Plant R genes

The genetically controlled induction of HR is triggered in plant-pathogen interactions only if the plant contains a disease-resistance protein (R) that recognises the correspondent avirulence (Avr) protein from the pathogen. In the absence of a functional R gene or avirulence gene product, no recognition occurs and disease ensues. This indicates that the factors controlling HR are quite specific and that they do not provide resistance to more than a limited number of races or pathotypes. To engineer broad-spectrum disease resistance relying on HR, two approaches have been employed, although they are still in the early stages: one is based on the transfer of an avirulence gene (i.e. the Cladosporium fulvum avr9 gene) into a plant containing the corresponding resistance gene (i.e. the tomato Cf9 gene) and its subsequent expression under the control of a promoter inducible by fungal pathogens. Pathogen-induced expression of the Avr gene will then provoke a resistance reaction manifested by a HR. A localised HR will be induced preventing further spread of any invading pathogen, followed by a general defence response. The other approach is based on the overexpression of R genes.

The first approach has been tested experimentally three times, twice in tomato using the AvrPto gene (Tobias et al. 1999; Melchers and Stuiver 2000) and the other in tobacco using an elicitin gene coupled to a pathogen-inducible promoter (Keller et al. 1999). In both cases, increased resistance to a broad spectrum of diseases, including both fungal and viral, was obtained. Nevertheless, the main limitation of this approach is still the limited number of Avr genes available.

The second approach has benefited from the cloning and analysis of over 20 R genes isolated from seven plant species, including both monocots and dicots (Martin 1999). Resistance genes involved in race-specific interactions often provide full disease resistance and are well-known from conventional breeding programmes (Rommens and Kishore 2000). In spite of the great diversity in lifestyles and pathogenic mechanisms of disease-causing organisms, it was somewhat surprising that R genes were found to encode proteins with sequence similarities and conserved motifs (Martin 1999). They have been classified into five classes according to the structural characteristics of their predicted protein products: intracellular protein kinases; receptor-like protein kinases with an extracellular leucine-rich repeat (LRR) domain; intracellular LRR proteins with a nucleotide binding site (NBS) and a leucine zipper motif; intracellular NBSLRR proteins with a region with similarity to the Toll and interleukin-1 receptor (TIR) proteins; and LRR proteins that encode membrane-bound extracellular proteins.

Overexpression of Pto (an R gene) in tomato elicited an array of defence responses including microscopic cell death, SA accumulation and PR gene expression, and the plants showed increased resistance to several pathogenic bacteria and fungi (Tang et al. 1999). Similarly, overexpression of the Bs2 pepper R gene in tomato allowed enhance resistance against Xantomonas campestris (Oldroyd and Staskawicz 1998) and bacterial spot disease (Tai et al. 1999). Isolation and analysis of plant R genes is an extremely active area of research and considering the large number of genes already available and the current work to isolate more genes in many laboratories around the world, there will be substantial progress in this field in the short term.

Overexpression of signalling components that lie downstream of R genes is another possible strategy to increase disease resistance. In the first successful example of this approach, the NPR1 gene was overexpressed in Arabidopsis and the resultant transgenic plants exhibited significant increases in resistance to Pseudomonas and Peronospora pathogens (Cao et al. 1998). Manipulation of downstream components such as NPR1 potentially allows activation of only specific defence pathways. This might be one way to avoid agronomic problems associated with constitutive activation of R gene-mediated pathways such as HR (see Antifungal proteins).