Gene transfer methods for plants and cell cultures

Agrobacterium-mediated gene transfer provides a routine and efficient gene transfer system for a variety of plant species. As this biological vector does not, however, function with important plant species, numerous alternative approaches have been studied. Of those, direct gene transfer into protoplasts, microinjection and biolistics have been demonstrated to be effective. Others, for example, viral vectors, agroinfection, liposome injection and electrophoresis may have special merits, although transgenic plants have not been produced by these techniques yet. From methods based on pollen transformation, the pollen tube pathway, pollen maturation, incubation of dry seeds, incubation of tissues, liposome fusion with tissues, macroinjection, laser treatment and electroporation of tissues no proof of integrative transformation is available, so far, and it is difficult to envisage how these approaches will ever produce transgenic cells and plants. We discuss (a) why Agrobacterium does not function with all plants, (b) what merits and disadvantages we see for the effective methods, (c) what possibilities we foresee for some of the other approaches, and (d) why we do not expect the remaining ones to be successful.