GERMLINE TRANSFORMATION OF MAIZE FOLLOWING MANIPULATION OF CHIMERIC SHOOT MERISTEMS

Genetic transformation of maize has been limited to a small number of genotypes that form embryogenic tissue in culture. We have investigated whether cells in the developing shoot meristem of immature zygotic embryos might provide an alternative, more universal target for production of transformed maize plants. Following DNA delivery mediated by microprojectile bombardment, immature embryos developed into chimeric plants with transgenic sectors containing an antibiotic resistance marker and the B-glucuronidase (GUS) gene at a high frequency. Because the majority of transgenic sectors were restricted in size, the probability of a transformation event contributing to the germline without further manipulation was low. To enlarge the transgenic sectors and increase the likelihood of germline transmission, the apical meristems of germinated plants were excised and cultured on cytokinin-containing medium with a selective agent. Transformed sectors were visualized by their non-bleached phenotype or by staining with a GUS histochemical stain. Hormonally-induced shoot multiplication produced plants with sectors that had a greater chance of contributing to the germline. Transmission to progeny was demonstrated both by transgene expression and by Southern analysis. This method has been used successfully with genotypes that include a sweet corn hybrid and an elite field corn inbred.