Transformation ofBrassica oleracea L.: a critical review

Brassica oleracea is a highly polymorphic species encompassing a wide range of important vegetable and fodder crops. Gene transfer into cultivated forms of this species requires reproducible and efficient methods for genetic transformation and plant regeneration. In this review, we have collated the research experience on transformation ofB. oleracea to highlight the problems encountered. Most research effort has been directed at developingAgrobacterium-mediated transformation methods with relatively little emphasis to date on direct gene transfer techniques. Common procedures for the transformation ofB. oleracea have not emerged, due to the inherent variability between and amongst genotypes. Future progress would be facilitated by the use of genetically fixed material, such as double-haploid or inbred lines, to reduce variation of response within genotypes and would avoid the need for cultivar-specific transformation protocols if responsive lines amenable to crossing with cultivated forms could be identified. The principal difficulties relate to combining efficient plant regeneration with gene transfer. Methods that enhance bacterial virulence and increase the proportion of cells susceptible to transformation and competent for regeneration are discussed. Inefficient selection is a major cause of poor transformation frequencies inB. oleracea and has resulted in the regeneration of chimeric plants uponAgrobacterium tumefaciens-mediated transformation. Promising results have been obtained withAgrobacterium rhizogenes-mediated transformation but the impact of therol genes on flowering of primary transformants has not yet been fully assessed. Strategies to reduce the deleterious effects of therol genes on flowering are discussed. Few agronomically useful characters have been introduced, the majority of research having been confined to the introduction of marker and reporter genes; possible candidate genes are discussed.