Single Nucleotide Polymorphisms in the Allene Oxide Synthase 2 Gene Are Associated with Field Resistance to Late Blight in Populations of Tetraploid Potato Cultivars

By Pajerowska-Mukhtar, Karolina; Stich, Benjamin et al. | Genetics, March 2009 | Go to article overview

Single Nucleotide Polymorphisms in the Allene Oxide Synthase 2 Gene Are Associated with Field Resistance to Late Blight in Populations of Tetraploid Potato Cultivars


Pajerowska-Mukhtar, Karolina, Stich, Benjamin, Achenbach, Ute, Ballvora, Agim, Lübeck, Jens, Strahwald, Josef, Tacke, Eckhard, Hofferbert, Hans-Reinhard, Ilarionova, Evgeniya, Bellin, Diana, Walkemeier, Birgit, Basekow, Rico, Kersten, Birgit, Gebhardt, Christiane, Genetics


ABSTRACT

The oomycete Phytophthora infestans causes late blight, the most relevant disease of potato (Solanum tuberosum) worldwide. Field resistance to late blight is a complex trait. When potatoes are cultivated under long day conditions in temperate climates, this resistance is correlated with late plant maturity, an undesirable characteristic. Identification of natural gene variation underlying late blight resistance not compromised by late maturity will facilitate the selection of resistant cultivars and give new insight in the mechanisms controlling quantitative pathogen resistance. We tested 24 candidate loci for association with field resistance to late blight and plant maturity in a population of 184 tetraploid potato individuals. The individuals were genotyped for 230 single nucleotide polymorphisms (SNPs) and 166 microsatellite alleles. For association analysis we used a mixed model, taking into account population structure, kinship, allele substitution and interaction effects of the marker alleles at a locus with four allele doses. Nine SNPs were associated with maturity corrected resistance (P < 0.001), which collectively explained 50% of the genetic variance of this trait. A major association was found at the StAOS2 locus encoding allene oxide synthase 2, a key enzyme in the biosynthesis of jasmonates, plant hormones that function in defense signaling. This finding supports StAOS2 as being one of the factors controlling natural variation of pathogen resistance.

THE oomycete Phytophthora infestans (''invasive plant destroyer'') causes the late blight disease of potato (Solanum tuberosum) and tomato (Solanum lycopersicum). Without chemical control, epidemics of P. infestans can lead to complete yield loss. The first outbreak of late blight in Europe in the 19th century was responsible for the infamous ''Irish famine'' (Salaman 1985). Today, potato ranks fourth among the world's most important crops after wheat, rice, and corn, and late blight remains a major threat to potato cultivation. The extensive use of pesticides in crop protection increases production costs, is damaging to the environment, and contributes to the emergence of resistant P. infestans isolates (Duncan 1999; Kamoun and Smart 2005).

Genetic resistance to P. infestans has been identified in wild, tuber-bearing Solanumspecies native to Mexico and Central and South America, which are closely related to the cultivated potato (Ross 1986; Hawkes 1990). Resistance factors were introgressed into cultivars by sexual hybridization with wild species and backcrossing to adapted germplasm. Resistance is expressed as local necrotic lesions around the infection sites [hypersensitive resistance, (HR)] or as retardation of the disease progression in infected leaves and tubers when compared to susceptible genotypes (quantitative or field resistance). The phenotypic classification in one or the other resistance type is not always clear cut. The HR type of resistance is triggered by single, dominant genes (R genes) and confers resistance to specific races of P. infestans, whereas field resistance is a complex trait controlled by multiple genes and environmental factors. Genetic analysis suggests that the defense mechanisms underlying both resistance phenotypes are similar. R genes as well as genes with sequence similarity to R genes, defense signaling genes as well as defense response genes, may play a role in quantitative resistance to P. infestans (Leonards-Schippers et al. 1992, 1994; Trognitz et al. 2002; Stewart et al. 2003; Pajerowska et al. 2005; Tan et al. 2008). Introgression of R genes from the wild species Solanum demissum into cultivated potato failed to equip cultivars with durable resistance in the field, because theHR type of resistance was soon overcome by new, virulent P. infestans races. Quantitative resistance to late blight holds the promise to be more durable, as it is conferred by multiple genes and is considered race unspecific (Ross 1986; Wastie 1991). …

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Single Nucleotide Polymorphisms in the Allene Oxide Synthase 2 Gene Are Associated with Field Resistance to Late Blight in Populations of Tetraploid Potato Cultivars
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