had been generated. Thus the primary goal of evolving intergeneric hybridisation products between these two plant types was fulfilled.

Identification of somatic hybrids in plant protoplast fusions with monoclonal antibodies to plasma-membrane antigens

Monoclonal antibodies generated by immunization with a plasma-membrane preparation from suspension-cultured cells of Nicotiana glutinosa L. were used in combination with fluoresceinor rhodamine-labeled goat anti-mouse immunoglobulins to identify heterokaryons in protoplast fusion procedures. Antibody labeling did not inhibit callus formation nor plantlet regeneration. The antibodies are non-invasive and surface labeling provides clear optical discrimination of true heterokaryons from unfused aggregates as well as from parental protoplasts and homokaryons. Labeling is stable throughout fusion and hence by pre-labeling parental protoplast populations the strategy is both versatile and of general applicability.

Transformation of cucumber (Cucumis sativus L.) plants with Agrobacterium rhizogenes

Transgenic cucumber plants (Cucumis sativus L., cv. 'Straight Eight') were regenerated from roots induced by inoculation of inverted hypocotyl sections with Agrobacterium rhizogenes containing the vector pARC8 in addition to the resident Ri-plasmid. The DNA transferred to the plant from the vector (T-DNA) included a gene which encoded the enzyme neomycin phosphotransferase II, and thus conferred on the plant cells resistance to kanamycin. The transgenic plants looked normal and were positive for the neomycin phosphotransferase II. Southern blot analysis of the transgenic plants revealed that all plants contained vector DNA, but only some of them contained DNA from the Ri plasmid.

Plant regeneration from protoplast-derived tissues of Linum usitatissimum L. (Flax)

Protoplasts were isolated enzymatically from seedling roots, hypocotyls and cotyledons of Linum usitatissimum L. which divided to form callus. Plant regeneration was obtained from protoplast-derived tissues of root and cotyledon, but only rhizogenesis was observed in the case of protoplasts derived from hypocotyls. The ability to isolate, culture, and regenerate plants from root and cotyledon protoplasts of Linum usitatissimum L. is discussed in relation to future attempts to produce somatic hybrids between Linum species.

Genetic manipulation in plant breeding: somatic versus generative

A comparison is made between molecular/ in vitro/somatic and plant-level/generative approaches in the reconstruction of genotypes and reproductive systems. Although classical methods will remain the basis of plant breeding, a number of new somatic as well as generative genetic manipulation techniques are definitely applicable in several special situations. The first are technically more demanding, the latter are often conceptually more difficult, and both are laborious. Choice of approach is determined by the plant species, the stage of development of the techniques, the amount of background genetic information and the genetic diversity available, and the capacity of the institution involved. In the final stages of the program traditional selection and testing procedures remain indispensable. Whether any particular breeding program will profit from the incorporation of sophisticated genetic manipulation techniques must be carefully analysed. This discussion is intended to provide a basis for this analysis.

Development of DNA-mediated transformation systems for plants

The genetic engineering of plants by DNA-mediated gene transfer requires that efficient transformation systems be developed. Considerable progress has been made in manipulating the Ti plasmid of Agrobacterium tumefaciens as a vehicle for delivery of foreign genes into protoplasts of dicotyle-donous plants. Part of the Ti plasmid, the T-DNA, can be incorporated into the genome of the host cell; the T-DNA can carry a foreign DNA sequence which co-integrates with it; under normal conditions, the tumorigenic-causing portion of the T-DNA can be inactivated so that transformed protoplasts can be regenerated and T-DNA with an inserted foreign gene can be stably maintained during regeneration, meiosis and gamete formation. A foreign gene has yet to be expressed in regenerated plants although a T-DNA gene for opine synthesis can function in regenerates. Developing a more ubiquitous transformation system for monocotyledons is further from fruition. Based on transformation systems for simple eukaryotic organisms, it is reasonable to expect that a DNA vector which is capable of amplifying a novel plant gene and which contains both a drug resistance marker to facilitate the selection of transformed plant protoplasts and a species-specific autonomously replicating sequence to ensure the stable maintenance of the input gene in the recipient cell can be constructed.

Crown Gall Disease and Prospects for Genetic Manipulation of Plants

Agrobacterium tumefaciens incites crown gall tumors when bacterial DNA integrates into plant nuclear DNA. Plant cells can express these integrated bacterial genes. Following insertion of desired genes into bacterial DNA using recombinant DNA techniques, this system permits introduction of these new genes into plant DNA. We discuss the potential for genetic manipulation of plants using Agrobacterium tumefaciens and the related organism Agrobacterium rhizogenes.