Functional Markers in Resistance Breeding

Abstract

Severe yield loss in plant crops caused by pathogens constantly demands plant breeders to improve disease resistance in crop species. To date, more than 90 plant resistance (R) genes have been isolated, many of them belonging to the NBSLRR class of R genes. These genes are often found in clusters along the chromosomes and frequently lack genetic synteny between species, which makes them difficult to identify based on sequence homology to previously isolated R genes. However, once identified, R genes provide the basis for the application of functional markers (FMs), which efficiently assist phenotypic selection in all phases of resistance breeding. FMs are derived from polymorphic sites within genes causally involved in phenotypic trait variation. The major advantage of FMs is that they are in complete linkage disequilibrium with causative genes. Consequently, they can be used in a number of different genetic backgrounds. In fact, only a few FMs for R genes have been published in crop species up to now, although several alleles have been identified for a number of cloned R genes. But the rapidly increasing number of cloned R genes, decreasing costs for allele sequencing and the increasing availability of TILLING populations (targeting-induced local lesions in genomes) for most crop species will significantly contribute to the establishment of FMs for most major disease R genes within the next decade. Thus, this review summarises the current status of FM development in R genes and discusses implications of the availability of FMs for resistance breeding.