Abstract
Immature zygotic embryo has been the widely used explant source to develop embryogenic callus lines, cell suspensions and protoplasts for transformation of cereal crops including maize, wheat, rice, oat, barley, sorghum, and millet. However, the lack of competence of immature embryos in certain elite lines is still a barrier to routine production of transgenic cereal crops in certain commercial cultivars. In addition, a great deal of effort is required to produce immature embryos, manipulate cultures of immature embryos or their cell suspensions, and cryopreserve cultures for further use. In addition, undifferentiated cells may have reduced regenerability after a few months of in vitro culture. Alternative explants and regeneration systems for efficient transformation of cereal crops are needed to avoid or reduce the above limitations. During the past decade, scientists have successfully manipulated the shoot apical meristems from seedlings of maize, oat, sorghum, millet, wheat, and barley in an effort to develop a less genotype-dependent and efficient cereal regeneration system that can be maintained in vitro for long periods of time without the need for cryopreservation. Furthermore, apical meristem regeneration systems were used to stably transform maize, wheat, rice, oat, barley, sorghum, and millet.