Fitness benefit plays a vital role in the retention of the Pi-ta susceptible alleles

In plants, large numbers of R genes, which segregate as loci with alternative alleles conferring different resistance to pathogens, have been maintained over a long evolutionary time. In theory, there seem to be no reason for hosts to harbor these susceptible alleles in view of their null contribution to resistance. As such, why should populations support disease-susceptible individuals along with disease-resistant individuals? In rice, a single copy R gene Pi-ta segregates for two expressed clades of alleles, one resistant and the other susceptible. We discovered that knockout of the Pi-ta susceptible alleles induced drastic fitness decline in the absence of pathogens. Gene expression profiling and endogenous hormones quantification showed that the susceptible alleles might serve as an off-switch to the downstream immune signaling, thus contributing to fine-tuning of plant defense response. The fitness benefit of carrying a susceptible Pi-ta allele provides a plausible explanation for their retention in the genome.