Rpi-blb2 gene from Solanum bulbocastanum confers extreme resistance to late blight disease in potato

The Rpi-blb2 gene (R gene) from the Mexican wild potato species, Solanum bulbocastanum, was introduced into the potato variety Desiree by agro-infection in order to confer resistance to late blight which is still the most important disease of the potato worldwide. A total of 148 putative transgenic events was obtained by selection based on hygromycin resistance of which 127 appeared to contain the Rpi-blb2 gene. To rapidly identify the highly resistant transgenic events, we used an in vitro assay with one isolate of Phytophthora infestans and a whole-plant assay with two isolates. Nine transgenic events were highly resistant to both isolates and four of these contained single copy T-DNA and did not contain backbone vector sequences. The transcript abundance of Rpi-blb2 was low in infected and uninfected leaves and did not correlate with the resistance level determined in whole plants. We proceeded with high dose inoculation of whole plants of three highly and moderately resistant transgenic events, which resulted in lesions with low levels of sporulation in a few cases. Reinfection was not successful in any of the cases. Hence our results show that a high level of resistance to LB, including extreme resistance, can be obtained through the Rpi-blb2 gene and that heavy inoculation did not readily increase virulence in our test conditions. These transgenic events will be used for either multi-line deployment or R gene stacking by conventional breeding in order to release late blight resistant varieties which will need limited fungicide use, if any.