Plant regeneration systems

The introduction of foreign genes by genetic engineering techniques as a means of plant improvement requires the development of an efficient regeneration system for the desired plant species. Such a system must be rapid, reliable and applicable to a broad range of genotypes. Until the early 1980s, efficient regeneration of plants from cultured cells and tissues of most of the important food crops had proven to be very difficult. The problem was solved by the culture of explants from immature tissues, which retain their morphogenetic potential, on nutrient media containing potent plant-growth regulators. Development of the leaf disk transformation system by Horsch and colleagues65 and the use of regenerable embryogenic cell cultures (so-called because they form somatic embryos) represented a technological breakthrough allowing almost routine transfer of foreign genetic material into a number of recalcitrant plant species. These techniques overcame many of the problems inherent in the protoplast transformation systems, particularly the extended culture period required and the limited regeneration of plants from protoplasts. However, the lack of efficient tissue culture systems generally applicable to agriculturally important crops is a major obstacle in the application of genetic engineering technology.

In tissue culture systems, it is important that a large number of in vitro culturable cells are accessible to the gene transfer treatment and they retain the capacity for regeneration of fertile plants during gene transfer and selection treatments. In some circumstances, especially in the design of gene transfer programs to produce desired commercial traits into elite vegetatively propagated cultivars, the need to avoid undesirable random genetic variation (somaclonal variation [66]) becomes the overriding consideration in the choice of tissue culture system. Minimizing the phase of tissue culture leading to the adventitious regeneration of plants is a factor favorably contributing to reduce the risk of somaclonal variation and morphological abnormality. This goal has been approached in several crops by particle bombardment into meristematic tissues, shoot proliferation and screening for transformed sexual progeny.67 The limiting factors remain the ability to prepare the explants, transfer genes into regenerable cells, and select or screen for transformants at an efficiency sufficient for practical use in crop improvement.