Future trends
The field of plant resistance control is undergoing a very active and exciting period, during which major breakthroughs are being made. Increased availability of cloned R genes will permit their testing in different plant backgrounds. In contrast with the success in the production of insect- and virus-resistant crops, the production of fungi-resistant crops with commercially useful levels of resistance has not been achieved. However, it is likely that commercial introduction of fungi-resistant crops can be expected within 4–8 years.In the same way that plant breeders are continually developing new varieties that contain the most effective combination of existing characteristics, there is a similar trend with transgenic crops. Many laboratories are experimenting with ‘pyramiding of genes’, which consists of the introduction of multiple genes conferring different characters. A good example of this is a potato line containing seven transgenes that will confer resistance to insects, fungi, virus, and will alter other phenotypic characteristics (Dunwell 2000).
Overexpression of signalling components that lie downstream of R genes is an interesting approach that is currently being tested. This may allow activation of only certain defence pathways and may avoid agronomic problems associated with constitutive activation of some R-mediated pathways (see Section Plant R genes). The use of antimicrobial peptides to engineer broad-spectrum resistance is a promising and powerful approach that will be used considerably in the near future.
The increased use of inducible promoters is another current trend to manipulate specific pathways or to express R genes in a much more controlled manner (Shen et al. 2000). Finally, in addition to the progress being made on the plant side of the equation, an understanding of the genetic make-up of pathogens and the critical genes involved in the pathogenesis process are expected to open new avenues in crop protection.