The genetics and mechanisms of resistance to tomato powdery mildew (Oidium neolycopersici) in Lycopersicon species

Oidium neolycopersici</span><span lang=EN-US>is the causal agent of the tomato powdery mildew disease. Besides a few recently released resistant cultivars, all tomato cultivars are susceptible to O. neolycopersici. Therefore, resistance to O. neolycopersici is an important trait in tomato breeding.<span style='layout-grid-mode:line'>The aim of this research is to study the</span>interaction between tomato ( Lycopersicon esculentum ) and tomato powdery mildew ( O. neolycopersici ). Firstly, we characterized the genetic basis for several resistances against <span style='layout-grid-mode:line'>O. neolycopersici</span><span style='layout-grid-mode:line'>originating from wild Lycopersicon species and mapped the corresponding</span>resistance genes on the tomato genome. Remarkably, all the mapped dominant Ol -genes are located in three loci on tomato Chromosome 6. On the long arm, one locus contains the resistance genes Ol-1 and Ol-3 , originating from L. hirsutum G1.1560 and G1.1290, respectively, while another closely linked locus harbors Ol-5 , a resistance gene originating from L. hirsutum PI247087. On the short arm of Chromosome 6, one locus contains the two resistance genes Ol-4 (originating from L. peruvianum LA2172) and Ol-6 (unknown origin). In addition to these Ol -loci, three quantitative trait loci (QTLs) have been identified governing the O. neolycopersici resistance in L. parviflorum G1. 1601. One QTL ( Ol-qtl1 ) interval overlaps with Ol-1 , Ol-3 and Ol-5 . Two other QTLs ( Ol-qtl2 and Ol-qtl3 ) are linked on Chromosome 12 with Ol-qtl2 in the vicinity of the Lv locus that confers resistance to another powdery mildew species, Leveillula taurica . Secondly, we generated a set of near isogenic lines (NILs), each of which contains an individual Ol -gene in a genetic background of L. esculentum cv. Moneymaker. These NILs represent well-defined plant material and are a very useful tool to study the interaction between Ol -genes and O. neolycopersici isolates. Thirdly, we determined the race-specificity of the resistance conferred by the Ol -genes by testing the NILs with local isolates of O. neolycopersici in different geographic places. In addition to the dominant Ol -genes, the recessive gene ol-2 that originates from L. esculentum var cerasiforme and is located on Chromosome 4 (Ciccarese et al., 1998; De Giovanni et al., 2004) was also included in our study. Our data suggested that the resistance to O. neolycopersici conferred by different Ol -genes ( Ol-1, ol-2, Ol-3, Ol-4, Ol-5 and Ol-6 ) can be race-specific. Finally, we studied the resistance mechanisms associated with different Ol -genes by using the NILs. Microscopically, the resistance conferred by the dominant Ol -genes is associated with a hypersensitive reaction (HR), while, the resistance governed by the recessive gene ol-2 is related to papilla formation