Genetic mapping of the Ph gene conferring disease resistance to black shank in tobacco

Black shank, caused by the hemibiotrophic oomycete Phytophthora parasitica var. nicotianae, is one of the most destructive tobacco diseases. Using host resistance is the most environmentally friendly and sustainable strategy for reducing potential crop losses from blue shank disease. To assist breeding for black shank resistance and to facilitate gene cloning, we genetically mapped and characterized the Ph gene that confers resistance to P. parasitica var. nicotianae race 0. The Ph gene, which originated in N. plumbaginifolia, was found to be located on the top of LG20 in a genomic region that is enriched with genes coding for receptor-like kinases. Expression analysis of pathogen-related (PR) genes revealed that the hypersensitive response (HR) was induced rapidly in resistant plants at the biotrophic infection stage, while in susceptible lines, an intensified HR-like reaction was activated during necrotrophy. The genotype race-specific resistance conditioned by the Ph gene may be triggered by recognition of a matching Avirulence (Avr) protein secreted by the pathogen during the early phase of infection. However, pathogen colonization in compatible hosts could be achieved by hijacking of resistance signaling and acquiring nutrients from the dead cells after it switches to necrotrophy.