Tomato Defense to Oldium neolycopersici: Dominant OI Genes Confer Isolate-Dependent Resistance Via a Different Mechanism Than Recessive oI-2

Abstract

Tomato powdery mildew caused by Oidium neolycopersici has become a globally important disease of tomato (Lycopersicon esculentum). To study the defense responses of tomato triggered by tomato powdery mildew, we first mapped a set of resistance genes to O. neolycopersici from related Lycopersicon species. An integrated genetic map was generated showing that all the dominant resistance genes (Ol-1, Ol-3, Ol-4, Ol-5, and Ol-6) are located on tomato chromosome 6 and are organized in three genetic loci. Then, near-isogenic lines (NIL) were produced that contain the different dominant Ol genes in a L. esculentum genetic background. These NIL were used in disease tests with local isolates of O. neolycopersici in different geographic locations, demonstrating that the resistance conferred by different Ol genes was isolate-dependent and, hence, may be race-specific. In addition, the resistance mechanism was analyzed histologically. The mechanism of resistance conferred by the dominant Ol genes was associated with hypersensitive response, which varies in details depending on the Ol-gene in the NIL, while the mechanism of resistance governed by the recessive gene ol-2 on tomato chromosome 4 was associated with papillae formation.

Genetics and molecular mechanisms of resistance to powdery mildews in tomato (Solanum lycopersicum) and its wild relatives

“…1). Ol-1, identified from S. habrochaites G1.1560 , has been mapped on the long arm of tomato chromosome 6 (Bai et al 2005). ol-2 is a recessive resistance gene found in S. lycopersicum var.…”

“…Ol-3, introgressed from S. habrochaites G1.1290, is located in the same chromosomal region as Ol-1. There is some evidence suggesting that Ol-1 and Ol-3 are allelic variants (Huang et al 2000b;Bai et al 2005). Ol-4, originating from S. peruvianum LA2172, is located on the short arm of chromosome 6 (Bai et al 2004).…”

“…Ol-4, originating from S. peruvianum LA2172, is located on the short arm of chromosome 6 (Bai et al 2004). Ol-5, introgressed from S. habrochaites PI247087, is closely linked to Ol-1 and Ol-3 on the long arm of chromosome 6 (Bai et al 2005). Ol-6, which was found in an advanced breeding line with unknown origin, is mapped in the same position as Ol-4 (Bai et al 2005).…”

“…These fungal pathogens produce discernible symptoms consisting of white colonies of mycelia on the surface of aerial green organs and sometimes on fruits upon heavy infection . The interaction of PMs with tomato, barley and Arabidopsis are well studied and, therefore, these pathosystems provide experimental models for understanding host and nonhost resistance to PMs (Bai et al 2005;Hückelhoven 2005;Li et al 2007;Schulze-Lefert and Vogel 2000).…”

“…We have identified five dominant resistance genes (Ol-genes) from wild tomato species and introgressed them into cultivated tomatoes, and cloned one recessive gene (ol-2) that confers mlo-based broadspectrum resistance (Bai et al 2005. In addition, we have mapped and introgressed three quantitative trait loci (QTLs) conferring different levels of resistance to O. neolycopersici.…”