Summary

We review here the results on gene delivery into pollen grains and transformation by particle bombardment. Most of the studies that have been done up to date on bombardment-mediated transformation of pollen are still confined to transient expression of several pollen- or anther-specific promoters. However, these studies do provide evidence that pollen grains can be transformed at least transiently, and suggest the potential use of these “transformed” pollen grains or those bearing foreign DNA for direct pollination to obtain transgenic seeds via natural reproduction system. The alternative way of using pollen transformation for crop improvement is to develop haploid plants via in vitro culture of bombarded immature pollen grains. Included here are the authors' own recent results on successful production of transgenic haploid plants derived from in vitro culture of bombarded pollen. These results provide a basis for a discussion of the usefulness of pollen transformation for crop improvement.

Significance of pollen transformation for crop improvement

Pollen transformation is an attractive approach for plant breeding and crop improvement. There have been reports of various techniques for gene delivery into pollen or microspores, including imbibition of pollen with DNA (Hess 1980), Agrobacterium-mediated transformation (Hess 1987; Pechan 1989), electroporation of pollen (Matthews et al. 1990; Fennell and Hauptmann 1992; Jardinaud et al. 1993), and polyethylene glycol-mediated transformation (Fennell and Hauptmann 1992).

However, all these techniques are problematic in their applicability or reproducibility. Electroporation is useful for introducing foreign DNA into germinating pollen (Matthews et al. 1990), but it is not applicable to plant species in which in vitro pollen germination is difficult.