of the known R genes, particularly those which confer protection from pathogen and nematode pests, are highly conserved in structure and function (Baker et al. 1997); that is, an R gene from one plant species will often function after transfer to another plant species. The N gene for resistance to tobacco mosaic virus in Nicotiana tabacum, for example, functions well after transfer to tomato (Whitham et al. 1996), and the Cf-9 gene for race-specific protection of tomato from the fungus Cladosporium fulvum is functional when transferred to tobacco and potato, as the gene triggers HR specifically in response to the C. fulvum avr9 avirulence protein (Hammond-Kosack et al. 1998).
Cloned R genes and pathogen avirulence genes make it possible to engineer natural resistance responses to a wide array of pathogens and pests. For example, combining an R gene with a corresponding avirulence gene under the control of appropriate regulatory genetic elements in transgenic pest-protected plants can facilitate activation of defense responses against pathogens that are normally not limited by that particular R gene.
Transfer of defense genes for specific degradative enzymes and inhibitors can also confer pest-protection. For example, constitutive or localized expression of a variety of genes that encode proteinase inhibitors, chitinases, and lectins in transgenic plants can provide protection against some chewing insects, sucking insects, or nematodes (Johnson et al. 1989; Kramer and Muthukrishnan 1997; Rao et al. 1998; Ryan 1990; Urwin et al. 1997). Transgenic modification of the production of defensive chemicals also will affect resistance to pests and pathogens (for example, Melanson et al. 1997).
Research focused on developing new ways to produce both conventional and transgenic pest-protected plants, is some of the most exciting in the field of plant biology. Through wide crosses and other nontransgenic techniques, plant resistance genes will continue to be transferred to crop species from species at greater and greater taxonomic distances. A number of genomics projects with model and crop plants are yielding data from which information about new R and defense genes can be obtained. That information could lead to identification and manipulation of resistance factors with unique specificities against important pests and pathogens. The signaling mechanisms whereby resistance responses are triggered by insects and pathogens are being unraveled. It might soon be possible to engineer plants with altered signaling components that result in resistance being triggered by a broader array of pests. Understanding how defensive secondary compounds and defense proteins are produced