Genetics of Disease Resistance

Abstract

Disease resistant plants are one of the prerequisites for sustainable agriculture. To understand and rationally use the naturally occurring disease resistance, its genetic basis has been investigated in great detail. These studies showed that there are two different genetic mechanisms for disease resistance: monogenic resistance is based on single genes whereas quantitative resistance depends on two or more genes. In most cases, single resistance genes confer complete resistance but are only active against certain races of the pathogen, i.e. they show a genetic interaction with genes from the pathogen. This resistance is based on an active recognition event between the product of the plant resistance gene and the product of the avirulence gene of the pathogen. Resistance genes are clustered at some loci in the genome or exist as different alleles conferring resistance towards specific pathogen races. Quantitative resistance shows no obvious genetic interaction with the pathogen and slows down the disease development by increasing latency period and other parameters related to the epidemic. Resistance breeding in crop plants depends on both types of resistance. Monogenic resistances are easy to work with but are frequently not durable. Consequently, quantitative resistance is preferred. The application of molecular markers has allowed the genetics of quantitative resistance to be determined and quantitative trait loci involved in resistance to be identified. Molecular markers have also contributed to improved breeding strategies for monogenic resistance genes in order to combine them in the “gene pyramiding” strategy for a more durable resistance. Finally, molecular markers have allowed the isolation of the first disease resistance genes. The cloning of such genes from crop plants and their wild relatives will open new possibilities for their sustainable use in breeding.