Genetic engineering of disease resistance in cereals

Transformation procedures have been developed for introducing cloned genes into all major cereals. These procedures are beginning to be used to enhance disease resistance, although progress is slowed by the 8 to 10 months required to obtain seed from the first generation of transformed plants. Genotype dependence, somaclonal variation, and lack of stability in inheritance and expression also limit output of useful transformants. Nevertheless, several types of genes with potential to enhance disease resistance, as demonstrated in tobacco or other easily transformed plants, are available for introduction into cereals. For virus diseases, these include pathogen-derived genes for coat protein and antisense versions of replicase or other factors required for virus replication. For fungal diseases, candidates include genes for chitinase and beta-1,3-glucanase, for several classes of low molecular weight antifungal proteins, for pathogenesis-related protein-1. Other candidate genes code for enzymes that detoxify pathogen toxins, disable hydrolytic enzymes of the pathogen, or enhance phytoalexin production. Isolation of race-specific genes for resistance from cereals and avirulence genes from cereal pathogens will offer new opportunities for engineering disease resistance using transgenes. To select among multiple candidate genes for stable transformation, a transient expression assay is described using barley epidermal cells inoculated with Blumeria (Erysiphe) graminis. For optimum efficiency and durability, gene expression in stable transformants needs to be targeted at infection sites in tissues and cells located in the invasion path of individual pathogens.