Transfer of foreign genes into intact maize cells with high-velocity microprojectiles

This report describes a process for delivering foreign genes into maize cells that does not require the removal of cell walls and is capable of delivering DNA into embryogenic and nonembryogenic tissues. Plasmid harboring a chimeric chloramphenicol acetyltransferase (CAT) gene was adsorbed to the surface of microscopic tungsten particles (microprojectiles). These microprojectiles were then accelerated to velocities sufficient for penetrating the cell walls and membranes of maize cells in suspension culture. High levels of CAT activity were consistently observed after bombardment of cell cultures of the cultivar Black Mexican Sweet, which were comparable to CAT levels observed after electroporation of protoplasts. Measurable increases in CAT levels were also observed in two embryogenic cell lines after bombardment. Gene expression was observed only when an intron from the alcohol dehydrogenase 1 gene of maize was ligated between the 35S promoter and the CAT coding region. CAT activity was detected in cell cultures bombarded with microprojectiles with an average diameter of 1.2 mum, but not after bombardment with microprojectiles 0.6 or 2.4 mum in diameter. Bombarding the same sample several times was found to markedly enhance CAT activity. These results demonstrate that the particle bombardment process can be used to deliver foreign DNA into intact cells of maize. Because this process circumvents the difficulties associated with regenerating whole plants from protoplasts, the particle bombardment process may provide significant advantages over existing DNA delivery methods for the production of transgenic maize plants. In addition, the process should be of value for studying transient and stable gene expression within intact cells and tissues.