Transformation of monocots

In the past 5 years, dramatic progress has been made toward the development of protocols for stably transforming agriculturally important monocotyledonous plant species. The first indication of gene transfer involved the introduction of a plant viral genome into a plant host via A. tumefaciens-mediated transfer of T-DNA carrying the viral genome. Once inside the plant host, the viral DNA excises from the T-DNA and infects the host, inciting disease symptoms that are characteristic of the virus. This process, termed agroinfection, supplied compelling evidence that A. tumefaciens transfers T-DNA to monocot plants such as maize. A notable feature of agroinfection is that the introduced viral DNA incites disease without incorporating into the plant genome. Early efforts to obtain stable transformation of monocot species were unsuccessful. The demonstration of agroinfection and the inability to demonstrate T-DNA integration together led to the suggestion that the T-DNA integration step was somehow blocked in monocots. However, protocols have been developed for the efficient and reproducible stable transformation of rice, corn, wheat, and other monocot species. Key to the success of these protocols was the use of actively dividing cells such as immature embryos. In addition, preinduction of A. tumefaciens with phenolic inducers appears to enhance T-DNA transfer efficiencies. Additional factors, such as plant genotype, the type and age of plant tissue, the kinds of vectors and bacterial strains, and the types of selectable genes delivered to plant cells, influence the transformation efficiencies. For rice and corn, most of these parameters have been optimized, so that the delivery of foreign DNA to these crop plants is a routine technique.

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