Other strategies

In addition to the strategies described above, other approaches to control fungal disease are based on the manipulation of the levels of phytoalexins which are small, broad-spectrum antimicrobial compounds whose synthesis and accumulation is frequently associated with HR. For example, increased resistance to Botrytis cinerea has been observed in tobacco expressing a grapevine stilbene synthase gene, which increased the level of the phytoalexin resveratrol (Hain et al. 1993). However, there are several caveats to engineering phytoalexin-mediated resistance. Many of these compounds are synthesised via complex pathways, which in order to alter existing phytoalexin structure or content, would require the manipulation of several genes, making the whole process technically more demanding. In addition, phytoalexins are often toxic to the pathogen as well as to the plant. Thus, some type of inducible expression system may be required. A novel and promising approach to engineer broad-spectrum resistance relies on the use of antimicrobial peptides. These peptides are ubiquitous (Gabay 1994) and show a strong antimicrobial activity. Expression in potato plants of a synthetic gene encoding an N-terminus-modified, cecropinmelittin cationic peptide chimera resulted in significant resistance against several bacterial and fungal phytopathogens (Osusky et al. 2000). It is likely that this type of approach will be employed considerably in the near future to engineer a range of disease-resistant plants.