In 1995, the Ronald laboratory isolated and characterized the rice XA21 genetic locus and showed that XA21 encodes a receptor kinase (aka pattern recognition receptor.)

Subsequent discoveries in flies, humans, mice, and Arabidopsis revealed that animals and other plant species also carry membrane-anchored receptors with striking structural similarities to XA21 and that these receptors also play key roles in the immune response.

 In the 1990s, rice geneticist Gurdev Khush and colleagues identified a wild species of rice, Oryza longistaminata, which conferred broad-spectrum resistance to Xoo. Khush showed that a single genetic locus, which he named XA21 (Xanthomonas resistance 21) was responsible for the resistance. Ronald hypothesized that Xa21 encoded a single protein that recognized a conserved microbial determinant.