Progress of cereal genome engineering mediated by Agrobacterium tumefaciens

Agrobacterium tumefaciens-mediated gene transfer is a method employed widely in many plant species. This bacterium can efficiently transfer large DNA segments with defined ends to plant chromosomes with few rearrangements. However, until about 20 years ago, it was generally believed that monocotyledons could not be transformed by Agrobacterium, because these plants are outside the host range of crown gall disease caused by this bacterium. In 1994, rice was the first cereal species to be transformed efficiently by A. tumefaciens, and maize, wheat and barley soon followed in the 1990s. Finally, it has now been clearly demonstrated that A. tumefaciens can transfer genes to both dicotyledons and monocotyledons by the same mechanism. Progress in transformation methodology has opened up new opportunities for plant biotechnology. Although monocotyledons were late-comers to the arena of genetic engineering, it is these plants that have been leading the latest developments. Genetically modified maize varieties dominate other commercialized biotechnology products in terms of volume. New techniques have been developed one after another in rice and other cereals. Arabidopsis has traditionally been the primary subject of the study of the genome, but it is rice that now dominates the field of plant genomics. Rice is a convenient model plant as well as a major crop in the world. The richest and most comprehensive sets of so-called genomic resources, such as libraries of cDNA and genomic DNA, sequence and expression profile databases, libraries of mutants and tagging lines, etc., have been accumulating in rice. Again, rice sets the main stage in research into gene targeting. This series of articles first briefly describes the research history of monocotyledon transformation before reviewing the progress and optimization of gene transfer technology in major cereals. Production and commercialization of genetically modified elite maize varieties is then reported by a representative of a leading company in the seed industry. New targets currently under development for genetically modified rice such as nutrient-fortification and stress tolerance are also reported. Diverse subjects including basic studies into the plant–bacteria interaction, functional genomics and targeted genetic/genomic modification are discussed by experts from leading laboratories. Through these discussions, the progress of cereal genetic engineering in the last two decades is celebrated and insights into future research provided.