QTL analysis of multigenic disease resistance in plant breeding

Abstract

Multigenic or quantitative disease resistance has challenged plant breeders working to develop disease resistant crop cultivars. The challenge to incorporate into new cultivars equivalent levels of resistance that existed in the original genetic resistance stock(s) is formidable, given the apparent complexity of quantitative resistance. Environmental factors, complex multigenic inheritance, plant avoidance, and escape mechanisms combine to hamper the efforts of breeders working to incorporate multigenic resistance into future cultivars. Breeding for quantitative resistance is more formidable than for qualitative resistance traits as more complex and lengthy breeding procedures are needed to effectively incorporate adequate levels of quantitative resistance into new crop cultivars. The expression of quantitative resistance in many instances is partial, not absolute, and the control of resistance appears to be governed by many genes acting cumulatively. The rating of genotypes for disease development in field or greenhouse becomes more subjective due to interactions with environmental and plant morphological factors, requiring additional testing and replications to validate their accuracy. In the literature, many nonspecific and complex resistance mechanisms associated with quantitative resistance have been grouped under the broad general headings of horizontal resistance, polygenic resistance, partial resistance, or durable resistance, which suggests complexity but contributes little to resistance breeding. Current analytical molecular tools, however, are making the breeding of quantitative resistance more effective and new insights on the magnitude and location of such resistance loci may assist plant breeders in better exploiting this type of resistance in future crop cultivars. Quantitative Trait Loci (QTL) analysis is a valuable tool for genome exploration and the investigation of multigenic traits. The focus of this chapter is to review the body of work devoted to the identification of QTL controlling quantitative disease resistance in crops and the exciting implications of the implementation of QTL analysis to dramatically enhance disease resistance breeding. QTL analysis is rapidly changing the way scientists view disease resistance and the time-held concepts and importance of major and minor gene resistance.