Transgenic Cereals — Zea mays (maize)

Abstract

Genetic transformation of maize is routine in several genotypes despite the many difficulties encountered in developing reliable transformation techniques in this major cereal species. Aspects of maize tissue culture, including the target expiant, subsequent rapid in vitro proliferation and dependable regeneration from competent cells were prerequisite developments for gene delivery into maize. Recovery of transgenic, fertile maize required high levels of gene expression and identification of new selectable markers, along with DNA delivery into competent maize cells. DNA delivery by particle bombardment, Agrobacterium, electroporation and silica fiber methods have been the most carefully documented, each of which can now be used for gene transfer into maize. Promoters such as those from the CaMV 35S or ubiquitin genes, together with various introns have been widely used to achieve high expression levels, while the herbicide resistance gene, bar, has served as an important selectable marker for numerous studies in maize transformation. Although tissue culture cells were instrumental in the development of maize transformation, the direct use of expiants such as the immature embryo and/or meristems has found favor in more recent applications. Gene delivery in maize has shifted from emphasis on technology development to evaluation of gene expression with various transgenes, some of which are already in large-scale commercial development (e.g. insect and herbicide resistance). Maize transformation is increasingly being used to address more sophisticated aspects of gene regulation, plant development and physiology. The stability of transgene expression in primary transgenic plants and subsequent generations is of obvious academic and commercial importance. The isolation of promoters with a variety of expression profiles that are tissue-specific and/or temporally regulated will become more important as trait modification strategies evolve. Technologies such as site-directed integration, homologous recombination, ‘chimeraplasty’, and others will likely become routine in higher plants such as maize as this research area, now in its infancy, continues to develop. These technologies have the potential to aid our understanding of gene regulation, and to more directly make changes in endogenous gene sequences or to permit targeting of new genes (or regulatory elements) into precise genomic locations. With an assortment of accompanying genetic tools such as reverse genetic methods, mapping, genome-scale analysis and gene expression information, maize transformation has evolved into an important tool for both basic and applied studies in plants.