Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 19, 2012
Publication Date: March 1, 2012
Citation: Halterman, D.A., Middleton, G.E. 2012. Presence of the potato late blight resistance gene RB does not promote adaptive parasitism of phytophthora infestans. American Journal of Plant Sciences. 3:360-367. Interpretive Summary: The gene Rpi-blb1, from the wild potato species S. bulbocastanum, confers partial resistance to late blight, caused by the pathogen Phytophthora infestans. The primary objective of this work was to determine whether continual exposure of the pathogen to resistant plants leads to the breakdown of resistance. We found that the late blight pathogen was not able to overcome Rpi-blb1 resistance after 20 rounds of selection on the partially resistant leaflets. We found that continual exposure to Rpi-blb1 containing leaflets resulted in reduced fitness of the pathogen since this strain produced fewer and smaller lesions at the end of the experiment. This indicates that adaption to cause disease when Rpi-blb1 is present imparts a fitness penalty on the pathogen. Our results are important because they indicate that deployment of the Rpi-blb1 gene on a large scale will not select for pathogen strains that can overcome resistance. This will impact potato breeders interested in incorporating this gene into newly developed cultivars.
Technical Abstract: The potato gene Rpi-blb1, from the wild potato species S. bulbocastanum, confers partial resistance to late blight, caused by the oomycete pathogen Phytophthora infestans. In order to determine whether a single strain of P. infestans can adapt to overcome this partial resistance source, we subjected Rpi-blb1 containing plants to multiple rounds of infection with P. infestans, with sporangia from a late blight lesion used to infect the next leaflet. A parallel line of inoculations was done using susceptible leaflets. At the end of the experiment, sporangia passaged through resistant or susceptible leaflets were compared for their ability to cause disease. Variants of the corresponding P. infestans effector IPI-O, which is recognized by Rpi-blb1 to elicit resistance, were also cloned and sequenced to determine whether variation occurred after selection on the partially resistant host. After 20 rounds of selection, no breakdown in Rpi-blb1 resistance was observed. In fact, the strain passaged through the partially resistant host produced smaller lesions on susceptible leaflets and had a lower infection frequency than the strain passaged through susceptible cultivar Katahdin. Our results indicate that continual selection by the Rpi-blb1 gene can reduce P. infestans virulence.